Method of monitoring subterranean reservoirs

Abstract

The present invention provides a method and system for monitoring a subterranean reservoir formation 16. The method comprises injecting at least one reactive tracer 20 into a well 14, injecting at lea

Description

The present invention relates to monitoring subterranean reservoirs and in particular monitoring characteristics and / or conditions of subterranean reservoirs. Aspects of the invention relates to a system and methods for obtaining information on subterranean reservoir conditions. The invention has particular, although not exclusive, application to hydrocarbon reservoirs and carbon storage formations. Background to the invention A hydrocarbon reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. A production well is drilled into or through the reservoir primarily for producing oil and gas. In an oil and gas system an injection well is drilled into or through the reservoir to inject fluids into the reservoir to primarily maintain reservoir pressure to assist production into the production well or displace oil by other fluids. Understanding conditions and / or characteristics of the reservoir such as fluid saturation levels are important for determining the amount of oil and other hydrocarbons present in the reservoir. The saturation level is a measure of how much pore space is occupied by a trapped phase. A carbon capture storage (CCS) reservoir is a subsurface formation used to store carbon dioxide (CO2) captured from industrial processes, or directly from the atmosphere. Captured CO2 is injected into subsurface formations such as oil, water or gas reservoirs, coal seams or salt dome reservoirs to store it permanently. Understanding conditions and / or characteristics of the CCS reservoir such as fluid saturation levels are important for determining the storage capacity, storage level and storage efficiency of the reservoir. The use of chemical tracers to monitor fluid transport in different type of subsurface reservoir is known including ground water studies, petroleum production and carbon capture applications. It is known to use chemical tracers to provide a limited understanding of oil and gas reservoir fluid flow pathways for example oil companies may use tracers to determine or monitor the connectivity between an injection and a production well in oil and gas reservoirs. Summary of the invention It is amongst the aims and objects of the invention to provide a system and method which obviates or mitigates one or more drawbacks or disadvantages of the prior art subterranean reservoir monitoring systems. There is a need to obtain information on characteristics and / or conditions of subterranean reservoirs and to understand characteristics and / or conditions of fluids in subterranean reservoirs. It is an object of the invention to provide a system and method for reproducibly and accurately determining characteristics and / or conditions of a subsurface formation. It is another object of the invention to provide a system and method for accurately controlling the position and activity of a tracer placed in a formation. It is further object of the invention to provide a system and method for reproducibly and accurately determining a fluid saturation of a hydrocarbon formation. It is an object of the invention to provide a system and method for reproducibly and accurately determining CO2 saturation of a CO2 storage formation. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the invention, there is provided a method of monitoring a subterranean reservoir formation, the method comprising: injecting at least one reactive tracer into a well; shutting in the well; inducing production from the well and sampling produced fluid to measure the presence and concentration of the reactive tracer and / or the at least one reaction product; based on the presence and / or concentration of the reactive tracer and / or the at least one reaction product determining at least one condition and / or characteristic of the subterranean reservoir formation. The reactive tracer may be configured to react to format least one reaction product. The method may comprise injecting at least one accelerating agent into the well. The at least one accelerating agent may be configured to enhance or accelerate the reaction of the at least one reactive tracer to form the at least one reaction product. The accelerating agent and the at least one reactive tracer may be injected into the well at different times. The accelerating agent and the at least one reactive tracer may have different migration rates though the reservoir or formation. The accelerating agent may have a faster migration rate than the at least one reactive tracer. The accelerating agent may be injected after the at least one reactive tracer. The accelerating agent may have a slower migration rate than the at least one reactive tracer. The accelerating agent may be injected before the at least one reactive tracer. The method may comprise determining at least one condition and / or characteristic of the subterranean reservoir formation based on the arrival time of the reactive tracer and / or the arrival time of the at least one reaction product in the samples. The method may comprise determining at least one condition and / or characteristic of the subterranean reservoir formation based on the concentration of first reactive tracer and / or the concentration of the at least one reaction product. The method may comprise injecting an injection fluid after the reactive tracer injection to push the reactive tracer towards or to the target location in the formation. The method may comprise injecting an injection fluid after the reactive tracer injection to push the reactive tracer and / or the accelerating agent towards or to the target location in the formation. The method may comprise injecting an injection fluid after the accelerating agent injection to push the accelerating agent towards or to a target location in the formation. The method may comprise injecting an injection fluid after the accelerating agent injection to push the accelerating agent and / or the reactive tracer towards or to the target location in the formation. The method may comprise injecting the reactive tracer into the well in a first injection medium. The method may comprise injecting the accelerating agent into the well in a second injection medium. The first and second injection mediums may be the same. The first and second injection mediums may be different. The method may comprise locating the reactive tracer in the target location in the formation. The method may comprise locating the accelerating agent in the target location in the formation. The method may comprise shutting in the well with the reactive tracer in the target location in the formation. The method may comprise shutting in the well with the reactive tracer and the accelerating agent in the target location in the formation. The method may comprise shutting in the well to allow reaction of the reactive tracer to generate at least one reaction product. The method may comprise shutting in the well to allow enhanced or accelerated reaction of the reactive tracer in the presence of or in contact with the accelerating agent to generate at least one reaction product. The method may comprise shutting in the well for 1 hour. The method may comprise shutting in the well for up to 1 day. The method may comprise shutting in the well for up to 1 week. The subterranean reservoir may be a hydrocarbon reservoir. The subterranean reservoir may be a carbon dioxide storage reservoir. The subterranean reservoir may be a water reservoir. The subterranean reservoir may be a carbon dioxide / water reservoir. The fluid may be selected from the group comprising oil, water, gas, carbon dioxide, or a mixture of any of oil, water, gas or carbon dioxide. The fluid may be a mixture of water and carbon dioxide. The method may comprise determining or estimating a saturation level for the reservoir. The method may comprise determining or estimating a residual fluid saturation level for the reservoir. The method may comprise determining a saturation level of the reservoir based on the presence and / or concentration of the reactive tracer and / or the at least one reaction product. The method may comprise determining an oil saturation level of the reservoir formation. The method may comprise determining a residual oil saturation level of the reservoir formation. The method may comprise determining a water saturation level of the reservoir formation. The method may comprise determining a gas saturation level of the reservoir formation. The method may comprise determining a carbon dioxide saturation level of the reservoir formation. The method may comprise determining a brine saturation level of the reservoir formation. The method may comprise assessing an effectiveness of the reservoir treatment. The at least one reactive tracer may be configured to react and / or interact under reservoir conditions. The at least one reactive tracer may be configured to react and / or interact to specific reservoir characteristics and / or conditions. The at least one reactive tracer may be configured to hydrolyse, react, degrade or at least partially hydrolyse, react or degrade on exposure to a condition or reactant in the reservoir. The at least one reactive tracer may be configured to hydrolyse, react, degrade or at least partially hydrolyse, react, degrade on contact or in the presence of a well or reservoir fluid. The at least one reactive tracer may be a partitioning tracer. The at least one reactive tracer may have a known oil / water partitioning coefficient. The method may comprise determining a condition and / or characteristic of the subterranean reservoir formation based on the partitioning tracer coefficient and / or the arrival time of the at least one reactive tracer and / or the at least one reaction product. The reactive tracer may be an ester. The reactive tracer may be an ester configured to partition to an oleic phase. The reactive tracer may be an amide. The at least one reactive tracer may be a liquid, solid or gas. The at least one reactive tracer may be configured to generate at least one reaction product. The at least one reaction product may comprise an alcohol. The at least one reaction product may comprise an acid. The at least one reaction product may be detectable in a production flow. The at least one reaction product may be a traceable product. The at least one reaction product may function as a tracer. The at least one reactive tracer may be selected based on the characteristics of the well or reservoir conditions. The accelerating agent may be an acid, base or buffered solution at a designed pH. The accelerating agent may be selected from the group comprising hydrochloric acid, formic acid, hydrofluoric acid, acetic acid, propionic acid, citric acid, benzoic acid, phosphoric acid, dihydrogenphosphate, ascorbic acid, sulphuric acid, sulfonic acid, ammonium salts (e.g. ammonium chloride, ammonium hydroxide, ammonium bisulphite), phoshonate buffers, citric acid buffers, ammonia, sodium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, pyridine, alkylamines, aromatic amines, iron chloride, iron sulphate, iron hydroxide. The accelerating agent may be a catalyst. The accelerating agent may be configured to catalyse the reactive tracer reaction to generate at least one reaction product. The accelerating agent may be a reactant. The accelerating agent may be configured to actively take part in the reaction to generate at least one reaction product. The accelerating agent may be configured to be consumed by the subterrain formation. The accelerating agent may be configured to be consumed by the subterrain formation in a competing reaction. The competing reaction may be a neutralization reaction between a carbonate reservoir and an acid. The competing reaction may be partly neutralized reaction by a naturally occurring buffer system. The method may comprise analysing the at least one sample to measure the presence and / or concentration of the at least one reactive tracer and / or at least one reaction product in the sample. The method may comprise analysing the at least one sample for type and / or concentration of the at least one reactive tracer and / or first reaction products as a function of sampling time. The method may comprise detecting and / or measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the at least one sample in real time. The method may comprise detecting and / or measuring the concentration of the at least one reactive tracer and / or at least one reaction products in the at least one sample using an online analyser. The method may comprise repeating the tracer test at a second or further time. The method may comprise repeating the tracer test at the same target location a second time. The method may comprise repeating the tracer test before and after