A method for obtaining shale gas saturation

Abstract

The present invention belongs to the field of oil and gas field exploration, and specifically discloses a method for obtaining shale gas saturation. The present invention uses a short-echo nuclear magnetic resonance online high-temperature and high-pressure device to test shale reservoir cores, obtains short-echo shale core T2 spectra under different states, and provides a method for analyzing and calculating the relative gas content of shale. The saturation of shale free gas, adsorbed gas, and bound water is analyzed and calculated, and the quantitative characterization of the gas saturation of shale gas reservoirs is achieved, which can be used for shale gas reservoir evaluation.

Description

Technical Field

[0001] The present invention belongs to the field of oil and gas field exploration, and more specifically, relates to a method for obtaining shale gas saturation. Background Art

[0002] Shale gas refers to unconventional natural gas that exists in reservoir rock systems dominated by organic-rich shales. It is continuously generated biochemical gas, thermogenic gas, or a mixture of the two. It can exist in natural fractures and pores in a free state, in kerogen and clay particle surfaces in an adsorbed state, and in very small amounts in kerogen and asphaltene in a dissolved state. Unlike conventional reservoir gas reservoirs, shale is both the source rock for natural gas generation and the reservoir and cap rock for the accumulation and preservation of natural gas.

[0003] Shale gas is mainly composed of methane. It is a clean and efficient energy resource and chemical raw material. It is mainly used for residential gas, urban heating, power generation, automobile fuel and chemical production, etc. It has a wide range of uses. In the process of shale gas production, there is generally no need to drain water. The production cycle is long, generally 30 to 50 years. The success rate of exploration and development is high, and it has high industrial and economic value. my country's shale gas resources have great potential. It is preliminarily estimated that the recoverable resources of shale gas in my country are 36.1 trillion cubic meters, which is equivalent to conventional natural gas and slightly less than the geological resources of shallow coalbed methane, which is about 36.8 trillion cubic meters. Shale gas reservoirs have low permeability and are difficult to mine. It requires a variety of methods such as horizontal well drilling and multi-stage fracturing technology to achieve effective development. Due to differences in geological conditions and process levels, the production capacity of gas wells varies, and there may be large differences in the same area.

[0004] The formation and enrichment of shale gas has its own unique characteristics. It is often distributed in thick and widely distributed shale source rock formations in the basin. Compared with conventional natural gas, shale gas development has the advantages of long mining life and long production cycle. Most of the shale gas production is widely distributed, thick, and generally contains gas, which enables shale gas wells to produce gas at a stable rate for a long time.

[0005] The gas saturation of shale is an important parameter for evaluating shale gas reservoirs, and is of great significance for optimizing shale gas reservoirs and designing shale gas development plans. CN201410249240.5 provides a method for obtaining shale gas content using well logging curves, that is, a method for calculating shale gas content using well logging curves based on shale hydrocarbon generation kinetics simulation experiments, methane isothermal adsorption experiments, and analytical gas content tests. The method is based on thermal simulation results, etc., to obtain the cumulative conversion rate of shale gas, gas generation potential, and the total amount of hydrocarbon gas generated; establish a calculation relationship between shale adsorption capacity and TOC and calculate the average gas content; obtain the gas content using field analysis and correct the average gas content; linearly regress the shale gas content obtained by analytical experiments with shale logging data to obtain a one-dimensional linear regression equation and a two-dimensional linear regression equation for shale gas content and acoustic time difference and resistivity, so as to calculate the shale gas content using well logging curves. CN201520524241.6 provides a device for measuring the gas content of shale matrix, which simulates the high temperature environment of the well through a constant temperature box and simulates the high pressure environment of the well through a confining pressure part, so that the error of the content of shale gas in the adsorbed state in the measured rock sample is small, and through the setting of the first pressure gauge, the third pressure gauge and the first valve, the shale gas content in the adsorbed state and the shale gas content in the free state can be calculated from the readings displayed by the first pressure gauge and the third pressure gauge, which is convenient for establishing the content ratio of shale gas in the adsorbed state and shale gas in the free state in the rock sample. CN201721187056.8 discloses a fully automatic shale gas content test system, which innovatively applies the measurement principle of the pressure difference method to the design of the shale gas content tester, uses a liquid level sensor, measures the pressure of the upper and lower liquid surfaces of the gas collecting device, calculates the pressure difference, converts the pressure difference into the water level, and thus calculates the volume of desorbed gas to obtain the shale desorbed gas content. CN201711466942.9 provides a shale gas content evaluation method and device, which first obtains multiple evaluation parameters of a horizontal well, including logging interpretation porosity, logging interpretation total gas content, total hydrocarbons × mud density / drilling time and pore pressure coefficient, then performs dimensionless processing on the multiple evaluation parameters to obtain multiple dimensionless evaluation parameters, and finally calculates the gas content index value of the horizontal well based on the multiple dimensionless evaluation parameters and the preset weight coefficients corresponding to each of the multiple dimensionless evaluation parameters.

