Coal measure reservoir in-situ gas content field evaluation method based on pressure-maintaining coring

Abstract

The invention provides a coal measure reservoir in-situ gas content field evaluation method based on pressure-maintaining coring, and belongs to the technical field of unconventional natural gas exploration, and the method comprises the following steps: calculating the drill lifting loss gas content VTL of all rock cores in a pressure-maintaining coring cylinder; the depressurized desorbed gas content VTD of all the rock cores in the pressure-maintaining coring cylinder is tested; selecting a rock core sample from the rock core taken out of the pressure-maintaining coring cylinder, and calculating the transfer loss gas content VEij when the rock core sample is exposed to the atmosphere in the process of transferring the selected rock core sample from the pressure-maintaining coring cylinder to the analyzer; testing the normal-pressure desorbed gas content VNij of the rock core sample; testing the residual gas content VRij of the core sample in the residual gas measuring instrument; and calculating to obtain the gas content Gij of the lithologic section coal reservoir corresponding to the rock core sample. According to the method, calculation and testing are carried out in five stages, the adsorption gas amount and the free gas amount of the coal measure reservoir can be accurately tested and obtained at the same time, and fine measurement and accurate evaluation of the in-situ gas content of each rock stratum of the coal measure are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of unconventional natural gas exploration, and in particular relates to a method for evaluating the in-situ gas content of coal-bearing reservoirs based on pressure-maintained coring. Background Art

[0002] Coal-measure gas, a low-carbon, clean, unconventional natural gas, boasts abundant reserves worldwide and is a key area for future natural gas production growth. However, coal-measure reservoirs are characterized by complex lithologic configurations, frequent thin interbeds, and a high proportion of free gas, leading to uncertainties in interpreting their gas content. Accurately acquiring coal-measure reservoir gas content data and enabling detailed and precise evaluation of coal-measure reservoir gas content are critical challenges facing the natural gas industry.

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

[0004] Disadvantages of existing technology:

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

[0006] Coal-bearing gas exists in diverse states, including adsorbed gas, primarily in organic reservoirs, and free gas, primarily in inorganic reservoirs. It is generally believed that the gas content of coal-bearing reservoirs is composed of both adsorbed and free gas. Existing technologies are unable to accurately measure and determine both adsorbed and free gas contents simultaneously. For example, gas content data obtained from gas logging primarily reflects the free gas content in coal-bearing reservoirs, while gas content data obtained from isothermal adsorption only reflects the adsorbed gas content in coal-bearing reservoirs.

[0007] Compared to indirect methods such as seismic interpretation, gas logging, well logging interpretation, isothermal adsorption, and geological analogy, direct field testing based on coring offers greater reliability and accuracy. However, coring in coal-bearing reservoirs requires a long drilling and hauling time. Conventional open-drill pipe or rope coring methods can distort core parameters and produce significant gas losses during this prolonged hauling process, leading to undetectable and inaccurate free gas measurements. Field analytical testing presents significant limitations, and the resulting coal-bearing reservoir gas content data often exhibits significant errors.

[0008] As can be seen, existing coal-measure reservoir gas content assessment technologies largely rely on methods used to assess gas content in shale or coal seams. However, due to the unique geological characteristics and complex gas content of coal-measure reservoirs, existing technologies cannot fully measure adsorbed and free gas in coal-measure reservoirs, cannot accurately measure gas loss during drilling, and struggle to precisely identify and assess the gas content of thin coal-measure layers. Consequently, the resulting gas content results are subject to significant errors. Currently, there is no direct, rapid, accurate, and reliable gas content assessment method for coal-measure reservoirs, making it difficult to meet the actual needs of coal-measure gas exploration and development sites. Summary of the Invention

[0009] An embodiment of the present invention provides a method for on-site evaluation of the in-situ gas content of coal-bearing reservoirs based on pressure-maintained coring, which aims to solve the problem that the existing technology cannot accurately test and obtain the adsorbed gas content and free gas content of coal-bearing reservoirs at the same time, and cannot achieve precise measurement and accurate evaluation of the in-situ gas content of various coal-bearing rock formations.

[0010] To achieve the above object, the present invention adopts a technical solution of providing a method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring, the method comprising:

[0011] During the pressure-maintaining coring and drilling stage, calculate the gas content V lost by drilling all the cores in the pressure-maintaining coring barrel. TL ;

[0012] During the decompression and desorption stage, the decompression and desorption gas content V of all cores in the pressure-maintaining coring barrel is tested. TD ;

[0013] During the core sample transfer phase, core samples are taken out from the pressure coring barrel and the gas content V lost when the core samples are exposed to the atmosphere is calculated during the process of transferring the selected core samples from the pressure coring barrel to the analyzer. Eij ;

[0014] At atmospheric pressure desorption stage, the atmospheric pressure desorption gas content V of the core sample is tested Nij ;

[0015] During the residual gas testing phase, the residual gas content V of the core sample is placed in the residual gas measuring instrument. Rij ;

[0016] According to formula (3) or (4), the gas content G of the coal-bearing reservoir corresponding to the lithologic section of the core sample is obtained: 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] Where G ij is the gas content of the jth core sample selected from the i-th lithologic section, in cm 3 / g;K ij V is the ratio of the test gas volume in the pressure-maintained coring barrel of the core sample to the test gas volume in the barrel of the full core; TL The gas content of all cores in the pressure-maintaining core barrel during the drilling process, cm 3 ; V TD is the decompression gas content of all cores in the pressure-maintaining coring barrel during the decompression test, cm 3 ; V Eij 、V Nij and V Rij represent the gas loss during transfer, the desorption gas content at normal pressure, and the residual gas content of the jth core sample selected from the i-th lithologic section, respectively, in cm 3 / g.

