System and method of monitoring subterranean reservoirs

Abstract

The invention provides a method for monitoring a subterranean reservoir. The method comprising injecting two or more tracers into a subterranean reservoir wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer. The method comprising collecting at least one sample from fluid produced from the subterranean reservoir and analysing the at least one sample to detect the presence or absence of the two or more tracers. The method comprising monitoring at least one characteristic of the subterranean reservoir based on measured tracer data.

Description

[0001] System and Method of Monitoring Subterranean Reservoirs

[0002] The present invention relates to monitoring subterranean reservoirs and in particular monitoring subterranean reservoirs using tracers. Aspects of the inventions relates a system and methods for obtaining information on subterranean reservoir conditions. The invention has particular, although not exclusive, application to geothermal reservoirs, hydrocarbon reservoirs and carbon storage formations.

[0003] Background to the invention

[0004] A geothermal reservoir is a naturally occurring area of hydrothermal resources. These reservoirs are deep underground and are largely undetectable above ground. A geothermal production well is drilled into a known geothermal reservoir and hot geothermal fluids flow through the production well to a power plant for use in generating electricity or the hot fluid is utilized through heat exchanger for heating or similar purposes. An injection well is drilled into the known geothermal reservoir to return used geothermal fluids to the geothermal reservoir or to introduce fluid into a dry heated rock. A geothermal system may comprise multiple injector and / or production wells.

[0005] An oil and gas reservoir is 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 or 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. An oil and gas system may comprise multiple injector and / production wells.

[0006] 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. Monitoring injected CO2 in its reservoir is crucial for predicting the risk of CO2 leakage, increasing efficiency, reducing the cost of CO2 storage, and reducing the risk of induced seismicity.

[0007] 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.

[0008] Summary of the invention

[0009] 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.

[0010] There is a need to obtain information on characteristics of subterranean reservoirs and to understand how fluids are affected by the characteristics of subterranean reservoirs as the fluid are stored in or migrate through the reservoir.

[0011] There is generally a need for a system and method to understand characteristics of flow paths and to understand how fluids are affected by flow path characteristics.

[0012] It is an object of the invention to provide a system and method to obtain conditions of a subterrain formation and / or flow paths into, through and / or from the subterrain formation.

[0013] It is an object of the invention to provide a system and method to obtain information on conditions of a geothermal reservoir and / or flow paths into, through and / or from the geothermal reservoir. This may facilitate optimisation of energy production from geothermal reservoirs.

[0014] It is an object of the invention to provide a system and method to obtain information on conditions of a hydrocarbon reservoir and / or flow paths into, through and / or from the hydrocarbon reservoir. This may facilitate optimisation of injection into and / or production from a hydrocarbon reservoir.

[0015] It is an object of the invention to provide a system and method to obtain information on conditions of a carbon capture storage reservoir and / or flow paths into, through and / or from the carbon capture storage reservoir. This may facilitate optimisation of injection into and / or storage in a carbon capture storage reservoir. Further aims and objects of the invention will become apparent from reading the following description.

[0016] According to a first aspect of the invention, there is provided a method for monitoring a subterranean reservoir, the method comprising: injecting two or more tracers into a subterranean reservoir; wherein the two or more tracers comprise at least one reactive tracer; collecting at least one sample from fluid produced from the subterranean reservoir; analysing the at least one sample to detect the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the subterranean formation.

[0017] The method may comprise measuring a concentration of the two or more tracers in the at least one sample. The reactive tracer may be a chemical tracer. The reactive tracer may be an unstable chemical tracer.

[0018] 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 degrade or at least partially degrade on exposure to a condition in the reservoir. The at least one reactive tracer may be configured to degrade or at least partially degrade on exposure to a condition in an injection well and / or a production well. The at least one reactive tracer may be configured to degrade at a known degradation rate under known conditions. The at least one reactive tracer may be configured to react or interact to specific reservoir conditions. The at least one reactive tracer may be configured to degrade or at least partially degrade on exposure to one or more selected chemical or pH in the reservoir. The at least one reactive tracer may be configured to degrade or at least partially degrade on exposure to a selected chemical or pH in an injection well and / or a production well. The at least one reactive tracer may be configured to interact with subterranean minerals and / or rocks. The at least one reactive tracer may be configured to be attracted to or repelled to subterranean minerals and / or rocks. The at least one reactive tracer may be configured to exhibit intermolecular forces of attraction or repulsion with subterranean minerals and / or rocks. The at least one reactive tracer may be a thermally unstable tracer. The thermally unstable tracer may be configured to degrade or at least partially degrade on exposure to a selected temperature or temperature range in the reservoir. The thermally unstable tracer may be configured to degrade or at least partially degrade on exposure to a specific temperature in an injection well and / or a production well.

[0019] The method may comprise injecting at least one stable tracer. The two or more tracers may comprise at least one stable tracer. The at least one stable tracer may be a stable chemical tracer. The at least one stable tracer may be configured to be stable in reservoir conditions. The at least one stable tracer may be configured to be stable in geothermal reservoir conditions. The at least one stable tracer may be configured to be stable in supercritical geothermal reservoir conditions. The at least one stable tracer may be configured to be stable in hydrocarbon reservoir conditions. The at least one stable tracer may be configured to be stable in carbon storage reservoir conditions.

[0020] The at least one stable tracer may be configured to resist degradation and / or resist full degradation on exposure to a condition in the reservoir. The at least one stable tracer may be configured to resist degradation and / or resist full degradation on exposure to a condition in an injection well and / or a production well. The at least one stable tracer may be chemically, physically and / or biologically stable in reservoir conditions. The at least one stable tracer may be configured to behave as close as possible to the traced fluid or phase. The at least one stable tracer may be configured to not adsorb to any surface or in other ways be chemically affected by reservoir rocks, minerals and / or chemicals. The at least one stable tracer may be configured to resist interaction with subterranean minerals and / or rocks. The at least one stable tracer may be configured to not be attracted to or repelled to subterranean minerals and / or rocks. The at least one stable tracer may be configured to exhibit no intermolecular forces of attraction or repulsion with subterranean minerals and / or rocks. The at least one stable chemical tracer may be a thermally stable tracer and / or chemical stable tracer.

[0021] The characteristics of the reservoirs may be selected from the group comprising: reservoir condition, injection flow paths, reinjection flow paths; rate of cooling, rate of heating; injection rates, production rates, reservoir residence time; fluid residence time, injection fluid temperature, produced fluid temperature; reservoir temperature; reservoir temperature gradients; swept pore volumes, interwell connections and / or connectivity; rock surface area; rock type, mineral type, enthalpy, enthalpy output, mixing capacity of cold injected fluid with hot fluid in the reservoir, heat equilibrium rate of the reservoir, rock mechanics, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity and / or fluid saturation.

[0022] The method may comprise monitoring and / or characterising reservoirs including geothermal reservoirs which may have chemically and physically distinct layers which may affect the capacity and productivity of the reservoir. The layers may be characterised by distinct reservoir rocks, mineral alteration, fluid chemistry, and temperature. The reactive tracer may be selected to characterise or monitor reservoir distinct layers rocks, mineral alteration, fluid chemistry, and / or temperature of the reservoir. The method may comprise simultaneous injection of the two or more tracers. The method may comprise injection of the two or more tracers at a known time. The method may comprise injection of each of the two or more tracers at known times. The method may comprise injection of the two or more tracers independently from one another or in synchrony with one another. The method may comprise injecting of each of the two or more tracers such that the tracer releases overlap. The method may comprise injecting the two or more tracers into at least one injection well. The method may comprise injecting the two or more tracers into at least one injection well.

