A method for characterizing gas storage space in shale gas reservoirs

Abstract

The present invention provides a method for characterizing the gas storage space of a shale gas reservoir, and relates to the field of unconventional natural gas. The method comprises the following steps: S1, preparing a shale sample; S2, placing the shale sample in a scattering chamber to perform a first small-angle neutron scattering experiment, and obtaining a first two-dimensional scattering pattern of the shale sample; S3, injecting a deuterated mixed gas into the scattering chamber to a target pressure, and then performing a second small-angle neutron scattering experiment, and obtaining a second two-dimensional scattering pattern of the shale sample; S4, respectively obtaining first pore structure information and second pore structure information of the shale sample before and after the deuterated mixed gas is injected according to the first two-dimensional scattering pattern and the second two-dimensional scattering pattern; S5, characterizing the gas storage space of the shale sample according to the first pore structure information and the second pore structure information. The present invention can more comprehensively characterize the gas storage space of a shale gas reservoir, and improve the accuracy of natural gas production potential prediction.

Description

Technical Field

[0001] The present invention relates to the field of unconventional natural gas, and in particular to a method for characterizing the gas storage space of a shale gas reservoir. Background Art

[0002] With the development of advanced horizontal well drilling and hydraulic fracturing technologies, abundant shale gas resources have been effectively developed. Even if other energy sources such as wind, solar or geothermal energy continue to contribute to the energy mix, natural gas from unconventional reservoirs will undoubtedly be an important energy source. However, unconventional natural gas development is not without technical difficulties. Currently, low recovery is one of its main problems, and most of the natural gas is still trapped in the reservoir. Improving the recovery rate of unconventional natural gas will be imperative. At present, domestic and foreign studies have found that the accessibility of pore space in unconventional reservoirs (coal and shale) is an important factor affecting methane storage capacity and methane desorption capacity. Quantifying open (accessible) and closed (inaccessible) porosity is very important for predicting natural gas production potential. Open pores form gas transmission paths, and they are responsible for fluid flow in the rock matrix. Therefore, shale pore accessibility is the key to the evaluation and development of shale oil and gas reservoirs, but due to the extremely low permeability and complexity of shale pore systems, accurate measurement of gas storage space in pores and analysis of controlling factors are challenging. Summary of the invention

[0003] The present invention aims to provide a method for characterizing the gas storage space in a shale gas reservoir, which can quantitatively characterize the gas storage space in the shale gas reservoir.

[0004] The present invention provides a method for characterizing the gas storage space of a shale gas reservoir, comprising the following steps:

[0005] S1. Prepare shale samples;

[0006] S2, placing the shale sample in a scattering chamber to perform a first small-angle neutron scattering experiment to obtain a first two-dimensional scattering pattern of the shale sample;

[0007] S3, injecting deuterated mixed gas into the scattering chamber to a target pressure, and then performing a second small-angle neutron scattering experiment to obtain a second two-dimensional scattering pattern of the shale sample;

[0008] S4, respectively obtaining first pore structure information and second pore structure information of the shale sample before and after the deuterated mixed gas is injected according to the first two-dimensional scattering pattern and the second two-dimensional scattering pattern;

[0009] S5. Characterize the gas storage space of the shale sample according to the first pore structure information and the second pore structure information.

[0010] Furthermore, in step S1, the shale sample is in the form of a sheet with a thickness of 0.2 to 0.5 mm.

[0011] Furthermore, step S1 also includes pre-treating the shale sample; the pre-treatment method is as follows: drying the shale sample at 45-65° C. to constant weight.

[0012] Furthermore, in step S2, before performing the first small-angle neutron scattering experiment, the scattering chamber is evacuated so that the shale sample is in a vacuum environment.

[0013] Further, in step S3, the deuterated mixed gas is a mixture of methane and deuterated methane, and the SLD value of the deuterated mixed gas is equal to the SLD value of the shale sample.

[0014] Furthermore, in step S3, the second small-angle neutron scattering experiment is performed after maintaining the target pressure for 15 to 30 minutes.

[0015] Further, in step S4, the method for obtaining the first pore structure information and the second pore structure information is as follows:

[0016] Importing the first two-dimensional scattering pattern and the second two-dimensional scattering pattern into fit 2D for one-dimensional processing respectively to obtain a first one-dimensional scattering intensity curve and a second one-dimensional scattering intensity curve, and performing background subtraction and normalization processing on the first one-dimensional scattering intensity curve and the second one-dimensional scattering intensity curve respectively;

[0017] The IRENA plug-in in the Igor Pro software is used to perform model fitting on the processed first one-dimensional scattering intensity curve and the second one-dimensional scattering intensity curve, and the first pore structure information and the second pore structure information are calculated respectively.

