In-situ gas content evaluation method for coal measure reservoir based on pressure-maintaining coring

Abstract

This invention provides a method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining core sampling, belonging to the field of unconventional natural gas exploration technology. The method includes: calculating the gas content V lost during drilling of all core samples within the pressure-maintaining core sampling cylinder. TL Test the depressurized desorbed gas content (V) of all core samples taken from the pressure-holding core tube. TD Core samples were selected from the core taken from the pressure-holding corer and calculated as a percentage of the gas content V lost during the transfer of the selected core samples from the pressure-holding corer to the analyzer when exposed to the atmosphere. Eij ; Test the atmospheric pressure desorbed gas content V of the core sample Nij The residual gas content V of the core sample was tested in the residual gas meter. Rij The gas content G of the coal-bearing reservoir in the corresponding lithological section of the core sample was calculated. ij This invention involves calculations and tests in five stages, enabling simultaneous and accurate testing and acquisition of adsorbed gas and free gas in coal-bearing reservoirs, thus achieving precise determination and evaluation of the in-situ gas content of various coal-bearing strata.

Description

Technical Field

[0001] This invention belongs to the field of unconventional natural gas exploration technology, specifically relating to a method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-preserving coring. Background Technology

[0002] Coal-series gas, as a low-carbon and clean unconventional natural gas, is abundant worldwide and has become an important area for future natural gas production growth. However, the lithological configuration of coal-series reservoirs is complex, with frequent thin interbedded layers and a large proportion of free gas, leading to uncertainty in gas content interpretation. How to accurately obtain gas content data of coal-series reservoirs and achieve a refined and accurate evaluation of gas content is a critical problem that urgently needs to be solved in the current natural gas industry.

[0003] Currently, there is no specific method for evaluating the gas content of coal-bearing reservoirs. Methods for evaluating the gas content of shale or coal seams are generally used, and can be divided into indirect and direct methods. Indirect methods include: seismic interpretation, gas logging, well logging interpretation, isothermal adsorption, charting, and geological analogy.

[0004] The shortcomings of existing technologies:

[0005] The accuracy of evaluation results from indirect methods such as seismic interpretation, well logging interpretation, chart methods, and geological analogy is highly dependent on the establishment of empirical relationship models or heavily influenced by the evaluator's personal experience. In actual exploration and development practice, these methods have been proven to have significant errors, and the evaluation results cannot reflect the true gas content of coal-bearing reservoirs under in-situ conditions.

[0006] Coal-series gas exists in various states, including adsorbed gas, which is the primary form of gas in organic reservoirs, and free gas, which is the primary form of gas in inorganic reservoirs. It is generally believed that the gas content of coal-series reservoirs consists of both adsorbed and free gas. Current technologies cannot simultaneously and accurately measure and obtain both adsorbed and free gas contents in coal-series reservoirs. For example, gas content results obtained from gas logging can only reflect the amount of free gas in coal-series reservoirs; gas content data obtained from isothermal adsorption can only reflect the amount of adsorbed gas.

[0007] Compared to indirect methods such as seismic interpretation, gas logging, well logging interpretation, isothermal adsorption, and geological analogy, direct methods based on in-situ testing using well core samples offer higher reliability and accuracy. However, drilling and core extraction in coal-bearing reservoirs involves long haul times. Conventional open-loop core extraction or wireline core extraction can lead to distortion of core parameters and significant gas loss during extended haul times, resulting in undetectable or inaccurate free gas measurements. These limitations in in-situ analytical testing often result in substantial errors in the obtained gas content data for coal-bearing reservoirs.

[0008] Therefore, existing technologies for evaluating the gas content of coal-bearing reservoirs largely follow the methods used for evaluating the gas content of shale or coal seams. However, coal-bearing reservoirs have unique geological characteristics and complex gas content. Existing technologies cannot fully test adsorbed and free gas in coal-bearing reservoirs, accurately measure gas loss during drilling, or precisely identify and evaluate the gas content of thin coal-bearing layers, resulting in significant errors in the obtained gas content results. Currently, there is no direct, rapid, accurate, and highly reliable method for evaluating the gas content of coal-bearing reservoirs, making it difficult to meet the actual needs of coal-bearing gas exploration and development sites. Summary of the Invention

[0009] This invention provides a method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring, aiming to solve the problem that existing technologies cannot simultaneously and accurately test and obtain the adsorbed gas content and free gas content of coal-bearing reservoirs, and cannot achieve precise measurement and evaluation of the in-situ gas content of each stratum in the coal-bearing system.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is: to provide a method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring, the method comprising:

[0011] During the pressure-maintaining coring and drilling stage, calculate the gas content V lost during drilling of all core samples from the pressure-maintaining coring cylinder. TL ;

[0012] During the depressurization desorption stage, the depressurization desorption gas content V of all core samples in the pressure-maintaining core tube was tested. TD ;

[0013] During the core sample transfer stage, core samples are selected from the core sample taken from the pressure-holding core cylinder. The gas content V lost during the transfer of the selected core sample from the pressure-holding core cylinder to the analyzer is calculated when the core sample is exposed to the atmosphere. Eij ;

[0014] During the atmospheric desorption stage, the atmospheric desorbed gas content V of the core sample was tested. Nij ;

[0015] During the residual gas testing phase, the residual gas content V of the core sample was measured in the residual gas meter. Rij ;

[0016] The gas content G of the coal-bearing reservoir in the corresponding lithological section of the core sample is obtained according to formula (3) or (4). ij ;

[0017] G ij =V Lij +V Dij +V Eij +V Nij +V Rij (3)

[0018] It can also be expressed as:

[0019] G ij =K ij (V TL +V TD ) / m ij +V Eij +V Nij +V Rij (4)

[0020] In the formula, G ij This represents the gas content of the j-th core sample selected from the i-th lithological segment, expressed in cm³. 3 / g;K ij V is the ratio of the test gas volume inside the pressure-holding core sample to the test gas volume inside the entire core sample; TL To ensure the gas content of all core samples within the pressure-maintaining core barrel is minimized during the drilling process, cm 3 V TD To measure the depressurization desorption gas content of all core samples within the pressure-maintaining core barrel during the depressurization test, cm 3 V Eij V Nij and V Rij These represent the gas content lost during transfer, the desorbed gas content under normal pressure, and the residual gas content of the j-th core sample selected from the i-th lithological section, respectively, during the transfer process (cm). 3 / g.

