Novel method for correcting water saturation of shale reservoir free gas under in-situ formation condition

Abstract

The invention discloses a novel method for correcting the water saturation of shale reservoir free gas under the in-situ formation condition. The novel method comprises the following steps: S1, preparing shale samples with different water saturations; s2, performing a saturated methane nuclear magnetic T2 spectrum test under different temperature and pressure conditions to obtain a saturated methane nuclear magnetic T2 spectrum under different temperature and pressure conditions; s3, counting nuclear magnetic signal intensity values of which the nuclear magnetic time T2 is greater than 1ms under different temperature and pressure conditions, and converting the nuclear magnetic signal intensity values into methane mass; s4, the methane volume under the landmark condition is obtained through calculation by means of the state equation; calculating free gas volumes under different temperature and pressure conditions; s5, according to the calculation result of the step S4, obtaining the correlation between the water saturation and the free gas volume under different temperature and pressure conditions; s6, establishing a water saturation correction formula of the free gas volume of the shale reservoir and determining a fitting coefficient in the water saturation correction formula; and S7, predicting the free gas volume of the target shale reservoir according to the correction formula after the fitting coefficient is determined. According to the method, the more accurate free gas amount under the in-situ stratum condition can be obtained.

Description

Technical Field

[0001] The invention relates to the technical field of shale gas development, and in particular to a new method for correcting the water saturation of free gas in a shale reservoir under in-situ formation conditions. Background Art

[0002] As an unconventional resource, shale gas is widely distributed and has huge resources. It is an important successor resource for oil and gas energy in the future. After more than ten years of exploration and development, industrial gas flows have been found in multiple strata, such as the Longmaxi Formation, Dalong Formation, Wujiaping Formation, Shanxi Formation, Longtan Formation, and Qiongzhusi Formation. In particular, the shale gas in the Longmaxi Formation in the Sichuan Basin has achieved scale-effective development, and the Qiongzhusi Formation has made breakthrough progress, with a single well test output of one million cubic meters per day. However, 86% of the shale gas resources in the Longmaxi Formation are concentrated in deep shale reservoirs with a burial depth greater than 3,500m, and the shale reservoirs in the Qiongzhusi Formation are also buried at a large depth. Affected by high temperature and high pressure, the occurrence states of shale gas and formation water under in-situ formation conditions are complex and diverse, resulting in challenges in the accurate assessment of shale gas resources. Exploration and development practices have confirmed that the natural gas extracted from shale and coal rock gas is mainly free methane in the formation. At the same time, free gas volume is also one of the key parameters for reserve calculation and reservoir evaluation, so it is extremely important to accurately evaluate the free gas volume in shale reservoirs.

[0003] At present, there are two main methods for obtaining the free gas content of shale gas reservoirs: field test acquisition and indoor experimental test calculation. For field testing, the gas content obtained by pressure coring is considered to be the most accurate method. The average gas content of high-quality shale reservoirs obtained by pressure coring is 15.31m 3 / t. Indoor experimental tests and calculations can be divided into two categories. One is to use the core saturated methane nuclear magnetic resonance method for direct testing, which is considered to be more reliable. The other is to use the volumetric method for calculation:

[0004]

[0005] Where: n free is the free gas volume, cm 3 / g; P is the formation pressure, MPa; T SC is the ground temperature, K; S W is water saturation, %; is the reservoir porosity, %; Z is the compression factor, constant; T is the formation temperature, K; P SC is the ground pressure, MPa; ρ r is the rock density, g / cm 3 .