[0018] Furthermore, the first pore structure information and the second pore structure information respectively include porosity, pore size distribution and pore density.

[0019] Furthermore, the characterization dimensions of the gas storage space include the porosity, porosity ratio, pore density and pore size distribution of the gas storage space.

[0020] The technical solution provided by the embodiment of the present invention has the following beneficial effects: the method for characterizing the gas storage space of shale gas reservoirs in the embodiment of the present invention makes full use of the non-destructive characteristics of small-angle neutron scattering technology for shale samples, and can detect the internal pore structure of shale samples at the nanoscale. Based on the same experimental principle, by comparing the pore structure information of the small-angle neutron scattering experiment before and after the same shale sample is filled with deuterated mixed gas, the porosity, porosity ratio, pore density and pore size distribution of the gas storage space of the shale gas reservoir are obtained. The gas storage space of the shale gas reservoir is characterized from the above four dimensions, which can more comprehensively characterize the gas storage space of the shale gas reservoir, thereby improving the accuracy of the prediction of natural gas production potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic flow chart of a method for characterizing the gas storage space of a shale gas reservoir in a certain embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of a shale sample preparation method in a certain embodiment of the present invention;

[0023] Figure 3 This is a comparison diagram of pore volume distribution-pore diameter curves of a small-angle neutron scattering experiment of a shale sample W201-31 in an embodiment of the present invention;

[0024] Figure 4 This is a comparison diagram of pore volume distribution-pore diameter curves of a small-angle neutron scattering experiment of a shale sample W201-8 in an embodiment of the present invention;

[0025] Figure 5 This is a graph of methane storage space proportion-scattering vector of shale sample W201-31 in an embodiment of the present invention;

[0026] Figure 6 This is a graph showing the methane storage space percentage-scattering vector curve of the shale sample W201-8 in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0028] The method for characterizing the gas storage space of shale gas reservoirs in the present invention is implemented based on the following principles:

[0029] When shale is regarded as an approximate two-phase system (pore space and solid matrix), neutrons will be elastically scattered due to the difference in scattering length density (SLD) between the two phases. The SLD of each component in shale depends on its chemical composition and density, reflecting the scattering ability of each component per unit volume, and its expression is:

[0030]

[0031] NA=6.022×10 23 is Avogadro's constant, d is the density of shale components (g / cm 3 ), M is the molecular mass (g / mol), pj is the proportion of molecules in phase j in the compound, sj is the abundance of nucleus i in phase j, and bi is the coherent scattering amplitude of nucleus i.

[0032] When shale is approximately a two-phase system, the shale porosity The relationship between the scattering intensity I(Q) and the scattering vector Q can be expressed as:

[0033]

[0034] ρ 1 is the SLD of the shale solid matrix, ρ 2 is the SLD of the shale pore space (usually 0).

[0035] When fluid is injected into the pore space, small-angle neutron scattering technology can characterize the fluid distribution in the pores; based on the sensitivity of neutrons to isotopic substances, deuterated methane can be used to characterize the accessibility of shale; when the SLD of deuterated methane is close to the SLD of the shale solid matrix, the scattering signal mainly comes from the pore space not filled with fluid. At the same time, methane is the main component of shale gas. Using this gas experiment can better match the sample with the in-situ environment; small-angle neutron scattering experiments in high-pressure deuterated methane environments can calculate the spatial proportion of methane in shale (F ap ) and the pore diameter (2R) / scattering vector (Q):

[0036]

[0037] Among them, I(Q i ,dry) and I(Q i , CM) represent the scattering intensities of the corresponding scattering vectors of the sample in a vacuum environment and in a high-pressure environment before and after deuterated methane injection, respectively.

[0038] Please refer to Figure 1 and Figure 2 The embodiment of the present invention provides a method for characterizing the gas storage space of a shale gas reservoir, comprising the following steps:

[0039] S1. Prepare shale samples;

[0040] In step S1, the shale sample is in the form of a sheet with a thickness of 0.2 to 0.5 mm. The method further includes pre-treating the shale sample. The pre-treating method is as follows: drying the shale sample at 45 to 65° C. to a constant weight;

[0041] Specifically, in this embodiment, the shale is first processed into 1 cm 3 A cube is prepared, and then a square slice with a thickness of 0.5 mm is taken out from the upper part of the sample parallel to the bedding direction of the cube, and vacuum dried in a vacuum drying oven at 60° C. until the mass of the shale sample no longer changes; by pretreating the shale sample, the stored water in the shale sample can be effectively removed, thereby avoiding inaccurate characterization of the gas storage space of the shale gas reservoir due to the presence of the stored water.