[0023] The method may comprise injecting an injection fluid with at least one tracer at a first injection time and injecting an injection fluid with a second tracer at a second injection time. The method may comprise analysing the measured concentration of tracer in the at least one sample with the sampling time, injection time and known degradation rate to determine and / or monitor characteristics of the reservoir. The method may comprise injecting a plurality of tracers into the reservoir. The method may comprise injecting a plurality of tracers into the injection well. The plurality of tracers may comprise tracer compounds which have different chemical and / or physical properties. The plurality of tracers may comprise at least one reactive tracer. The plurality of tracers may comprise at least one stable tracer. The plurality of tracers may comprise two or more reactive tracers. The plurality of tracers may comprise two or more reactive tracers and at least one stable tracer. The plurality of tracers may comprise two or more reactive tracers and two or more stable tracers. The plurality of tracers may comprise a combination of reactive tracers each configured to react or interact with a different reservoir condition. The plurality of tracers may comprise two or more reactive tracers each configured to degrade or partially degrade at a different temperature. The plurality of tracers may comprise two or more thermal reactive tracers each configured to degrade or partially degrade at a different temperature. The plurality of tracers may comprise at least one reactive tracer configured to react or interact with rock types, rock surface, minerals or reservoir chemicals. The plurality of tracers may comprise reactive tracers which have a range of chemical, rock, mineral and / or thermal sensitivities. The two or more thermal unstable tracers may be selected to cover a range of temperatures. The plurality of tracers may comprise tracers which may degrade or partially degrade over a selected range of reservoir chemicals or pH’s.

[0024] The plurality of tracers may comprise a combination of stable tracers and reactive tracers. The plurality of tracers may comprise a range of reactive tracers which are configured to degrade or partially degrade over a specific range of reservoir temperatures.

[0025] The plurality of tracers may comprise range of tracers which may degrade or partially degrade over a selected range of injection well or production well temperatures. The plurality of tracer may comprise at least one stable tracer.

[0026] The method may comprise a multi-stage tracer injection. The method may comprise injecting two or more tracers over time to map thermal evolution and / or changes in stages. The method may comprise injecting two or more tracers over time to map changes in the reservoir in stages. The method may comprise injecting two or more tracers at known times. The method may comprise injecting two or more tracers sequentially at known times. The method may comprise injecting two or more tracers sequentially, each at a known time. The method may comprise injecting a plurality of tracers over time to map thermal evolution and / or changes in stages. The method may comprise injecting a plurality of tracers over time to map changes in the reservoir in stages. The method may comprise injecting a plurality tracers at known times. The method may comprise injecting a plurality tracers sequentially at known times. The method may comprise injecting a plurality tracers sequentially, each at a known time. Each stage may be optimised for different temperature ranges and / or reaction rates. The method may comprise analysing changes in the reservoir over time. The method may comprise analysing thermal changes in the reservoir over time. The method may comprise capturing tracer data of the reservoir from multiple time points. The method may comprise using two or more tracers injected at different time points to capture data of the reservoir from two or more different time points. The method may comprise using a plurality of tracers each injected at different time points to capture data of the reservoir from different time points. The tracer data may provide a temperature distribution profile by capturing data from multiple time points. The tracer data may provide a profile of the reservoir by capturing data from multiple time points. The tracer data may provide a profile of changes in the reservoir over time by capturing data from multiple time points. The tracer data may be analysed post-collection. The tracer data may be analysed to track the evolution of a thermal front. The tracer data may be analysed to track the evolution of a changes in the reservoir. The method may comprise injecting two or more tracers into an injection well in fluid communication with the subterranean formation. The at least one reactive tracer may be a non-radioactive tracer.

[0027] The subterranean formation may be selected from the group comprising a geothermal reservoir, a supercritical geothermal reservoir, a hydrocarbon reservoir, a carbon capture storage reservoir, a carbon dioxide storage reservoir, a subterranean hydrogen storage reservoir, a water reservoir and / or a shale reservoir. The method may comprise controlling and / or optimising the rate of injection and / or rate of producing fluid from the subterranean formation based on measured concentration of the two or more tracers. The injection fluid may be a liquid, a gas and / or a supercritical fluid. 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 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 method may comprise injecting or releasing tracer continuously. The method may comprise injecting or releasing tracer continuously for a sustained period of time. The method may comprise actuating a tracer injection device to allow continuous release of tracer.

[0028] The method may comprise injecting a known amount and / or concentration of the two or more tracers. The method may comprise injecting or releasing tracer as a pulse. The method may comprise injecting or releasing tracer for a short period of time to form a tracer pulse. The method may comprise actuating a tracer injection device to allow pulsed release of tracer. The method may comprise adjusting the volume, duration, release rate and / or frequency of the two or more tracers. The method may comprise releasing the two or more tracers periodically for example hourly, weekly or monthly. The method may comprise releasing the two or more tracers manually or automatically. The method may comprise releasing or injecting the two or more tracers on command. The method may comprise releasing or injecting the two or more tracers in response to a timer and / or a control signal. The method may comprise releasing or injecting the two or more tracers in response to a pre-set programme and / or timer. The method may comprise releasing or injecting tracer in response to a trigger event. The method may comprise selectively releasing the two or more tracers into the injection well to allow characteristics of the formation to be monitored. The method may comprise measuring flow rates, flow paths and / or transport paths. The method may comprise identifying, calculating and / or monitoring flow rates, flow paths and / or transport paths.

[0029] The method may comprise pre-mixing the two or more tracers. The method may comprise premixing the injection fluid and the two or more tracers before injection of the injection fluid. The method may comprise premixing the injection fluid and the two or more tracers during injection of the injection fluid. The method may comprise providing the two or more tracers in a release apparatus or tracer source. The method may comprise locating the tracer release apparatus or tracer source in a flow path of the injection fluid. The method may comprise releasing the two or more tracer from the tracer release apparatus or tracer source on command or on contact with the injection fluid.

[0030] The method may comprise releasing 50% of the tracer from the tracer release apparatus or tracer source over a period of 1 hour to 10 years. The method may comprise releasing 50% of the two or more tracers from the tracer release apparatus or tracer source over a period of 1 hour to 5 years. The method may comprise releasing 50% of the two or more tracers from the tracer release apparatus or tracer source over a period of 1 hour to 24 months. The method may comprise releasing 50% of the two or more tracers from the tracer release apparatus or tracer source over a period of 1 hour to 6 months. The method may comprise releasing 50% of the two or more tracers from the tracer release apparatus or tracer source over a period of 1 hour to 3 weeks. The method may comprise releasing 50% of the two or more tracers from the tracer release apparatus or tracer source over a period of 1 hour to 1 day.

[0031] The two or more tracers may be a liquid, solid or gas. The two or more tracers may be a powdered solid. The two or more tracers may be water tracers.

[0032] The at least one reactive tracer may be selected from the group comprising dyes, halogenated benzoates, DNA based tracer, nanoparticles, halogenated hydrocarbons, metal oxides perfluorinated hydrocarbons, perfluoroethers, partly fluorinated compounds, perfluoro buthane (PB), perfluoro methyl cyclopentane (PMCP), perfluoro methyl cyclohexane (PMCH), organofluorine, perfluorocarbon, polyaromatic sulfonate, sulfonates, naphthalene, naphthalene sulphonic acid and / or quantum dot. The at least one reactive tracer may be configured, selected and / or designed to degrade or at least partially degrade on exposure to a condition in the reservoir.

[0033] The at least one stable tracer may be selected from the group comprising dyes, halogenated benzoates, DNA based tracer, nanoparticles, halogenated hydrocarbons, metal oxides perfluorinated hydrocarbons, perfluoroethers, partly fluorinated compounds, perfluoro buthane (PB), perfluoro methyl cyclopentane (PMCP), perfluoro methyl cyclohexane (PMCH), organofluorine, perfluorocarbon, polyaromatic sulfonate, sulfonates, naphthalene, naphthalene sulphonic acid and / or quantum dot. The at least one stable tracer may be configured, selected and / or designed to resist degradation and / or resist full degradation on exposure to a condition in the reservoir.