[0006] In both indoor experimental tests and calculations, water saturation needs to be used to correct the free gas volume to obtain accurate free gas volume. It is usually believed that the sum of water saturation and gas saturation is 1, and the pore volume occupied by water needs to be deducted during the free gas calculation process. However, under real formation conditions, water exists in three states in the nanopores of shale reservoirs: adsorbed water, bound water, and free water. Especially in high-temperature and high-pressure confined spaces, the density of gas and water will change with the size of the nanopores. Therefore, it is inaccurate to directly use the mass of formation water divided by the density of formation water to obtain the pore volume occupied by formation water, which leads to large errors in the calculation results of free gas volume under shale reservoir formation conditions and miscalculations of resources. Therefore, it is urgent to establish a new method for correcting free gas volume with water saturation to obtain more accurate free gas volume under in-situ formation conditions. Summary of the invention

[0007] In view of the above problems, the present invention aims to provide a new method for correcting the water saturation of free gas in shale reservoirs under in-situ formation conditions.

[0008] The technical solution of the present invention is as follows:

[0009] A new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions comprises the following steps:

[0010] S1: Preparation of shale samples with different water saturations;

[0011] S2: Perform saturated methane nuclear magnetic resonance T2 spectrum tests on shale samples with different water saturations under different temperature and pressure conditions to obtain nuclear magnetic resonance T2 spectra of shale samples with different water saturations under different temperature and pressure conditions;

[0012] S3: Count the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1ms under different temperature conditions, and convert the nuclear magnetic signal intensity values ​​into methane mass by using the conversion relationship between methane mass and nuclear magnetic signal intensity under each temperature condition;

[0013] S4: According to the methane mass, the volume of each methane mass under the landmark conditions corresponding to different temperature and pressure conditions is calculated using the state equation; and the volume under the landmark conditions is divided by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions;

[0014] S5: based on the calculation result of step S4, a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn, and the correlation between water saturation and free gas content is obtained;

[0015] S6: establishing a water saturation correction formula for the free gas content of the shale reservoir according to the correlation between the water saturation and the free gas content and determining a fitting coefficient in the water saturation correction formula for the free gas content of the shale reservoir;

[0016] S7: predicting the free gas volume of the target shale reservoir according to the water saturation correction formula of the free gas volume of the shale reservoir after the fitting coefficient is determined.

[0017] Preferably, in step S1, preparing shale samples with different water saturations specifically includes the following sub-steps:

[0018] S11: Obtain a shale sample and prepare it into a standard plunger sample, which is then dried and weighed and recorded as m;

[0019] S12: placing the shale sample in a vacuum saturation device to saturate with water, removing floating water on the surface of the sample after it is completely saturated with water, weighing it and recording it as M;

[0020] S13: obtain the total mass M1 filled into the core pores by subtracting the dry weight m from the saturated weight M;

[0021] S14: Calculate different target water saturations S using the total mass M1 of the saturated core pores and the dry weight m wn The corresponding core mass m n ;

[0022] S15: Dry the shale sample until the mass m n The water saturation S is obtained wn shale samples.

[0023] Preferably, in step S12, the saturation conditions for water saturation are vacuum time 24 hours, pressure 25 MPa, and saturation time 48 hours.

[0024] Preferably, in step S15, the shale sample is first dried to a mass corresponding to the maximum target water saturation, and then the process proceeds to step S2 to perform saturated methane nuclear magnetic resonance T2 spectrum testing under different temperature and pressure conditions. After completion, the shale sample is continued to be dried to a mass corresponding to the second largest target water saturation, and this process is repeated to obtain nuclear magnetic resonance T2 spectra of shale samples of each target water saturation under different temperature and pressure conditions.

[0025] Preferably, in step S2, when performing a saturated methane nuclear magnetic resonance T2 spectrum test, the pressure is increased to above 4 MPa before the experiment, and then the temperature is increased. After reaching the target temperature and target pressure required for the experiment, the nuclear magnetic resonance T2 spectrum is collected; after each experimental test is completed, the temperature is first lowered and then the pressure is reduced.

[0026] Preferably, in step S2, the nuclear magnetic resonance T2 spectrum is tested continuously for multiple times under the same conditions. When the repetition rate of the test results is higher than the target threshold, it is considered that methane has reached an equilibrium state in the core under these conditions, and the result in the equilibrium state is used as the nuclear magnetic resonance T2 spectrum result under these conditions.