[0006] However, in the literature reports on the research of shale gas content determination and calculation methods in the prior art, what is determined and calculated is the absolute gas content of shale, and there is no report on the relative gas content of shale core, especially the test and analysis and calculation of the relative gas content of shale using nuclear magnetic resonance technology. Summary of the invention

[0007] The purpose of the present invention is to use the short echo nuclear magnetic resonance online high temperature and high pressure device technology to test shale reservoir cores, obtain short echo shale core T2 spectra under different states, and use this to analyze and calculate the shale free gas, adsorbed gas, and bound water saturation, thereby achieving quantitative characterization of the gas saturation of shale gas reservoirs.

[0008] In order to achieve the above object, the present invention provides a method for obtaining shale gas saturation, which comprises:

[0009] 1) Scanning the shale sample in the core holder using a nuclear magnetic resonance device to obtain a real-time short echo resonance T2 spectrum, and calculating the spectral area S1 of the T2 spectrum;

[0010] 2) vacuuming the core holder, applying confining pressure to the core holder and heating the core sample;

[0011] 3) After the confining pressure of the shale sample reaches the target reservoir pressure and the shale sample is heated to the target reservoir temperature, high-pressure methane gas is injected to maintain the pressure difference between the confining pressure and the shale sample at 2-4 MPa. After the pressure, temperature and confining pressure of the shale sample are stable, the nuclear magnetic resonance device is started again to scan the shale sample to obtain a real-time short echo resonance T2' spectrum, and the spectrum area S2 of the T2' spectrum is calculated;

[0012] 3) Calculate the relative free gas saturation G of shale using equations 1-2 1 and relative adsorption gas saturation G 2 :

[0013]

[0014]

[0015] The core holder selected in the present invention is a high-temperature, high-pressure non-metallic material core holder. According to the present invention, in the above method for obtaining shale gas saturation, preferably, in step 1), the sampling time of the nuclear magnetic resonance is 50 to 70 μs.

[0016] The vacuum treatment in the present invention is a conventional technique, and those skilled in the art can perform reasonable operations according to actual conditions. Generally, in step 2), the vacuum treatment is continued for 48 to 72 hours, and the vacuum degree is 10 -5 ~10 - 6 mbar.

[0017] In step 2) of the present invention, the core holder is preferably subjected to confining pressure and the core sample is heated by injecting confining pressure fluid. In the implementation of the present invention, there are usually special restrictions on the confining pressure fluid, and hydrogen-free oil can be selected. Preferably, the confining pressure fluid is fluorine oil.

[0018] The temperature and pressure of the confining pressure fluid are adjusted by a constant temperature heating element and a high pressure pump respectively. In order to better apply confining pressure to the core sample and heat the core sample, preferably, the injection rate of the confining pressure fluid is 0.5-60 ml / min.

[0019] The method for obtaining shale gas saturation of the present invention has a wide range of applications. Generally, in step 3), the target reservoir pressure is 15-45 MPa, and the target reservoir temperature is 40-90° C., and the method can be used for calculation.

[0020] According to the present invention, preferably, the pressure of the high-pressure methane gas is 20-40 MPa, and the temperature is 10-20° C.; the injection rate of the high-pressure methane gas is 0.5-2 ml / min.

[0021] In the present invention, the sampling parameters of the T2' spectrum are consistent with the sampling parameters of the T2 spectrum. Preferably, in step (3), the sampling time of the nuclear magnetic resonance is 50 to 70 μs.