[0034] 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 sample 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.

[0035] The method may comprise analysing the at least one sample to measure the presence and / or concentration of the two or more one tracer in the sample. The method may comprise analysing the at least one sample for type and / or concentration of the two or more tracers as a function of sampling time. The method may comprise detecting and / or measuring the concentration of the two or more tracers in the at least one sample in real time. The method may comprise detecting and / or measuring the concentration of the two or more tracers in the at least one sample using an online analyser. The method may comprise detecting and / or measuring the concentration of the plurality of tracers in at least one sample. The method may comprise measuring a tracer concentration signature of the two or more tracers in the at least one sample. The method may comprise measuring a tracer concentration signature of the at least one reactive tracer and the at least one stable tracer in the at least one sample. The method may comprise comparing the measuring tracer concentration signatures of the at least one reactive tracer and the at least one stable tracer in the at least one sample with the tracer concentration signatures of the at least one reactive tracer and the at least one stable tracer injected in the well.

[0036] The method may comprise comparing the ratio of each of the injected two or more tracers with the ratio of each of the two or more tracers in the at least one sample. The method may comprise using the tracer concentration signatures of the at least one stable tracer as reference data.

[0037] The method may comprise comparing the tracer concentrations signatures of the two or more tracers in the at least one sample with the tracer concentration signature of the two or more tracers injected into the injection well. The method may comprise interpretating differences or changes between injected tracer concentrations signatures and produced tracer concentrations signatures to determine and / or monitor characteristics of the reservoir. The method may comprise comparing the tracer concentrations signatures of at least one thermally unstable tracers in the at least one sample with the tracer concentration signature of the at least one thermally unstable tracers injected into the injection well. The method may comprise comparing the tracer concentrations signatures of two or more thermally unstable tracers in the at least one sample with the tracer concentration signature of the two or more thermally unstable tracers injected into the injection well. The method may comprise interpretating differences or changes between injected thermally unstable tracer concentrations signatures and produced thermally unstable tracer concentrations signatures to determine, monitor and / or map thermal characteristics of the reservoir.

[0038] The method may comprise measuring and / or calculating a baseline of tracer in the injection fluid. The method may comprise reinjecting produced fluid into the injection well. The method may comprise taking one or more samples of the reinjection fluid. The method may comprise measuring and / or calculating a baseline of tracer in the reinjection fluid. The method may comprise collecting samples of the produced fluid. The method may comprise collecting samples of the injection fluid. The sampling may be conducted at one or more sampling times. The sampling may be conducted downhole in a production well. The sampling may be conducted at surface. The sampling may be conducted at a location in a direction towards the surface of the production well.

[0039] The method may comprise conducting optical monitoring for the detection and / or concentration of the two or more tracers in the produced fluid, injection fluid and / or reinjection fluid. The method may comprise determining the type of tracer. The method may comprise the measuring and / or monitoring the concentration of tracer. The method may comprise the measuring and / or monitoring the concentration of tracer in the at least one sample. .The method may comprise the measuring and / or monitoring the transport time of the two or more tracer. The method may comprise the measuring and / or monitoring the transport time of the two or more tracer from injection to detection in the producing fluid.

[0040] Tracer in the at least one sample may be detected, and its concentration measured by different techniques such as optical detection, optical fibers, spectrophotometric methods , chromatographic methods or radioactivity analysis. The method may comprise analysing at least one characteristic of the tracer release, injection, injection volume, injection location, sampling time, sampling location, transport time, tracer concentration, temperature of injection fluid and / or production fluid and / or produced volume. The method may comprise analysing the arrival and / or tracer concentration of each tracer in the produced fluid. The method may comprise analysing the rate of decline of the tracer concentration in the produced fluid to determine and / or monitor reservoir flow, flow rates and / or flow paths.

[0041] The method may comprise modelling the formation, tracer concentration, transport time, injection flow rate and / or production rates in a model. The formation model profile, tracer concentration, transport time, injection flow rate and / or production rate may be adjusted until calculated concentrations of model tracers compare with the measured concentrations of produced tracers to estimate formation characteristics. The model may comprise parameters selected from the group including: the number of injection wells, temperature, temperature of fluid at each injection well, temperature of fluid produced, number of production wells, temperature of fluid at each production well, transport time, transport time of tracer-containing fluid from injection to production for one or more injection wells, transport time of tracer-containing fluid from injection to production for one or more production wells, transport time of tracer-containing fluid through the reservoir, tracer type; tracer combination, concentration of tracer, concentration of tracer as a function of time, fluid flow path from one or more injection well to one or more production wells in communication to the geothermal reservoir, injection rate, production rate, mixing capacity of cold injected fluid with hot fluid in the reservoir, heat equilibrium rate of the reservoir, rock mechanics, rock chemistry, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity, fluid saturation, injection amount, injection volumes and / or migration path of the at least one tracer in and / or through the formation.

[0042] The method may be a computer-implemented method. The method may be a computer- implemented history matching method. The method may comprise storing the measurement data to a database. The method may comprise storing the model data to a database.

[0043] The method may comprise updating the model based upon measured and / or calculated data. The method may comprise history matching. The method may comprise comparing historical parameter measurements to calculated data. The method may comprise adjusting one or more parameter measurements of the model until a reasonable match is achieved between the measured and calculated data. The model may simulate characteristics of the formation and / or pathways of injected fluid and at least one tracer. The method may comprise comparing modelled tracer sample data to measured tracer sample data.

[0044] The method may comprise integrating tracer data with at least one geothermal reservoir simulation model. The method may comprise integrating tracer data with at least one geothermal reservoir simulation model configured to determine and / or predict a long-term thermal behaviour of the reservoir.

[0045] The method may comprise predictive modelling of a reservoir performance. The method may comprise integrating tracer data with at least one geothermal reservoir simulation model for predictive modelling of reservoir performance. The method may comprise integrating tracer data with at least one geothermal reservoir simulation model which may extend the lifespan of the geothermal resource and / or optimise energy extraction.

[0046] The method may comprise updating the model with measured tracer data. The method may comprise updating the model with measured tracer data to refine predictions. The method may comprise updating the model with measured tracer data to optimize heat extraction strategies.

[0047] According to a second aspect of the invention, there is provided a method for characterising a subterranean reservoir, the method comprising: injecting two or more tracers into a subterranean reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the subterranean reservoir; analysing the at least one sample to determine the presence or absence of the two or more tracers; based on measured tracer data determining at least one characteristic of the subterranean formation.

[0048] The at least one reactive tracer may be configured to react or interact under reservoir conditions. The method may comprise injecting at least one stable tracer into the subterranean reservoir. The method may comprise measuring the concentration of the two or more tracers injected into the subterranean reservoir. The method may comprise obtaining a tracer injection profile or signature based on the concentration of the two or more tracers injected into the subterranean reservoir. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise obtaining a tracer produced profile or signature based on the concentration of the two or more tracers in the at least one sample. The method may comprise comparing the tracer injection profile or signature and the tracer produced profile or signature to determine and / or monitor at least one characteristic of the subterranean formation.