[0021] In one feasible approach, during the pressure-maintaining coring and drilling stage, the gas content V lost during drilling of all core samples from the pressure-maintaining coring cylinder is... TL The calculation is obtained from formula (5):

[0022] V TL =(V TD +V TE +V TN +V TR (1 / PHR-1) (5)

[0023] In the formula, PHR is the ratio of the pore pressure of the core sample to the in-situ pressure of the formation when the pressure-holding core tube reaches the surface.

[0024] In the formula, V TE V TN and V TR These figures represent the depressurization desorption gas content, transfer loss gas content, atmospheric pressure desorption gas content, and residual gas content of all core samples within the pressure-holding core cylinder during the depressurization test, respectively. The units are cm³. 3 Among them, the depressurization desorption gas content V in the whole core is... TD The measured value is the gas content V of the transferred core sample.TE Atmospheric pressure desorption gas content V TN and residual gas content V TR The calculation method is as shown in formula (6):

[0025]

[0026] In the formula, M i V represents the total mass of all core samples from the i-th lithological segment within the pressure-maintaining core barrel, expressed in grams (g). TD V Nij and V Rij Direct measurement, V TL and V Eij It is then obtained through calculation;

[0027] n is the total number of all core samples in the i-th lithological segment;

[0028] q represents the total number of all lithological sections divided within the pressure-maintaining core.

[0029] In one feasible approach, during the core sample transfer stage, the transfer loss gas content V when the core sample is exposed to the atmosphere is... Eij Calculated using formulas (7), (8), and (9):

[0030]

[0031] In the formula, Q t —Measured value of desorbed gas content at the initial stage of desorption under normal pressure, cm 3 / g;Q l —Gas content lost during core transfer, cm 3 / g; k—slope of the straight segment; t0—transfer loss time, min; t—measured depressurization and desorption time, min; then the gas loss of the core sample during transfer is calculated by formula (8):

[0032]

[0033] Where t1 is the time lost without considering the transfer of core samples, min; t2 is the time lost considering the transfer of core samples, min; the values ​​of t1 and t2 are calculated according to formula (9):

[0034]

[0035] In the formula, T test This is the time used for the depressurization and desorption test; T retrieve This refers to the time from the start of drilling into the rock formation to the transport of the pressure-maintaining core to the depressurization desorption test; T transfer It is the time during which the core sample is exposed to the atmosphere from the time it is taken out of the pressure-holding core sampler to the time it is transferred to the desorption instrument;

[0036] In formula (8), Q t The value is the measured value of the desorbed gas content of the j-th core sample selected from the i-th lithological section during the initial stage of desorption at normal pressure, and the calculated V is the value of the desorbed gas content. E This is the gas content V that is transferred when the core sample is exposed to the atmosphere. Eij .

[0037] In one feasible manner, the test gas volume V inside the core sample cylinder Dij +V Lij Calculation:

[0038] The free-to-adsorption ratio is determined by the three-stage desorption results consisting of desorption at room temperature, desorption at reservoir temperature, and desorption at high temperature. The calculation formula (10) is as follows:

[0039] k=(V NH +V NS ) / (V NS +V NN (10)

[0040] In the formula, k represents the ratio of free gas volume to adsorbed gas volume; V NN V NS V NH These represent the desorption gas volume at room temperature, reservoir temperature, and high temperature of the core sample during the atmospheric pressure desorption stage, respectively; the free gas ratio k is then calculated using formula (11). free :

[0041]

[0042] Where, k free The percentage of free gas is dimensionless; k adsorbed The percentage of adsorbed gas is dimensionless; therefore, the amount of gas N measured in the core sample tube is... ij The calculation expression is as follows:

[0043] N ij =K 1i K 2ij (V TL +V TD ) / m ij (12)

[0044] In the formula, N ij That is, V Lij +V Dij ;K 1i K is the ratio of the gas volume measured inside the cylinder in the i-th lithological section to the gas volume measured inside the cylinder in all lithological sections; 2ij It is the ratio of the gas volume measured inside the cylinder of the j-th core sample selected in the i-th lithological section to the gas volume measured inside the cylinder of the corresponding lithological section; m ijLet g be the mass of the j-th core sample selected from the i-th lithological segment; K be the mass of the core sample. 1i K 2ij Calculated using formula (13):

[0045]

[0046] In one feasible approach, the gas content G of the coal-bearing reservoir in the corresponding lithological section of the core sample is... ij The calculation process is as follows:

[0047] The test gas volume inside the core sample cylinder is:

[0048] V Lij +V Dij =N ij =K 1i K 2ij (V TL +V TD ) / m ij (14)

[0049] The proportion of test gas volume inside the core sample cylinder to the total test gas volume inside the core cylinder (K) ij for:

[0050] K ij =K 1i K 2ij (15)

[0051] Right now:

[0052]

[0053] According to formula (4), the gas content G of the j-th core sample selected from the i-th lithological segment is... ij It can be represented as:

[0054]

[0055] In one feasible manner, the depressurization desorption gas volume V of the core sample inside the cylinder during the depressurization desorption stage is... TD The testing process is as follows:

[0056] Step 1: Seal one end of the gas collecting cylinder filled with saturated saline solution and inverted, and connect it to one end of the guide hose filled with saturated saline solution. Connect the other end of the guide hose to the gas collection port reserved on the pressure-holding core sampler.

