Deep coal seam gas content testing method based on pressure-maintaining coring

Abstract

The invention provides a deep coal seam gas content testing method based on pressure-maintaining coring. The method comprises the steps that a target coal seam sample is obtained in a pressure-maintaining coring mode; the pressure maintaining cylinder is wrapped with a heating blanket and heated to 100 DEG C for 1 hour, the steps are repeated, the deflation valve is closed, and the gas volume VS2 of the adsorption gas is obtained; all the coal rock samples in the cylinder are taken out and weighed to obtain the total mass of the coal rock, the coal rock samples are loaded into a desorption tank to be desorbed until the average desorption amount of each gram of sample in one week is smaller than 0.05 ml / d, and the gas total volume VS3 of the coal rock samples in the desorption tank is obtained; a coal rock sample with the mass of m is taken out and smashed, and the residual gas volume VS4 is measured; and calculating the free gas content, the adsorbed gas content and the total gas content of the coal bed gas. According to the pressure-maintaining coring technology based on the deep coal seam, sealing can be kept, the taken-out coal rock sample can be kept under the original formation pressure, the loss of methane gas is greatly avoided, and the proportion of free gas and the total gas content of the coal seam can be accurately and effectively evaluated.

Description

Technical Field

[0001] The invention relates to the technical field of geological exploration, and in particular to a method for testing the gas content of deep coal seams based on pressure-maintaining coring. Background Art

[0002] As the exploration and development of coalbed methane moves deeper, the disadvantages of conventional coring methods and gas loss calculation methods are becoming more and more obvious. Accurately evaluating the gas content characteristics of deep coal seams is of great significance for the calculation of deep coalbed methane resources. Compared with shallow coal seams, deep coal seams have the characteristics of large burial depth, complex geological structure, high temperature and pressure, and coexistence of adsorbed gas and free gas. This also leads to problems such as difficult sampling and low test accuracy during deep coalbed methane exploration and development experiments.

[0003] The commonly used coring method at present is rope coring, which is an open coring method. Due to the characteristics of free gas in deep coal, the gas will escape during the coring process of rope coring, making it impossible for the extracted cores to maintain the original formation state. The test data is small, and the free gas part cannot be measured for deep coal seams, so it is impossible to directly and accurately obtain the gas content parameters of the reservoir under formation conditions. Therefore, the method based on rope coring and calculating the lost gas using mathematical models is no longer applicable to deep coal seams. Summary of the invention

[0004] In view of the shortcomings and defects of the prior art, the present invention provides a method for testing the gas content of deep coal seams based on pressure-maintained coring, aiming to establish a coalbed methane content calculation method that is compatible with pressure-maintained coring, thereby minimizing the loss of gas volume and effectively solving the problem of large errors in conventional gas content testing.

[0005] To achieve the above object, the technical solution of the present invention is implemented as follows: A method for testing the gas content of deep coal seams based on pressure-maintaining coring comprises the following steps:

[0006] S10, obtain the target coal seam sample by pressure-maintaining coring; connect the metering cylinder with the pressure-maintaining cylinder, open the pressure-maintaining cylinder venting valve, and record the gas volume V in the metering cylinder S1 At the same time, the gas is collected by the gas collecting bottle through the gas collecting hole above the metering cylinder;

[0007] S20, wrap the pressure-maintaining cylinder with a heating blanket and heat the heating blanket to 100°C for 1 hour, repeat step S10 until the pressure in the pressure-maintaining cylinder is completely released, close the vent valve, and obtain the adsorbed gas volume V S2 ;

[0008] S30, open the pressure-maintaining cylinder, take out all the coal and rock samples in the cylinder and weigh them to obtain the total mass M of the coal and rock, put the coal and rock samples into the desorption tank for desorption, until the daily average desorption amount of each gram of sample is less than 0.05 ml / d within one week, and obtain the total volume V of the gas in the desorption tank S3 ;

[0009] S40, the desorption tank is opened, a coal rock sample with a mass of m is taken out and crushed, and the residual gas volume V is measured. S4 ;

[0010] S50, based on the weight of the core taken, calculate the free gas content, adsorbed gas content and total gas content of the coalbed methane.

[0011] Furthermore, in step S10, the step of obtaining the target coal seam sample by pressure-maintaining coring includes:

[0012] S101, coring and drilling: the drilling speed should be controlled within 1m / s, the drilling tool should be lowered smoothly, and forced drilling by reaming is not allowed until the target coal seam is encountered;

[0013] S102, ball-throwing and core cutting: a steel ball with a diameter of 50.8 mm is thrown in and pumped at the same displacement as that during coring drilling;

[0014] S103, pressure measurement after drilling: After drilling to the ground, check whether the ball valve of the pressure-maintaining cylinder is closed, and transport the pressure-maintaining cylinder to the designated area; the pressure-maintaining cylinder includes an inner cylinder and an outer cylinder, of which the part where the core enters is the inner cylinder. Separate the inner and outer cylinders and use a pressure gauge to detect the pressure inside the pressure-maintaining cylinder.

[0015] Furthermore, in step S10, the metering cylinder is filled with water and placed horizontally, and the gas volume is obtained by the water drainage and gas collection method.