[0049] The method may comprise creating a model of the reservoir comprising parameters selected from the group including: tracer injection profile or signature, tracer produced profile or signature, temperature of fluid injected, the number of injection wells, temperature, temperature of fluid at each injection well, temperature of fluid produced, number of production wells, temperature of fluid at each production well, transport time, transport time of tracer-containing fluid from injection to production for one or more injection wells, transport time of tracer-containing fluid from injection to production for one or more production wells, transport time of tracer-containing fluid through the reservoir, tracer type; tracer combination, concentration of tracer, concentration of tracer as a function of time, fluid flow path from one or more injection well to one or more production wells in communication to the geothermal reservoir, injection amount, injection rate, production rate, enthalpy, enthalpy output, residence time, mixing capacity of cold injected fluid with hot fluid in the reservoir, heat equilibrium rate of the reservoir, rock mechanics, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity, fluid saturation, injection volumes and / or migration path of the at least one tracer in and / or through the formation.

[0050] The model may be adjusted until calculated concentrations of model tracers compare, substantially match or best fit with the measured concentrations of produced tracers to estimate the reservoir characteristics.

[0051] 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.

[0052] According to a third aspect of the invention, there is provided a method for monitoring a geothermal reservoir, the method comprising: injecting two or more tracers into a geothermal reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the geothermal reservoir; analysing the at least one sample to determine the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the geothermal formation.

[0053] The at least one reactive tracer may be configured to react or interact under geothermal reservoir conditions. The at least one stable tracer may be a thermally stable tracer. The at least one reactive tracer may be a thermally unstable tracer. The method may comprise injecting a plurality of thermally unstable tracers. Each of thermally unstable tracers may have a different thermal stability and each may be configured to degrade or partially degrade at a different temperature. The method may comprise measuring the concentration of the two or more tracers injected into the subterranean reservoir. The method may comprise obtaining a tracer injection profile or signature based the concentration of the two or more tracers injected into the subterranean reservoir. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise obtaining a tracer produced profile or signature based the concentration of the two or more tracers in the at least one sample. The method may comprise comparing the tracer injection profile or signature and the tracer produced profile or signature to determine and / or monitor at least one characteristic of the subterranean formation.

[0054] The at least one reactive tracer may be configured to react and / or interact under geothermal reservoir conditions. The at least one reactive tracer may be configured to degrade or partially degrade when exposed to temperatures within the range of 100°C to 374°C. The at least one reactive tracer may be configured to degrade or partially degrade when exposed to temperatures within the range of 200°C to 374°C. Each reactive tracer may be configured to degrade or partially degrade when exposed to a different temperature or different temperature range within the range of 100°C to 374°C. Each reactive tracer may be configured to degrade or partially degrade when exposed to a known temperature or known temperature range within the range of 100°C to 374°C. Each reactive tracer may be configured to degrade or partially degrade when exposed to a known temperature or known temperature range within the range of 200°C to 374°C. The at least one stable tracer may be configured to physically and / or chemically stable when exposed to temperatures within the range of 100°C to 374°C. The at least one stable tracer may be configured to physically and / or chemically stable when exposed to temperatures within the range of 100°C to 450°C. The at least one stable tracer may be configured to physically and / or chemically stable up to 450°C.

[0055] The method may comprise creating a model of the reservoir comprising parameters selected from the group including: tracer injection profile or signature, tracer produced profile or signature, temperature of fluid injected, the number of injection wells, temperature, temperature of fluid at each injection well, temperature of fluid produced, number of production wells, temperature of fluid at each production well, transport time, transport time of tracer-containing fluid from injection to production for one or more injection wells, transport time of tracer-containing fluid from injection to production for one or more production wells, transport time of tracer-containing fluid through the reservoir, tracer type; tracer combination, concentration of tracer, concentration of tracer as a function of time, fluid flow path from one or more injection well to one or more production wells in communication to the geothermal reservoir, injection amount, injection rate, production rate, enthalpy, enthalpy output, residence time, mixing capacity of cold injected fluid with hot fluid in the reservoir, heat equilibrium rate of the reservoir, rock mechanics, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity, fluid saturation, injection volumes and / or migration path of the at least one tracer in and / or through the formation. The model may be used to optimise and / or control the flow of fluid into, through and from the geothermal reservoir for power generation.

[0056] The method may comprise optimising power generation based on the tracer data. The method may comprise optimising power generation from the geothermal reservoir by adjusting and / or controlling the rate of fluid injection into the reservoir. The method may optimise power generation by adjusting and / or controlling the rate of fluid produced from the reservoir. The method may optimise power generation by adjusting and / or controlling the rate of pumping hot fluid out of the reservoir. By tracing the flow into, through and out from the reservoir the heating efficiency of the geothermal reservoir can be estimated or calculated. By identifying variation in temperature from the injector well, through the reservoir and / or to the production wells the temperature gradient can be determined and / or monitored. The migration of cold and hot fluid through the reservoir and / or the production well can be understood and monitored maximising the heating capacity of the geothermal system.

[0057] The method may comprise determining a heat equilibrium rate of the geothermal well. The rate of injected fluid may be adjusted based on the model data. The rate of produced fluid may be adjusted based on the model data. The methods may comprise maximising energy recovery from a geothermal reservoir. The method may comprise calculating fluid transport, volumes and / or energy outtake from the geothermal reservoir.

[0058] The method may comprise controlling the fluid injection flow rate at one or more injection well. The method may comprise controlling the fluid injection flow rate into the reservoir. The method may comprise controlling one or more pumps located at one or more injection well to control the fluid injection flow rate at one or more injection well. The method may comprise controlling the fluid injection flow rate to control the rate of cold fluid being injected into the reservoir. The method may comprise controlling fluid production flow rate at one or more production well. The method may comprise controlling the fluid production flow rate to control the rate of fluid extracted from the reservoir. The method may comprise controlling one or more pumps located at one or more production well to control the fluid production flow rate at one or more production well. The method may comprise controlling the fluid production flow rate to control the rate of heated fluid being extracted from the reservoir. The method may comprise maintaining a constant injection flow rate and / or production flow rate once a heat equilibrium rate for the reservoir has been achieved. The method may comprise maintaining a balanced injection flow rate and / or production flow rate once a heat equilibrium rate for the reservoir has been achieved.

[0059] The method may comprise balancing the injection flow rate at one or more injection wells with the production rate at one or more production wells to obtain a heat equilibrium rate for the geothermal reservoir based on the tracer data. The method may comprise balancing the heated fluid extracted via the one or more production well with the rate of heating injected fluid via the one or more injection wells. The method may comprise monitoring the injection rates and / or production rates. The method may comprise automatically adjusting the injection rates and / or production rates to substantially maintain a heat equilibrium rate for the geothermal reservoir. The method may comprise automatically adjusting the injection rates and / or production rates to optimise the heating capacity of the geothermal reservoir. The method may comprise automatically adjusting the injection rates and / or production rates to control and / or optimise the temperature of fluid produced from the geothermal reservoir.

[0060] Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or its embodiments, or vice versa.

[0061] According to a fourth aspect of the invention, there is provided a method for monitoring a supercritical geothermal reservoir, the method comprising: injecting two or more tracers into a supercritical geothermal reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the supercritical geothermal reservoir; analysing the at least one sample to detect the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the supercritical geothermal formation.

[0062] The at least one reactive tracer may be configured to react and / or interact under supercritical geothermal reservoir conditions. The at least one reactive tracer may be configured to degrade or partially degrade when exposed to temperatures within the range of 100°C to 450°C. The at least one reactive tracer may be configured to degrade or partially degrade when exposed to temperatures within the range of 374 to 450°C.

[0063] Each reactive tracer may be configured to degrade or partially degrade when exposed to a different temperature or different temperature range within the range of 100°C to 450°C. Each reactive tracer may be configured to degrade or partially degrade when exposed to a known temperature or known temperature range within the range of 100°C to 450°C. Each reactive tracer may be configured to degrade or partially degrade when exposed to a known temperature or known temperature range within the range of 374 to 450°C.

