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

Abstract

The present application relates to the technical field of oil and gas development, and particularly relates to a method for determining the occurrence state of pore water in a shale gas reservoir based on ion displacement efficiency, comprising: obtaining a target reservoir shale core, removing water in the core and determining the water saturation of the core; making the core adsorb brine until saturation, and displacing the brine in the core by using a displacement medium containing a tracer until the displacement medium completely replaces the brine in the core; recording the change of the concentration value of the tracer with time during the displacement process; and determining the occurrence state of the pore water in the shale gas based on the slope of the change of the concentration value of the tracer with time and the water saturation. The present application uses the tracer to track the occurrence water in the rock sample, and uses the slope of the change of the concentration of the tracer with time and the water saturation to more accurately obtain the absolute content of the water in different occurrence states in the shale reservoir, thereby providing technical support for the evaluation and exploration and development of the shale gas content.

Description

Technical Field

[0001] This invention relates to the field of oil and gas development technology, and in particular to a method for determining the pore water occurrence state of shale gas reservoirs based on ion displacement efficiency. Background Technology

[0002] During the exploration and development of shale gas, with the increased efforts in deep shale gas exploration and development, the fracturing and flowback rate of shale gas wells is relatively high. In some cases, shale gas wells with low gas production and high water production have even appeared. This phenomenon indicates that the water occurrence state and its migration and accumulation laws in shale gas reservoirs are not well understood, and it has seriously affected the selection of favorable shale gas areas and efficient exploration and development.

[0003] Compared to the quantitative characterization of gas adsorption and ionization in pores, the fundamental theory and evaluation techniques for the microscopic occurrence of liquid fluids are relatively weak. Shale reservoirs have well-developed nanoscale pores and exhibit significant nano-effects, with a high proportion of adsorbed fluids that cannot be ignored. Currently, both domestic and international research mainly evaluates shale water-bearing capacity through macroscopic parameters such as initial water saturation, bound water saturation, and mobile water saturation. Ignoring differences in pore water occurrence states will lead to an overestimation of water saturation.

[0004] Currently, the main characterization methods for the occurrence state of pore water in shale gas reservoirs include high-speed centrifugation, nuclear magnetic resonance (NMR) T1-T2 spectroscopy, and equilibrium water adsorption. High-speed centrifugation inherits the traditional methods for characterizing mobile water in reservoirs; however, if the centrifugation speed is low, the fluid within the nanopores cannot be effectively characterized; if the centrifugation speed is too high, it can easily damage the core, leading to significant errors in the experimental results. NMR T1-T2 spectroscopy relies primarily on NMR experiments, but the accuracy of NMR characterization is only 3 nm, making it impossible to characterize a large number of micropores. Furthermore, establishing T1-T2 charts requires a NMR computational model, making quantitative characterization difficult. Equilibrium water adsorption experiments mainly use powder samples, characterizing water adsorption characteristics through adsorption volume. However, water vapor is often adsorbed into the gaps between particles, leading to an overestimation of adsorption volume. Therefore, it is necessary to develop new methods for characterizing the occurrence state of pore water in shale gas reservoirs, clarifying the water content in different occurrence states, and thus guiding shale gas content assessment and exploration and development. Summary of the Invention

[0005] To overcome at least one of the aforementioned defects, this invention proposes a method for determining the pore water occurrence state of shale gas reservoirs based on ion displacement efficiency. By studying reservoir samples and calculating the gas and water content in the reservoir through displacement, this method can assist in the exploration and exploitation of shale gas.

[0006] To achieve the above objectives, the method for determining the pore water occurrence state disclosed in this invention can adopt the following technical solution:

[0007] A method for determining the pore water occurrence state 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] The core is saturated with brine, and the brine in the core is displaced by a displacement medium containing a tracer until the displacement medium completely replaces the brine in the core.

[0010] Record the changes in tracer concentration over time during the displacement process;

[0011] The state of shale gas pore water occurrence is determined based on the slope of the tracer concentration value changing over time and the water saturation, specifically 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] Where Q is the volume of water stored; K is the slope of the time-tracer concentration change curve; T is the displacement time; subscript a indicates the tracer injection stage; subscript b indicates the stage in the tracer flushing stage where the tracer concentration begins to decrease with increasing time; subscript e indicates the stage in the tracer flushing stage where the tracer concentration stabilizes; subscripts c and d are both stages between stage b and stage e; S w This represents the water saturation level.