[0021] In a feasible method, during the pressure-maintaining coring and drilling stage, the gas loss content V of all the cores in the pressure-maintaining coring barrel is TL The calculation of is obtained by formula (5):

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

[0023] Where PHR is the ratio of the pore pressure of the core sample to the in-situ formation pressure when the pressure-maintaining coring barrel reaches the surface;

[0024] Where V TE 、V TN and V TR Respectively represent the decompression gas content of all cores in the pressure-maintaining coring barrel during the decompression test, the transfer loss gas content during the transfer process, the atmospheric desorption gas content under normal pressure, and the residual gas content, in cm 3 ; Among them, the decompression gas content of the whole core V TD is the measured value, the transfer loss gas content V of the whole coreTE , atmospheric pressure desorbed gas content V TN and residual gas content V TR The calculation method is as follows:

[0025]

[0026] Where M i V is the total mass of all cores in the ith lithologic section in the pressure-maintaining coring barrel, in g; TD 、V Nij and V Rij Direct determination, V TL and V Eij It is obtained by calculation;

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

[0028] q is the total number of all lithologic sections divided in the pressure-maintaining core barrel.

[0029] In one achievable method, during the core sample transfer phase, the core sample is exposed to the atmosphere and the transfer loss gas content V Eij Calculated by formulas (7), (8), and (9):

[0030]

[0031] Where Q t —Measured value of desorbed gas content at the initial stage of desorption at normal pressure, cm 3 / g;Q l —Transfer loss gas content of core, cm 3 / g; k—slope of the straight line; t0—transfer loss time, min; t—measured decompression desorption time, min; the gas loss of the core sample during the transfer period is calculated by formula (8):

[0032]

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

[0034]

[0035] Where, T test is the time used for the decompression desorption test; T retrieve T is the time from the start of drilling the rock formation to the time when the pressure-maintaining core barrel is transported to the decompression desorption test; transfer It is the time the core sample is exposed to the atmosphere from the time it is taken out from the pressure-maintaining coring barrel to the time it is transferred to the desorber;

[0036] In formula (8), Q t The value is the measured value of the desorbed gas content of the jth core sample selected from the i-th lithologic section at the initial stage of desorption at normal pressure. The calculated V E is the transfer loss gas content V when the core sample is exposed to the atmosphere Eij .

[0037] In one achievable method, the test gas volume V in the core sample tube is Dij +V Lij Calculation:

[0038] The free absorption ratio is determined by calculating the three-stage desorption results consisting of normal temperature desorbed gas, reservoir temperature desorbed gas, and high temperature desorbed gas. The calculation formula (10) is as follows:

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

[0040] Where, k represents the ratio of free gas volume to adsorbed gas volume; V NN 、V NS 、V NH They represent the desorption volume of the core sample at normal temperature, the desorption volume at reservoir temperature, and the desorption volume at high temperature during the desorption stage at normal pressure, respectively; thus, the free gas ratio k is obtained according to formula (11): free :

[0041]

[0042] Among them, k free is the proportion of free gas, dimensionless; k adsorbed is the adsorbed gas ratio, dimensionless; then the test gas volume in the core sample tube N ij The calculation expression is as follows:

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

[0044] Where N ij That is, V Lij +V Dij ;K 1i K is the ratio of the gas volume tested in the cylinder of the ith lithologic segment to the gas volume tested in the cylinder of all lithologic segments; 2ij is the ratio of the gas volume tested in the core sample j selected in the ith lithologic segment to the gas volume tested in the core sample of the corresponding lithologic segment; m ijis the mass of the jth core sample selected from the i-th lithologic section, g; K 1i , K 2ij Calculated by formula (13):

[0045]

[0046] In one achievable method, the gas content G of the coal-bearing reservoir corresponding to the lithologic section of the core sample is ij The calculation process is as follows:

[0047] The test gas volume in the core sample tube is:

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

[0049] The ratio of the gas volume tested in the core sample to the total gas volume tested in the core 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 jth core sample selected from the i-th lithologic segment is ij It can be expressed as:

[0054]

[0055] In one achievable method, the decompression desorption gas volume V of the core sample in the cylinder during the decompression desorption stage is TD The testing process is as follows:

[0056] Step 1: Connect an inverted gas collecting graduated cylinder filled with saturated salt water to one end of a diversion hose filled with saturated salt water, and connect the other end of the diversion hose to the gas collection interface reserved on the pressure-maintaining core barrel;