[0064] The at least one stable tracer may be configured to physically and / or chemically stable when exposed to temperatures within the range of 100°C to 450°C. The at least one stable tracer may be configured to physically and / or chemically stable when exposed to temperatures within the range of 374 to 450°C. The method may comprise measuring the concentration of the two or more tracers injected. The method may comprise determining a tracer injection profile or signature. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise determining a produced tracer profile or signature in the at least one sample. The method may comprise comparing the produced tracer profile or signature with the tracer injection profile or signature.

[0065] 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

[0066] According to a fifth aspect of the invention, there is provided a method for controlling energy output from a geothermal system comprising injecting two or more tracers into a geothermal reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; analysing fluid produced from the geothermal reservoir for the concentration of the two or more tracers; based on measured concentration of the two or more tracers adjusting an injection rate of the injection fluid and / or adjusting a production rate of the produced fluid.

[0067] The at least one reactive tracer may be configured to react or interact under geothermal reservoir conditions. The geothermal reservoir may be a supercritical geothermal reservoir. The at least one reactive tracer may be configured to react or interact under supercritical geothermal reservoir conditions. The method may comprise injecting tracer continuously. This may allow continuous monitoring of the geothermal reservoir. The method may comprise injecting tracer as a pulse. The method may comprise analysing characteristics of the tracer release, injection, injection volume, injection location, sampling time, sampling location, transport time, tracer concentration, temperature of injection fluid and / or production fluid and / or produced volume.

[0068] The method may comprise measuring the concentration of the two or more tracers injected. The method may comprise determining a tracer injection profile or signature. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise determining a produced tracer profile or signature in the at least one sample. The method may comprise comparing the produced tracer profile or signature with the tracer injection profile or signature. The method may comprise modelling the geothermal reservoir, tracer concentration, transport time, injection flow rate and / or production rates in a model. The geothermal reservoir model profile, tracer concentration, transport time, injection flow rate and / or production rate may be adjusted until calculated concentrations of model tracers compare with the measured concentrations of produced tracers to estimate geothermal reservoir characteristics. The model may be used to optimise and / or control the flow of fluid into, through and from the geothermal reservoir for power generation.

[0069] The method may comprise controlling the fluid injection flow rate at one or more injection well. The method may comprise controlling the fluid injection flow rate into the reservoir. The method may comprise controlling one or more pumps located at one or more injection well to control the fluid injection flow rate at one or more injection well. The method may comprise controlling the fluid injection flow rate to control the rate of cold fluid being injected into the reservoir. The method may comprise controlling fluid production flow rate at one or more production well. The method may comprise controlling the fluid production flow rate to control the rate of fluid extracted from the reservoir. The method may comprise controlling one or more pumps located at one or more production well to control the fluid production flow rate at one or more production well. The method may comprise controlling the fluid production flow rate to control the rate of heated fluid being extracted from the reservoir. The method may comprise maintaining a constant injection flow rate and / or production flow rate once a heat equilibrium rate for the reservoir has been achieved. The method may comprise maintaining a balanced injection flow rate and / or production flow rate once a heat equilibrium rate for the reservoir has been achieved. The method may comprise balancing the injection flow rate at one or more injection well with the production rate at one or more production well to obtain a heat equilibrium rate for the geothermal reservoir. The method may comprise balancing the heated fluid extracted via the one or more production well with the rate of heating injected fluid via the one or more injection wells. The method may comprise monitoring the reservoir temperature, injection rates and / or production rates. The method may comprise automatically adjusting the injection rates and / or production rates to substantially maintain a heat equilibrium rate for the geothermal reservoir. The method may comprise automatically adjusting the injection rates and / or production rates to optimise the heating capacity of the geothermal reservoir. The method may comprise automatically adjusting the injection rates and / or production rates to control and / or optimise the temperature of fluid produced from the geothermal reservoir. The method may comprise optimising power generation from the geothermal reservoir by adjusting and / or controlling the rate of fluid injection into the reservoir. The method may comprise optimising power generation from the geothermal reservoir by adjusting and / or controlling the rate of fluid injection into the reservoir based on the tracer data. The method may optimise power generation by adjusting and / or controlling the rate of fluid produced from the reservoir. The method may optimise power generation by adjusting and / or controlling the rate of pumping hot fluid out of the reservoir based on the tracer data. The method may comprise adjusting and / or controlling the rate of fluid produced from the reservoir based on measured concentration of the two or more tracers.

[0070] The method may comprise determining a heat equilibrium rate of the geothermal well. The rate of injected fluid may be adjusted based on the model data. The rate of produced fluid may be adjusted based on the model data. The methods may comprise maximizing energy recovery from a geothermal reservoir. The method may comprise calculating fluid transport, volumes and / or energy outtake from the geothermal reservoir.

[0071] 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.

[0072] According to a sixth aspect of the invention, there is provided a method of monitoring flow into, through and / or from a subterranean reservoir comprising the steps of: injecting a fluid and two or more tracers into subterranean reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample of fluid produced from the subterranean reservoir; and analysing the at least one sample to measure the type and / concentration of the at least one tracer.

[0073] The at least one reactive tracer may be configured to react or interact under reservoir conditions.

[0074] The subterranean formation may be selected from the group comprising a geothermal reservoir, a hydrocarbon reservoir; a carbon capture storage reservoir; a carbon dioxide storage reservoir, a subterranean hydrogen storage reservoir, a water reservoir and / or a shale reservoir. The method may comprise monitoring flow into, through and / or from the subterranean reservoir. The method may comprise measuring the concentration of the two or more tracers injected. The method may comprise determining a tracer injection profile or signature. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise determining a produced tracer profile or signature in the at least one sample. The method may comprise comparing the produced tracer profile or signature with the tracer injection profile or signature.

[0075] 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.

[0076] According to a seventh aspect of the invention, there is provided a system for monitoring a reservoir, the system comprising: a tracer injection device configured to inject two or more tracers into the reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; at least one pump configured to pump injection fluid and the two or more tracers into at least a portion of a reservoir; a collection device configure to collect samples of fluid produced from the reservoir.

[0077] The at least one reactive tracer may be configured to react or interact under reservoir conditions. The system may comprise at least one tracer analyser device configured to detect the concentration of the two or more tracers in fluid produced from the reservoir.

[0078] The system may comprise at least one probe. The at least one probe may be configured to detect the concentration of the two or more tracers 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 tracer analyser for analysing presence, type and / or concentration of the at least one tracer.

[0079] The system may comprise a processor. The processor may be configured to compare a tracer concentration profile of the injected two or more tracers with a tracer concentration profile of the two or more tracers in a sample as a function of time. The processor may be configured to determine and / or monitor a characteristic of the reservoir based on the presence and / or concentration of the two or more tracers in the samples as a function of time. The subterranean reservoir may be selected from the group comprising a geothermal reservoir, supercritical geothermal reservoir; a hydrocarbon reservoir; a carbon capture storage reservoir; a carbon dioxide storage reservoir, a subterranean hydrogen storage reservoir, a water reservoir and / or a shale reservoir.

[0080] 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.

[0081] According to an eighth aspect of the invention, there is provided a method for monitoring a hydrocarbon reservoir; the method comprising; injecting two or more tracers into a subterranean reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the reservoir; analysing the at least one sample to determine the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the hydrocarbon reservoir.

[0082] The at least one reactive tracer may be configured to react or interact under hydrocarbon reservoir conditions. The method may comprise measuring the concentration of the two or more tracers injected. The method may comprise determining a tracer injection profile or signature. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise determining a produced tracer profile or signature in the at least one sample. The method may comprise comparing the produced tracer profile or signature with the tracer injection profile or signature.

[0083] The at least one tracer may be a water tracer. The at least one tracer may be a hydrocarbon tracer. The at least one tracer may be an oil tracer. The at least one tracer may be a gas tracer.