[0014] The aforementioned method for determining the pore water occurrence state uses displacement tracing to determine the water storage capacity of the pores in the core after obtaining the core, thereby calculating the pore water occurrence state in the shale gas reservoir. This provides a more accurate and reliable reference for shale gas exploration and development, and can further determine a more reasonable construction and development plan.

[0015] Furthermore, in this invention, removing water from the core before saturating it with brine facilitates accurate determination of the core's water retention capacity and improves the accuracy of subsequent displacement tracing. Various methods can be used to remove water from the core, and this is not a single, limited method. Here, we optimize and propose one feasible option: removing water from the core by drying. During the drying process, the weight of the core is continuously recorded, and when the core weight decreases to a constant weight, it is determined that the water has been removed. Using this method, a low-temperature drying method can be employed, which ensures effective removal of water from the core while avoiding damage to the core structure from high temperatures.

[0016] Furthermore, when the core is dried and saturated with brine, various brine solutions can be used, and the specific solution is not limited to one. Here, we optimize the process and propose one feasible option: the brine is a sodium chloride solution, and the concentration of sodium chloride is equal to the concentration of the production water from the target reservoir gas well. Using this approach, sodium chloride is readily available, inexpensive, and non-acidic / alkali-corrosive, thus avoiding damage to the core structure and ensuring the accuracy of subsequent testing.

[0017] Furthermore, various displacement media can be used in this invention, and they are not limited to a single one. Here, we optimize and propose one feasible option: the displacement media is prepared using distilled water. When this scheme is adopted, the distilled water is composed of H2O and does not contain other impurities. Therefore, the prepared media will not cause corrosion or damage to the core, and the measured results are closer to the actual occurrence.

[0018] Furthermore, in this invention, the tracer can be a variety of substances and is not limited to one. Here, we optimize and propose one feasible option: the tracer of the displacement medium is an anionic tracer or a cationic tracer.

[0019] Furthermore, to better calculate the occurrence state and process the measured data, one feasible option is proposed here: While recording the tracer concentration change over time, a time-concentration curve of the tracer is generated. Using this approach, the displacement process is divided into an injection stage, a flushing stage, and a displacement termination stage using the tracer's time-concentration curve. The tracer concentration change varies in each stage, and the concentration change rate for the corresponding stage can be determined based on the curve.

[0020] Furthermore, the method of saturating the core with brine is optimized, and one feasible option is proposed: when the core is saturated with brine, the core is fixed by a clamp, and brine is continuously poured onto the core. With this approach, a weight sensor can be installed on the clamp to continuously monitor the weight of the core as it absorbs brine; a constant weight indicates saturation.

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

[0022] This invention utilizes tracers to track the water content in rock samples. By using the slope of the tracer concentration change over time and the water saturation, the absolute water content in different occurrence states of shale reservoirs can be obtained more accurately, providing technical support for the assessment and exploration and development of shale gas content. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the process of the group topology control method. Detailed Implementation

[0025] The present invention will be further explained below with reference to 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 and determine the amount of water, which affects shale gas extraction, the following embodiments are optimized to overcome the defects of the existing technology.

[0027] Example

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

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

[0030] S02: To adsorb brine into the core until it is saturated. The inlet end of the core is a solution with a tracer, and the outlet end is saturated brine. Displacement is carried out 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. The ion concentration at the outlet end tends to stabilize, which is considered as complete displacement.

[0031] S03: The concentration of the tracer in the brine at the outlet end was measured using laser ablation-inductively coupled plasma mass spectrometry (ICP-MS). The change in tracer concentration over time during the displacement process was recorded; specifically, the tracer concentration in the displaced solution was continuously sampled and monitored, and the displacement was considered complete when the tracer concentration reached its highest level.

[0032] S04: The state of shale gas pore water occurrence is determined based on the slope of the tracer concentration value changing over time and the water saturation, specifically calculated 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] Where Q is the volume of water stored; K is the slope of the time-tracer concentration change curve; T is the displacement time; subscript a indicates the tracer injection stage; subscript b indicates the stage in the tracer flushing stage where the tracer concentration begins to decrease with increasing time; subscript e indicates the stage in the tracer flushing stage where the tracer concentration stabilizes; subscripts c and d are both stages between stage b and stage e; S w This represents the water saturation level.