[0057] Step 2: Open the vent valve and the gas in the pressure-maintaining core barrel enters the gas collecting graduated cylinder through the diversion hose;

[0058] Step 3: When the saturated salt water in the gas collecting cylinder is completely discharged, close the vent valve and replace it with another gas collecting cylinder filled with saturated salt water. Record the gas volume VDi ;

[0059] Repeat steps 1 and 2 until the liquid level in the nth gas collecting cylinder does not change significantly within 3-5 minutes. Record the time T used for the decompression desorption test. test ; Accumulate the gas in each gas collecting cylinder to obtain the decompression desorption gas volume V of the entire core TD ; The calculation formula (1) is as follows:

[0060]

[0061] Where V TD The total volume of all core desorbed gas in the pressure-maintaining coring barrel during the decompression test, in cm 3 ; V Di is the volume of gas collected in the i-th gas collecting cylinder, in cm 3 .

[0062] In one achievable method, the total mass M of all core samples in the ith lithologic section in the pressure-maintained coring barrel is i The calculation formula (2) is as follows:

[0063]

[0064] Where M i is the total mass of all cores in the ith lithologic section in the pressure-maintaining coring barrel, in g; L i is the total length of all cores in the ith lithologic section in the pressure-maintaining core barrel, in m; m ij is the mass of the jth core sample selected from the i-th lithologic section, in g; l ij is the length of the jth core sample selected from the i-th lithologic section, in meters.

[0065] In one achievable method, during the atmospheric pressure desorption stage, the atmospheric pressure desorption gas content V of the core sample is Nij The testing process is as follows:

[0066] Gradient temperature-controlled desorption: desorption is carried out step by step under normal temperature, ground temperature and high temperature conditions. When the curve of the cumulative desorbed gas volume over time is close to a horizontal state, the desorption of the temperature section is terminated and the next temperature section is entered until the desorption is completed. The normal pressure desorption volume V of the core sample at normal pressure is recorded. Nij .

[0067] In one achievable method, the normal temperature is 20-25°C, the ground temperature is 65-70°C, and the high temperature is 85-90°C.

[0068] The in-situ gas content evaluation method for coal-bearing reservoirs based on pressure-maintained coring provided by the present invention has the following advantages compared with the prior art:

[0069] The use of a highly sealed pressure-maintaining coring barrel can keep the core at the in-situ formation pressure state, which can minimize the loss of oil and gas components in the core during the drilling process. During the pressure-maintaining coring and drilling stage, rock drilling and coring need to face high-temperature and high-pressure geological conditions and the drilling time is long. The sealing ability and triggering stability of pressure-maintaining coring are often not ideal. In fact, due to the complexity of pressure-maintaining coring technology, pressure-maintaining coring tools can only minimize but not completely avoid the loss of gas in the core. There will be a trace amount of gas loss. This part of the lost gas is restored and calculated based on the pressure-maintaining rate data recorded in real time by the pressure sensor built into the pressure-maintaining coring tool, combined with the gas content data tested in other stages. The pressure-maintaining rate is used to estimate the amount of natural gas lost during the drilling stage, thereby improving the accuracy of the lost gas content of the entire core during the drilling stage.

[0070] Both the pressure-maintained coring and drill-lifting phases and the pressure-reducing and desorption phases are considered in-situ gas content assessments. The total in-situ gas content of all cores, including the gas lost during drill-lifting and the gas desorption during pressure reduction, can be directly calculated or measured. However, the gas content of different thin coal-bearing strata differs fundamentally, and cores from different lithologic intervals contribute significantly different amounts to the in-situ gas content. Therefore, the in-situ gas content of core samples requires further calculation. Free gas, as the primary component of in-situ gas content, accounts for a significant portion of the overall gas content in cores. Therefore, the accuracy of the calculated in-situ gas content of core samples directly impacts the precision and reliability of in-situ gas content assessments of thin interbedded coal-bearing strata. To minimize calculation errors and ensure the timeliness and validity of test results, the in-situ gas content of core samples is calculated based on the gas content of the thin strata as reflected by field core desorption data. This paper proposes a method for calculating the in-core test gas content of core samples based on the core's gas content. The out-of-core test gas content of the core sample, already determined during the aforementioned testing and calculation process, is the primary component of the core's gas content, largely reflecting the core's gas-bearing capacity and serving as a key parameter for characterizing the core's gas content. Using the out-of-core test gas content and the free gas fraction as key parameters, the in-core test gas content of the core sample is rationally calculated, ensuring a precise evaluation of the gas content of coal-bearing reservoirs with frequent thin interbeds.

[0071] During testing, the pressure-maintaining coring barrel must be opened to select samples from each lithologic section for the next stage of testing. During this period, gas loss from the core sample is inevitable. To address this easily overlooked loss of gas content in the core sample during this process, this paper proposes a method for calculating the gas loss during the core sample transfer phase based on the gas escape patterns in the core and the sample transfer time recorded on-site, effectively improving the accuracy of gas content assessment in coal-bearing reservoirs.

[0072] In the normal pressure desorption stage, the accuracy of gas content evaluation in coal-bearing reservoirs was effectively improved by adopting gradient temperature-controlled desorption.