[0057] Step 2: Open the vent valve, and the gas in the pressure-holding core sampler enters the gas collecting cylinder through the guide hose;

[0058] Step 3: When the saturated saline solution in the gas collecting cylinder has been completely drained, close the vent valve, replace it with another gas collecting cylinder filled with saturated saline solution, and record the gas volume V.Di ;

[0059] Repeat steps one and two until the liquid level in the nth gas collecting cylinder shows no significant change within 3-5 minutes. Record the time T taken for the depressurization desorption test. test The gas from each gas collecting cylinder is summed to obtain the depressurization desorption gas volume V of the entire core sample. TD The calculation formula (1) is as follows:

[0060]

[0061] In the formula, V TD The total volume of all desorbed gas in the core sampler during the depressurization test, expressed in cm³. 3 V Di The volume of gas collected in the i-th gas collecting cylinder is expressed in cm³. 3 .

[0062] In one feasible manner, the total mass M of all core samples from the i-th lithological segment within the pressure-holding core cylinder is... i The calculation formula (2) is as follows:

[0063]

[0064] In the formula, M i The total mass of all core samples from the i-th lithological segment within the pressure-maintaining core barrel, expressed in grams (g); L i The total length of all core samples from the i-th lithological segment within the pressure-maintaining core sampler, in meters (m). ij The mass of the j-th core sample selected from the i-th lithological segment, in grams; ij The length of the j-th core sample selected from the i-th lithological segment is expressed in meters.

[0065] In one feasible approach, during the atmospheric desorption stage, the atmospheric desorbed gas content V of the core sample... Nij The testing process is as follows:

[0066] Gradient-temperature controlled desorption was performed, with desorption occurring in stages at ambient temperature, geothermal temperature, and high temperature. When the cumulative desorbed gas volume over time approached a horizontal state, desorption at that temperature was terminated, and the process moved to the next temperature stage until desorption was complete. The atmospheric pressure desorbed gas volume V of the core sample was recorded. Nij .

[0067] In one feasible approach, the ambient temperature is 20-25℃, the ground temperature is 65-70℃, and the high temperature is 85-90℃.

[0068] The in-situ gas content evaluation method for coal-bearing reservoirs based on pressure-maintaining coring provided by this invention has the following advantages compared with existing technologies:

[0069] High-sealing pressure-maintaining core barrels can maintain the core under in-situ formation pressure, minimizing the loss of oil and gas components during core retrieval. During the core retrieval phase, drilling and core extraction face high-temperature, high-pressure geological conditions and long retrieval times, often resulting in less than ideal sealing capabilities and trigger stability. In fact, due to the complexity of pressure-maintaining core technology, these tools can only minimize, but not completely eliminate, gas loss from the core; a small amount of gas loss will occur. This lost gas is reconstructed using pressure-maintaining rate data recorded in real-time by the pressure sensor built into the tool, combined with gas content data from other stages of testing. The pressure-maintaining rate is used to estimate the natural gas loss during retrieval, improving the accuracy of the overall core loss during this phase.

[0070] Both the pressure-maintaining coring and drilling stages and the pressure-reducing desorption stages are part of the evaluation of in-cylinder gas content. The in-cylinder gas volume of all core samples, including the gas lost during drilling and the gas content from pressure reduction desorption, can be directly calculated or tested. However, the gas content of different thin coal-bearing strata differs fundamentally, and the contribution of cores located in different lithological sections to the in-cylinder gas volume varies significantly. Therefore, the in-cylinder gas content of core samples needs further calculation. Free gas, as the main component of in-cylinder gas, occupies a significant proportion of the overall gas content of the core. Thus, the accuracy of the calculated in-cylinder gas content directly affects the precision and reliability of the in-situ gas content evaluation of thin interbedded coal-bearing strata. To minimize calculation errors while ensuring the timeliness and effectiveness of test results, the in-cylinder gas content of core samples is calculated based on the gas content of the thin strata reflected by the in-situ desorption data. This invention proposes a method for calculating the gas content inside the core sample cylinder based on the gas content of the core sample. The gas content outside the cylinder of the core sample has been determined in the above-mentioned testing and calculation process and is a major component of the gas content of the core, largely reflecting the gas-bearing capacity of the core and serving as a key parameter for characterizing the gas content of the core. By using the gas content outside the core sample cylinder and the proportion of free gas as key parameters, the gas content inside the core sample cylinder is reasonably calculated, providing a guarantee for achieving a precise evaluation of the gas content of coal-bearing reservoirs with frequent thin interlayer stacking.

[0071] During testing, the pressure-holding core sampler needs to be opened, and samples are selected from each lithological section for the next stage of testing. During this process, gas loss from the core sample is inevitable. To address the easily overlooked gas loss in the core sample during this process, this invention proposes a method for calculating the gas loss during the core sample transfer stage, based on the gas escape patterns in the core and the sample transfer time recorded on-site. This effectively improves the accuracy of evaluating the gas content of coal-bearing reservoirs.

[0072] During the atmospheric pressure desorption stage, the accuracy of evaluating the gas content of coal-bearing reservoirs was effectively improved by adopting gradient temperature-controlled desorption.

[0073] Therefore, this invention divides the evaluation process into five stages: pressure-maintaining core sampling and drilling stage, pressure-reducing desorption stage, core sample transfer stage, atmospheric pressure desorption stage, and residual gas testing stage. Correspondingly, five test results are obtained: drilling loss gas content, pressure-reducing desorption gas content, transfer loss gas content, atmospheric pressure desorption gas content, and residual gas content. The gas content of the core sample is obtained by summing these results, and then the gas content of the coal-bearing reservoir in the corresponding lithology section is obtained.

