Method for determining pore water occurrence state of shale gas reservoir based on ion displacement efficiency

Abstract

The invention relates to the technical field of oil and gas development, in particular to a shale gas reservoir pore water occurrence state determination method based on ion displacement efficiency, which comprises the following steps: acquiring a target reservoir shale core, removing moisture in the core and determining the water saturation of the core; enabling the rock core to adsorb the saline water until the rock core is saturated, and displacing the saline water in the rock core through a displacement medium containing a tracer agent until the displacement medium completely replaces the saline water in the rock core; recording the change condition of the concentration value of the tracer agent along with time in the displacement process; and calculating and determining the shale gas void water occurrence state according to the slope of each stage of the tracer concentration value along with the time change and the water saturation. According to the method, the occurrence water in the rock sample is tracked by utilizing the tracer agent, the absolute content of the water in different occurrence states of the shale reservoir can be more accurately obtained by utilizing the slope of the concentration of the tracer agent along with the time change and the water saturation, and technical support is provided for evaluation, exploration and development of the gas content of the shale gas.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas development, and in particular to a method for determining the occurrence state of pore water in a shale gas reservoir based on ion displacement efficiency. Background Art

[0002] During the shale gas exploration and development process, with the intensification of deep shale gas exploration and development, the fracturing flowback rate of shale gas wells is relatively high, and even shale gas wells with low gas production and high water production have appeared. This phenomenon shows that the water storage state and its migration and accumulation rules in shale gas reservoirs are not clearly understood, and has seriously affected the optimization of shale gas favorable areas and efficient exploration and development.

[0003] Compared with the quantitative characterization of gas adsorption and release in pores, the basic theory and evaluation technology of liquid fluid microscopic occurrence are relatively weak. Shale reservoirs have developed nanoscale pores and significant nano effects. The proportion of adsorbed fluid is high and cannot be ignored. At present, macroscopic parameters such as initial water saturation, bound water saturation, and movable water saturation are mainly used to evaluate the water content of shale at home and abroad. If the difference in the occurrence state of pore water is not taken into account, the water saturation will be overestimated.

[0004] At present, the main methods for characterizing the state of pore water in shale gas reservoirs include high-speed centrifugation, nuclear magnetic resonance T1-T2, equilibrium water adsorption and other methods. The high-speed centrifugation method inherits the characterization method of movable water in traditional conventional reservoirs. However, if the speed is low during the centrifugation process, the fluid in the nanopores cannot be effectively characterized; if the centrifugation speed is too high, it is easy to cause damage to the core, resulting in large errors in the experimental results. The nuclear magnetic resonance T1-T2 spectrum test mainly relies on nuclear magnetic resonance experiments. The accuracy of nuclear magnetic resonance experimental characterization is 3nm, and a large number of micropores cannot be characterized; secondly, the establishment of T1-T2 plates requires the nuclear magnetic calculation model as the basis, and quantitative characterization is difficult. The equilibrium water adsorption experiment mainly uses powder samples for experiments, and the adsorption characteristics of water are characterized by the adsorption amount. Often, the gaps between particles will also adsorb water vapor, resulting in an overestimation of the adsorption amount. Therefore, it is necessary to develop new methods to characterize the state of pore water in shale gas reservoirs, clarify the content of water in different states of occurrence, and then guide the evaluation and exploration and development of shale gas content. Summary of the invention

[0005] In order to overcome at least one of the defects mentioned above, the present invention proposes a method for determining the pore water state of shale gas reservoirs based on ion displacement efficiency. The method studies the reservoir by sampling and determines the gas and water content in the reservoir by displacement, thereby assisting the exploration and exploitation of shale gas.