[0073] Therefore, the present invention divides the evaluation process into five stages: pressure-maintaining coring and drilling stage, pressure-reducing desorption stage, core sample transfer stage, normal pressure desorption stage, and residual gas test. The five test results, namely, the gas content lost during drilling, the gas content lost during pressure-reducing desorption, the gas content lost during transfer, the gas content lost during normal pressure desorption, and the residual gas content, are obtained. The gas content of the core sample is obtained by summing them up, and then the gas content of the coal-bearing reservoir in the corresponding lithologic section is obtained.

[0074] Compared with the conventional coal seam or shale gas content evaluation method, the calculation and testing method proposed in the present invention targets the characteristics of different stages, takes into account the testing characteristics of each stage, minimizes gas loss to the greatest extent, and almost completely tests the free gas and adsorbed gas in the coal reservoir core, establishing a set of fast, direct, practical and highly reliable evaluation methods, which greatly reduces the test error, effectively improves the accuracy and reliability of the coal reservoir gas content data, and can provide timely and effective coal reservoir in-situ gas content data for the detailed exploration and efficient development of coal gas.

[0075] Aiming at the complex gas-bearing characteristics of coal-bearing strata, the present invention designs and establishes an evaluation method for coal-bearing reservoir gas content based on pressure-maintaining coring technology, which combines on-site testing and calculation. This method effectively solves the practical problem that free gas and adsorbed gas cannot be measured or cannot be measured accurately, and realizes the fine measurement and precise evaluation of the in-situ gas content of each coal-bearing stratum. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 A flow chart for testing and calculating the gas content of coal-bearing reservoirs provided in an embodiment of the present invention; DETAILED DESCRIPTION

[0077] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0078] See also Figure 1 The in-situ gas content field evaluation method for coal-bearing reservoirs based on pressure-maintained coring provided by the present invention is now described. The in-situ gas content field evaluation method for coal-bearing reservoirs based on pressure-maintained coring is sequentially divided into five stages: the pressure-maintained coring drill lifting stage, the pressure reduction and desorption stage, the core sample transfer stage, the atmospheric pressure desorption test stage, and the residual gas test. The five test results, namely, the gas content lost during drill lifting, the pressure reduction and desorption gas content, the gas content lost during transfer, the atmospheric pressure desorption gas content, and the residual gas content, are obtained by summing the results to obtain the gas content of the core sample, and thus the gas content of the coal-bearing reservoir in the corresponding lithologic section.

[0079] The embodiment of the present invention describes 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 of all the cores in the pressure-maintaining coring barrel when drilling is lifted is calculated. TL :

[0081] After the pressure-maintaining coring operation is completed, the pressure-maintaining coring barrel will be lifted to the surface. During this period, there will be a small amount of gas loss. This part of the lost gas needs to be recovered and calculated based on the pressure-maintaining rate data recorded in real time by the pressure sensor built into the pressure-maintaining coring barrel, combined with the gas content data tested in other stages.

[0082] Since rock drilling and coring need to face high temperature and high pressure geological conditions and the drilling time is long, the sealing ability and triggering stability of the pressure-maintaining core barrel are often not ideal. In fact, due to the complexity of pressure-maintaining coring technology, the pressure-maintaining core barrel can only minimize but not completely avoid the loss of gas in the core. The pressure holding rate (PHR) refers to the ratio of the pore pressure of the core to the in-situ pressure of the formation when the core barrel reaches the surface. It is a key parameter for estimating the amount of natural gas lost during the drilling stage. The amount of gas lost during the drilling stage of the entire core barrel V TL It can be calculated by the following formula:

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

[0084] Where V TD 、V TE 、V TN and V TR Respectively represent the decompression gas content of all cores in the pressure-maintaining coring barrel during the decompression test, the transfer loss gas content during the transfer process, the atmospheric desorption gas content under normal pressure, and the residual gas content, in cm 3 Among them, the decompression gas content of the whole core is V TD is the measured value, the transfer loss gas content V of the whole core TE , atmospheric pressure desorption gas content V TN , residual gas content V TR The calculation method is as follows:

[0085]

[0086] Where M i V is the total mass of all cores in the ith lithologic section in the pressure-maintaining coring barrel, in g; Eij 、V Nij and VRij They represent the gas loss during transfer, the desorption gas content at atmospheric pressure, and the residual gas content of the j-th core sample selected from the i-th lithologic section, respectively, in cm 3 / g;V TD 、V Nij and V Rij Direct determination, V TL and V Eij It is obtained by calculation;

[0087] Where n is the total number of all core samples in the ith lithologic section;

[0088] q is the total number of all lithologic sections divided in the pressure-maintaining core barrel.

[0089] The second stage is the decompression and desorption stage. The decompression and desorption gas content V of all the cores in the cylinder is obtained by testing. TD , the test process is as follows:

[0090] Step 1: After the pressure-maintained coring barrel is delivered to the test site after the coring operation is completed, the time T from the time the rock formation is encountered to the time when the rock formation is encountered is recorded. retrieve . The inverted high-pressure gas collecting graduated cylinder filled with saturated salt water is tightly connected to one end of the diversion hose filled with saturated salt water through a quick-connect plug, and the other end of the diversion hose is connected to the gas collection interface reserved on the pressure-maintaining core barrel through a connecting sleeve.