a reservoir is treated. The method may comprise repeating the tracer test multiple times before and / or after a reservoir is treated. The method may comprise repeating the tracer test at the same target location in the formation. The method may comprise repeating the tracer test with a different tracer. The method may comprise repeating the tracer test with a different tracer having different properties. The method may comprise repeating the test with a different tracer and / or a different accelerating agent. The method may comprise repeating the tracer test with a different tracer having a different partitioning coefficient. The method may comprise repeating the tracer test with a different tracer at the same target location in the formation. The method may comprise injecting a first reactive tracer into the well in a first injection medium in a first test. The method may comprise injecting a second reactive tracer into the well in a second injection medium in a second test. The first and second injection mediums may be the same. The first and second injection mediums may be different. The first injection medium may be a high salinity water. The second injection medium may be a low salinity water. The first injection medium may be a low salinity water. The second injection medium may be a high salinity water. The at least one reactive tracer may be selected based on adjusted conditions or characteristics in a treated well or treated reservoir. The at least one reactive tracer may be selected based on adjusted conditions or characteristics in a treated well or treated reservoir to locate the second reactive tracer at substantially the same location in the formation as the first reactive tracer. The method may comprise selecting at least one reactive tracer having characteristics compatible with the adjusted conditions and / or characteristics in the treated well or treated reservoir to locate the at least one reactive tracer at substantially the same target location in the formation before and after the reservoir treatment. The characteristics of the at least one reactive tracer may be selected from chemical composition, partitioning coefficient and / or reactivity. The method may comprise collecting samples. The method may comprise collecting samples of produced fluid. The method may comprise obtaining produced fluid from at least one production well. The at least one production well may be in fluid communication with the formation. The method may comprise collecting the at least one sample at one or more sampling times. The at least one sample may be collected for later analysis onsite or offsite. The method may comprise preserving the tracer data in the samples. The method may comprise stopping or quenching the reaction in the samples. The method may comprise chemically treating the sample to stop or quench the reaction in the samples. The method may comprise removing a reactant from the samples. The method may comprise chemically treating the samples to remove a reactant. The method may comprise dehydrating the samples. The method may comprise removing water from the samples. The method may comprise freezing the samples. The method may comprise storing the samples. The method may comprise transporting the samples. The samples be measured in real time. Samples may be collected and / or measured downstream of a production influx zone at known sampling times. The method may comprise collecting at least one sample at a pre-determined time sequence or pre-determined profile. The method may comprise adjusting the sample volume and / or sampling time. The sampling sequence, duration and / or frequency may be modified during the sampling operation. The sampling may be achieved by a sampling device or probe arranged in the flow of produced fluid and / or injection fluid. The sampling device or probe may be located downhole or at surface. The sampling may be conducted at the one or more of said sampling times. The at least one tracer may be detected by a detection device such a sensor. The detection device may facilitate real time monitoring and / or analysis of the tracer in the flow of produced fluid and / or injection fluid. The real time monitoring and / or analysis may be achieved by a detector device or probe. The detector device or probe may be arranged in the flow of produced fluid and / or injection fluid. The detector device or probe may be located downhole or at surface. The method may comprise controlling and / or optimising the time of tracer injection, rate of tracer injection, duration of tracer injection, time of injection fluid injection, rate of injection fluid injection and / or duration of injection fluid injection to locate the reactive tracer in a target location in the formation. The method may comprise controlling and / or optimising the time of accelerating agent injection, rate of accelerating agent injection, duration of the accelerating agent injection, time of injection fluid injection, rate of injection fluid injection, and / or duration of injection fluid injection to locate the accelerating agent in the target location in the formation. The method may comprise controlling and / or optimising the tracer injection, accelerating agent and injection fluid injections such that the tracer and accelerating agent are both present in the target location at the same time. The injection fluid may be a liquid or a gas. The injection fluid may be water, wastewater, brine (salt water), or water mixed with chemicals. The injection fluid may be carbon dioxide, nitrogen and / or hydrocarbon gas. The at least one tracer may be injected into the well via an injection device. The at least one accelerating agent may be injected into the well via an injection device. The injection device may be permanently installed in a well or injection site. The method may comprise adjusting and / or controlling the duration and / or frequency of the injection of tracer into the well. The reactive tracer may be a liquid, solid or gas. According to a second aspect of the invention, there is provided a method for monitoring a subterranean reservoir formation, the method comprising: injecting at least one reactive tracer into a well; injecting at least one accelerating agent into the well; shutting in the well; inducing production from the well and sampling produced fluid; measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples; based on the presence and / or concentration of the reactive tracer and / or the at least one reaction product determining at least one condition and / or characteristic of the subterranean reservoir formation. The method may comprise determining at least one condition and / or characteristic of the subterranean reservoir formation based on the arrival time of the reactive tracer and / or the arrival time of the at least one reaction product in the samples. The method may comprise determining at least one condition and / or characteristic of the subterranean reservoir formation based on the concentration of reactive tracer and / or the concentration of the at least one reaction product. The method may comprise injecting an injection fluid (push fluid) after the at least one reactive tracer injection to push the at least one reactive tracer into the formation. The method may comprise injecting an injection fluid (push fluid) after the injecting the at least one accelerating agent to push the at least one accelerating agent into the formation. The method may comprise injecting an injection fluid (push fluid) after the injecting the at least one accelerating agent and / or at least one reactive tracer to push the at least one accelerating agent and / or at least one reactive tracer into the formation. The method may comprise injecting an injection fluid (push fluid) to locate the accelerating agent and the at least one reactive tracer at a target location in the formation. The method may comprise injecting an injection fluid (push fluid) to locate the accelerating agent and the at least one reactive tracer at a target location in the formation such that they occupy the target location in the formation at substantially the same time. The method may comprise shutting in the well after the reactive tracer and the accelerating agent have reached the target location in the formation. The method may comprise shutting in the well after the reactive tracer and the accelerating agent have reached the target location to allow reaction of the reactive tracer to generate at least one reaction product. The method may comprise shutting in the well for 1 hour. The method may comprise shutting in the well for up to 1 day. The method may comprise shutting in the well for up to 1 week. The method may comprise injecting the reactive tracer into a wellbore before injecting an accelerating agent into the wellbore. The method may comprise injecting the accelerating agent into the wellbore before injecting the reactive tracer. The subterranean reservoir may be a hydrocarbon reservoir. The subterranean reservoir may be a carbon dioxide storage reservoir. The subterranean reservoir may be a water reservoir. The subterranean reservoir may be a carbon dioxide / water reservoir. The fluid may be selected from the group comprising oil, water, gas, carbon dioxide, or a mixture of any of oil, water, gas or carbon dioxide. The fluid may be a mixture of water and carbon dioxide. The method may comprise determining or estimating a saturation level for the reservoir. The method may comprise determining or estimating a residual fluid saturation level for the reservoir. The method may comprise determining a saturation level of the reservoir based on the presence and / or concentration of the reactive tracer and / or the at least one reaction product. The method may comprise determining an oil saturation level of the reservoir formation. The method may comprise determining a residual oil saturation level of the reservoir formation. The method may comprise determining a water saturation level of the reservoir formation. The method may comprise determining a gas saturation level of the reservoir formation. The method may comprise determining a carbon dioxide saturation level of the reservoir formation. The method may comprise determining a brine saturation level of the reservoir formation. The reactive tracer may be configured to react and / or interact under reservoir conditions. The reactive tracer may be configured to react and / or interact to specific reservoir characteristics and / or conditions. The reactive tracer may be configured to hydrolyse, react, degrade or at least partially hydrolyse, react or degrade on exposure to a condition or reactant in the reservoir. The reactive tracer may be configured to hydrolyse, react, degrade or at least partially hydrolyse, react, degrade on contact or in the presence of a well fluid. The at least one reactive tracer may be a partitioning tracer. The at least one reactive tracer may have a known oil / water partitioning coefficient. The at least one reactive tracer may be an ester. The at least one reactive tracer may be an ester configured to partition to an oleic phase. The at least one reactive tracer may be an amide. The method may comprise analysing the arrival time of the at least one reactive tracer and / or the at least one reaction product in the samples. The method may comprise determining a condition or characteristic of the subterranean reservoir formation based on the partitioning tracer coefficient and the arrival times of the at least one reactive tracer and / or the at least one reaction product. The at least one reactive tracer may be configured to generate at least one reaction product. The at least one reaction product may comprise an alcohol. The at least one reaction product may comprise an acid. The at least one reaction product may be detectable in samples collected from the production flow. The at least one reaction product may be a traceable product. The at least one reaction product may function as a tracer. The at least one accelerating agent may be an acid, base or buffered solution at a designed pH. The at least one accelerating agent may be selected from the group comprising hydrochloric acid, formic acid, hydrofluoric acid, acetic acid, propionic acid, citric acid, benzoic acid, phosphoric acid, dihydrogenphosphate, ascorbic acid, sulphuric acid, sulfonic acid, ammonium salts (e.g. ammonium chloride, ammonium hydroxide, ammonium bisulphite), phoshonate buffers, citric acid buffers, ammonia, sodium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, pyridine, alkylamines, aromatic amines, iron chloride, iron sulphate, iron hydroxide. The at least one accelerating agent may be a catalyst. The at least one accelerating agent may be configured to catalyse the reactive tracer reaction to generate at least one reaction product. The at least one accelerating agent may be a reactant. The at least one accelerating agent may be configured to actively take part in the reaction to generate at least one reaction product. The at least one accelerating agent may be configured to be consumed by the subterrain formation. The accelerating agent may be configured to be consumed by the subterrain formation in a competing reaction. The competing reaction may be a neutralization reaction between a carbonate reservoir and an acid. The competing reaction may be partly neutralized reaction by a naturally occurring buffer system. The at least one reactive tracer and the accelerating agent may have different migration rates or retention times through the formation. The at least one reactive tracer and at least one accelerating agent may be configured, selected or designed to have different migration rates or retention times through the formation. The method may comprise controlling the injection order, injection time and / or injection flow rate of the reactive tracer, accelerating agent and / or injection fluid (push fluid) so that the at least one reactive tracer and at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation overlap or align at the target location in the formation. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation only overlap or align at the target location in the formation. The method may comprise controlling the injection order, injection time and / or injection flow rate of the at least one reactive tracer and at least one accelerating agent so that they are exposed to or contact each other only at the target location in the formation. The at least one reactive tracer may have a slower migration rate through the formation than the at least one accelerating agent. The method may comprise injecting the at least one reactive tracer before injecting the at least one accelerating agent such that the at least one reactive tracer and the at least one accelerating agent are present at the target location in the formation at substantially the same time. The at least one reactive tracer may have a faster migration rate than the at least one accelerating agent. The method may comprise injecting the at least one accelerating agent before injecting the at least one reactive tracer such that the at least one reactive tracer and the at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise analysing the at least one sample to measure the presence and / or concentration of the reactive tracer and / or reaction products in the sample. The method may comprise analysing the at least one sample for type and / or concentration of the at least one reactive tracer and / or reaction products as a function of sampling time. The method may comprise detecting and / or measuring the concentration of the at least one reactive tracer and / or the at least one reaction products in the at least one sample in real time. The method may comprise detecting and / or measuring the concentration of the at least one reactive tracer and / or the at least one reaction products in the at least one sample using an online analyser. The method may comprise selecting at least one reactive tracer having characteristics suitable to locate the at least one reactive tracer at substantially the same time at the target location in the formation as the at least one accelerating agent. The method may comprise selecting at least one accelerating agent having characteristics suitable to locate the at least one accelerating agent at substantially the same time at the target location in the formation as the at least one reactive tracer. The at least one reactive tracer may have a higher affinity for various phases in the reservoir in comparison to the at least one accelerating agent which may slow the migration of the at least one reactive tracer in comparison to the at least one accelerating agent. The at least one reactive tracer may have a lower affinity for various phases in the reservoir in comparison to the at least one the accelerating agent which may slow the migration of the at least one reactive tracer in comparison to the at least one accelerating agent. The characteristics of the reactive tracer may be selected from chemical composition, partitioning coefficient and / or reactivity. The characteristics of the accelerating agent may be selected from chemical composition, partitioning coefficient and / or reactivity. The reactive tracer may be configured, selected or designed to have a lower reactivity in the absence of the accelerating agent than in the presence of the accelerating agent. The accelerating agent configured, selected or designed not to partition in oil. The injection fluid may be a liquid or a gas. The injection fluid may be water, wastewater, brine (saltwater), or water mixed with chemicals. The injection fluid may be carbon dioxide, nitrogen and / or hydrocarbon gas. The at least one tracer may be released or injected into the injection fluid via a tracer injection device. The tracer injection device may be permanently installed in a well or injection site. The method may comprise adjusting and / or controlling the duration and / or frequency of the injection or release of tracer into the injection fluid. The method may comprise controlling and / or adjusting the release of tracer into at least one injection fluid for a desired duration and / or frequency. The first reactive tracers may be a liquid, solid or gas. The second reactive tracers may be a liquid, solid or gas. The method may comprise obtaining produced fluid from at least one production well. The at least one production well may be in fluid communication with the formation. The method may comprise collecting the at least one sample at one or more sampling times. The at least one sample may be collected for later analysis onsite or offsite. The method may comprise preserving the tracer data in the samples. The method may comprise stopping or quenching the reaction in the samples. The method may comprise chemically treating the sample to stop or quench the reaction in the samples. The method may comprise removing a reactant from the samples. The method may comprise chemically treating the samples to remove a reactant. The method may comprise dehydrating the samples. The method may comprise removing water from the samples. The method may comprise freezing the samples. The method may comprise storing the samples. The method may comprise transporting the samples. The samples may be measured in real time. Samples may be collected and / or measured downstream of a production influx zone at known sampling times. The method may comprise collecting at least one sample at a predetermined time sequence or pre-determined profile. The method may comprise adjusting the sample volume and / or sampling time. The sampling sequence, duration and / or frequency may be modified during the sampling operation. The sampling may be achieved by a sampling device or probe arranged in the flow of produced fluid and / or injection fluid. The sampling device or probe may be located downhole or at surface. The sampling may be conducted at the one or more of said sampling times. The at least one tracer may be detected by a detection device such a sensor. The detection device may facilitate real time monitoring and / or analysis of the tracer in the flow of produced fluid and / or injection fluid. The real time monitoring and / or analysis may be achieved by a detector device or probe. The detector device or probe may be arranged in the flow of produced fluid and / or injection fluid. The detector device or probe may be located downhole or at surface. Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa. According to a third aspect of the invention, there is provided a method for monitoring a subterranean reservoir formation, the method comprising: injecting at least one accelerating agent into the well; subsequently injecting at least one reactive tracer into a well; shutting in the well; inducing production from the well and sampling produced fluid; measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples; based on the presence and / or concentration of the at least one reactive tracer and / or the least one reaction product determining at least one condition or characteristic of the subterranean reservoir formation. The method may comprise injecting an injection fluid (push fluid) after injecting the accelerating agent to push the accelerating agent into the formation. The method may comprise injecting an injection fluid after the accelerating agent injection to push the accelerating agent towards or to the target location in the formation. The method may comprise injecting an injection fluid (push fluid) after the reactive tracer injection to push the reactive tracer and / or the at least one accelerating agent into the formation. The method may comprise injecting an injection fluid after the reactive tracer injection to push the at least one reactive tracer towards or to the target location in the formation. The method may comprise injecting an injection fluid (push fluid) to push the reactive tracer and / or the at least accelerating agent towards or to the target location in the formation. The at least one reactive tracer may have a faster migration rate than the at least one accelerating agent. The method may comprise injecting the at least one accelerating agent before injecting the at least one reactive tracer such that the at least one reactive tracer and the at least one accelerating agent are present at a target location in the formation at substantially the same time. Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or their embodiments, or vice versa. According to a fourth aspect of the invention, there is provided a method for monitoring a subterranean reservoir formation, the method comprising: injecting at least one reactive tracer into a well; subsequently injecting at least one accelerating agent into the well; shutting in the well; inducing production from the well and sampling produced fluid; measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples; based on the presence and / or concentration of the at least one reactive tracer and / or the least one reaction product determining at least one condition or characteristic of the subterranean reservoir formation. The method may comprise injecting an injection fluid (push fluid) after the injecting the accelerating agent to push the accelerating agent into the formation. The method may comprise injecting an injection fluid after the accelerating agent injection to push the accelerating agent towards or to the target location in the formation. The method may comprise injecting an injection fluid (push fluid) after the reactive tracer injection to push the reactive tracer and / or the at least one accelerating agent into the formation. The method may comprise injecting an injection fluid after the reactive tracer injection to push the at least one reactive tracer towards or to the target location in the formation. The method may comprise injecting an injection fluid (push fluid) to push the reactive tracer and / or the at least accelerating agent towards or to the target location in the formation. The at least one reactive tracer may have a slower migration rate through the formation than the at least one accelerating agent. The method may comprise injecting the at least one reactive tracer before injecting the at least one accelerating agent such that the at least one reactive tracer and the at least one accelerating agent are present at the target location in the formation at substantially the same time. Embodiments of the fourth aspect of the invention may include one or more features of the first to third aspects of the invention or their embodiments, or vice versa. According to a fifth aspect of the invention, there is provided a method for monitoring a subterranean reservoir formation, the method comprising: injecting at least one reactive tracer into a well at a first time; injecting at least one accelerating agent into the well at a second time; wherein the at least one accelerating agent and at least one reactive tracer have different migration rates into and / or through the formation; shutting in the well; inducing production in the well and sampling produced fluid; based on the presence and / or concentration of the at least one reactive tracer and / or the least one reaction product determining at least one condition or characteristic of the subterranean reservoir formation. The at least one reactive tracer may have a slower migration rate through the formation than the at least one accelerating agent. The method may comprise injecting the at least one reactive tracer before injecting the at least one accelerating agent such that the at least one reactive tracer and the at least one accelerating agent are present at the target location in the formation at substantially the same time. The at least one reactive tracer may have a faster migration rate than the at least one accelerating agent. The method may comprise injecting the at least one accelerating agent before injecting the at least one reactive tracer such that the at least one reactive tracer and the at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise controlling the injection order, injection time and / or injection flow rate of the reactive tracer, accelerating agent and / or injection fluid (push fluid) so that the at least one reactive tracer and at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation overlap or align at the target location in the formation. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation only overlap or align at the target location in the formation. The method may comprise controlling the injection order, injection time and / or injection flow rate of the at least one reactive tracer and at least one accelerating agent so that they are exposed to or contact each other only at the target location in the formation. The method may comprise shutting in the well after the reactive tracer and the accelerating agent have reached the target location in the formation. The method may comprise shutting in the well after the reactive tracer and the accelerating agent have reached the target location to allow reaction of the reactive tracer to generate at least one reaction product in the presence of the accelerating agent. The method may comprise shutting in the well for 1 hour. The method may comprise shutting in the well for up to 1 day. The method may comprise shutting in the well for up to 1 week. The first time may be before the second time. The second time may be before the first time. Embodiments of the fifth aspect of the invention may include one or more features of the first to fourth aspects of the invention or their embodiments, or vice versa. According to a sixth aspect of the invention, there is provided a method for monitoring a subterranean reservoir formation, the method comprising: injecting at least one reactive tracer and at least one accelerating agent at different times into a well to locate the at least one reactive tracer and the at least one accelerating agent at a target location in the formation; shutting in the well; inducing production in the well and sampling produced fluid; measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples; based on the presence and / or concentration of the at least one reactive tracer and / or the least one reaction product determining at least one condition or characteristic of the subterranean reservoir formation. The at least one reactive tracer may have a slower migration rate through the formation than the at least one accelerating agent. The method may comprise injecting the at least one reactive tracer before injecting the at least one accelerating agent such that the at least one reactive tracer and the at least one accelerating agent are present at the target location in the formation at substantially the same time. The at least one reactive tracer may have a faster migration rate than the at least one accelerating agent. The method may comprise injecting the at least one accelerating agent before injecting the at least one reactive tracer such that the at least one reactive tracer and the at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise controlling the injection order, injection time and / or injection flow rate of the reactive tracer, accelerating agent and / or injection fluid (push fluid) so that the at least one reactive tracer and at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation overlap or align at the target location in the formation. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation only overlap or align at the target location in the formation. The method may comprise controlling the injection order, injection time and / or injection flow rate of the at least one reactive tracer and at least one accelerating agent so that they are exposed to or contact each other only at the target location in the formation. The method may comprise shutting in the well after the reactive tracer and the accelerating agent have reached the target location in the formation. The method may comprise shutting in the well after the reactive tracer and the accelerating agent have reached the target location to allow reaction of the reactive tracer to generate at least one reaction product in the presence of the accelerating agent. The method may comprise shutting in the well for 1 hour. The method may comprise shutting in the well for up to 1 day. The method may comprise shutting in the well for up to 1 week. Embodiments of the sixth aspect of the invention may include one or more features of the first to fifth aspects of the invention or their embodiments, or vice versa. According to a seventh aspect of the invention, there is provided a method for measuring fluid saturation of a subterranean reservoir, the method comprising: injecting at least one reactive tracer into a well; injecting at least one accelerating agent into the well; injecting fluid into the wellbore to locate the at least one reactive tracer and the at least one accelerating agent at a target location in the formation; shutting in the well; inducing production from the well and sampling produced fluid; measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples; based on the presence and / or concentration of the at least one reactive tracer and / or the least one reaction product determining or estimating fluid saturation of a subterranean reservoir. The method may comprise determining or estimating fluid saturation of a subterranean reservoir based on the arrival time of the reactive tracer and / or the arrival time of the reaction product. The method may comprise determining or estimating fluid saturation of the subterranean reservoir formation based on the concentration of reactive tracer and / or the concentration of the reaction product in the samples. The subterranean reservoir may be a hydrocarbon reservoir. The subterranean reservoir may be a carbon dioxide storage reservoir. The subterranean reservoir may be a water reservoir. The subterranean reservoir may be a carbon dioxide / water reservoir. The fluid may be selected from the group comprising oil, water, gas, carbon dioxide, or a mixture of any of oil, water, gas or carbon dioxide. The fluid may be a mixture of water and carbon dioxide. The method may comprise controlling the injection order, injection time and / or injection flow rate of the reactive tracer, accelerating agent and / or injection fluid (push fluid) so that the at least one reactive tracer and at least one accelerating agent are present at a target location in the formation at substantially the same time. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation overlap or align at the target location in the formation. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation only overlap or align at the target location in the formation. The method may comprise controlling the injection order, injection time and / or injection flow rate of the at least one reactive tracer and at least one accelerating agent so that they are exposed to or contact each other only at the target location in the formation. Embodiments of the seventh aspect of the invention may include one or more features of the first to sixth aspects of the invention or their embodiments, or vice versa. According to an eighth aspect of the invention, there is provided a system for monitoring a reservoir, the system comprising: an injection device configured to inject at least one reactive tracer into the wellbore; an injection device configured to inject at least one accelerating agent into the wellbore; and a collection device configured to collect samples of fluid produced from the reservoir. The injection device for injecting the at least one reactive tracer and the injection device for injecting at least one accelerating agent may be the same injection device. The injection device for injecting the at least one reactive tracer, the injection device for injecting at least one accelerating agent and the injection device for injecting injection fluid may be the same injection device. The injection device may be configured to inject the one reactive tracer into the wellbore at a first time. The injection device may be configured to inject the at least one accelerating agent into the wellbore at a second time. The first time may be before the second time. The first time may be after the second time. The injection device may be configured to inject an injection fluid (push fluid) into the well after the injecting the accelerating agent to push the accelerating agent into the formation. The injection device may be configured to inject an injection fluid (push fluid) into the well after the accelerating agent injection to push the accelerating agent towards or to the target location in the formation. The injection device may be configured to inject an injection fluid (push fluid) into the well after the reactive tracer injection to push the reactive tracer and / or the at least one accelerating agent into the formation. The injection device may be configured to inject an injection fluid (push fluid) into the well after the reactive tracer injection to push the reactive tracer towards or to the target location in the formation. The injection device may be configured to inject an injection fluid (push fluid) into the well to push the reactive tracer and / or the at least accelerating agent towards or to the target location in the formation. The system may comprise at least one analyser device configured to detect the presence of the at least one reactive tracer in fluid produced from the reservoir. The system may comprise at least one analyser device configured to detect the presence of the at least one reaction product in fluid produced from the reservoir. The system may comprise at least one analyser device configured to detect the concentration of the at least one reactive tracer in fluid produced from the reservoir. The system may comprise at least one analyser device configured to detect the concentration of the at least one reaction product in fluid produced from the reservoir. The system may comprise at least one probe. The at least one probe may be configured to detect the presence and / or concentration of the at least one tracer in fluid produced from the reservoir. The at least one probe may be a sample collection probe, a detector probe and / or a real time detector probe. The system may comprise a processor. The processor may be a computer processor. The processor may be configured to analyse concentration data of reactive tracer and / or the at least one reaction product in the samples. The processor may be configured to analyse the arrival time of the reactive tracer and / or the at least one reaction product in the samples. The processor may be configured to determine a condition and / or characteristic of the subterranean reservoir formation based on the partitioning tracer coefficient and / or concentration of the reactive tracer and / or the at least one reaction product. The processor may be configured to determine a condition and / or characteristic of the subterranean reservoir formation based on the partitioning tracer coefficient and / or the arrival times of the reactive tracer and / or the at least one reaction product. The processor may be configured to control the injection order, injection time and / or injection flow rate of the reactive tracer, accelerating agent and / or injection fluid (push fluid) so that the reactive tracer and accelerating agent are present at a target location in the formation at substantially the same time. The processor may be configured to control the injection migrations of the reactive tracer and the accelerating agent so that their respective migrations through the formation overlap or align at the target location in the formation. The processor may be configured to control the injection migrations of the reactive tracer and the accelerating agent so that their respective migrations through the formation only overlap or align at the target location in the formation. The processor may be configured to control the injection order, injection time and / or injection flow rate of the reactive tracer and accelerating agent so that they are exposed to or contact each other only at the target location in the formation. Embodiments of the eighth aspect of the invention may include one or more features of the first to seventh aspects of the invention or their embodiments, or vice versa. According to a ninth aspect of the invention, there is provided a method for monitoring a subterranean formation, the method comprising: analysing tracer data from at least one sample previously collected from a fluid produced from a subterranean formation; wherein the subterranean formation comprises a reactive tracer and an accelerating agent injected into the formation; based on the tracer data monitoring at least one characteristic and / or condition