[0042] S2, placing the shale sample in a scattering chamber to perform a first small-angle neutron scattering experiment to obtain a first two-dimensional scattering pattern of the shale sample;

[0043] In step S2, before performing the first small-angle neutron scattering experiment, the scattering chamber is evacuated so that the shale sample is in a vacuum environment to avoid affecting the characterization result of the gas storage space of the shale gas reservoir due to the presence of air;

[0044] In this embodiment, before officially starting the small-angle neutron experiment, the staff is required to perform a series of configurations on the experimental instruments, including detector efficiency calibration, wavelength calibration, background calibration, transmittance measurement, etc.; then the dried shale sample is loaded into a high-pressure neutron scattering chamber, and in the absence of deuterated methane gas injection, the entire sample environment is placed in a vacuum state by a vacuum pump, and the scattering chamber isolation valve is closed, and the pore structure information of the shale sample in a vacuum state is obtained by small-angle neutron scattering, and the scattering signal comes from all pore spaces within the measurement range in the sample; the neutron wavelength λ used in the experiment is The distances from the shale sample to the detector are set to 20 meters and 4 meters respectively, so the range of the scattering vector Q can be obtained as For elastic scattering, the scattering vector Q is related to the wave vector 2πλ -1 The relationship can be expressed as:

[0045] Q=4πλ -1 sin(θ) (4)

[0046] At the same time, the parameters of the shale sample need to be recorded during the test, including sample thickness, test time, dark current intensity, etc., for subsequent experimental data processing. After the experiment is completed, the first two-dimensional scattering pattern of the shale sample is obtained.

[0047] S3, injecting deuterated mixed gas into the scattering chamber to a target pressure, and then performing a second small-angle neutron scattering experiment to obtain a second two-dimensional scattering pattern of the shale sample;

[0048] In step S3, the deuterated mixed gas is a mixture of methane and deuterated methane, and the SLD value of the deuterated mixed gas is equal to the SLD value of the shale sample, the deuterated mixed gas enters the supercharger, and the temperature of the supercharger is lowered by a liquid nitrogen cooling device, so that the deuterated mixed gas is liquefied and stored in the supercharger to meet the amount of this experiment; after the pressure in the scattering chamber reaches the target pressure, the target pressure is maintained for 15 to 30 minutes, and then the second small-angle neutron scattering experiment is carried out;

[0049] Illustratively, in this embodiment, the second small-angle neutron scattering experiment is performed after maintaining the target pressure for 20 minutes.

[0050] S4, respectively obtaining first pore structure information and second pore structure information of the shale sample before and after the deuterated mixed gas is injected according to the first two-dimensional scattering pattern and the second two-dimensional scattering pattern;

[0051] In step S4, the method for obtaining the first pore structure information and the second pore structure information is as follows:

[0052] Importing the first two-dimensional scattering pattern and the second two-dimensional scattering pattern into fit 2D for one-dimensional processing respectively to obtain a first one-dimensional scattering intensity curve and a second one-dimensional scattering intensity curve, and performing background subtraction and normalization processing on the first one-dimensional scattering intensity curve and the second one-dimensional scattering intensity curve respectively according to experimental environment data;

[0053] Using the IRENA plug-in in the Igor Pro software to perform model fitting on the processed first one-dimensional scattering intensity curve and the second one-dimensional scattering intensity curve, and respectively calculate the first pore structure information and the second pore structure information;

[0054] The first pore structure information and the second pore structure information include porosity, pore size distribution and pore density respectively.

[0055] S5. Characterize the gas storage space of the shale sample according to the first pore structure information and the second pore structure information.

[0056] In this embodiment, the characterization dimensions of the gas storage space include the porosity, porosity ratio, pore density and pore size distribution of the gas storage space.

[0057] In this embodiment, characterization experiments were performed by setting two shale samples with different physical property parameters. The names of the shale samples were W201-31 and W201-8. Among them, the W201-31 sample was a sample with low organic matter content, and the W201-8 sample was a sample with high organic matter content. The experimental results are shown in Table 1.

[0058] Table 1 shows the pore structure information of two shale samples after the experiment

[0059]

[0060] It can be seen from Table 1 that the methane storage space of the two types of samples is significantly different, and the methane storage space of the two samples accounts for 53.08% and 64.29% respectively; at the same time, the closed pore density of the two shale samples is much greater than the open pore density, while the methane storage space of the two samples is not much different, which indicates that the number of closed pores is large but concentrated in the range of small pore diameters; the pore size distribution diagram of the two shale samples ( Figure 3 and Figure 4 ) also showed the same result. The curve showed that there were a large number of closed pores in the interval with a pore diameter of less than 20nm (mainly composed of pores inside organic matter). The main reason was the lack of an effective throat connecting this part of the pore network, which directly affected the space where methane could be stored in the sample pores; therefore, in this embodiment, methane gas was mainly stored in the pore space with a diameter greater than 20nm.