[0022] A specific method for obtaining shale gas saturation of the present invention is implemented as follows:

[0023] (1) Shale sampling;

[0024] (2) Sample preparation;

[0025] (3) placing the prepared shale sample into a core holder, scanning the shale sample in the core holder using a nuclear magnetic resonance device to obtain a real-time short echo resonance T2 spectrum, and calculating the spectral area S1 of the T2 spectrum;

[0026] (4) evacuating the core holder, applying confining pressure to the core holder and heating the core sample;

[0027] (5) After the confining pressure of the shale sample reaches the target reservoir pressure and the shale sample is heated to the target reservoir temperature, high-pressure methane gas is injected to maintain the pressure difference between the shale sample and the confining pressure at 2-4 MPa. After the pressure, temperature and confining pressure of the shale sample are stabilized, the nuclear magnetic resonance device is started again to scan the shale sample to obtain a real-time short echo resonance T2' spectrum, and the spectral area S2 of the T2' spectrum is calculated;

[0028] (6) The relative free gas saturation G of shale is calculated by equations 1 and 2. 1 and relative adsorption gas saturation G 2 :

[0029]

[0030]

[0031] The present invention uses a short-echo nuclear magnetic resonance online high-temperature and high-pressure device to test shale reservoir cores, obtains short-echo shale core T2 spectra under different states, and provides a method for analyzing and calculating the relative gas content of shale. The saturation of shale free gas, adsorbed gas, and bound water is analyzed and calculated, and the quantitative characterization of the gas saturation of shale gas reservoirs is achieved, which can be used for shale gas reservoir evaluation.

[0032] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention in conjunction with the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0034] Figure 1 A schematic diagram of an online high temperature and high pressure nuclear magnetic resonance device in a specific embodiment of the present invention is shown.

[0035] Figure 2 The T2 spectra of the shale samples in Example 1 of the present invention under different conditions are shown.

[0036] Figure 3 The T2 spectra of shale samples in different states in Example 2 of the present invention are shown.

[0037] Figure 4 The T2 spectra of shale samples in different states in Example 3 of the present invention are shown.

[0038] Figure 5 The T2 spectra of the shale samples in Example 4 of the present invention under different conditions are shown.

[0039] Description of reference numerals:

[0040] 1. Core sample; 2. Magnet; 3. Fluorine oil; 4. Probe; 5. High-pressure pump; 6. Pressure gauge; 7. Heating element; 8. High-pressure methane gas; 9. Intermediate container; 10. Back pressure valve; 11. Vacuum pump; 12. Non-magnetic core holder; 13. Nuclear magnetic resonance spectrometer; 14. Computer. DETAILED DESCRIPTION

[0041] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0042] Example 1

[0043] (1) Shale sampling: Shale was collected from: Jiaoye K31 in Fuling shale gas field;

[0044] (2) Sample preparation: The shale was prepared into a cylindrical shale sample 1 with a diameter of 2.5 cm and a length not exceeding 7 cm;

[0045] (3) The prepared shale sample 1 is placed in a core holder, and a nuclear magnetic resonance device is used to scan the shale sample in the core holder, wherein the nuclear magnetic resonance parameters are set as follows: sampling interval of 2 μs, number of samples of 2048, sampling time of 60 μs, waiting time of 2s, number of scans of 64, and main frequency of 9.8 MHz. A real-time short echo resonance T2 spectrum (such as Figure 2 As shown), the spectral area S1 of the T2 spectrum is calculated to be 185.8;

[0046] (4) The core holder was vacuumed for 72 hours with a vacuum degree of 10 -6 mbar, then apply confining pressure to the core holder and heat the core sample, wherein the confining pressure fluid is fluorine oil, and the injection rate is 0.5-60ml / min;

[0047] (5) After the confining pressure of the shale sample reaches 24 MPa and the shale sample is heated to 60°C, high-pressure methane gas is injected. The pressure of the high-pressure methane gas is 20 MPa and the injection rate is 0.5-2 ml / min. After the pressure, temperature and confining pressure of the shale sample are stable, the nuclear magnetic resonance device is started again to scan the shale sample. The nuclear magnetic resonance parameters are set as follows: sampling interval is 2 μs, number of samples is 2048, sampling time is 60 μs, waiting time is 2s, number of scans is 64, main frequency is 9.8 MHz, and real-time short echo resonance T2' spectrum (such as Figure 2 As shown), the spectrum area S2 of the T2' spectrum is calculated to be 597.6;