of the subterranean formation. The subterranean formation may comprise a reactive tracer and an accelerating agent injected into the formation at different times. The method may comprise measuring a concentration of the reactive tracer in the at least one sample. The method may comprise measuring concentration of at least one reaction product in the at least one sample. The method may comprise analysing arrival times of the reactive tracer and / or the at least one reaction product. Embodiments of the ninth aspect of the invention may include one or more features of the first to eighth aspects of the invention or their embodiments, or vice versa. According to a tenth aspect of the invention, there is provided a method of collecting samples for analysis in monitoring a subterranean reservoir formation, wherein the subterranean formation comprises a reactive tracer and an accelerating agent injected into the formation at different times; producing fluid from the reservoir, collecting at least one sample from the produced fluid. The at least one reactive tracer may be configured to react or interact under reservoir conditions. The reactivity of the at least one reactive tracer may be configured to increase or accelerate in the presence of the accelerating agent. The method may comprise preserving the tracer data in the samples. The method may comprise stopping or quenching the reaction in the samples. The method may comprise chemically treating the sample to stop or quench the reaction in the samples. The method may comprise removing a reactant from the samples. The method may comprise chemically treating the samples to remove a reactant. The method may comprise dehydrating the samples. The method may comprise removing water from the samples. The method may comprise freezing the samples. The method may comprise storing the samples. The method may comprise transporting the samples. Embodiments of the tenth aspect of the invention may include one or more features of the first to ninth aspects of the invention or their embodiments, or vice versa. According to an eleventh aspect of the invention, there is provided a method of storing and / or transporting collected samples for later analysis in monitoring a subterranean reservoir formation, wherein the formation comprises a reactive tracer and an accelerating agent injected into the formation at different times; the method comprising; quenching a reaction in the at least one sample; storing and / or transporting the at least one sample. The method may comprise preserving the tracer data in the samples. The method may comprise chemically treating the sample to stop or quench the reaction in the samples. The method may comprise removing a reactant from the samples. The method may comprise chemically treating the samples to remove a reactant. The method may comprise dehydrating the samples. The method may comprise removing water from the samples. The method may comprise freezing the samples. The method may comprise storing the samples. The method may comprise transporting the samples. The method may comprise transporting the samples to a laboratory for analysis. Embodiments of the eleventh aspect of the invention may include one or more features of the first to tenth aspects of the invention or their embodiments, or vice versa. According to a twelfth aspect of the invention, there is provided a method for monitoring a subterranean reservoir formation, the method comprising: injecting at least one reactive tracer into a well; injecting at least one accelerating agent into the well; shutting in the well; inducing production from the well and sampling produced fluid; measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples; based on the presence and / or concentration of the reactive tracer and / or the at least one reaction product determining at least one condition and / or characteristic of the subterranean reservoir formation. The at least one accelerating agent may be configured to enhance or accelerate the reaction of the at least one reactive tracer to form the at least one reaction product. The at least one accelerating agent and the at least one reactive tracer may be injected into the well at different times. The at least one accelerating agent and the at least one reactive tracer may have different migration rates though the reservoir or formation. The at least one accelerating agent may be injected before and / or after the at least one reactive tracer. The method may comprise determining at least one condition and / or characteristic of the subterranean reservoir formation based on the arrival time of the reactive tracer, partitioning tracer coefficient and / or the arrival time of the least one reactive tracer and / or the at least one reaction product. The method may comprise injecting an injection fluid after the reactive tracer injection and / or the accelerating agent injection to push the at least one reactive tracer and / or at least one accelerating agent towards or to a target location in the formation. The method may comprise shutting in the well with the at least one reactive tracer and the at least one accelerating agent in a target location in the formation. The subterranean reservoir may be selected from the group comprising hydrocarbon reservoir, carbon dioxide storage reservoir, and / or a water reservoir. The injection fluid may be selected from the group comprising oil, water, gas, carbon dioxide, nitrogen, hydrocarbon gas, waste water, brine and / or water mixed with chemicals. The method may comprise determining or estimating a saturation level for the reservoir, a residual fluid saturation level for the reservoir, an oil saturation level of the reservoir formation, a residual oil saturation level of the reservoir formation, a water saturation level of the reservoir formation, a gas saturation level of the reservoir formation, a carbon dioxide saturation level of the reservoir formation, a brine saturation level of the reservoir formation and / or an effectiveness of the reservoir treatment. The reactive tracer may be an ester or an amide. The at least one reaction product may be selected from the groups comprising an alcohol, or an acid. The at least one accelerating agent may be selected from the group comprising a catalyst, a reactant, hydrochloric acid, formic acid, hydrofluoric acid, acetic acid, propionic acid, citric acid, benzoic acid, phosphoric acid, dihydrogenphosphate, ascorbic acid, sulphuric acid, sulfonic acid, ammonium salts (e.g. ammonium chloride, ammonium hydroxide, ammonium bisulphite), phoshonate buffers, citric acid buffers, ammonia, sodium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, pyridine, alkylamines, aromatic amines, iron chloride, iron sulphate and / or iron hydroxide. The method may comprise repeating the tracer test before and / or after a reservoir is treated. The method may comprise repeating the test with a different tracer and / or a different accelerating agent. The method may comprise preserving the tracer data in the samples by stopping or quenching a reaction in the samples, removing a reactant from the samples, dehydrating the samples and / or freezing the samples. The method may comprise controlling and / or optimising the time of tracer injection, accelerating agent injection, rate of tracer injection, rate of accelerating agent injection duration of tracer injection, duration of the accelerating agent injection, time of injection fluid injection, rate of injection fluid injection and / or duration of injection fluid injection to locate the reactive tracer and / or the accelerating agent in a target location in the formation. The method may comprise controlling the migration of the at least one reactive tracer and the at least one accelerating agent so that their respective migration through the formation substantially overlap or substantially align at the target location in the formation. The method may comprise controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation substantially overlap or substantially align only at the target location in the formation. The method may comprise controlling the injection order, injection time and / or injection flow rate of the at least one reactive tracer and at least one accelerating agent so that they are substantially exposed to each other or substantially contact each other only at the target location in the formation. Embodiments of the twelfth aspect of the invention may include one or more features of the first to eleventh aspects of the invention or their embodiments, or vice versa. According to a thirteenth aspect of the invention, there is provided a system for monitoring a reservoir, the system comprising: at least one injection device configured to inject at least one reactive tracer and at least one accelerating agent into the wellbore; and a collection device configured to collect samples of fluid produced from the reservoir. The system may comprise two or more injection device wherein at least one injection device is configured to inject at least one reactive tracer into the wellbore and at least one different injection device is configured to inject at least one accelerating agent into the wellbore. The system may comprise at least one injection device is configured to inject the one reactive tracer into the wellbore at a first time and the at least one accelerating agent into the wellbore at a second time. Embodiments of the thirteenth aspect of the invention may include one or more features of the first to twelfth aspects of the invention or their embodiments, or vice versa. Brief description of the drawings There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which: Figure 1A is a simplified top sectional view of a reservoir formation showing migration of an injected tracer from the wellbore to a target location in the formation; Figure 1B and 1C are simplified side sectional views of a reservoir formation showing injection of tracer into the formation and production of fluids from the formation respectively in accordance with an aspect of the invention; Figure 2A to 2E are simplified side sectional views of a reservoir formation showing different stages of tracer injections and production in accordance with an aspect of the invention; Figure 3A to 3F are simplified side sectional views of a reservoir formation showing different stages of controlled reaction of a partitioning tracer in a target location of a formation in accordance with an aspect of the invention; and Figure 4 is a graph of hydrolysis rate as function of pH for Ethyl acetate at a temperature of 75 C. Detailed description of preferred embodiments Figure 1A, 1B and 1C are top and cross-sectional diagrams of a well showing the migration and reaction of a partitioning tracer in a formation. Figures 1A, 1B and 1C show a reservoir section 10, having a wellbore 14 in the formation 16. As best shown in Figure 1B a reactive partitioning tracer 20 is injected into the wellbore. After the tracer is injected additional fluid 22 is injected into the wellbore to push the tracer 20 into the formation 16 away from the well bore 14 to a target location (test zone) 15 in the formation. The target location in this example is defined as between radius Rmaxand Rminfrorn the wellbore centre as shown by arrows in Figure 1 A. In this example the reactive tracer 20 is used to determine oil saturation at the target location (test zone) 15 in the formation. In this example the tracer 20 is an ester (e.g. Ethyl Acetate) which hydrolyses or partitions in the presence of an oleic phase (oil) to a form a reaction product 17 which in this example is an alcohol. When the tracer is located in the target location 15 the well is then shut-in or closed, and the tracer 20 is left to react in-situ with oil present in the formation to form a reaction product (alcohol). Production fluid is then produced from the formation into the well as best shown by arrows 25 in Figure 1C. Samples of produced fluids are collected and concentrations of the unreacted reactive tracer (ester) and the reaction product (alcohol) are measured and recorded as function of produced volume and / or time. The transport time for reactive tracer and the reaction product may also be analysed. As shown in Figure 1C, the arrival time of the reactive tracer 20 in the produced fluid is delayed compared with the arrival time of the reaction product 17 because the reactive tracer has affinity to stagnant oil in the formation slowing its migration to the wellbore during production. In contrast the reaction product 17 has no (or low) affinity to the stagnant oil so the reaction product 17 arrives at the wellbore first. Analysis of the production of unreacted tracer (ester) and reaction products (alcohol) may be used to establish tracer production curves to find the oil saturation. Symbolically we can write such reactions as RrC00R2 + H2O R20H + R^OOH (1) For the specific example of ethyl acetate, the