[0084] 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 carbon capture storage reservoir; the method comprising; injecting two or more tracers into a subterranean reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the reservoir; analysing the at least one sample to detect the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the reservoir.

[0085] The two or more tracers may comprise at least one stable tracer. The method may comprise measuring the concentration of the two or more tracers injected. The method may comprise determining a tracer injection profile or signature. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise determining a produced tracer profile or signature in the at least one sample. The method may comprise comparing the produced tracer profile or signature with the tracer injection profile or signature.

[0086] The at least one reactive tracer may be configured to react or interact under carbon capture storage conditions. The method may comprise establishing a carbon quota offset value based on the amount of carbon dioxide remaining in the storage reservoir. The method may comprise establishing a carbon quota offset value based on the amount of carbon dioxide released from the storage reservoir. The injection fluid may be carbon dioxide. The method may comprise quantifying the amount of injected carbon dioxide remaining in the reservoir. The method may comprise quantifying the amount of injected carbon dioxide remaining in the storage formation based on the measured tracer concentration. The method may comprise quantifying the amount of carbon dioxide leaking from the reservoir based on the measured tracer concentration.

[0087] A model may simulate characteristics of the formation and / or pathways of injected carbon dioxide. The model may comprise parameters selected from the group comprising rock mechanics, temperature, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity, fluid saturation, physical behaviour of carbon dioxide, tracer and / or carbon dioxide injection amounts, injection volumes, injection rates, cap rock characteristics, migration path of the tracer and carbon dioxide in and / or through the storage formation and / or chemical behaviour of carbon dioxide.

[0088] The method may comprise updating the model based upon measured and / or calculated data. The method may comprise history matching. The method may comprise comparing historical parameter measurements to calculated data. The method may comprise adjusting one or more parameter measurements of the model until a reasonable match is achieved between the measured and calculated data. The model may simulate characteristics of the reservoir and / or pathways of injected carbon dioxide and at least one tracer. The method may comprise comparing modelled tracer sample data to measured tracer sample data. The reservoir may be an underground storage formation, a subsurface reservoir, an oil and / or gas reservoir, a saline formation, an abandoned coal seam, an organic-rich shale and / or a basalt formation. The method may comprise identifying the source of a leak of carbon dioxide stored in a storage reservoir based on the characteristics of the at least one tracer in the at least one sample.

[0089] 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.

[0090] According to a tenth aspect of the invention, there is provided a method for monitoring a subterranean hydrogen storage reservoir; the method comprising; injecting two or more tracers into the reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the reservoir; analysing the at least one sample to detect the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the reservoir.

[0091] The at least one reactive tracer may be configured to react or interact under hydrogen storage reservoir conditions. The two or more tracers may comprise at least one stable tracer.

[0092] The method may comprise measuring the concentration of the two or more tracers injected. The method may comprise determining a tracer injection profile or signature. The method may comprise measuring the concentration of the two or more tracers in the at least one sample. The method may comprise determining a produced tracer profile or signature in the at least one sample. The method may comprise comparing the produced tracer profile or signature with the tracer injection profile or signature.

[0093] The injection fluid may be hydrogen. The method may comprise quantifying the amount of injected hydrogen remaining in the reservoir. The method may comprise quantifying the amount of injected hydrogen remaining in the storage formation based on the measured tracer concentration. The method may comprise quantifying the amount of hydrogen leaking from the reservoir based on the measured tracer concentration.

[0094] 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.

[0095] According to an eleventh aspect of the invention, there is provided a method for monitoring a subterranean formation, the method comprising: analysing measured concentrations and type of tracer data from at least one sample previously collected from a fluid produced from a subterranean formation; wherein the subterranean formation comprises two or more tracers injected into the formation and wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; and based on the tracer data monitoring at least one characteristic of the subterranean formation.

[0096] The method may comprise comparing a tracer profile of the produced tracer in the sample with a tracer profile of the injected tracers.

[0097] The method may comprise comparing a ratio of two or more injected tracers with the ratio of the two or more tracers in the at least one sample. The method may comprise comparing a tracer concentrations signatures of the two or more tracers in the at least one sample with the tracer concentration signature of the two or more tracers injected into the injection well. The method may comprise comparing measured tracer concentration signatures of at least one reactive tracer and at least one stable tracer in the at least one sample with the tracer concentration signatures of the at least one reactive tracer and the at least one stable tracer injected into the injection well and / or reservoir. The subterranean formation may be selected from the group comprising a geothermal reservoir, a supercritical geothermal reservoir, a hydrocarbon reservoir; a carbon capture storage reservoir; a carbon dioxide storage reservoir, a subterranean hydrogen storage reservoir, a water reservoir and / or a shale reservoir.

[0098] 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.

[0099] According to a twelfth aspect of the invention, there is provided a method of collecting samples for analysis in monitoring a subterranean formation, wherein the reservoir comprises two or more tracers injected into the reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; producing fluid from the reservoir, collecting at least one sample from the produced fluid.

[0100] The at least one reactive tracer may be configured to react or interact under reservoir conditions.

[0101] The subterranean formation may be selected from the group comprising a geothermal reservoir, a supercritical geothermal reservoir, a hydrocarbon reservoir; a carbon capture storage reservoir; a carbon dioxide storage reservoir, a subterranean hydrogen storage reservoir, a water reservoir and / or a shale reservoir.

[0102] 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.

[0103] Brief description of the drawings

[0104] There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which:

[0105] Figure 1 is a simplified section of a geothermal system in accordance with an aspect of the invention; and Figure 2 is a simplified section of a supercritical geothermal system in accordance with another aspect of the invention; and

[0106] Figure 3 is a flow chart showing steps for the optimization of a model of a reservoir in accordance with an aspect of the invention.

[0107] Detailed description of preferred embodiments

[0108] Figure 1 is a simplified section of a geothermal system according to the invention shown generally as 10. The geothermal system has a geothermal reservoir 12 in communication with an injection well 14 and a production well 16. In this example the geothermal reservoir is in communication with one injection well and one production well. However, it will be appreciated that the geothermal reservoir may be in communication with multiple injection wells and / or multiple production well. The geothermal reservoir comprises a geothermal fluid disposed within the geothermal reservoir. The geothermal fluid flows within the geothermal reservoir.

[0109] Typically, geothermal reservoirs have a temperature in the range of 100°C to 374°C. In this example the temperature parameters of the geothermal reservoir are unknown. In order to determine and / or monitor the temperature of the geothermal reservoir a plurality of tracers are injected into the reservoir. A tracer injection device 18 is located at an upper end of the injection well. The injection device controls the amounts and / or types of the plurality of tracers of the tracer combination injected into the injection fluid in the injection well. The injection device may also control the duration and / or frequency the tracer combination is injected into the injection well.

[0110] In this example the different tracers in the tracer combination are selected to have different thermal stabilities. In this example five tracers are used including four reactive tracers designed to degrade or partially degrade at different temperature ranges. First tracer “A” has a thermal stability limit of 130°C, second tracer “B” has a thermal stability limit of 170°C, third tracer “C” has a thermal stability limit of 220°C and fourth tracer “D” has a thermal stability limit of 240°C. The tracer combination includes a thermally stable tracer designed with a high temperature stability, fifth tracer “E” with a thermal stability limit of 380°C. The fifth tracer is included as it is considered to be stable in typical geothermal temperature conditions and provide reference tracer data basis for differential interpretation. Each of the tracers in the tracer combination are injected simultaneously at known concentrations which determines a tracer injection profile or signature for tracer combination.