[0027] Preferably, in step S3, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step S2 on the dried shale sample.

[0028] Preferably, in step S5, the correlation between the water saturation and the free gas content is:

[0029] For shale samples with porosity less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation;

[0030] For shale samples with porosity greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation.

[0031] Preferably, in step S6, the water saturation correction formula of the free gas content in the shale reservoir is:

[0032]

[0033] Where: Q f is the free gas volume of shale sample, cm 3 / g; A, B, C, D are all fitting coefficients, dimensionless; S w is water saturation, %; is the reservoir porosity, %.

[0034] Preferably, each fitting coefficient is obtained by the following steps:

[0035] The correlation between the fitting coefficients A, B, C, D and temperature and pressure were analyzed respectively;

[0036] According to the correlation analysis results, the correlation diagram between parameter B and pressure, and the correlation diagram between C and temperature are drawn, and nonlinear fitting is performed on A and D to obtain the following expressions for each fitting coefficient:

[0037] B=a*P+b (4)

[0038] C=c*Td (5)

[0039] A=-e+f*Tg*P+h*P 2 (6)

[0040] D=ij*Ln(T) / Tk*P (7)

[0041] Where: P is pressure, T is temperature; ak is the fitting constant;

[0042] According to the expressions of formula (4)-formula (7), the values ​​of the fitting coefficients under different pressure and temperature conditions are obtained, and the fitting correlation coefficient R 2 Greater than 0.9.

[0043] The beneficial effects of the present invention are:

[0044] The present invention can obtain more accurate free gas volume under in-situ formation conditions, with the accuracy improved by 12% to 63.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0046] Figure 1 It is a schematic diagram of the results of saturated methane nuclear magnetic T2 spectrum under different water saturation, different temperature and different pressure conditions in a specific embodiment;

[0047] Figure 2 It is a schematic diagram of the correlation results between water saturation and free gas content at different temperatures and pressures in a specific embodiment;

[0048] Figure 3 It is a schematic diagram of the correlation results between pressure and parameter B, and between temperature and parameter C in a specific embodiment;

[0049] Figure 4 It is a schematic diagram showing the comparison of the free gas volume and the pressure-maintaining coring results obtained by different experimental methods in a dry state in a specific embodiment;

[0050] Figure 5 It is a schematic diagram showing the comparison results of different correction methods for water saturation of free gas under in-situ formation conditions in an experimental test of the present invention in a specific embodiment. DETAILED DESCRIPTION

[0051] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the disclosure of the present invention mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0052] The present invention provides a new method for correcting the water saturation of free gas in a shale reservoir under in-situ formation conditions, comprising the following steps:

[0053] S1: Preparation of shale samples with different water saturations.

[0054] In a specific embodiment, preparing shale samples with different water saturations specifically includes the following sub-steps:

[0055] S11: Obtain a shale sample and prepare it into a standard plunger sample, which is then dried and weighed and recorded as m;

[0056] S12: placing the shale sample into a vacuum saturation device for water saturation, removing floating water on the sample surface after it is completely saturated with water, weighing it and recording it as M; optionally, the saturation conditions for water saturation are vacuum time of 24h, pressure of 25MPa, and saturation time of 48h.

[0057] S13: obtain the total mass M1 filled into the core pores by subtracting the dry weight m from the saturated weight M;

[0058] S14: Calculate different target water saturations S using the total mass M1 of the saturated core pores and the dry weight m wn The corresponding core mass m n ;

[0059] S15: Dry the shale sample until the mass m n The water saturation S is obtained wn shale samples.

[0060] In a specific embodiment, the shale sample is first dried to a mass corresponding to the maximum target water saturation, and then the process proceeds to step S2 to perform a saturated methane nuclear magnetic resonance T2 spectrum test under different temperature and pressure conditions. After the test, the shale sample is dried to a mass corresponding to the second largest target water saturation, and this process is repeated to obtain the nuclear magnetic resonance T2 spectra of shale samples of each target water saturation under different temperature and pressure conditions.