relevant reaction is ch2cooch2ch2 + h2o CH2CH2OH + CH2C00H (2) i.e. ethyl acetate (CH2COOCH2CH2) hydrolyses (reacts with water) and form ethanol (CH2CH2OH) and acetic acid (CH2C00H). The ratio of unreacted tracer to reaction product may also be used to identify EOR potential as well as to evaluate the effect of EOR in numerous applications. Additionally, or alternative the ratio of unreacted reactive tracer to reaction product may be used to investigate conditions of the reservoir such as low-salinity water flooding on residual oil saturation. A range of compounds and reactions may be used as a basis for the tracer. In one example esters which partition to an oleic phase may be used. Esters that hydrolyses during a shut-in period, producing alcohol as a secondary tracer may be used. Alternatively, the reaction product used as reaction product (secondary tracer) may be an acid rather than an alcohol. Figure 2A to 2E are cross-sectional diagrams of a reservoir 100 showing the different stages of a method of assessing the effectiveness of a reservoir treatment which in this example is an Enhanced Oil Recovery (EOR) treatment. The EOR treatment chemical may be any chemical designed to change the well chemistry and / or the mobility of the displacing fluid. The injected chemicals may be designed to adjust the viscosity, wettability, salinity, pH and / or tension of the displaced fluid and / or well conditions. In this example the purpose of the EOR operation is to reduce oil saturation by injecting low salinity water in a reservoir with a highly saline water (salinity of 20-25%). Figures 2A, 2B and 2C show a reservoir 100, having a wellbore 114 in the formation 116. As best shown in Figure 2A a first reactive partitioning tracer 120 having a first partitioning coefficient for oil is injected into the wellbore. In this example the first tracer is methyl acetate (MeAc) which has a partitioning coefficient (K~3) which is suitable for the high salinity of the formation 116. After the first reactive tracer 120 is injected additional fluid 122 is injected into the wellbore to push the tracer 120 into the formation 116 away from the well bore 114 to a target location 115 (test zone) at a desired location in the formation. In this example the first reactive tracer 120 is used to determine oil saturation at the target location 115 (test zone) in the formation. In this example the first reactive tracer is an ester which hydrolyses or partitions in the presence of an oleic phase (oil) to a form a reaction product 117 which in this example is an alcohol. When the first tracer 120 is located at the target location the well is then shut-in or closed, and the first tracer 120 is left to react in-situ with oil present in the formation to form a reaction product (alcohol). Production fluid is produced as best shown by arrows 125 in Figure 2B. Samples of produced fluid are collected and the concentrations of the unreacted reactive tracer (ester) and the reaction product (alcohol) are measured and recorded as function of produced volume and / or time. As shown in Figure 2C an EOR operation or treatment is performed by injecting an EOR chemical 140 into the well. It will be appreciated that the EOR treatment may be performed or injected into a different well to treat the reservoir. In order to effectively test and evaluate the effect of the EOR operation or treatment it is important to test a similar reservoir volume by locating a tracer at the same target location (test zone) in the formation as the first test. By locating a tracer in the same target location (test zone) an accurate comparison may be made. This way the only factor which has changed is the change of reservoir conditions due to the EOR operation or treatment. The final location of the tracer in the formation is dependent on the partitioning coefficient of the tracer. The partitioning coefficient is dependent on the properties of the formation such as temperature and salinity of the formation which may have changed due to the EOR operation. As shown in Figure 2D, a second tracer 150 is selected and injected into the well. The second partitioning tracer is selected based on the optimal partitioning coefficient for the conditions of the treated formation 116a. In this example, as the EOR process is designed to reduce the salinity environment of the reservoir, a suitable the second tracer is used which is Ethyl Acetate (EtAc) with a partitioning coefficient in the order of 10-15 which is suitable for a low salinity environment test. This will facilitate the second tracer being located in the same target location 115 in the formation. The second reactive partitioning tracer 150 having a second partitioning coefficient for oil is injected into the wellbore. After the second tracer is injected additional fluid 122 is injected in the well to push the second tracer 150 to the same target location (test zone) 115. The volume of the second tracer and the push-volume used to displace the tracer into the formation is closely controlled the inject the second tracer into the target location (test zone). In this example the second tracer 150 is an ester which hydrolyses or partitions in the presence of an oleic phase (oil) to a form a reaction product 152 which in this example is an alcohol. The well is then shut-in or closed again, and the second tracer 150 is left to react in-situ with oil present in the formation to form a reaction product (alcohol). Production fluid is then produced as best shown by arrows 125a in Figure 2E. Samples of the production fluid are collected and the concentrations of the unreacted second tracer 150 (ester) and the reaction product (alcohol) are measured and recorded as function of produced volume and / or time. The pre-EOR treatment first tracer data may be compared with the post-EOR treatment second tracer data to assess the effectiveness of the EOR treatment in reducing oil saturation. By using two different tracers each selected and adapted to the different well conditions the tracers may be located in the same target location (test zone) before and after the reservoir treatment (EOR) allowing comparable results to be produced. As shown in Figures 2B and 2E, the arrival time of the reactive tracers 120, 150 in the produced fluid are delayed when compared with the arrival time of the reaction product 117, 152 because the reactive tracer 120, 150 has affinity to stagnant oil in the formation slowing its migration to the wellbore during production. In contrast the reaction products 117, 152 have no (or low) affinity to the stagnant oil so they arrive at the wellbore first. In the above examples samples of the produced fluid are taken and analysed for the presence of tracer and / or reaction products. Due to the reactive nature of the reactive tracers, the analysis of the sample to determine concentration values in a sample is time sensitive due to the presence of reactants in the samples. The reactions may continue after the fluid have been produced and collected from the well. The concentrations of tracer and reaction products may change while the samples are being stored or transported. One option may be to analyse the samples on site. This may involve collecting samples for later analysis on site or real time analysis. Due to harsh conditions onsite in remote locations it may be difficult on occasion to perform analysis on site or obtain accurate results onsite. This may limit the distance from the sampling point at the well-site to the lab facility where analysis is performed and may require the lab to be at or very close to the well site. Alternatively robust field equipment and methodologies may be applied to analysis the samples. Robust equipment suitable for field conditions may be less sensitive than instrumentation and methodology applicable in an off-site lab. For this reason, the minimal concentrations that can be detected in onsite analysis operations may be significantly higher (1000000 above) than those achievable in an off-site laboratory. Additionally or alternatively, once samples have been collected the reaction process may be quenched or stopped. Quenching or stopping a reaction in the samples may be achieved by various means such as temperature quenching or chemical quenching. Temperature quenching may involve quickly lowering the temperature to freeze the sample. This will slow or halt the reaction. Chemical quenching may involve removing one or more required chemicals involved in the reaction process. In this example when the reaction is hydrolysis, water is required to keep the process going. If water can be completely removed from the samples, the reaction can be stopped and the tracer data in the samples preserved. The stabilized preserved sample may be stored and / or transported to an offsite laboratory for analysis. The detection of the tracer type, concentration of tracer, concentration of reaction products, injection rate, production rate and / or the transport time may be used to determine characteristics and / or conditions of the reservoir. Tracer data from a first tracer test performed before a reservoir treatment or event may be used to determine characteristics and / or conditions of the reservoir, formation or the well before the reservoir treatment or event. Tracer data from a second tracer test performed after a reservoir treatment or event may be used to determine characteristics and / or conditions of the reservoir after a reservoir treatment or event. A comparison of tracer data from a first tracer test before a reservoir treatment may be compared with tracer data from a second tracer test performed after the reservoir treatment to assess the effectiveness of the reservoir treatment. It will be appreciated that in other examples other reactive tracer types may be used to determine, assess and / or characterise other parameters of the reservoir, well or formation. As an example reactive tracers may be selected which exhibit different portioning coefficients at different well, reservoir or formation conditions. It will be appreciated that in other examples other reactive tracer types may be used to locate a first tracer in a location or zone in a first test before a reservoir treatment and a second tracer test in substantially the same location or same zone in a second test after the reservoir treatment to facilitate accurate assessment of the well treatment. The type of the first tracer and / or the second tracer may be selected depending on the reservoir conditions, type of well treatment and the expected changes to the chemistry and / or conditions of the well, reservoir and / or formation. The type of the first tracer and / or the second tracer may be selected based on how the expected changes to the chemistry and / or conditions of the well, reservoir and / or formation will affect the migration of the tracer in the formation. It will be appreciated that in other examples other reactive tracer types may be used to locate a first tracer in a location or zone in a first test before a EOR operation and a second tracer test in substantially the same location or same zone in a second test after the EOR operation to facilitate accurate assessment of the well treatment. The type of the first tracer and / or the second tracer may be selected depending on the expected changes to the chemistry and / or conditions of the well, reservoir and / or formation due to the EOR operation. Figure 3A to 3F are simplified side sectional views of a reservoir formation showing the controlled reaction of a partitioning tracer in a target location of a formation. In this example a reactive partitioning tracer is first injected into the well followed by a faster travelling accelerating agent. The partitioning tracer, accelerating agent and the injection programme are designed such that the tracer and accelerating agent are in contact with one another only in the target location in the formation. Figures 3A to 3F show a reservoir 210, having a wellbore 214 in the formation 216. In a first step a reactive partitioning tracer 220 is injected into the wellbore for specific duration, (4 hours) as best shown in Figure 3A. In this example the partitioning tracer 220 is used to determine oil saturation at the target location (test zone) 215 in the formation (see Figure 3D). In this example the tracer is an ester which hydrolyses or partitions in the presence of an oleic phase (oil) to a form a reaction product which in this example is an alcohol. In this example the tracer is Ethyl Acetate (EtAc) which has a partitioning coefficient to oil of 3. After the partitioning tracer 220 is injected an injecting fluid 222 is injected into the wellbore for 20 hours to push the tracer 220 into the formation 216 away from the well bore 214. Having an oil saturation of 0.25 the ester is pushed to a radius of 2.5 to 2.7 meters from the wellbore centre. After a specific period of time an accelerating agent 230 injected into the wellbore as