[0035] The method for determining the pore water occurrence state provided in this embodiment determines the water storage capacity of the pores in the core by displacement tracing after obtaining the core, thereby calculating the pore water occurrence state in the shale gas reservoir. This provides a more accurate and reliable reference for shale gas exploration and development, and can further determine a more reasonable construction and development plan.

[0036] In this embodiment, removing water from the core before saturating it with brine facilitates accurate determination of the core's water retention capacity and improves the accuracy of subsequent displacement tracing. Various methods can be used to remove water from the core, and this embodiment is not limited to a single one. This embodiment optimizes and adopts one feasible option: removing water from the core by drying. During the drying process, the weight of the core is continuously recorded, and when the core weight decreases to a constant weight, it is determined that the water has been removed. Using this method, a low-temperature drying method can be employed, which ensures effective removal of water from the core while avoiding damage to the core structure from high temperatures.

[0037] After the core is dried, it absorbs brine to reach saturation. Various brine solutions with different compositions can be used, and the specific solution is not limited to one. This embodiment optimizes the process and adopts one feasible option: the brine is a sodium chloride solution, and the concentration of sodium chloride is equal to the concentration of the production water from the target reservoir gas well. Using this approach, sodium chloride is readily available, inexpensive, and non-acidic / alkali-corrosive, thus avoiding damage to the core structure and ensuring the accuracy of subsequent testing.

[0038] This embodiment can use various displacement media, and is not limited to one. This embodiment optimizes and adopts one feasible option: the displacement media is prepared using distilled water. When this scheme is adopted, the distilled water is composed of H2O and does not contain other impurities. Therefore, the prepared media will not cause corrosion or damage to the core, and the measured results are closer to the actual occurrence.

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

[0040] To better calculate the assigned state and process the measured data, this embodiment employs one feasible option: such as... Figure 1 As shown, a time-concentration curve of the tracer is generated when recording the change in tracer concentration over time. Using this method, the displacement process is divided into an injection stage, a flushing stage, and a displacement termination stage based on the tracer's time-concentration curve. The concentration change of the tracer differs in each stage, and the concentration change rate for the corresponding stage can be determined from the curve.

[0041] The method of saturating the core with brine was optimized, and one feasible option was adopted: when the core was saturated with brine, the core was fixed by a clamp, and brine was continuously poured onto the core. With this approach, a weight sensor can be installed on the clamp to continuously monitor the weight of the core as it absorbs brine, with constant weight indicating saturation.

[0042] The above are the embodiments listed in this example. However, this example is not limited to the optional embodiments described above. Those skilled in the art can arbitrarily combine the above methods to obtain other various embodiments. Anyone can derive other various forms of embodiments under the guidance of this example. The above specific embodiments should not be construed as limiting the scope of protection of this example. The scope of protection of this example should be defined in the claims.

Claims

1. A method for determining the pore water occurrence state in shale gas reservoirs based on ion displacement efficiency, characterized in that, include: S01: Obtain the shale core of the target reservoir, remove the water in the core and determine the water saturation of the core; S02: The core is saturated with brine. The inlet end of the core is a solution with a tracer, and the outlet end is saturated brine. Displacement is carried out under a certain pressure, which 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. The ion concentration at the outlet end tends to stabilize, which is considered as complete displacement. S03: The concentration of tracer in the brine at the outlet end is measured using a laser ablation-inductively coupled plasma mass spectrometer, and the change of tracer concentration over time during the displacement process is recorded; the concentration of tracer in the displaced solution is continuously sampled and monitored, and the displacement is completed when the tracer concentration reaches its highest value. S04: The state of shale gas pore water occurrence is determined based on the slope of the tracer concentration value changing over time at each stage and the water saturation. Specifically, the calculation is performed as follows: Where Q is the volume of water stored; K is the slope of the time-tracer concentration change curve; T is the displacement time; subscript a indicates the tracer injection stage; subscript b indicates the stage in the tracer flushing stage where the tracer concentration begins to decrease with increasing time; subscript e indicates the stage in the tracer flushing stage where the tracer concentration stabilizes; subscripts c and d are both stages between stage b and stage e; S w This represents the water saturation level.

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

3. The method for determining the pore water occurrence state of 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 the production water in the target reservoir gas well.

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

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

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

7. The method for determining the pore water occurrence state of 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 in place by a clamp and brine is continuously poured onto the core.

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