[0061] S2: Saturated methane nuclear magnetic resonance T2 spectrum tests are performed on shale samples with different water saturations under different temperature and pressure conditions to obtain nuclear magnetic resonance T2 spectra of shale samples with different water saturations under different temperature and pressure conditions.

[0062] In a specific embodiment, when performing a saturated methane nuclear magnetic T2 spectrum test, the pressure is increased to more than 4MPa before the experiment, and then the temperature is increased. After reaching the target temperature and target pressure required for the experiment, the nuclear magnetic T2 spectrum is collected. In this embodiment, by first increasing the pressure, then increasing the temperature, and then reaching the target temperature and target pressure before testing, the water in the core can be prevented from evaporating, thereby ensuring the accuracy of the test results under the corresponding water saturation conditions.

[0063] In a specific embodiment, when performing a saturated methane nuclear magnetic resonance T2 spectrum test, after each experimental test is completed, the temperature is first lowered and then the pressure is lowered. In this embodiment, by first lowering the temperature and then lowering the pressure, it is possible to prevent water loss in the core and prevent the shale sample from failing to reach the next target water saturation.

[0064] In a specific embodiment, the nuclear magnetic resonance T2 spectrum is tested multiple times continuously under the same conditions. When the repetition rate of the test results is higher than the target threshold, it is considered that the methane in the core has reached an equilibrium state under the conditions, and the result in the equilibrium state is used as the nuclear magnetic resonance T2 spectrum result under the conditions.

[0065] S3: Count the NMR signal intensity values ​​with NMR T2 time greater than 1ms under different temperature conditions, and convert the NMR signal intensity values ​​into methane mass using the conversion relationship between methane mass and NMR signal intensity under each temperature condition.

[0066] In a specific embodiment, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step S2 on a dry shale sample.

[0067] S4: Based on the methane mass, the state equation is used to calculate the volume of each methane mass under the landmark conditions under different temperature and pressure conditions; and the volume under the landmark conditions is divided by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions.

[0068] S5: According to the calculation result of step S4, a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn, and the correlation between water saturation and free gas content is obtained.

[0069] In a specific embodiment, the correlation between the water saturation and the free gas content is:

[0070] For shale samples with porosity less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation;

[0071] For shale samples with porosity greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation.

[0072] S6: According to the correlation between the water saturation and the free gas content, a water saturation correction formula for the free gas content of the shale reservoir is established and a fitting coefficient in the water saturation correction formula for the free gas content of the shale reservoir is determined.

[0073] In a specific embodiment, the water saturation correction formula for the free gas content in the shale reservoir is:

[0074]

[0075] Where: Q f is the free gas volume of shale sample, cm 3 / g; A, B, C, D are all fitting coefficients, dimensionless; S w is water saturation, %; is the reservoir porosity, %.

[0076] In a specific embodiment, each fitting coefficient is obtained by the following steps:

[0077] The correlation between the fitting coefficients A, B, C, D and temperature and pressure were analyzed respectively;

[0078] According to the correlation analysis results, the correlation diagram between parameter B and pressure, and the correlation diagram between C and temperature are drawn, and nonlinear fitting is performed on A and D to obtain the following expressions for each fitting coefficient:

[0079] B=a*P+b (4)

[0080] C=c*Td (5)

[0081] A=-e+f*Tg*P+h*P 2 (6)

[0082] D=ij*Ln(T) / Tk*P (7)

[0083] Where: P is pressure, T is temperature; ak is the fitting constant;

[0084] According to the expressions of formula (4)-formula (7), the values ​​of the fitting coefficients under different pressure and temperature conditions are obtained, and the fitting correlation coefficient R 2 Greater than 0.9.