[0091] Step 2: Slowly unscrew the vent valve and open the pressure regulator. A large amount of gas will flow through the diversion hose into the gas collecting cylinder. The pressure regulator controls the gas flow rate. The gas occupies the upper space of the gas collecting cylinder, and an equal volume of saturated salt water is forced out of the drain port by the gas above.

[0092] Step 3: When the gas collecting cylinder is about to be completely discharged, close the vent valve and replace it with a new gas collecting cylinder filled with saturated salt water. Record the volume of gas measured at this time V Di .

[0093] Repeat steps 1 and 2 until the liquid level in the nth gas collecting cylinder does not change significantly within 3 consecutive minutes. Record the time T used for the decompression desorption test. test The gas measured in each gas collecting cylinder is accumulated to obtain the decompression and desorption gas volume V of the entire core TD .

[0094]

[0095] Where V TD is the total volume of all core desorbed gases in the pressure-maintaining coring barrel during the decompression test, cm 3 ; V Diis the volume of gas collected in the i-th measuring cylinder, cm 3 .

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

[0097] After the decompression and desorption test, open the pressure-maintaining core barrel, take out the coal reservoir core sample, and clean the mud on the surface of the core sample. Measure the length L of each lithologic segment i , and select at least one representative core sample in each lithologic section, measure its length and weigh it, and record the length l of the jth rock sample taken in the i-th lithologic section ij , mass m ij Assuming that the density of each lithologic segment is equal, the mass M of the ith lithologic segment in the pressure-maintaining core barrel is i Calculated by formula (2):

[0098]

[0099] Where M i is the total mass of all cores in the ith lithologic section in the pressure-maintaining coring barrel, g; L i is the total length of all cores in the ith lithologic section in the pressure-maintaining core barrel, m; m ij is the mass of the jth core sample selected from the i-th lithologic section, g; l ij is the length of the jth core sample selected from the i-th lithologic section, m. After recording the sample number, the core sample is promptly loaded into the high-precision desorption instrument and the time T during its exposure to the atmosphere during the transfer process is recorded. transfer .

[0100] The entire core barrel is removed to select representative core samples from each lithologic section for the next stage of testing. Obviously, from the time the pressure-maintaining core barrel is opened until the core sample is loaded 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, and the only way to recover and calculate it is by substituting the field desorption data into a mathematical model. During the initial stage of gas release, the cumulative desorbed gas volume of the core sample is proportional to the square root of time, as expressed in the following mathematical expression:

[0101]

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

[0103]

[0104] Where t1 is the time lost without considering sample transfer, in minutes; t2 is the time lost with considering sample transfer, in minutes. The values ​​of t1 and t2 are calculated according to formula (9):

[0105]

[0106] Where, T test is the time used for the decompression desorption test; T retrieve T is the time from the start of drilling the rock formation to the time when the pressure-maintaining core barrel is transported to the decompression desorption test; transfer It is the time the core sample is exposed to the atmosphere from the time it is taken out from the pressure-maintaining coring barrel to the time it is transferred to the desorber;

[0107] In formula (8), Q t The value is the measured value of the desorbed gas content of the jth core sample selected from the i-th lithologic section at the initial stage of desorption at normal pressure. The calculated V E is the transfer loss gas content V when the core sample is exposed to the atmosphere Eij .

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

[0109] Start desorption with gradient temperature control, desorbing in steps at room temperature (20°C), ground temperature (70°C), and high temperature (90°C). The desorption time depends on the amount of desorbed gas. When the curve of the cumulative desorbed gas amount versus time approaches a horizontal state, terminate the desorption in that temperature section and enter the next temperature section until the desorption is complete. Record the atmospheric pressure desorbed gas content V at atmospheric pressure of the core sample. Nij .

[0110] The fifth stage is the residual gas test of core samples, which tests the residual gas content V Rij :

[0111] The core sample is taken out from the high-precision desorption instrument and placed in the residual gas measuring instrument to collect and measure the desorbed residual gas to obtain the residual gas content V Rij .

[0112] The in-cylinder test gas content (V Dij +V Lij ) is calculated as follows:

[0113] The in-barrel test gas volume of all cores includes the gas loss during drilling and the gas desorption during pressure reduction, which can be directly calculated or tested. However, the gas content of different thin coal-bearing strata is essentially different, and cores located in different lithologic sections contribute significantly to the in-barrel test gas volume. Therefore, the in-barrel test gas content of core samples needs to be further calculated. This paper proposes a method for calculating the in-barrel test gas content of core samples based on the gas content of the cores. With the core sample's out-of-barrel test gas content and the proportion of free gas as key parameters, the in-barrel test gas content of the core samples is reasonably calculated, providing a guarantee for the precise evaluation of the gas content of coal-bearing reservoirs with frequent thin interbeds.