[0074] Compared with the existing methods for evaluating the gas content of coal seams or shale, the calculation and testing method proposed in this invention takes into account the characteristics of different stages and the testing features of each stage, minimizing gas loss and almost completely testing the free and adsorbed gas in the coal-bearing reservoir core. It establishes a rapid, direct, practical, and highly reliable evaluation method, greatly reducing testing errors and effectively improving the accuracy and reliability of coal-bearing reservoir gas content data. It can provide timely and effective in-situ gas content data of coal-bearing reservoirs for the fine exploration and efficient development of coal-bearing gas.

[0075] This invention addresses the complex gas-bearing characteristics of coal-bearing strata by designing and establishing an evaluation method that combines on-site testing and calculation of gas content in coal-bearing reservoirs based on pressure-preserving coring technology. This method effectively solves the practical problems of undetectable and inaccurate measurement of free and adsorbed gas, enabling precise determination and accurate evaluation of in-situ gas content in various coal-bearing strata. Attached Figure Description

[0076] Figure 1 A flowchart for testing and calculating the gas content of coal-bearing reservoirs provided in an embodiment of the present invention; Detailed Implementation

[0077] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0078] Please see Figure 1 The present invention describes the in-situ gas content evaluation method for coal-bearing reservoirs based on pressure-preserving core sampling. This method comprises five stages: pressure-preserving core sampling and drilling stage, depressurization desorption stage, core sample transfer stage, atmospheric pressure desorption test stage, and residual gas test stage. Correspondingly, five test results are obtained: drilling loss gas content, depressurization desorption gas content, transfer loss gas content, atmospheric pressure desorption gas content, and residual gas content. These results are summed to obtain the gas content of the core sample, and thus the gas content of the corresponding lithological section of the coal-bearing reservoir.

[0079] The embodiments of the present invention describe the calculation or testing process of each stage in sequence, and the specific implementation process is as follows:

[0080] In the first stage, the pressure-maintaining coring and drilling stage, the gas loss V during drilling of all core samples from the pressure-maintaining coring cylinder was calculated. TL :

[0081] After the pressure-holding coring operation is completed, the pressure-holding coring cylinder will be raised to the ground. During this period, there will be a small amount of gas loss. This lost gas needs to be recovered by calculating the pressure rate data recorded in real time by the pressure sensor built into the pressure-holding coring cylinder, combined with the gas content data from other stages of testing.

[0082] Because core drilling in rock formations involves high-temperature and high-pressure geological conditions and long extraction times, the sealing capacity and triggering stability of pressure-holding core samples are often less than ideal. In fact, due to the complexity of pressure-holding core drilling technology, pressure-holding core samples can only minimize, but not completely prevent, gas loss from the core. Pressure holding ratio (PHR) refers to the ratio of the pore pressure of the core to the in-situ formation pressure when the core reaches the surface; it is a key parameter for estimating the amount of natural gas lost during extraction. The total gas loss (V) of the entire core sample during extraction is... TL It can be calculated using the following formula:

[0083] V TL =(V TD +V TE +V TN +V TR (1 / PHR-1) (5)

[0084] In the formula, V TD V TE V TN and V TR These figures represent the depressurization desorption gas content, transfer loss gas content, atmospheric pressure desorption gas content, and residual gas content of all core samples within the pressure-holding core cylinder during the depressurization test, respectively. The units are cm³. 3 Among them, the depressurized desorption gas content V in the whole core is... TD The measured value is the gas content V of the transferred core sample. TE Atmospheric pressure desorption gas content V TN Residual gas content V TR The calculation method is as follows:

[0085]

[0086] In the formula, M i V represents the total mass of all core samples from the i-th lithological segment within the pressure-maintaining core barrel, expressed in grams (g). Eij V Nij and VRij These represent the gas content lost during transfer, the desorbed gas content at atmospheric pressure, and the residual gas content of the j-th core sample selected from the i-th lithological section, respectively, during the transfer process, in cm³. 3 / g;V TD V Nij and V Rij Direct measurement, V TL and V Eij It is then obtained through calculation;

[0087] In the formula, n is the total number of all core samples in the i-th lithological segment;

[0088] q represents the total number of all lithological sections divided within the pressure-maintaining core.

[0089] The second stage, the depressurization desorption stage, involved testing the depressurization desorption gas content (V) of all core samples inside the cylinder. TD The testing process is as follows:

[0090] Step 1: After the pressure-holding coring cylinder, which has completed the coring operation, is transported to the test site, record the time T taken from the start of drilling into the rock strata until this point. retrieve A high-pressure gas collecting cylinder filled with saturated saline solution and inverted is connected to one end of a flow guide hose filled with saturated saline solution via a quick-connect plug. The other end of the flow guide hose is connected to the gas collection port reserved on the pressure-holding core sampler via an adapter sleeve.

[0091] Step two: Slowly open the vent valve to turn on the pressure regulator. A large amount of gas will flow into the gas collecting cylinder through the guide hose. The gas flow rate is controlled by the pressure regulator. The gas occupies the upper space of the gas collecting cylinder, and an equal volume of saturated saline solution is forced out of the drain outlet by the gas in the upper part.

[0092] Step 3: When the gas collecting cylinder is about to be completely emptied, close the vent valve, replace it with a new gas collecting cylinder filled with saturated saline solution, and record the volume V of gas measured at this point. Di .

[0093] Repeat steps one and two until the liquid level in the nth gas collecting cylinder shows no significant change within 3 consecutive minutes. Record the time T taken for the depressurization desorption test. test The gas measured by each gas collecting cylinder is summed to obtain the total depressurization and desorption gas volume V of the entire core. TD .