[0085] S7: predicting the free gas volume of the target shale reservoir according to the water saturation correction formula of the free gas volume of the shale reservoir after the fitting coefficient is determined.

[0086] In a specific embodiment, taking a shale reservoir as an example, the new method for correcting water saturation of free gas in a shale reservoir under in-situ formation conditions of the present invention is used to predict the free gas in the shale reservoir under in-situ formation conditions, specifically comprising the following steps:

[0087] (1) Preparing shale samples with different water saturations;

[0088] Obtain a shale sample and prepare it into a standard plunger sample, weigh it after drying and record it as m; put the shale sample into a vacuum saturation device for water saturation (saturation conditions are vacuum time 24h, pressure 25MPa, saturation time 48h), remove the floating water on the surface of the sample after it is completely saturated with water, weigh it and record it as M; subtract the dry weight m from the saturated weight M to obtain the total mass M1 saturated into the core pores; calculate different target water saturations S using the total mass M1 saturated into the core pores and the dry weight m wn (10%, 30%, 50%) corresponding to the core mass m n (m1, m2, m3); put the shale sample into a constant temperature box at 40°C, detect the mass change of the sample every 2s, and when the mass is reduced to m3, obtain a shale sample with a water saturation of 50%, and enter step (2); after the saturated methane nuclear magnetic resonance T2 spectrum test of the shale sample with a water saturation of 50% is completed, put the sample into the constant temperature box again, and after the mass is reduced to m2, continue to complete the saturated methane nuclear magnetic resonance T2 spectrum test; samples with different water saturations only need to repeat the above operations.

[0089] (2) Shale samples with different water saturations were subjected to saturated methane NMR T2 spectrum tests under different temperature and pressure conditions;

[0090] In order to prevent the evaporation of water in the core, the pressure needs to be quickly increased to above 4MPa before the experiment, and then the temperature is increased. After reaching the temperature and pressure required for the experiment, the NMR T2 spectrum is collected. After the temperature and pressure are stable, the NMR T2 spectrum is tested three times in succession, and the repetition rate is higher than 98%. It is considered that methane reaches equilibrium in the core under this pressure. Repeat the above operation to obtain the NMR T2 spectra of samples with different water saturations under different temperature and pressure conditions. After each experimental test, the temperature is lowered first and then the pressure is reduced to prevent the loss of water in the core. In this embodiment, methane NMR T2 spectra at three water saturations (10%, 30%, 50%), three temperature points (60°C, 80°C, 100°C) and three pressure points (15MPa, 25MPa, 35MPa) are tested, and the results are as follows Figure 1 shown.

[0091] (3) Counting the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1 ms under different temperature conditions, and converting the nuclear magnetic signal intensity values ​​into methane mass using the conversion relationship between methane mass and nuclear magnetic signal intensity under different temperature conditions;

[0092] In this embodiment, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step (2) on the dried shale sample.

[0093] (4) according to the methane mass, using the state equation to calculate the volume of each methane mass under the landmark conditions under different temperature and pressure conditions; and dividing the volume under the landmark conditions by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions;

[0094] (5) Based on the calculation results of step (4), a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn. The results are as follows: Figure 2 As shown, the correlation between water saturation and free gas content was obtained;

[0095] from Figure 2 It can be seen that the correlation between the water saturation and the free gas content is: for shale samples with a porosity of less than 4.5%, there is a negative logarithmic relationship between the free gas content and the water saturation; for shale samples with a porosity of greater than or equal to 4.5%, there is a negative linear relationship between the free gas content and the water saturation.

[0096] (6) Based on the correlation between the water saturation and the free gas content, a water saturation correction formula for the free gas content of the shale reservoir shown in equations (2)-(3) is established;

[0097] (7) determining the fitting coefficient of the water saturation correction formula for the free gas content of the shale reservoir;

[0098] For shale samples with porosity less than 4.5%: under the same pressure, parameter B remains unchanged. It is believed that parameter B is mainly affected by pressure, while parameter A is affected by both temperature and pressure.