shown in Figure 3B. In order for the accelerating agent to catch up with the tracer, the injection of accelerating agent is started 6 hours after completing the injection of the tracer. The accelerating agent is injected for 3 hours. In this example the accelerating agent is an acid, specifically diluted hydrochloric acid (1M) which does not partition to oil. It will be appreciated that other acids may be used. After the accelerating agent is injected an injection fluid 222 is injected into the wellbore for 11 hours to push the accelerating agent 230 and the tracer 220 into the formation 216 away from the well bore 214 towards the target location (test zone) in the formation. The accelerating agent is selected not to partition into the stagnant oil phase compared to the reactive tracer. The accelerating agent 230 therefore migrates through the formation faster than the reactive tracer 220. As shown in Figures 3B to 3D the accelerating agent 230 catches up with the reactive tracer 220 at the target location 215. In this example the partitioning tracer is designed to have a slow to moderate reaction rate in the absence of the accelerating agent but will react significantly faster in the presence of the accelerating agent. During the displacement of the partitioning tracer through the formation towards the target location reactions between the partitioning tracer and fluid in the formation is reduced as the accelerating agent is not present. Similarly during the displacement of the partitioning tracer through the formation from the target location to the wellbore during production reactions between the partitioning tracer and fluid in the formation are also reduced as the accelerating agent is not present. The reaction of the tracer as it is displaced on the way to the target location during injection and from the target location during production skew both the reacting tracer curve and the generated reaction product curve. By mitigating or reducing reactions outside the target location, reactions occurring in the target location (test zone) in the formation may be accurately determined. The partitioning tracer 220 and accelerating agent 230 and their relative injection volumes, injection times, push fluid volumes and / or push fluid times are designed so that the migration of rate of the accelerating agent 230 is faster than the tracer 220 so that their migration paths overlap at the target location (test zone) 215. Figure 3C shows that the accelerating agent 230 has caught up with the tracer 220 at the target location (test zone) 215 in the formation thereby enhancing and accelerating the hydrolysis of the reactive tracer in contact with oil located in the target location 215. By providing an accelerating agent which migrates through the formation faster than the tracer may facilitate the tracer and accelerating agent being kept separate from one another until they reach the target location of the formation. The reaction (hydrolysis) rates for esters are strongly dependent on pH. Figure 4 provides a graph of hydrolysis rate as function of pH for Ethyl acetate at a temperature of 75 C. For values above a minimum, the hydrolysis rate increases significantly with increasing pH. As an example, for EtAc the hydrolysis rate at the minimum value of pH ~ 5.1 is about 0.007 days. At pH of 7 the hydrolysis rate is almost 50 times larger. For values below a minimum, the hydrolysis rate increases significantly with decreasing pH. As an example, for EtAc the hydrolysis rate at the minimum value of pH ~ 5.1 is about 0.007 days. At pH of 3 the hydrolysis rate is almost 50 times larger. As shown in Figure 3D the well is shut-in or closed and the tracer 220 is left to react in-situ with oil present in the formation in the presence of the accelerating agent to form a reaction product 217 in this example alcohol. In the presence of the accelerating agent at the target location the reaction may occur rapidly in the presence of the reactants in this case oil, reducing the time required for the test. This may avoid the necessity of a shut in or reduce the shut in time required. For example, without an accelerating agent a significant shut in period would be required to expose tracer to oil in the formation (three days). This is in addition to the time for injecting tracer and pushing to reach depth of the target testing location (typically one day), and production period (two days) to produce any unreacted tracer and the reaction products. In cases were the reaction rate is slow the shut-in time may need to be prolonged significantly. Providing an accelerating agent enhances the reaction and reaction rate of the tracer at the target testing location. Production fluid is then produced from the well as best shown by arrows 225 in Figure 3E, the unreacted tracer 220 and reaction product 217 move from the target location (test zone) towards the wellbore. The reaction product 217 does not partition into the stagnant oil phase and moves at a higher speed than the unreacted tracer through the formation. Consequently the arrival time of the unreacted tracer in the produced fluid is delayed compared with the arrival time of the reaction product because the unreacted tracer 220 has affinity to the stagnant oil during production. The reaction product 217 arrives at the wellbore 214 first as shown in Figure 3F. The transport time for reactive tracer 220 and the reaction product 217 may be analysed. The concentrations of the reactive tracer 220 and the reaction product 217 may be measured and analysed as function of produced volume and / or time. In the above example hydrochloric acid is used as accelerating agent. It will be appreciated that additionally or alternatively several acids can be used as reacting agents, as they are generally not (or moderately) partitioning to other phases than water. Figure 4 also demonstrates that the hydrolyse of the tracer Ethyl acetate may also be accelerated by increasing pH above~5.1. This may be achieved by injecting a base as an accelerating agent instead of an acid. It will be appreciated that the accelerating agent may be an acid, base or buffered solution at a designed pH. It will be appreciated that other accelerating agent types may be used which increase reaction rate by other mechanism other than pH adjustment. The accelerating agent may be any component or compound capable of enhancing or accelerating the reaction of the reactive tracer and capable of traveling at different migration speed through the reservoir than the reactive tracer. The accelerating agent may be selected based on its ability to travel at a different rate through the reservoir than the reactive tracer. In the above example the accelerating agent is selected based on its ability to travel faster through the formation than the reactive tracer. It will be appreciated that the accelerating agent may be selected based on its ability to travel slower through the formation than the reactive tracer. In such an example the reactive tracer may be injected after the accelerating agent so that the slower accelerating agent and the faster reactive tracer reach the target location (test location) at the same time or significantly overlap at the target test location. In this arrangement the accelerating agent and reactive tracer are kept separate until they reach the target location. The accelerating agent may be of both catalytic and reactive type. The accelerating agent may either catalyse the reactive tracer reaction or actively take part in the reaction. The accelerating agent may be consumed by the subterrain formation, e.g. in a competing reaction exemplified by the neutralization reaction between a carbonate reservoir and hydrochloric acid, or by e.g. partly being neutralized by a naturally occurring buffer system. It will be appreciated that the accelerating agent may alternatively be selected from the group comprising formic acid, hydrofluoric acid, acetic acid, propionic acid, citric acid, benzoic acid, phosphoric acid, dihydrogenphosphate, ascorbic acid, sulphuric acid, sulfonic acid, ammonium salts (e.g. ammonium chloride, ammonium hydroxide, ammonium bisulphite), phoshonate buffers, citric acid buffers, ammonia, sodium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, pyridine, alkylamines, aromatic amines, iron chloride, iron sulphate, iron hydroxide. It will be appreciated that the tracer systems described in Figures 1A to 2E may also include an accelerating agent designed to enhance or accelerate the reaction of the reactive tracers 20, 120 and 150 at the target locations. It will be appreciated that a suitable accelerating agent may be injected at a different time to the tracers 20,120, 150 so that the accelerating agent and tracer are not in contact with each other until they are at the target location in the formation. It will be appreciated that the tracers 20, 120, 150 may have a slower migration than an accelerating agent in which case the slower tracer is injected first and the faster accelerating agent catches up with the tracer at the target location. Alternatively the tracers 20, 120, 150 may have a faster migration than an accelerating agent in which case the slower accelerating agent is injected first and the faster tracer catches up with the tracer at the target location. Alternatively, two or more reactive compounds may be injected at various pre-designed times. The two or more reactive compounds designed to react with each other in situ to form secondary tracers. The secondary tracers and two or more reactive compounds may be used together to infer the saturation of separate phases. At least some of these compounds must have different affinity to the different phases. In the above examples the application have been focused on assessing characteristics of a hydrocarbon well. However, it will be appreciated that the invention may be applied for other applications such as Carbon Capture Utilisation and Storage (CCUS) applications. The principle is the same comprising injecting a reactive tracer and an accelerating agent with different migration rates at different times so that they contact one another at a target location in the reservoir and producing or recovering the injected tracer and / or reaction products. The method may comprise recording of a time lag. The method may comprise recording of ratio of unreacted tracer to reaction products. The ratio of unreacted tracer to reaction products and / or time-lag may provide information on the at least one phase. The ratio of unreacted tracer to reaction products and / or time-lag may provide information such as saturation of the at least one phase (stagnant phase). One example application may be the assessment of CO2 storage potential in saline aquifers, where the objective is to find how much CO2 is retained through capillary trapping. By providing an accelerating agent which migrates at a different rate through the formation than the tracer may facilitate the tracer and accelerating agent being kept separate from one another until they reach the target location of the formation. In the absence of the accelerating agent the reaction may be designed to occur very slowly. This may reduce reactions occurring during movement of the tracer into and out from the formation. In the presence of the accelerating agent at the target location the reaction may occur rapidly in the presence of the reactants in this case oil. This has the benefit of providing accurate information on the conditions of the target location (test zone) and reducing the time required for the test. This may also avoid the necessity of a shut in or reduce the shut in time required. Throughout the specification, unless the context demands otherwise, the terms 'comprise' or 'include', or variations such as 'comprises' or 'comprising', 'includes' or 'including' will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. Furthermore, relative terms such as “up”, “down”, “top”, “bottom”, “upper”, “lower”, “upward”, “downward”, “horizontal”, “vertical”, “extend”, “retract” and the like are used herein to indicate directions and locations as they apply to the appended drawings and will not be construed as limiting the invention and features thereof to particular arrangements or orientations. The foregoing description of the invention has been presented for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for monitoring a subterranean reservoir formation, the method comprising:injecting at least one reactive tracer into a well;injecting at least one accelerating agent into the well;shutting in the well;inducing production from the well and sampling produced fluid;measuring the concentration of the at least one reactive tracer and / or at least one reaction product in the samples;based on the presence and / or concentration of the reactive tracer and / or the at least one reaction product determining at least one condition and / or characteristic of the subterranean reservoir formation.