[0094]

[0095] In the formula, V TD The total volume of all desorbed gas in the core sampler during the depressurization test is expressed in cm³. 3 V DiLet be the volume of gas collected in the i-th graduated cylinder, in cm³. 3 .

[0096] The third stage is the core sample transfer stage, during which the gas loss V during transfer is recorded. E The calculation process:

[0097] After the depressurization desorption test is completed, the pressure-maintaining core sampler is opened, and the coal-bearing reservoir core sample is removed. The mud on the surface of the core sample is then washed away. The length L of each lithological section is measured. i At least one representative core sample was selected from each lithological segment, its length was measured and its weight was weighed, and the length l of the j-th core sample taken from the i-th lithological segment was recorded. ij mass m ij Assuming the density is constant throughout the lithological section, the mass M of the i-th lithological section in the pressure-holding core is... i The result can be calculated from equation (2):

[0098]

[0099] In the formula, M i The total mass of all cores from the i-th lithological segment within the pressure-maintaining core barrel is given by g; L i The total length of all core samples from the i-th lithological segment within the pressure-maintaining core sampler is given in m. ij Let g be the mass of the j-th core sample selected from the i-th lithological segment; l ij Let be the length (m) of the j-th core sample selected from the i-th lithological segment. After recording the sample number, promptly load the core sample into a high-precision desorption instrument and record the exposure time T to the atmosphere during its transfer. transfer .

[0100] The entire core cylinder is then removed to selectively choose representative core samples from each lithological segment for the next stage of testing. Clearly, from the opening of the pressure-holding core cylinder to the loading of the core sample into the high-precision desorption instrument, the core sample will be exposed to the atmosphere. During this period, the amount of gas released cannot be directly measured; instead, on-site desorption data must be substituted into a mathematical model for reconstruction. In the initial stage of gas release, the cumulative amount of desorbed gas from the core sample is proportional to the square root of time, as expressed mathematically below:

[0101]

[0102] In the formula, Q t —Measured value of desorbed gas content at the initial stage of desorption under normal pressure, in cm³. 3 / g;Q l —Gas loss from the core, in cm³ 3 / g; k—slope of the straight segment; t0—loss time, in min; t—measured desorption time, in min. The gas loss of the core sample during transfer is calculated by the following formula (8):

[0103]

[0104] Where t1 is the time before sample transfer loss, in minutes; t2 is the time before sample transfer loss, in minutes. The values ​​of t1 and t2 are calculated according to equation (9):

[0105]

[0106] In the formula, T test This is the time used for the depressurization and desorption test; T retrieve This refers to the time from the start of drilling into the rock formation to the transport of the pressure-maintaining core to the depressurization desorption test; T transfer It is the time during which the core sample is exposed to the atmosphere from the time it is taken out of the pressure-holding core sampler to the time it is transferred to the desorption instrument;

[0107] In formula (8), Q t The value is the measured value of the desorbed gas content of the j-th core sample selected from the i-th lithological section during the initial stage of desorption at normal pressure, and the calculated V is the value of the desorbed gas content. E This is the gas content V that is transferred when the core sample is exposed to the atmosphere. Eij .

[0108] The fourth stage, the atmospheric pressure desorption stage, involves the atmospheric pressure desorption gas content V of the core sample. Nij The testing process is as follows:

[0109] Gradient temperature-controlled desorption was initiated, with desorption performed stepwise at ambient temperature (20℃), ground temperature (70℃), and high temperature (90℃). The desorption time depended on the amount of desorbed gas. When the cumulative desorbed gas volume over time approached a horizontal state, desorption at that temperature was terminated, and the process moved to the next temperature range until desorption was complete. The atmospheric pressure desorbed gas content (V) of the core sample under normal pressure was recorded. Nij .

[0110] The fifth stage is the residual gas testing stage for core samples, which tests the residual gas content V. Rij :

[0111] The core sample was removed from the high-precision desorption instrument and placed in a residual gas measuring instrument to collect and measure the desorbed residual gas, thus obtaining the residual gas content V. Rij .

[0112] In-cylinder test gas content (V) of coal-series reservoir core samples obtained by pressure-holding coring Dij +V Lij The calculation process is as follows:

[0113] The gas content inside the core sample cylinder, including gas loss during drilling and desorption due to pressure reduction, can be directly calculated or tested. However, the gas content of different thin coal-bearing strata varies significantly, and cores located in different lithological sections contribute vastly different amounts to the gas content inside the cylinder. Therefore, the gas content inside the core sample cylinder needs further calculation. This invention proposes a method for calculating the gas content inside the core sample cylinder based on the gas content of the core sample cylinder. Using the gas content outside the cylinder and the proportion of free gas as key parameters, the gas content inside the cylinder of the core sample cylinder is reasonably calculated, providing a guarantee for achieving a precise evaluation of the gas content in coal-bearing reservoirs with frequent thin interbedded layers.

[0114] During the core sampling test, as the temperature and pressure conditions of the coal core change, the free gas in the compressed state has a strong seepage capacity and does not require slow desorption. It always expands and dissipates before the adsorbed gas. In addition, due to the short depressurization desorption test time, the adsorbed gas in the core inside the cylinder has not yet desorbed and diffused in time. Therefore, the free gas constitutes the main body of the test gas inside the cylinder.