[0006] In order to achieve the above purpose, the method for determining the pore water occurrence state disclosed in the present invention can adopt the following technical scheme:

[0007] The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency includes:

[0008] Obtain shale cores from the target reservoir, remove water from the cores and determine the water saturation of the cores;

[0009] Allow the core to absorb the brine until it is saturated, and displace the brine in the core with a displacement medium containing a tracer until the displacement medium completely replaces the brine in the core;

[0010] Record the change of tracer concentration over time during the displacement process;

[0011] The occurrence state of shale gas pore water is determined based on the slope of each stage of the tracer concentration value changing with time and the water saturation. The specific calculation method is as follows:

[0012] Q b =K b *T b / (K a *T a +K b *T b +K c *T c +K d *T d +K e *T e )*S w

[0013] Wherein, Q is the amount of stored water; K is the slope of the time-tracer concentration curve; T is the displacement time; subscript a indicates the tracer injection stage; subscript b indicates the stage in the tracer flushing stage when the tracer concentration begins to decrease with time; subscript e indicates the stage in the tracer flushing stage when the tracer concentration is stable; subscript c and subscript d are both stages between stage b and stage e; S w is the water saturation.

[0014] The above-disclosed method for determining the occurrence state of pore water determines the water storage capacity of the voids in the core by means of displacement tracing after obtaining the core, thereby inferring the occurrence state of pore water in the shale gas reservoir, which is convenient for providing a more accurate and reliable reference for shale gas exploration and exploitation, and can further determine a more reasonable construction and exploitation plan.

[0015] Furthermore, in the present invention, after removing the water in the core, the core is allowed to absorb brine to reach saturation, which is convenient for accurately determining the water storage capacity of the core and the accuracy of subsequent displacement tracing. The water in the core can be removed in a variety of ways, which are not limited to the only way. Here, optimization is performed and one of the feasible options is proposed: remove the water in the core by drying, continuously record the weight of the core during the drying process, and judge that the water is removed when the weight of the core is reduced to a constant weight. When such a scheme is adopted, low-temperature drying can be adopted, which can not only ensure the effective removal of water in the core, but also prevent the core structure from being damaged by high temperature.

[0016] Furthermore, when the core is dried and absorbs brine to reach a saturated state, brine with multiple components can be absorbed, which is not limited to the specific one. Here, optimization is made and one feasible option is proposed: the brine is a sodium chloride solution, and the concentration of sodium chloride is equal to the concentration of produced water from the target reservoir gas well. When such a solution is adopted, sodium chloride is easy to obtain, the cost is low, and it is not acid-base corrosive, and will not damage the core structure, which can ensure the accuracy of subsequent detection.

[0017] Furthermore, the present invention can use a variety of displacement media, which are not limited to the only one. Here, optimization is made and one feasible option is proposed: the displacement medium is prepared with distilled water. When such a solution is adopted, the distilled water component is H2O and does not contain other impurities. Therefore, the prepared body medium will not cause corrosion damage to the core, and the measured result is closer to the actual occurrence.

[0018] Furthermore, in the present invention, the tracer can be a variety of substances, which are not limited to a single substance. Here, optimization is performed and one feasible option is proposed: the tracer of the displacement medium is an anionic tracer or a cationic tracer.

[0019] Furthermore, in order to better calculate the occurrence state and process the measured data, one feasible option is proposed here: when recording the change of tracer concentration over time, a tracer time-concentration change curve is made. When such a scheme is adopted, the displacement process is divided into the injection stage, the flushing stage and the end of displacement stage by the tracer time-concentration change curve. The concentration change of the tracer in each stage is different, and the concentration change rate of the corresponding stage can be determined according to the curve.

[0020] Furthermore, the method of absorbing brine to saturation of the core is optimized and one feasible option is proposed: when the core absorbs brine to saturation, the core is fixed by a clamp and brine is continuously poured onto the core. When such a scheme is adopted, a weight sensor can be set on the clamp, and the weight of the core changes continuously as the core absorbs brine, and finally a constant weight indicates adsorption saturation.