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

[0115] Given the unique geological characteristics of coal-bearing reservoirs, a single pressure-maintained coring operation simultaneously harvests cores from different lithologic formations. Gas content data for the corresponding formations is obtained by sampling and testing each lithologic interval. The in-core test gas data for all cores within the pressure-maintained coring barrel, as well as the out-of-core test gas data for the core samples, have been obtained through established testing and calculation processes. However, the in-core test gas content of the core samples requires further calculation. It is worth noting that free gas, as the primary component of the in-core test gas, accounts for a significant proportion of the overall core gas content. Therefore, the accuracy of the calculated in-core test gas content directly impacts the precision and reliability of the in-situ gas content assessment of thin interbeds in the coal measures.

[0116] To minimize calculation errors and ensure the timeliness and validity of test results, the core sample's in-barrel test gas is calculated based on the gas content of thin layers as reflected by field core desorption data. The core sample's out-of-barrel test gas, already determined during the aforementioned testing and calculation process, is the primary component of the core's gas content, largely reflecting the core's gas-bearing capacity and serving as a key parameter for characterizing the core's gas content. Coal-bearing reservoirs contain both adsorbed and free gas. Even formations or cores with the same mass and total gas content contribute differently to the in-barrel free gas due to significant differences in their gas-bearing structures. In other words, a core with a high mass and total gas content but a low free gas fraction may not necessarily contain a high proportion of the in-barrel free gas. To further ensure the rationality of the calculation results, the free gas fraction should be considered as another important parameter.

[0117] During the atmospheric desorption test, gradient temperature-controlled desorption was performed. The changes in desorbed gas under normal temperature conditions are more reflected in the characteristics of free gas, the changes in desorbed gas under formation temperature conditions are more reflected in the recoverability characteristics, and the desorbed gas volume under high temperature conditions is more reflected in the characteristics of adsorbed gas. That is, the free-absorption ratio can be determined by calculating the three-stage desorption results consisting of normal 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] Where, k represents the ratio of free gas volume to adsorbed gas volume; V NN 、V NS 、V NH They represent the desorbed gas content at room temperature, the desorbed gas content at reservoir temperature, and the desorbed gas content at high temperature of the core sample during the desorption stage at normal pressure. free :

[0120]

[0121] Among them, k free is the proportion of free gas, dimensionless; k adsorbed is the adsorbed gas ratio, dimensionless. Then the test gas N in the core sample tube ij The calculation expression is as follows:

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

[0123] Where K 1i K is the ratio of the test gas volume in the cylinder of the ith lithologic segment to the test gas volume in the cylinder of all lithologic segments. 2ij K is the ratio of the test gas in the core sample j selected in the i-th lithologic segment to the test gas in the core sample of the corresponding lithologic segment. 1i , K 2ij It can be calculated by the following formula:

[0124]

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

[0126] The test gas in the core sample tube is known to be:

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

[0128] The ratio of the gas volume tested in the core sample to the total gas volume tested in the core ij for:

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

[0130] Right now:

[0131]

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

[0133] The present invention divides the evaluation process into five stages: pressure-maintaining and drilling-lifting stage, pressure-reducing and desorption stage, core sample selection and transfer stage, normal pressure gradient temperature control test of core sample desorption gas and core sample residual gas test. Direct testing or calculation is performed in each stage to obtain the gas content of the corresponding stage, and the sum is performed to obtain the total gas content of the coal-bearing core sample.

[0134] The gas content G of the jth rock sample selected from the ith lithologic section in the pressure-maintaining core is ij It can be expressed as the sum of the gas content obtained in the five evaluation stages:

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

[0136] Where G ij is the gas content of the jth core sample selected from the i-th lithologic section, in cm 3 / g;V Lij 、V Dij 、V Eij 、V Nij and V Rij represent the gas loss during the drilling phase, the gas desorption during the decompression phase, the gas loss during the transfer process, the desorption at normal pressure, and the residual gas content of the j-th core sample selected from the i-th lithologic section, respectively, cm 3 / g.

[0137] According to formula (3) and formula (14), the gas content G of the jth rock sample selected from the ith lithologic section in the pressure-maintaining core barrel can be obtained: ij According to formula (3), the gas content of the coal-bearing reservoir in the corresponding lithologic section of the core sample can be expressed as: G = V L +V D +V E +V N +V R .

[0138] Gas content V of core sample after drilling Lij , decompression desorption gas content V Dij is the test gas in the cylinder; the transfer loss gas content 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 test gas inside the cylinder and the test gas outside the cylinder.

[0139] The gas content G of the jth rock sample selected from the ith lithologic section in the pressure-maintaining core is ij It can be expressed 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 Through the above steps, V TL and V Eij It can also be calculated through mathematical methods.

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

[0143]

[0144] Existing evaluation technologies have significant limitations when applied to coal-bearing reservoirs with frequent thin interbeds and high free gas contents. To address the geological specificities and gas-bearing complexity of these reservoirs, the present invention proposes a systematic evaluation method that combines gas content testing with calculations. This method minimizes gas loss and provides a nearly complete assessment of both free and adsorbed gas in coal-bearing reservoir cores. This method is fast, direct, highly practical, and highly reliable, providing timely and effective in-situ gas content data for the detailed exploration and efficient development of coal-bearing gas.