[0048] (5) The relative free gas saturation G of shale is calculated by equations 1 and 2. 1 The relative adsorption gas saturation G is 68.9%. 2 31.1%:

[0049]

[0050]

[0051] Example 2

[0052] (1) Shale sampling: Shale was collected from: Jiaoye K43 in Fuling shale gas field;

[0053] (2) Sample preparation: The shale was prepared into a cylindrical shale sample 2 with a diameter of 2.5 cm and a length not exceeding 7 cm;

[0054] (3) The prepared shale sample 2 is placed in a core holder, and a nuclear magnetic resonance device is used to scan the shale sample in the core holder, wherein the nuclear magnetic resonance parameters are set as follows: sampling interval of 2 μs, number of samples of 2048, sampling time of 60 μs, waiting time of 2s, number of scans of 64, main frequency of 9.8 MHz, and real-time short echo resonance T2 spectrum (such as Figure 3 As shown), the spectral area S1 of the T2 spectrum is calculated to be 209.3;

[0055] (4) The core holder was vacuumed for 72 hours with a vacuum degree of 10 -6 mbar, then apply confining pressure to the core holder and heat the core sample, wherein the confining pressure fluid is fluorine oil, and the injection rate is 0.5-60ml / min;

[0056] (5) After the confining pressure of the shale sample reaches 24 MPa and the shale sample is heated to 60°C, high-pressure methane gas is injected. The pressure of the high-pressure methane gas is 20 MPa and the injection rate is 0.5-2 ml / min. After the pressure, temperature and confining pressure of the shale sample are stable, the nuclear magnetic resonance device is started again to scan the shale sample. The nuclear magnetic resonance parameters are set as follows: sampling interval is 2 μs, number of samples is 2048, sampling time is 60 μs, waiting time is 2s, number of scans is 64, main frequency is 9.8 MHz, and real-time short echo resonance T2' spectrum (such as Figure 3 As shown), the spectrum area S2 of the T2' spectrum is calculated to be 641;

[0057] (5) The relative free gas saturation G of shale is calculated by equations 1 and 2. 1 The relative adsorption gas saturation G is 67.4%. 2 32.6%:

[0058]

[0059]

[0060] Example 3

[0061] (1) Shale sampling: Shale was collected from: Jiyang Liye 5-20-27;

[0062] (2) Sample preparation: The shale was prepared into a cylindrical shale sample 3 with a diameter of 2.5 cm and a length not exceeding 7 cm;

[0063] (3) The prepared shale sample 3 is placed in a core holder, and the shale sample in the core holder is scanned using a nuclear magnetic resonance device, wherein the nuclear magnetic resonance parameters are set as follows: sampling interval of 2 μs, number of samples of 2048, sampling time of 60 μs, waiting time of 2s, number of scans of 64, main frequency of 9.8 MHz, and real-time short echo resonance T2 spectrum (such as Figure 4 As shown), the spectral area S1 of the T2 spectrum is calculated to be 940.5;

[0064] (4) The core holder was vacuumed for 72 hours with a vacuum degree of 10 -6 mbar, then apply confining pressure to the core holder and heat the core sample, wherein the confining pressure fluid is fluorine oil, and the injection rate is 0.5-60ml / min;

[0065] (5) After the confining pressure of the shale sample reaches 24 MPa and the shale sample is heated to 45°C, high-pressure methane gas is injected. The pressure of the high-pressure methane gas is 20 MPa and the injection rate is 0.5-2 ml / min. After the pressure, temperature and confining pressure of the shale sample are stable, the nuclear magnetic resonance device is started again to scan the shale sample. The nuclear magnetic resonance parameters are set as follows: sampling interval of 2 μs, number of samples of 2048, sampling time of 60 μs, waiting time of 2s, number of scans of 64, main frequency of 9.8 MHz, and real-time short echo resonance T2' spectrum (such as Figure 4 As shown), the spectrum area S2 of the T2' spectrum is calculated to be 3690.9;

[0066] (5) The relative free gas saturation G of shale is calculated by equations 1 and 2. 1 The relative adsorption gas saturation G is 74.5%. 2 25.5%:

[0067]

[0068]

[0069] Example 4

[0070] (1) Shale sampling: Shale was collected from: Jiyang Liye 18-9-38;

[0071] (2) Sample preparation: The shale was prepared into a cylindrical shale sample 4 with a diameter of 2.5 cm and a length not exceeding 7 cm;

[0072] (3) The prepared shale sample 4 is placed in a core holder, and the shale sample in the core holder is scanned using a nuclear magnetic resonance device, wherein the nuclear magnetic resonance parameters are set as follows: sampling interval of 2 μs, number of samples of 1048, sampling time of 60 μs, waiting time of 2s, number of scans of 64, main frequency of 9.8 MHz, and real-time short echo resonance T2 spectrum (such as Figure 5 As shown), the spectral area S1 of the T2 spectrum is calculated to be 417.8;

[0073] (4) The core holder was vacuumed for 72 hours with a vacuum degree of 10 -6 mbar, then apply confining pressure to the core holder and heat the core sample, wherein the confining pressure fluid is fluorine oil, and the injection rate is 0.5ml / min-60ml / min;

[0074] (5) After the confining pressure of the shale sample reaches 24 MPa and the shale sample is heated to 45°C, high-pressure methane gas is injected. The pressure of the high-pressure methane gas is 20 MPa and the injection rate is 0.5-2 ml / min. After the pressure, temperature and confining pressure of the shale sample are stable, the nuclear magnetic resonance device is started again to scan the shale sample. The nuclear magnetic resonance parameters are set as follows: sampling interval of 2 μs, number of samples of 1048, sampling time of 60 μs, waiting time of 2s, number of scans of 64, main frequency of 9.8 MHz, and real-time short echo resonance T2' spectrum (such as Figure 5 As shown), the spectrum area S2 of the T2' spectrum is calculated to be 1416.7;

[0075] (5) The relative free gas saturation G of shale is calculated by equations 1 and 2. 1 is 70.5% and the relative adsorption gas saturation G 2 29.5%:

[0076]

[0077]

[0078] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for obtaining shale gas saturation, characterized in that: The acquisition method includes: 1) Scanning the shale sample in the core holder using a nuclear magnetic resonance device to obtain a real-time short echo resonance T2 spectrum, and calculating the spectral area S1 of the T2 spectrum; 2) evacuating the core holder, applying confining pressure to the core holder and heating the core sample; 3) After the confining pressure of the shale sample reaches the target reservoir pressure and the shale sample is heated to the target reservoir temperature, high-pressure methane gas is injected to maintain the pressure difference between the confining pressure and the shale sample at 2-4 MPa. After the pressure, temperature and confining pressure of the shale sample are stable, the nuclear magnetic resonance device is started again to scan the shale sample to obtain a real-time short echo resonance T2' spectrum, and the spectrum area S2 of the T2' spectrum is calculated; 3) The relative free gas saturation G1 and relative adsorbed gas saturation G2 of shale are calculated by formula 1-2:

2. The acquisition method according to claim 1, wherein: In step 1), the sampling time of the nuclear magnetic resonance is 50 to 70 μs.

3. The acquisition method according to claim 1, wherein: In step 2), the vacuum treatment is continued for 48 to 72 hours, and the vacuum degree is 10 -5 ~10 -6 mbar.

4. The acquisition method according to claim 1, wherein: In step 2), confining pressure is applied to the core holder and the core sample is heated by injecting confining pressure fluid.

5. The acquisition method according to claim 4, wherein: The confining pressure fluid is fluorine oil.

6. The acquisition method according to claim 4, wherein: The injection speed of the confining pressure fluid is 0.5-60 ml / min.

7. The acquisition method according to claim 1, wherein: In step 3), the target reservoir pressure is 15-45 MPa, and the target reservoir temperature is 40-90°C.

8. The acquisition method according to claim 1, wherein: In step 3), the pressure of the high-pressure methane gas is 20-40 MPa and the temperature is 10-20°C.

9. The acquisition method according to claim 1, wherein: In step 3), the injection rate of the high-pressure methane gas is 0.5-2 ml / min.

10. The acquisition method according to claim 1, wherein: In step 3), the sampling time of the nuclear magnetic resonance is 50 to 70 μs.

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