[0021] Compared with the prior art, some beneficial effects of the technical solution disclosed in the present invention include:

[0022] The present invention uses tracers to track the stored water in rock samples, and uses the slope of the tracer concentration change over time and the water saturation to more accurately obtain the absolute content of water in different storage states of shale reservoirs, providing technical support for the evaluation, exploration and development of shale gas content. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only represent some embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 Schematic diagram of the process of group topology control method. DETAILED DESCRIPTION

[0025] The present invention will be further explained below in conjunction with the accompanying drawings and specific embodiments.

[0026] In view of the fact that pore water exists in existing shale gas reservoirs, but it is difficult to accurately explore to determine the amount of water, which in turn affects shale gas extraction, the following embodiments are optimized to overcome the defects in the prior art.

[0027] Example

[0028] This embodiment provides a method for determining the occurrence state of pore water in a shale gas reservoir based on ion displacement efficiency, including:

[0029] S01: Obtain the target reservoir shale core, remove the water in the core and determine the water saturation of the core;

[0030] S02: Allow the core to absorb brine until it is saturated. The core inlet is a solution with a tracer, and the outlet is saturated brine. Displacement is performed under a certain pressure (the pressure is designed according to actual needs); the displacement medium displaces the brine in the core until the displacement medium completely replaces the brine in the core, and the ion concentration at the outlet tends to be stable, which is considered complete displacement;

[0031] S03: Use laser ablation-inductively coupled plasma mass spectrometry to measure the tracer concentration in the brine at the outlet. Record the change of the tracer concentration over time during the displacement process; specifically, continuously sample and monitor the tracer concentration in the displaced solution, and when the tracer concentration reaches the maximum, it indicates that the displacement is complete.

[0032] S04: The occurrence state of shale gas pore water is determined based on the slope of each stage of the tracer concentration value changing with time and the water saturation. The specific calculation is as follows:

[0033] Q b =K b *T b / (K a *T a +K b *T b +K c *T c +K d *T d +K e *T e )*S w

[0034] Wherein, Q is the amount of stored water; K is the slope of the time-tracer concentration curve; T is the displacement time; subscript a indicates the tracer injection stage; subscript b indicates the stage in the tracer flushing stage when the tracer concentration begins to decrease with time; subscript e indicates the stage in the tracer flushing stage when the tracer concentration is stable; subscript c and subscript d are both stages between stage b and stage e; S w is the water saturation.

[0035] The method for determining the occurrence state of pore water provided in this embodiment determines the water storage capacity of the voids in the core by a displacement tracing method after obtaining the core, thereby inferring the occurrence state of pore water in the shale gas reservoir, which is convenient for providing a more accurate and reliable reference for shale gas exploration and exploitation, and can further determine a more reasonable construction and exploitation plan.

[0036] In this embodiment, after removing the water in the core, the core is allowed to absorb brine to reach saturation, which is convenient for accurately determining the water storage capacity of the core and the accuracy of subsequent displacement tracing. The water in the core can be removed in a variety of ways, which are not limited to the only one. This embodiment is optimized and adopts one of the feasible options: removing the water in the core by drying, continuously recording the weight of the core during the drying process, and judging that the water is removed when the weight of the core is reduced to a constant weight. When such a scheme is adopted, low-temperature drying can be used, which can not only ensure the effective removal of water in the core, but also prevent the core structure from being damaged by high temperature.

[0037] When the core is dried and absorbs brine to reach a saturated state, brine with multiple components can be absorbed, which is not limited to the specific one. This embodiment is optimized and adopts one of the feasible options: the brine is a sodium chloride solution, and the concentration of sodium chloride is equal to the concentration of produced water from the target reservoir gas well. When such a solution is adopted, sodium chloride is easy to obtain, the cost is low, and it is not acid-base corrosive, and will not damage the core structure, which can ensure the accuracy of subsequent detection.

[0038] In this embodiment, a variety of displacement media can be used, which is not limited to the only one. This embodiment is optimized and adopts one of the feasible options: the displacement medium is prepared with distilled water. When such a solution is adopted, the distilled water component is H2O and does not contain other impurities. Therefore, the configured body medium will not cause corrosion damage to the core, and the measured result is closer to the actual occurrence.