[0111] It will be appreciated that alternative tracer release methods and mechanisms may be used to release tracers into the injection fluid. As an example, tracer source may be connected to or encapsulated by a carrier. The tracer source may be located in a flow path of the injection fluid and may be configured to selectively release tracer from the carrier on contact with the injection fluid. The tracer and injection fluid are injected using pumps capable of operating at the pressure of the injection line. In this example the pump is connected to a flowline. It will be appreciated that it may alternatively be connected to a wellhead or other suitable injection point using a hose with suitable pressure rating. Typically, pneumatic pumps or electrical pumps are used for injection, but other pumping methods including manually operated pumps may be used. The tracer injection device may comprise a tracer injection pump configured to operate at a desired injection rate suitable to achieve the desired tracer concentration for each of the tracers in the tracer combination in the injection fluid and / or to efficiently inject the tracer in a short time period. Pumps must be rigged up ensuring integrity of the wellhead or flowline in case of hose or pump failure by e.g., installing check valves on the connection to the injection point.

[0112] As shown in Figure 1, the geothermal operation follows a cycle denoted by the arrows in Figure 1. The injection fluid migrates through the geothermal reservoir, as the fluid passes through the geothermal reservoir it is heated via a heat exchange with the subterranean formation of the geothermal reservoir. Heated fluid is pumped up the production well from the reservoir to the sampling location.

[0113] An upper end 16a of the production well is a sampling device 20. In this example samples of the produced geothermal fluid from the geothermal reservoir are collected at known sampling times. The sampling device is configured to take samples which are analysed to determine the presence and concentrations of tracers in the produced geothermal fluid. The analysis of the samples in this example are carried out using a HPLC combined with a fluorescent detector. It will be appreciated that alternative analysis techniques may be used depending on the tracer type. In this example all five tracers were injected at the same concentration. If the temperature of the geothermal reservoir is higher than the thermal stability limit of a tracer the tracer starts to degrade.

[0114] A tracer profile or signature of the sample of the produced fluid was determined using measured concentration of each tracer in the samples. The tracer profile or signature of the samples was compared with the injected tracer profile or signature. The resulted showed a significant reduction in the ratio of tracers “A” and “B” and the ratio of “C” was lower than “D” and “E”. This suggests that the temperature of the reservoir is high enough to significantly degrade tracers "A” and “B” and partially degrade tracer “C”, but the temperature is not high enough to affect tracers “D” and “E”. The temperature can be determined to be in the range of between 220°C and 240°C. It will be appreciated that by providing more tracers with overlapping thermal stability limits a more precise temperature level can be determined.

[0115] A model of the reservoir, temperature gradients and / or flow through the reservoir may be established. By monitoring the presence and / or concentration of the tracers in the samples produced from the reservoir temperature characteristics of the reservoir can be determined. It will be appreciated that the model may be adjusted for multiple injection points and / or multiple sampling points. It will be appreciated that the operational steps described above in relation to Figure 1 are examples of the invention and that one or more steps may be omitted or added and / or that the sequence of the steps may be different and / or steps may overlap in time.

[0116] The system uses a combination of thermally unstable tracer and at least one thermally stable tracer with the use of data interpretation to calculate the temperature of a reservoir and flow pathways. This information may be modelled and the model may include parameters including inject tracer profile, produced sample tracer profile, tracer degradation temperatures, tracer degradation rates, rock mechanics, injected fluid temperature, produced fluid temperature, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity, residence time distribution, fluid saturation, physical behaviour of geothermal fluid and / or chemical behaviour of geothermal fluid. The model may be updated based upon measured and / or calculated data. The reservoir model may employ history matching. History matching may use historical parameter measurements compared to calculated data. The parameters of the model may be adjusted until a reasonable match is achieved between the measured and calculated data.

[0117] By understanding the temperature gradients of the geothermal system may provide information on the heating efficiency, volume and / or enthalpy of the geothermal reservoir. To monitor changes over time a new tracer injection can be manually or automatically carried out and the changes in flow through the reservoir monitored. Monitoring tracer concentrations of each tracer over time and / or modelling may enable an understanding of the thermal characteristics of the geothermal reservoir system.

[0118] In this example the tracer combination is mixed, infused or co-injecting to monitor the fluid conditions as it passes through the reservoir. However, it will be appreciated that the tracer combination may alternatively be used to tag or label the fluid.

[0119] Figure 2 is a simplified section of a supercritical geothermal system according to the invention shown generally as 100. The geothermal system has a geothermal reservoir 112 which in this example is located at a depth near a brittle-ductile transition zone in the earth crust where magmatic intrusions 111 in the earth crust heat water in the reservoir to a supercritical state. Supercritical water is a state of water that occurs at extremely high temperatures and pressures, typically above 374 degrees Celsius and 22000 kPa (221 bar) resulting in the formation of a fluid that has unique properties where it behaves like a mixture of gas and liquid phases. The supercritical geothermal reservoir in communication with an injection well 114 and a production well 116. In this example the geothermal reservoir is in communication with one injection well and one production well. However, it will be appreciated that the supercritical geothermal reservoir may be in communication with multiple injection wells and / or multiple production well. It will be appreciated that the injection well may be a network of injection wells extending from the surface 18 to the geothermal reservoir and / or the production well may be a network of production wells extending from the surface 18 to the geothermal reservoir.

[0120] In this example five different tracers in a tracer combination are selected which have different thermal stabilities. In this example five tracers are used including a first tracer “A’ with a thermal stability limit of 380°C, a second tracer “B” with a thermal stability limit of 400°C, a third tracer “C” with a thermal stability limit of 420°C, a fourth tracer “D” with a thermal stability limit of 440°C and a fifth tracer “E” with a thermal stability limit of 500°C. The fifth tracer is included as it is considered to be stable in supercritical geothermal conditions and provide reference tracer data basis for differential interpretation.

[0121] Each of the tracers in the tracer concentration are injected using a tracer injection device 118 at known concentrations which is the tracer injection profile or signature. In this example all five tracers are injected at the same concentration. If the temperature of the supercritical geothermal reservoir is higher than the thermal stability limit of a tracer the tracer starts to degrade which will affect the tracer profile or signature.

[0122] In this example each of the five tracers are perfluorocarbon tracers and are detected and analysed using Gas chromatography-mass spectrometry (GC-MS). The ratio of the injected tracer concentrations were compared the ratio of the tracer concentrations in the samples. The results showed that all tracers were detected in the samples. Tracer “A” had a very low ratio, and tracer B has a low ratio compared to tracers “C”, “D” and “E”. This suggests that the temperature of the reservoir is high enough to significantly degrade tracer "A” and partially degrade tracer “B”, but the temperature is not high enough to affect tracers “C” “D” and “E”. The temperature can be determined or estimated to be in the range of between 400°C and 420°. It will be appreciated that by providing more tracers with overlapping thermal stability limits a more precise temperature level can be determined.

[0123] The detection of the tracer type, the concentration of tracer, injection rate, production rate and / or the transport time may be used to determine thermal characteristics of the reservoir and / or flow paths. A model of the reservoir and / or flow through the reservoir may be established.

[0124] It will be appreciated that in other examples other reactive tracers may be used to determine and / or characterise other parameters of the reservoir. As an example reactive tracers may be selected which exhibit intermolecular forces of attraction or repulsion with subterranean minerals and / or rocks. By monitoring the relative transport times of these reactive tracers compared with stable (unreactive) tracers will provide information on subterranean minerals and / or rocks in the reservoir. Figure 3 is a flow diagram 200 of the optimization of a model of a geothermal reservoir. Tracer data may be used to add resolution to a model of the reservoir model and calibrate the predicted behaviour model based on future thermal changes, injection rates and volumes.