[0115] Given the unique geological characteristics of coal-bearing reservoirs, a single pressure-maintaining coring operation simultaneously yields cores from different lithological strata. Gas content data for each stratum is obtained by sampling and testing within each lithological section. While the gas content data for both inside and outside the coring cylinder of all core samples have been obtained through established testing and calculation procedures, the gas content data for the core samples inside the cylinder requires further calculation. It is noteworthy that free gas, as a major component of the gas content inside the cylinder, constitutes a significant proportion of the overall gas content in the core. Therefore, the accuracy of the calculated gas content data inside the cylinder directly impacts the precision and reliability of the in-situ gas content assessment for thin interbedded coal-bearing layers.

[0116] To minimize calculation errors while ensuring the timeliness and validity of test results, the in-core test gas of the core sample was calculated based on the gas content of the thin strata reflected by the in-situ desorption data. The out-of-core test gas of the core sample, as determined in the aforementioned testing and calculation process, is a major component of the core's gas content and largely reflects its gas-bearing capacity, serving as a key parameter characterizing the core's gas content. Adsorbed gas and free gas coexist in coal-bearing reservoirs. Even strata or cores with the same mass and total gas content can contribute differently to the in-core free gas due to significant differences in their gas-bearing structure. In other words, a core with a large mass and high total gas content but a low proportion of free gas may not necessarily have a high proportion of in-core free gas. To further ensure the rationality of the calculation results, the proportion of free gas should be considered as another important parameter.

[0117] During the atmospheric pressure desorption test, gradient temperature-controlled desorption was performed. Changes in desorbed gas under ambient temperature conditions primarily reflect free gas characteristics, changes under formation temperature conditions primarily reflect recoverability characteristics, and changes under high temperature conditions primarily reflect adsorbed gas characteristics. In other words, the free-to-adsorbed gas ratio can be calculated and determined using the three-stage desorption results comprised of ambient temperature desorbed gas, reservoir temperature desorbed gas, and high temperature desorbed gas, as shown in the following formula:

[0118] k=(V NH +V NS ) / (V NS +V NN (10)

[0119] In the formula, k represents the ratio of free gas volume to adsorbed gas volume; V NN V NS V NH These represent the desorbed gas content of the core sample at ambient temperature, reservoir temperature, and high temperature during the atmospheric pressure desorption stage, respectively. The free gas percentage k can then be easily calculated from this. free :

[0120]

[0121] Where, k free The percentage of free gas is dimensionless; k adsorbed The percentage of adsorbed gas is dimensionless. The N2 content of the test gas inside the core sample cylinder is... ij The calculation expression is as follows:

[0122] N ij =K 1i K 2ij (V TL +V TD ) / m ij (12)

[0123] In the formula, K 1i K is the ratio of the test gas volume in the cylinder of the i-th lithological segment to the test gas volume in the cylinders of all lithological segments. 2ij K is the ratio of the test gas inside the cylinder of the j-th core sample selected in the i-th lithological section to the test gas inside the cylinder of the corresponding lithological section. 1i K 2ij It can be calculated using the following formula:

[0124]

[0125] The variables in equation (13) have been clearly explained above.

[0126] The test gas inside the core sample cylinder is known to be:

[0127] V Lij +VDij =N ij =K 1i K 2ij (V TL +V TD ) / m ij (14)

[0128] The proportion of test gas volume inside the core sample cylinder to the total test gas volume inside the core cylinder (K) ij for:

[0129] K ij =K 1i K 2ij (15)

[0130] Right now:

[0131]

[0132] Finally, the gas content G of the core sample ij Calculation:

[0133] This invention divides the evaluation process into five stages: pressure holding and drilling stage, pressure reduction and desorption stage, core sample selection and transfer stage, atmospheric pressure gradient temperature control test of core sample desorbed gas, and core sample residual gas test. The gas content of each stage is obtained by direct testing or calculation, and the total gas content of the coal-bearing core sample is obtained by summing them.

[0134] Then the gas content G of the j-th rock sample selected from the i-th lithological section in the pressure-holding core is... ij It can be expressed as the sum of the gas content obtained from the five evaluation stages:

[0135] G ij =V Lij +V Dij +V Eij +V Nij +V Rij (3)

[0136] In the formula, G ij This represents the gas content of the j-th core sample selected from the i-th lithological segment, expressed in cm³. 3 / g;V Lij V Dij V Eij V Nij and V Rij The values ​​represent the gas content lost during drilling, the depressurization desorption gas content during depressurization, the transfer loss gas content during transfer, the atmospheric pressure desorption gas volume, and the residual gas content of the j-th core sample selected from the i-th lithological section, respectively, in cm⁻¹. 3 / g.

[0137] Then, according to formulas (3) and (14), the gas content G of the j-th rock sample selected from the i-th lithological section in the pressure-holding core can be obtained. ij According to formula (3), the gas content of the coal-bearing reservoir corresponding to the lithology of the core sample can be expressed as: G = V L +V D +V E +V N +V R .

[0138] Gas content V lost during core drilling Lij Depressurized desorption gas content V Dij For the test gas inside the cylinder; the content of the transfer loss gas is V. Eij Atmospheric pressure desorption gas content V Nij Residual gas content V Rij The gas content of the core sample is the sum of the gas content of the gas inside and outside the cylinder.

[0139] Then the gas content G of the j-th rock sample selected from the i-th lithological section in the pressure-holding core is... ij It can be represented as:

[0140] G ij =K ij (V TL +V TD ) / m ij +V Eij +V Nij +V Rij (4)

[0141] V TD V Nij and V Rij The steps described above can be used to directly determine V. TL and V Eij It can also be calculated using mathematical methods.

[0142] According to formulas (4) and (16) above, the gas content G of the j-th rock sample selected from the i-th lithological section in the pressure-maintaining core tube is... ij It can also be expressed as:

[0143]

[0144] Existing evaluation techniques have significant limitations when applied to coal-bearing reservoirs with frequent thin interbedded layers and high free gas content. To address the unique geological characteristics and complex gas-bearing properties of these reservoirs, this invention proposes a systematic evaluation method combining gas content testing and calculation. This method minimizes gas loss and provides a near-complete test of free and adsorbed gas in coal-bearing reservoir cores. It establishes a rapid, direct, practical, and highly reliable evaluation method that can provide timely and effective in-situ gas content data for the precise exploration and efficient development of coal-bearing gas.