[0039] In this embodiment, the tracer can be a variety of substances, which are not limited to the only one. This embodiment is optimized and adopts one of the feasible options: the tracer of the displacement medium is an anion tracer or a cationic tracer (such as Br - ,I - ,ReO4 - ,Sb - Complex, Li + ,Cs + ,Co 2+ ).

[0040] In order to better calculate the occurrence state and process the measured data, this embodiment adopts one of the feasible options: Figure 1 As shown, when recording the tracer concentration change over time, a tracer time-concentration change curve is prepared. When such a scheme is adopted, the displacement process is divided into the injection stage, the flushing stage and the displacement end stage by the tracer time-concentration change curve. The tracer concentration change in each stage is different, and the concentration change rate of the corresponding stage can be determined according to the curve.

[0041] The method of absorbing brine to saturation of the core is optimized and one of the feasible options is adopted: when the core absorbs brine to saturation, the core is fixed by a clamp and brine is continuously poured onto the core. When such a scheme is adopted, a weight sensor can be set on the clamp, and the weight of the core changes continuously as the core absorbs brine, and the final constant weight indicates saturation of adsorption.

[0042] The above are the implementation methods listed in this embodiment, but this embodiment is not limited to the above optional implementation methods. Those skilled in the art can arbitrarily combine the above methods to obtain other various implementation methods. Anyone can derive other various implementation methods under the inspiration of this embodiment. The above specific implementation methods should not be understood as limiting the protection scope of this embodiment. The protection scope of this embodiment should be based on the definition in the claims.

Claims

1. A method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency, characterized in that: include: Obtain shale cores from the target reservoir, remove water from the cores and determine the water saturation of the cores; Allow the core to absorb the brine until it is saturated, and displace the brine in the core with a displacement medium containing a tracer until the displacement medium completely replaces the brine in the core; The inlet of the core is a solution with a tracer, and the outlet is a saturated brine, which is displaced under a certain pressure (the pressure is designed according to actual needs). The concentration of the tracer in the brine at the outlet is measured using a laser ablation-inductively coupled plasma mass spectrometer. The concentration of the tracer in the brine at the outlet changes with time during the displacement process. The occurrence state of shale gas pore water is determined based on the slope of each stage of the tracer concentration value changing with time and the water saturation. The specific calculation method is as follows: Q b =K b *T b / (K a *T a +K b *T b +K c *T c +K d *T d +K e *T e )*S w Wherein, Q is the amount of stored water; K is the slope of the time-tracer concentration curve; T is the displacement time; subscript a indicates the tracer injection stage; subscript b indicates the stage in the tracer flushing stage when the tracer concentration begins to decrease with time; subscript e indicates the stage in the tracer flushing stage when the tracer concentration is stable; subscript c and subscript d are both stages between stage b and stage e; S w is the water saturation.

2. The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency according to claim 1, characterized in that: The moisture in the core is removed by drying. The weight of the core is continuously recorded during the drying process. When the weight of the core is reduced to a constant weight, it is determined that the moisture has been removed.

3. The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency according to claim 1, characterized in that: The brine is a sodium chloride solution, and the concentration of sodium chloride is equal to the concentration of produced water from the target reservoir gas well.

4. The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency according to claim 1, characterized in that: The displacement medium is prepared with distilled water.

5. The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency according to claim 1 or 4, characterized in that: The displacement medium tracer is an anion tracer or a cation tracer.

6. The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency according to claim 1, characterized in that: When recording the tracer concentration change over time, a time-concentration change curve of the tracer is prepared.

7. The method for determining the occurrence state of pore water in shale gas reservoirs based on ion displacement efficiency according to claim 1, characterized in that: When the core is saturated with brine, the core is fixed by a clamp and brine is continuously poured onto the core.

Images