[0061] refer to Figure 5 and Figure 6 Before being affected by the nanopore confined space effect, the proportion of methane storage space is relatively stable. As the pore size into which methane enters decreases, the nanopore confined space effect is significantly enhanced, and the proportion of methane storage space in shale (F ap ) decreases as the pore size decreases and even becomes negative, which is caused by the increase in gas density due to the confined space effect of nanopores; while F ap The continued decrease in the value also reflects that methane has entered into pores with smaller pore sizes, resulting in an increase in the methane density in the limited space; the injected pore pressure in this example is 56.4 MPa (corresponding to a density of deuterated methane of 0.34 g / cm 3 ), after the density of deuterated methane exceeds the critical point, when F ap When the value is 0, the density of deuterated methane is 0.68 g / cm 3, and the density of methane gas will further increase with the decrease of pore size; in some high-pressure deep shale gas reservoirs, the high-pressure methane environment and nanoscale confined pore space effect will lead to an increase in gas density, which is greater than the density of methane in the ideal gas phase under the same pressure conditions; therefore, this microscopic mechanism also explains the phenomenon that the volume of methane produced in some abnormally high-pressure shale gas reservoirs is much larger than the pore volume of the shale itself; by studying the influence of nanopore confined space effect on methane storage space, the gas storage space of shale gas reservoirs can be more comprehensively characterized, thereby accurately predicting the natural gas production potential.

[0062] The method for characterizing the gas storage space of shale gas reservoirs in this embodiment can also be used to quantitatively evaluate the preservation of the pore network of shale reservoirs and the reservoir development potential; at the same time, it can be used to evaluate the pore structure of other porous media besides shale.

[0063] For matters not mentioned above, the prior art applies.

[0064] In this document, the directional words such as front, back, top, and bottom are defined by the positions of the components in the drawings and the positions of the components relative to each other, and are only for the sake of clarity and convenience in expressing the technical solution. It should be understood that the use of the directional words should not limit the scope of protection claimed in this application.

[0065] In the absence of conflict, the above embodiments and features in the embodiments may be combined with each other.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for characterizing the gas storage space of a shale gas reservoir, characterized in that: The steps include: S1. Prepare shale samples; S2, placing the shale sample in a scattering chamber to perform a first small-angle neutron scattering experiment to obtain a first two-dimensional scattering pattern of the shale sample; S3, injecting deuterated mixed gas into the scattering chamber to a target pressure, and then performing a second small-angle neutron scattering experiment to obtain a second two-dimensional scattering pattern of the shale sample; S4, respectively obtaining first pore structure information and second pore structure information of the shale sample before and after the deuterated mixed gas is injected according to the first two-dimensional scattering pattern and the second two-dimensional scattering pattern; S5. Characterizing the gas storage space of the shale sample according to the first pore structure information and the second pore structure information; In step S4, the method for obtaining the first pore structure information and the second pore structure information is as follows: Importing the first two-dimensional scattering pattern and the second two-dimensional scattering pattern into fit 2D for one-dimensional processing respectively to obtain a first one-dimensional scattering intensity curve and a second one-dimensional scattering intensity curve, and performing background subtraction and normalization processing on the first one-dimensional scattering intensity curve and the second one-dimensional scattering intensity curve respectively; Using the IRENA plug-in in the Igor Pro software to perform model fitting on the processed first one-dimensional scattering intensity curve and the second one-dimensional scattering intensity curve, and respectively calculate the first pore structure information and the second pore structure information; The first pore structure information and the second pore structure information respectively include porosity, pore size distribution and pore density; The characterization dimensions of the gas storage space include the porosity, porosity ratio, pore density and pore size distribution of the gas storage space.

2. The method for characterizing the gas storage space of a shale gas reservoir according to claim 1, characterized in that: In step S1, the shale sample is in the form of a sheet with a thickness of 0.2-0.5 mm.

3. The method for characterizing the gas storage space of a shale gas reservoir according to claim 1, characterized in that: Step S1 also includes pre-treating the shale sample; the pre-treatment method is as follows: drying the shale sample at 45-65° C. to constant weight.

4. The method for characterizing the gas storage space of a shale gas reservoir according to claim 1, characterized in that: In step S2, before performing the first small-angle neutron scattering experiment, the scattering chamber is evacuated so that the shale sample is in a vacuum environment.

5. The method for characterizing the gas storage space of shale gas reservoir according to claim 1, characterized in that: In step S3, the deuterated mixed gas is a mixture of methane and deuterated methane, and the SLD value of the deuterated mixed gas is equal to the SLD value of the shale sample.

6. The method for characterizing the gas storage space of a shale gas reservoir according to claim 1, characterized in that: In step S3, the target pressure is maintained for 15 to 30 minutes before the second small-angle neutron scattering experiment is performed.

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