[0145] Compared with the existing methods for evaluating the gas content of coal seams or shale, the in-situ gas content evaluation method for coal-bearing reservoirs proposed in this invention greatly reduces testing errors. The results obtained are consistent with the geological characteristics and gas content complexity of coal-bearing strata, effectively improving the accuracy and reliability of gas content data in coal-bearing reservoirs.

[0146] The evaluation methods mentioned in the background section are explained as follows:

[0147] Seismic interpretation: Hydrocarbon detection based on seismic data easily obtains the gas content of formations and has been widely applied to areas covered by existing seismic data. This method is based on the anomalous changes in seismic wave attribute parameters, seeking the relationship between these anomalous changes and gas content to predict the spatial distribution of gas content.

[0148] Gas logging: During drilling, formation fluids enter the wellbore and return to the surface with the drilling fluid, carrying with them information about the gas content of the formation. This evaluation method mainly relies on logging anomaly curves. By utilizing anomalies in total hydrocarbon and methane content, the gas content of coal-bearing formations, especially the free gas content, can be directly reflected.

[0149] Well logging interpretation: Well logging interpretation makes it easy to obtain parameters such as formation porosity, gas saturation, and organic carbon. The free gas content in the formation is calculated using porosity and gas saturation parameters, and the adsorbed gas content is calculated using the empirical relationship between organic carbon content and adsorbed gas content. The sum of the free gas and adsorbed gas contents is the formation gas content.

[0150] Isothermal adsorption: Under a given formation temperature, the amount of methane adsorbed in the sample is measured by changing the pressure. Based on the relationship between pressure and the amount of adsorbed gas, the adsorbed gas content in the formation under the corresponding temperature conditions is calculated.

[0151] The theoretical chart method: When available data is limited, the theoretical chart method can be used for prediction. This method uses natural gas enrichment geology theory to calculate the gas content of strata under various pre-defined geological conditions, and compiles the results into charts that are easy to query and use.

[0152] Geological analogy: This method is a prediction and evaluation method based on similar geological conditions. By selecting a known gas field with similar geological characteristics to the area to be evaluated as an analogy object, and based on the gas content data of the analogy object, combined with the differences in geological characteristics of the area to be evaluated, the gas content of the reservoir in the area to be evaluated is estimated.

[0153] The direct method, also known as the field analysis method, refers to an evaluation method that uses on-site analysis experiments conducted at the drilling site based on reservoir cores obtained through open drill pipe coring or wireline coring to determine the actual gas content.

[0154] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0155] 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, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring, characterized in that, The method includes: During the pressure-maintaining coring and drilling stage, calculate the gas content lost during drilling of all core samples from the pressure-maintaining coring cylinder. V TL ; During the depressurization desorption stage, the depressurization desorption gas content of all core samples within the pressure-maintaining core tube was tested. V TD ; During the core sample transfer stage, core samples are selected from the core sample taken from the pressure-holding core cylinder. The gas content loss during the transfer of the selected core sample from the pressure-holding core cylinder to the analyzer is calculated when the core sample is exposed to the atmosphere. V Eij ; During the atmospheric desorption stage, the atmospheric desorption gas content of the core sample was tested. V Nij ; During the residual gas testing phase, the residual gas content of the core sample is measured in the residual gas meter. V Rij ; The gas content of the coal-bearing reservoir in the corresponding lithological section of the core sample can be calculated according to formula (3) or (4). G ij ; （3） It can also be expressed as: （4） In the formula, G ij For from the first i The first lithological section selected j Gas content of each core sample, in cm³ / g; K ij This is the ratio of the test gas volume inside the pressure-holding core sample to the test gas volume inside the entire core sample. V TL The gas content lost during drilling of all core samples in the pressure-maintaining core barrel during the drilling process is measured in cm³. V TD The depressurization desorption gas content (cm³) of all core samples taken from the pressure-maintaining core cylinder during the depressurization test; V Eij , V Nij and V Rij Representing from the first i The first lithological section selected j The content of gas lost during the transfer process, the content of desorbed gas at normal pressure, and the content of residual gas in each core sample, in cm³ / g; V Lij For the first i The first lithological section selected j Gas content lost during drilling of each core sample; V Dij For the first i The first lithological section selected j Depressurized desorbed gas content of each core sample; Test gas volume inside core sample cylinder V Dij + V Lij Calculation: The free-to-adsorption ratio is determined by the three-stage desorption results consisting of desorption at room temperature, desorption at reservoir temperature, and desorption at high temperature. The calculation formula (10) is as follows: （10） In the formula, k This indicates the ratio of free gas volume to adsorbed gas volume; V NN , V NS , V NH These represent the desorption gas volume at room temperature, reservoir temperature, and high temperature of the core sample during the atmospheric pressure desorption stage, respectively; the free gas ratio can then be calculated using formula (11). k free : （11） in, k free The percentage of free gas is dimensionless. k adsorbed The percentage of adsorbed gas is dimensionless; therefore, the amount of test gas inside the core sample tube is... N ij The calculation expression is as follows: （12） In the formula, N ij That is, V Lij +V Dij ; K 1i It is the first i The ratio of the test gas volume in the cylinder of a single lithological section to the test gas volume in the cylinder of all lithological sections; K 2ij It is in the i The first lithological section selected j The ratio of the amount of gas tested in the cylinder of a core sample to the amount of gas tested in the cylinder of the corresponding lithological section. m ij For from the first i The first lithological section selected j Mass of each core sample, in grams; K 1i , K 2ij Calculated using formula (13): （13）； In the formula, k freeij For the first i The first lithological section selected j The percentage of free gas in each core sample.

2. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in claim 1, characterized in that, During the pressure-maintaining coring and drilling stage, the gas content lost during drilling of all core samples from the pressure-maintaining coring cylinder. V TL The calculation is obtained from formula (5): （5） In the formula, PHR The ratio of pore pressure to in-situ formation pressure in the core sample when the pressure-maintaining core tube reaches the surface; In the formula, V TE , V TN and V TR These figures represent the depressurization desorption gas content, transfer loss gas content, atmospheric pressure desorption gas content, and residual gas content of all core samples within the pressure-holding core cylinder during the depressurization test, respectively. The units are cm³. 3 Among them, the content of depressurized desorbed gas in the whole core sample V TD The measured value represents the gas content lost during transfer from the entire core sample. V TE Atmospheric pressure desorption gas content V TN and residual gas content V TR The calculation method is as shown in formula (6): （6） In the formula, M i To maintain pressure inside the core tube i Total mass of all core samples from a lithological section, in grams; V TD , V Nij and V Rij Direct measurement, V TL and V Eij It is then obtained through calculation; n For the first i The total number of all core samples in each lithological section; q The total number of all lithological sections divided within the core for pressure maintenance.

3. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in claim 2, characterized in that, During the core sample transfer stage, the gas content lost during core sample exposure to the atmosphere. V Eij Calculated using formulas (7), (8), and (9): （7） In the formula, Q t —Measured value of desorbed gas content at the initial stage of desorption under normal pressure, cm 3 / g; Q l —Gas content lost during core transfer, cm 3 / g; k —Slope of the straight line segment; t 0 — Transfer loss time, in minutes; t —Measured depressurization and desorption time, min; then the gas loss of the core sample during transfer is calculated by formula (8): （8） in, t 1 represents the time lost during core sample transfer, in minutes, without considering the time lost during transfer. t 2 represents the time lost during core sample transfer, expressed in minutes. t 1 and t The value of 2 is calculated according to formula (9): （9） In the formula, T test This is the time taken for the depressurization and desorption test; T retrieve It is the time from the start of drilling into the rock formation to the transportation of the pressure-maintaining core sample to the depressurization desorption test. T transfer It is the time during which the core sample is exposed to the atmosphere from the time it is taken out of the pressure-holding core sampler to the time it is transferred to the desorption instrument; In formula (8), Q t The value is the first i The first lithological section selected j Measured values ​​of atmospheric pressure desorbed gas content in each core sample V Nij Calculated V E This refers to the gas content lost due to transfer when the core sample is exposed to the atmosphere. V Eij .

4. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in claim 1, characterized in that, Gas content of coal-bearing reservoirs in corresponding lithological sections of core samples G ij The calculation process is as follows: The test gas volume inside the core sample cylinder is: （14） The proportion of test gas volume inside the core sample cylinder to the total test gas volume inside the core cylinder. K ij for: （15） Right now: （16） From formula (4), then the first i The first lithological section selected j Gas content of each core sample G ij It can be represented as: （17）。 5. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in any one of claims 1-4, characterized in that, Depressurization desorption gas volume of core samples in the depressurization desorption stage V TD The testing process is as follows: Step 1: Seal one end of the gas collecting cylinder filled with saturated saline solution and inverted, and connect it to one end of the guide hose filled with saturated saline solution. Connect the other end of the guide hose to the gas collection port reserved on the pressure-holding core sampler. Step 2: Open the vent valve, and the gas in the pressure-holding core sampler enters the gas collecting cylinder through the guide hose; Step 3: When the saturated saline solution in the gas collecting cylinder has been completely drained, close the vent valve, replace it with another gas collecting cylinder filled with saturated saline solution, and record the gas volume. V Di ; Repeat steps one and two until the first [item] appears within 3-5 minutes. n Record the time taken for the depressurization and desorption test until the liquid level in each gas collecting cylinder shows no significant change. T test The gas from each gas collecting cylinder is summed to obtain the depressurization and desorption gas volume of the entire core sample. V TD The calculation formula (1) is as follows: （1） In the formula, V TD The total volume of all desorbed gas in the core sampler during the depressurization test, expressed in cm³. 3 ; V Di For the first i The volume of gas collected in each gas collecting cylinder, in cm³. 3 .

6. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in any one of claims 1, 2, or 4, characterized in that, The first pressure-holding core sampler i Total mass of all core samples from each lithological segment M i The calculation formula (2) is as follows: （2） In the formula, M i To maintain pressure inside the core tube i Total mass of all core samples from a lithological section, in grams; L i To maintain pressure inside the core tube i The total length of all core samples in a lithological segment, in meters; m ij For from the first i The first lithological section selected j Mass of each core sample, in grams; l ij For from the first i The first lithological section selected j The length of each core sample, in meters.

7. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in any one of claims 1-4, characterized in that, During the atmospheric desorption stage, the atmospheric desorption gas content of the core sample V Nij The testing process is as follows: Gradient-temperature controlled desorption was performed, with desorption occurring in stages at ambient temperature, geothermal temperature, and high temperature. When the cumulative desorbed gas volume over time approached a horizontal state, desorption at that temperature was terminated, and the process moved to the next temperature stage until desorption was complete. The amount of gas desorbed at atmospheric pressure in the core sample was recorded. V Nij .

8. The method for in-situ evaluation of gas content in coal-bearing reservoirs based on pressure-maintaining coring as described in claim 7, characterized in that, The ambient temperature is 20-25℃, the ground temperature is 65-70℃, and the high temperature is 85-90℃.

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