2. The method according to claim 1 wherein the at least one accelerating agent is configured to enhance or accelerate the reaction of the at least one reactive tracer to form the at least one reaction product.

3. The method according to any preceding claim wherein the at least one accelerating agent and the at least one reactive tracer are injected into the well at different times.

4. The method according to any preceding claim wherein the at least one accelerating agent and the at least one reactive tracer have different migration rates though the reservoir or formation.

5. The method according to any preceding claim wherein the at least one accelerating agent is injected before or after the at least one reactive tracer.

6. The method according to any preceding claim comprising determining at least one condition and / or characteristic of the subterranean reservoir formation based on the arrival time of the reactive tracer, partitioning tracer coefficient and / or the arrival time of the least one reactive tracer and / or the at least one reaction product.

7. The method according to any preceding claim comprising injecting an injection fluid after the at least one reactive tracer injection and / or the at least oneaccelerating agent injection to push the at least one reactive tracer and / or at least one accelerating agent towards or to a target location in the formation.

8. The method according to any preceding claim comprising shutting in the well with the at least one reactive tracer and the at least one accelerating agent in a target location in the formation.

9. The method according to any preceding claim wherein the subterranean reservoir is selected from the group comprising hydrocarbon reservoir, carbon dioxide storage reservoir, and / or a water reservoir.

10. The method according to any preceding claim wherein the injection fluid is selected from the group comprising oil, water, gas, carbon dioxide, nitrogen, hydrocarbon gas, waste water, brine, and / or water mixed with chemicals.

11. The method according to any preceding claim comprising determining or estimating a saturation level for the reservoir, a residual fluid saturation level for the reservoir, an oil saturation level of the reservoir formation,, a residual oil saturation level of the reservoir formation, a water saturation level of the reservoir formation, a gas saturation level of the reservoir formation, a carbon dioxide saturation level of the reservoir formation, a brine saturation level of the reservoir formation and / or an effectiveness of the reservoir treatment.

12. The method according to any preceding claim wherein the at least one reactive tracer is an ester or an amide.

13. The method according to any preceding claim wherein the at least one reaction product is selected from the group comprising an alcohol, or an acid.

14. The method according to any preceding claim wherein the at least one accelerating agent is selected from the group comprising a catalyst, a reactant, hydrochloric acid, formic acid, hydrofluoric acid, acetic acid, propionic acid, citric acid, benzoic acid, phosphoric acid, dihydrogenphosphate, ascorbic acid, sulphuric acid, sulfonic acid, ammonium salts (e.g. ammonium chloride, ammonium hydroxide, ammonium bisulphite), phoshonate buffers, citric acid buffers, ammonia, sodium hydroxide, calcium hydroxide, lithium hydroxide,magnesium hydroxide, pyridine, alkylamines, aromatic amines, iron chloride, iron sulphate and / or iron hydroxide.

15. The method according to any preceding claim comprising repeating the tracer test before and / or after a reservoir is treated.

16. The method according to any preceding claim comprising repeating the test with a different tracer and / or a different accelerating agent.

17. The method according to any preceding claim comprising preserving the tracer data in the samples by stopping or quenching a reaction in the samples, removing a reactant from the samples, dehydrating the samples and / or freezing the samples.

18. The method according to any preceding claim comprising controlling and / or optimising the time of tracer injection, accelerating agent injection, rate of tracer injection, rate of accelerating agent injection duration of tracer injection, duration of the accelerating agent injection, time of injection fluid injection, rate of injection fluid injection and / or duration of injection fluid injection to locate the reactive tracer and / or the accelerating agent in a target location in the formation.

19. The method according to any preceding claim comprising controlling the migration of the at least one reactive tracer and the at least one accelerating agent so that their respective migration through the formation substantially overlap or substantially align at the target location in the formation.

20. The method according to any preceding claim comprising controlling the injection migrations of the at least one reactive tracer and the at least one accelerating agent so that their respective migrations through the formation substantially overlap or substantially align only at the target location in the formation.

21. The method according to any preceding claim comprising controlling the injection order, injection time and / or injection flow rate of the at least one reactive tracer and at least one accelerating agent so that they aresubstantially exposed to each other or substantially contact each other only at the target location in the formation.

22. A system for monitoring a reservoir, the system comprising:at least one injection device configured to inject at least one reactive tracer and at least one accelerating agent into the wellbore; anda collection device configured to collect samples of fluid produced from the reservoir.

23. The system according to claim 22 comprising two or more injection device wherein at least one injection device is configured to inject at least one reactive tracer into the wellbore and at least one different injection device is configured to inject at least one accelerating agent into the wellbore.

24. The system according to claim 22 or 23 comprising at least one injection device is configured to inject the one reactive tracer into the wellbore at a first time and the at least one accelerating agent into the wellbore at a second time.

25. A method for monitoring a subterranean formation, the method comprising: analysing tracer data from at least one sample previously collected from a fluid produced from a subterranean formation; wherein the subterranean formation comprises a reactive tracer and an accelerating agent injected into the formation;based on the tracer data monitoring at least one characteristic and / or condition of the subterranean formation.

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