[0145] Compared with the conventional coal seam or shale gas content evaluation method, the in-situ coal reservoir gas content evaluation method proposed in the present invention greatly reduces the test error. The results obtained are consistent with the geological particularity and gas content complexity of the coal formation, effectively improving the accuracy and credibility of the coal reservoir gas content data.

[0146] The evaluation method mentioned in the background technology is explained as follows:

[0147] Seismic interpretation: Hydrocarbon detection based on seismic data facilitates the determination of gas content in formations and has been widely applied in areas covered by existing seismic data. This method, based on abnormal changes in seismic wave attribute parameters, identifies the relationship between these changes and gas content, thereby predicting 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 formation's gas content. This evaluation method primarily relies on logging anomaly curves. Using anomalies in total hydrocarbon and methane content, it can directly reflect the gas content of coal-bearing formations, particularly the free gas content.

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

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

[0151] Chart method: When limited data is available, the theoretical chart method can be used for prediction. This method uses the geological theory of natural gas enrichment to calculate the gas content of various pre-defined geological conditions and compile the results into a chart for easy reference.

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

[0153] The direct method, also known as the field analysis method, refers to an evaluation method that determines the actual gas content by conducting field analysis experiments on reservoir cores obtained by open drill pipe coring or rope coring at the drilling site.

[0154] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant description 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 in the scope of protection of the present invention.

Claims

1. A method for evaluating the in-situ gas content of coal-bearing reservoirs based on pressure-maintained coring, characterized in that: The method comprises: During the pressure-maintaining coring and drilling stage, calculate the gas content V lost by drilling all the cores in the pressure-maintaining coring barrel. TL ; During the decompression and desorption stage, the decompression and desorption gas content V of all cores in the pressure-maintaining coring barrel is tested. TD ; During the core sample transfer phase, core samples are taken out from the pressure coring barrel and the gas content V lost when the core samples are exposed to the atmosphere is calculated during the process of transferring the selected core samples from the pressure coring barrel to the analyzer. Eij ; At atmospheric pressure desorption stage, the atmospheric pressure desorption gas content V of the core sample is tested Nij ; During the residual gas testing phase, the residual gas content V of the core sample is placed in the residual gas measuring instrument. Rij ; The gas content G of the coal-bearing reservoir corresponding to the lithologic section of the core sample is calculated according to formula (3) or (4): ij ; G ij =V Lij +V Dij +V Eij +V Nij +V Rij (3) It can also be expressed as: G ij =K ij (V TL +V TD ) / m ij +V Eij +V Nij +V Rij (4) Where G ij is the gas content of the jth core sample selected from the i-th lithologic section, in cm 3 / g;K ij V is the ratio of the test gas volume in the pressure-maintained coring barrel of the core sample to the test gas volume in the barrel of the full core; TL The gas content of all cores in the pressure-maintaining core barrel during the drilling process, cm 3 ; V TD V is the decompression gas content of all cores in the pressure-maintaining core barrel during the decompression test, cm3; Eij 、V Nij and V Rij represent the gas loss during transfer, the desorption gas content at normal pressure, and the residual gas content of the jth core sample selected from the i-th lithologic section, respectively, in cm 3 / g.

2. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to claim 1, characterized in that: During the pressure-maintaining coring and drilling stage, the gas content V lost by drilling all the cores in the pressure-maintaining coring barrel TL The calculation of is obtained by formula (5): In TL =(V TD +V TE +V TN +V TR (1 / PHR-1) (5) Where PHR is the ratio of the pore pressure of the core sample to the in-situ formation pressure when the pressure-maintaining coring barrel reaches the surface; Where V TE 、V TN and V TR Respectively represent the decompression gas content of all cores in the pressure-maintaining coring barrel during the decompression test, the transfer loss gas content during the transfer process, the atmospheric desorption gas content under normal pressure, and the residual gas content, in cm 3 ; Among them, the decompression gas content of the whole core V TD is the measured value, the transfer loss gas content V of the whole core TE , atmospheric pressure desorption gas content V TN and residual gas content V TR The calculation method is as follows: Where M i V is the total mass of all cores in the ith lithologic section in the pressure-maintaining coring barrel, in g; TD 、V Nij and V Rij Direct determination, V TL and V Eij It is obtained by calculation; n is the total number of all core samples in the i-th lithologic section; q is the total number of all lithologic sections divided in the pressure-maintaining core barrel.

3. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to claim 2, characterized in that: During the core sample transfer phase, the transfer loss gas content V when the core sample is exposed to the atmosphere Eij Calculated by formulas (7), (8), and (9): Where Q t —Measured value of desorbed gas content at the initial stage of desorption at normal pressure, cm 3 / g;Q l —Transfer loss gas content of core, cm 3 / g; k—slope of the straight line; t0—transfer loss time, min; t—measured decompression desorption time, min; the gas loss of the core sample during the transfer period is calculated by formula (8): Where t1 is the time lost without considering the core sample transfer, min; t2 is the time lost with considering the core sample transfer, min; the values ​​of t1 and t2 are calculated according to formula (9): Where, T test is the time used for the decompression desorption test; T retrieve T is the time from the start of drilling the rock formation to the time when the pressure-maintaining core barrel is transported to the decompression desorption test; transfer It is the time the core sample is exposed to the atmosphere from the time it is taken out from the pressure-maintaining coring barrel to the time it is transferred to the desorber; In formula (8), Q t The value is the measured value of the desorbed gas content at normal pressure V of the jth core sample selected from the i-th lithologic section Nij , the calculated V E is the transfer loss gas content V when the core sample is exposed to the atmosphere Eij .

4. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to claim 3, characterized in that: Test gas volume V in core sample tube Dij +V Lij Calculation: The free absorption ratio is determined by calculating the three-stage desorption results consisting of normal temperature desorbed gas, reservoir temperature desorbed gas, and high temperature desorbed gas. The calculation formula (10) is as follows: k=(V NH +V NS ) / (V NS +V NN ) (10) Where, k represents the ratio of free gas volume to adsorbed gas volume; V NN 、V NS 、V NH They represent the desorption volume of the core sample at normal temperature, the desorption volume at reservoir temperature, and the desorption volume at high temperature during the desorption stage at normal pressure, respectively; thus, the free gas ratio k is obtained according to formula (11): free : Among them, k free is the proportion of free gas, dimensionless; k adsorbed is the adsorbed gas ratio, dimensionless; then the test gas volume in the core sample tube N ij The calculation expression is as follows: N ij =K 1i K 2ij (V TL +V TD ) / m ij (12) Where N ij That is V Lij +V Dij ;K 1i K is the ratio of the gas volume tested in the cylinder of the ith lithologic segment to the gas volume tested in the cylinder of all lithologic segments; 2ij is the ratio of the gas volume tested in the core sample j selected in the ith lithologic segment to the gas volume tested in the core sample of the corresponding lithologic segment; m ij is the mass of the jth core sample selected from the i-th lithologic section, g; K 1i , K 2ij Calculated by formula (13):

5. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to claim 4, characterized in that: Gas content G of coal-bearing reservoir in corresponding lithologic section of core sample ij The calculation process is as follows: The test gas volume in the core sample tube is: V Lij +V Dij =N ij =K 1i K 2ij (V TL +V TD ) / m ij (14) The ratio of the gas volume tested in the core sample to the total gas volume tested in the core ij for: K ij =K 1i K 2ij (15) Right now: According to formula (4), the gas content G of the jth core sample selected from the i-th lithologic segment is ij It can be expressed as:

6. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to any one of claims 1 to 5, characterized in that: The decompression and desorption gas volume V of the core sample in the cylinder during the decompression and desorption stage TD The testing process is as follows: Step 1: Connect an inverted gas collecting graduated cylinder filled with saturated salt water to one end of a diversion hose filled with saturated salt water, and connect the other end of the diversion hose to the gas collection interface reserved on the pressure-maintaining core barrel; Step 2: Open the vent valve and the gas in the pressure-maintaining core barrel enters the gas collecting graduated cylinder through the diversion hose; Step 3: When the saturated salt water in the gas collecting cylinder is completely discharged, close the vent valve and replace it with another gas collecting cylinder filled with saturated salt water. Record the gas volume V Di ; Repeat steps 1 and 2 until the liquid level in the nth gas collecting cylinder does not change significantly within 3-5 minutes. Record the time T used for the decompression desorption test. test ; Accumulate the gas in each gas collecting cylinder to obtain the decompression desorption gas volume V of the entire core TD ; The calculation formula (1) is as follows: Where V TD The total volume of all core desorbed gas in the pressure-maintaining coring barrel during the decompression test, in cm 3 ; V Di is the volume of gas collected in the i-th gas collecting cylinder, in cm 3 .

7. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to any one of claims 2, 4 or 5, characterized in that: The total mass M of all core samples in the ith lithologic section in the pressure-maintaining coring barrel i The calculation formula (2) is as follows: Where M i is the total mass of all cores in the ith lithologic section in the pressure-maintaining coring barrel, in g; L i is the total length of all cores in the ith lithologic section in the pressure-maintaining core barrel, in meters; m ij is the mass of the jth core sample selected from the i-th lithologic section, in g; l ij is the length of the jth core sample selected from the i-th lithologic section, in meters.

8. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to any one of claims 1 to 5, characterized in that: During the atmospheric desorption stage, the atmospheric desorption gas content V of the core sample is Nij The testing process is as follows: Gradient temperature-controlled desorption: desorption is carried out step by step under normal temperature, ground temperature and high temperature conditions. When the curve of the cumulative desorbed gas volume over time is close to a horizontal state, the desorption of the temperature section is terminated and the next temperature section is entered until the desorption is completed. The normal pressure desorption volume V of the core sample at normal pressure is recorded. Nij .

9. The method for in-situ gas content evaluation of coal-bearing reservoirs based on pressure-maintained coring according to claim 8, characterized in that: Normal temperature is 20-25℃, ground temperature is 65-70℃, and high temperature is 85-90℃.

Images