[0125] In a first step an initial tracer test (step 201) such as described in Figure 1 may be carried out to determine or measure the thermal characteristics of geothermal reservoir and flow paths from the injection well, through the geothermal reservoir to the production well. The initial tracer test may establish thermal gradient map of the reservoir for an injector I producer pair. The method comprises repeating or continuous tracer testing to calibrate the model (step 202) and / or optimise the energy extraction from the geothermal reservoir. This may include optimisation or improvement of the injection rates, injection volumes, production rates and / or production volumes. Subsequent tracer tests (step 203) on the geothermal reservoir may further calibrate the model and identify issues with the geothermal reservoir at an early stage. A comparison of measured and modelled tracer data may identify or diagnose changes in the geothermal system such as a loss or reduction of enthalpy by injected water overcooling the reservoir.

[0126] In the above examples geothermal reservoirs are monitored. It will be appreciated that the system and methods may be adapted to tracer based monitoring of any subterranean formation including a hydrocarbon reservoir; a carbon capture storage reservoir; a subterranean hydrogen storage reservoir; a carbon dioxide storage reservoir, a water reservoir and / or a shale reservoir.

[0127] 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.

[0128] 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

Claims1. A method for monitoring a subterranean reservoir, the method comprising: injecting two or more tracers into a subterranean reservoir; wherein the two or more tracers comprise at least one reactive tracer and at least one stable tracer; collecting at least one sample from fluid produced from the subterranean reservoir; analysing the at least one sample to detect the presence or absence of the two or more tracers; based on measured tracer data monitoring at least one characteristic of the subterranean reservoir.

2. The method according to claim 1 comprising measuring a concentration of the two or more tracers in the at least one sample.

3. The method according to claim 1 or claim 2 comprising measuring a tracer concentration signature of the two or more tracers in the at least one sample.

4. The method according to any preceding claim comprising measuring a tracer concentration signature of the at least one reactive tracer and the at least one stable tracer in the at least one sample.

5. The method according to any preceding claim comprising comparing a ratio of each of the injected two or more tracers with the ratio of each of the two or more tracers in the at least one sample.

6. The method according to any of claims 3 to 5 comprising comparing the tracer concentrations signatures of the two or more tracers in the at least one sample with the tracer concentration signature of the two or more tracers injected into the injection well.

7. The method according to any of claims 3 to 6 comprising interpretating differences or changes between injected tracer concentrations signatures and produced tracer concentrations signatures to monitor at least one characteristic of the reservoir.

8. The method according to any preceding claim comprising injecting a plurality of tracers into the reservoir wherein the plurality of tracers comprises two or morereactive tracers each configured to react and / or interact with a different reservoir condition and / or a different reservoir condition parameter.

9. The method according to claim 8 wherein the plurality of tracers comprises two or more thermal reactive tracers each configured to degrade or partially degrade at a different temperature.

10. The method according to claim 9 comprising comparing tracer concentrations signatures of two or more thermal reactive tracers in the at least one sample with a tracer concentration signature of the two or more thermal reactive tracers injected into reservoir.

11. The method according to claim 9 or 10 comprising interpretating differences or changes between the concentration signatures of two or more thermal reactive tracers injected into the reservoir and the concentrations signatures of two or more thermal reactive tracers produced to monitor and / or map thermal characteristics of the reservoir.

12. The method according to claim 8 wherein the plurality of tracers comprises at least one reactive tracer configured to react and / or interact with rock types, rock surface, minerals or reservoir chemicals.

13. The method according to any preceding claim wherein the at least one reactive tracer and the at least one stable are tracer chemical tracers.

14. The method according to any preceding claim wherein the at least one reactive tracer is selected from the group comprising dyes, halogenated benzoates, DNA based tracer, nanoparticles, halogenated hydrocarbons, metal oxides perfluorinated hydrocarbons, perfluoroethers, partly fluorinated compounds, perfluoro buthane (PB), perfluoro methyl cyclopentane (PMCP), perfluoro methyl cyclohexane (PMCH), organofluorine, perfluorocarbon, polyaromatic sulfonate, sulfonates, naphthalene, naphthalene sulphonic acid and / or quantum dot.

15. The method according to any preceding claim wherein the at least one stable tracer is selected from the group comprising dyes, halogenated benzoates, DNA based tracer, nanoparticles, halogenated hydrocarbons, metal oxides perfluorinatedhydrocarbons, perfluoroethers, partly fluorinated compounds, perfluoro buthane (PB), perfluoro methyl cyclopentane (PMCP), perfluoro methyl cyclohexane (PMCH), organofluorine, perfluorocarbon, polyaromatic sulfonate, sulfonates, naphthalene, naphthalene sulphonic acid and / or quantum dot.

16. The method according to any preceding claim wherein the at least one reactive tracer is configured to react and / or interact to specific reservoir characteristics and or conditions and the at least one stable tracer is configured to be stable in reservoir conditions.

17. The method according to any preceding claim comprising collecting the at least one sample at one or more sampling times.

18. The method according to any preceding claim wherein the at least one characteristic of the reservoir is selected from the group comprising: reservoir condition, injection flow paths, reinjection flow paths; rate of cooling, rate of heating; injection rates, production rates, reservoir residence time; fluid residence time, injection fluid temperature, produced fluid temperature; reservoir temperature; reservoir temperature gradients, swept pore volumes, interwell connections and / or connectivity; rock surface area; rock type, mineral type, enthalpy, enthalpy output, mixing capacity of cold injected fluid with hot fluid in the reservoir, heat equilibrium rate of the reservoir, rock mechanics, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity and / or fluid saturation.

19. The method according to any preceding claim wherein the subterranean reservoir is selected from the group comprising a geothermal reservoir, a supercritical geothermal reservoir a hydrocarbon reservoir; a carbon capture storage reservoir; a carbon dioxide storage reservoir, a subterranean hydrogen storage reservoir, a water reservoir and / or a shale reservoir.

20. The method according to any preceding claim comprising creating a model of the reservoir comprising parameters selected from the group including: tracer injection profile or signature, tracer produced profile or signature, temperature of fluid injected, the number of injection wells, temperature, temperature of fluid at each injection well, temperature of fluid produced, number of production wells,temperature of fluid at each production well, transport time, transport time of tracercontaining fluid from injection to production for one or more injection wells, transport time of tracer-containing fluid from injection to production for one or more production wells, transport time of tracer-containing fluid through the reservoir, tracer type; tracer combination, concentration of tracer, concentration of tracer as a function of time, fluid flow path from one or more injection well to one or more production wells in communication to the geothermal reservoir, injection amount, injection rate, production rate, enthalpy, enthalpy output, residence time, mixing capacity of cold injected fluid with hot fluid in the reservoir, heat equilibrium rate of the reservoir, rock mechanics, gravity, density, viscosity; reservoir permeability, reservoir heterogeneities, solubility, fluid chemistry, porosity, fluid saturation, injection volumes and / or migration path of the at least one tracer in and / or through the formation.

21. The method according to claim 20 comprising adjusting the model until calculated concentrations of model tracers substantially match or best fit with the measured concentrations of produced tracers to estimate the reservoir characteristics.

22. A system for monitoring a reservoir, the system comprising: two or more tracer sources wherein the two or more tracer sources comprise at least one reactive tracer and at least one stable tracer; a tracer injection device configured to inject the two or more tracers into a reservoir; a collection device configured to collect samples of fluid produced from the reservoir.

23. The system according to claim 22 comprising at least one pump configured to pump injection fluid and the at least two or more tracers into an injection well or at least a portion of a reservoir.

24. The system according to claim 22 or 23 comprising at least one tracer analyser device configured to detect the concentration of the two or more tracers in fluid produced from the reservoir.

25. The system according to any of claims 22 to 24 comprising a processor configured to compare a tracer concentration profile of the injected two or more tracers with a tracer concentration profile of the two or more tracers in a sample.

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