[0099] For shale samples with porosity greater than 4.5%: at the same temperature, parameter C remains basically unchanged, while parameter D changes. It is believed that parameter C is mainly affected by temperature, and parameter D is affected by both temperature and pressure.

[0100] Plot the correlation between parameter B and pressure and the correlation between parameter C and temperature, and then obtain the values ​​of B and C under different pressure / temperature conditions. The results are as follows Figure 3 shown.

[0101] B=0.0618*P+0.0328 (8)

[0102] C=0.0342*T-6.972 (9)

[0103] Where: P is pressure and T is temperature.

[0104] A and D are fitted nonlinearly to obtain the A and D values ​​under different pressure / temperature conditions:

[0105] A=-0.4591+0.006*T-0.0168*P+0.0001*P 2 (10)

[0106] D=1.1577-71.6746*Ln(T) / T-0.0481*P (11)

[0107] (8) predicting the free gas volume of the target shale sample by using the water saturation correction formula of the free gas volume of the shale reservoir after determining the fitting coefficient;

[0108] In this embodiment, the free gas volume in a dry state measured by the experimental test method of the present invention is first compared with the calculation results of the traditional volumetric method and the pressure-maintaining coring results. Figure 4 As shown. Figure 4 It can be seen that the free gas volume of the present invention is greater than the pressure coring result and greater than the result of the traditional free gas volume calculation method (without considering the water saturation). The dry state experimental test results of the present invention are 0.47-1.62 m larger than those of the pressure coring. 3 / t, mainly because the result is not corrected for water saturation, while pressure coring is the result under formation conditions. The range of water saturation is 14% to 40%, which is the main reason why the free gas volume in the dry state of the present invention is greater than that of pressure coring. The result of the traditional free gas calculation method (without considering water saturation) is 23% to 45% smaller than that of pressure coring, which laterally proves that the experimental test method of the present invention is more accurate and reliable.

[0109] Then, based on the test results of the free gas experimental test method of the present invention, the free gas amount was corrected using the traditional water saturation correction method and the new correction method of the present invention, and compared with the pressure-maintaining coring results. The results are as follows: Figure 5 As shown. Figure 5 It can be seen that the new correction method of the present invention is slightly smaller than the pressure-maintaining coring result, and the absolute value of the difference is 0.2 to 1.69 m. 3 / t, this is because the pressure-maintained coring results include adsorbed gas. The results of the traditional correction method are much smaller than the pressure-maintained coring results, with an absolute difference of 1.78 to 3.35 m 3 / t, especially in low porosity and low gas content sections. The ground temperature of deep shale gas reservoirs exceeds 120°C, and the adsorbed gas volume is usually less than 2m 3 / t. It is therefore believed that the new method for correcting the free gas water saturation provided by the present invention improves the prediction accuracy of the free gas by 12% to 63.5%.

[0110] In summary, the present invention can more accurately obtain the free gas content of shale reservoirs under in-situ formation conditions. Compared with the prior art, the present invention is a significant improvement.

[0111] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A new method for correcting the water saturation of free gas in shale reservoirs under in-situ formation conditions, characterized in that: The following steps are involved: S1: Preparation of shale samples with different water saturations; S2: Perform saturated methane nuclear magnetic resonance T2 spectrum tests on shale samples with different water saturations under different temperature and pressure conditions to obtain nuclear magnetic resonance T2 spectra of shale samples with different water saturations under different temperature and pressure conditions; S3: Count the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1ms under different temperature conditions, and convert the nuclear magnetic signal intensity values ​​into methane mass by using the conversion relationship between methane mass and nuclear magnetic signal intensity under each temperature condition; S4: According to the methane mass, the volume of each methane mass under the landmark conditions corresponding to different temperature and pressure conditions is calculated using the state equation; and the volume under the landmark conditions is divided by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions; S5: based on the calculation result of step S4, a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn, and the correlation between water saturation and free gas content is obtained; S6: establishing a water saturation correction formula for the free gas content of the shale reservoir according to the correlation between the water saturation and the free gas content and determining a fitting coefficient in the water saturation correction formula for the free gas content of the shale reservoir; S7: predicting the free gas volume of the target shale reservoir according to the water saturation correction formula of the free gas volume of the shale reservoir after the fitting coefficient is determined.

2. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S1, preparing shale samples with different water saturations specifically includes the following sub-steps: S11: Obtain a shale sample and prepare it into a standard plunger sample, which is then dried and weighed and recorded as m; S12: placing the shale sample in a vacuum saturation device to saturate with water, removing floating water on the surface of the sample after it is completely saturated with water, weighing it and recording it as M; S13: obtain the total mass M1 filled into the core pores by subtracting the dry weight m from the saturated weight M; S14: Calculate different target water saturations S using the total mass M1 of the saturated core pores and the dry weight m wn The corresponding core mass m n ; S15: Dry the shale sample until the mass m n The water saturation S is obtained wn shale samples.

3. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 2, characterized in that: In step S12, the saturation conditions for water saturation are vacuum time 24 hours, pressure 25 MPa, and saturation time 48 hours.

4. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 2, characterized in that: In step S15, the shale sample is first dried to a mass corresponding to the maximum target water saturation, and then the process proceeds to step S2 to perform saturated methane nuclear magnetic resonance T2 spectrum testing under different temperature and pressure conditions. After the test, the shale sample is continued to be dried to a mass corresponding to the second largest target water saturation, and this process is repeated to obtain the nuclear magnetic resonance T2 spectra of shale samples of each target water saturation under different temperature and pressure conditions.

5. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S2, when performing a saturated methane nuclear magnetic resonance T2 spectrum test, the pressure is increased to above 4 MPa before the experiment, and then the temperature is increased. After reaching the target temperature and target pressure required for the experiment, the nuclear magnetic resonance T2 spectrum is collected; after each experimental test is completed, the temperature is first lowered and then the pressure is reduced.

6. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S2, the nuclear magnetic resonance T2 spectrum is tested continuously for multiple times under the same conditions. When the repetition rate of the test results is higher than the target threshold, it is considered that methane has reached an equilibrium state in the core under the conditions, and the result in the equilibrium state is used as the nuclear magnetic resonance T2 spectrum result under the conditions.

7. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S3, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step S2 on the dried shale sample.

8. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to any one of claims 1 to 7, characterized in that: In step S5, the correlation between the water saturation and the free gas content is: For shale samples with porosity less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation; For shale samples with porosity greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation.

9. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 8, characterized in that: In step S6, the water saturation correction formula for the free gas content in the shale reservoir is: Where: Q f is the free gas volume of shale sample, cm 3 / g; A, B, C, and D are all fitting coefficients, dimensionless; S w is water saturation, %; is the reservoir porosity, %; The fitting coefficients are obtained by analyzing the correlation between each fitting coefficient and temperature and pressure, and then fitting the parameters under different temperatures and pressures.

10. The new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 9, characterized in that: The fitting coefficients are obtained by the following steps: Analyze the correlation between the fitting coefficients A, B, C, D and temperature and pressure respectively; According to the correlation analysis results, the correlation diagram between parameter B and pressure, and the correlation diagram between C and temperature are drawn, and nonlinear fitting is performed on A and D to obtain the following expressions for each fitting coefficient: B=a*P+b (4) C=c*Td (5) A=-e+f*T-g*P+h*P 2 (6) D=ij*Ln(T) / Tk*P (7) Where: P is pressure, T is temperature; ak is the fitting constant; According to the expressions of formula (4)-formula (7), the values ​​of the fitting coefficients under different pressure and temperature conditions are obtained, and the fitting correlation coefficient R 2 Greater than 0.9.

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