[0016] Furthermore, in step S10, a mud filter barrel is connected between the pressure-maintaining barrel and the metering barrel; the mud filter barrel is filled with water, and a one-way check valve is left on the top, and the one-way check valve is connected to the metering barrel through a pipeline.

[0017] Further, step S10 includes:

[0018] S11, recording the initial parameter P0 of the pressure gauge in step S103, and calculating the core pressure holding rate B using formula (1); connecting the pressure holding cylinder to the mud filter barrel through a pipeline, and the mud filter barrel is connected to the metering cylinder through a pipeline;

[0019]

[0020] Wherein, B is the core pressure holding rate, expressed as a percentage, %;

[0021] P0 is the measured core pressure on the ground, MPa;

[0022] P d is the formation pressure, MPa;

[0023] S12, slowly open the vent valve of the pressure-maintaining cylinder, and methane carries the mud through the mud filter barrel into the metering cylinder. Use the staged pressure reduction method to record the volume V of the gas desorbed each time. S1i , and use a gas collecting bottle to collect gas; where i is the number of measurements;

[0024] S13, record the ambient temperature T for each measurement n and atmospheric pressure P n , use formula (2) to calculate the standard volume of a single measurement:

[0025]

[0026] Among them, V S1n is the gas volume under standard conditions, cm 3 ;

[0027] n and i are the number of measurements and take the same value;

[0028] T n is the atmospheric temperature, °C;

[0029] P n is the atmospheric pressure, kPa;

[0030] S14, using a gas chromatograph to measure the gas components in the gas collection bottle in step S12, and respectively calculating the volume proportion A of the coalbed methane in each gas collection bottle n ;

[0031] S15, repeat steps S12-S14 until no bubbles are generated in the metering cylinder, and close the pressure-maintaining cylinder vent valve; free gas volume V S1 It is the sum of the gas volumes measured multiple times, as shown in formula (3):

[0032] V S1 =V S11 ×A1+V S12 ×A2+· · ·+V S1n ×A n (3).

[0033] Furthermore, in step S12, the method of staged pressure reduction and gas collection includes:

[0034] S121, open the pressure-maintaining cylinder vent valve. When the gas enters the metering cylinder and the reading is 10L-15L, close the pressure-maintaining cylinder vent valve and install a pressure gauge to read the remaining pressure P in the pressure-maintaining cylinder. m , where m is the number of tests;

[0035] S122, fill the gas collecting bottle with water and invert it into the water tank, making sure that the mouth of the gas collecting bottle is facing downwards and is placed below the water surface. Use a hose to connect the air hole on the metering cylinder to the gas inlet of the gas collecting bottle. After the air in the hose is exhausted, insert the end of the hose connected to the gas inlet of the gas collecting bottle into the gas collecting bottle, and let the gas flow into the gas collecting bottle through the hose. When the gas is collected to 120cm 3 ~150cm 3 Finally, unplug the hose in the sink and cover the gas inlet of the gas collection bottle with a rubber stopper, then stick a label on the outside of the bottle, turn it upside down in the box, and wait to be sent to the laboratory for gas composition analysis;

[0036] S123, repeat step S121 and step S122 until no bubbles are generated in the measuring cylinder.

[0037] Further, step S30 includes:

[0038] S31, after the pressure in the pressure-maintaining cylinder is completely released, the instrument cabin at the bottom of the pressure-maintaining cylinder is opened to take out the core inside;

[0039] S32, select all coal and rock samples from the core, wipe off the surface mud, take photos and quickly load them into the desorption tank, record the weight of the desorption tank before and after loading, and obtain the weight M of the coal and rock of this coring;

[0040] S33, continue desorption until the daily average desorption amount of each gram of sample is less than 0.05 ml / d within one week, obtain the total volume of gas in the desorption tank and calibrate it to the standard condition using formula (2) to obtain the volume V S3 .

[0041] Further, step S40 includes:

[0042] S41, after the coal sample in the desorption tank is desorbed, the desorption tank is opened, a sample with a mass of m is quickly weighed, and then loaded into a sealed ball mill tank;

[0043] S42, grind the sample to a particle size of 2 cm to 3 cm, then perform sealed residual gas measurement, measure the residual gas volume and calibrate it to V under standard conditions S4 .

[0044] Furthermore, the mass m of the sample weighed in step S41 ranges from 500 g to 700 g.

[0045] Further, in step S50, the free gas content, the adsorbed gas content and the total gas content are calculated using the following formula:

[0046]

[0047] G=G f +G a (6)

[0048] In the above formula, M is the weight of the coal sample actually obtained in the pressure-maintaining cylinder and the desorption tank, g;

[0049] m is the weight of the coal sample taken for residual gas measurement, g;

[0050] V S1 is the volume of gas in the pressure-maintaining cylinder when not heated, cm 3 ;

[0051] V S2 is the volume of gas desorbed by heating in the pressure-maintaining cylinder, cm 3 ;

[0052] V S3 is the volume of gas desorbed in the desorption tank, cm 3 ;

[0053] V S4 is the volume of gas desorbed during residual gas measurement, cm 3 ;

[0054] G f is the free gas content, cm 3 / g;

[0055] G a is the adsorbed gas content, cm 3 / g;

[0056] G is the total gas content, cm 3 / g.

[0057] Compared with the prior art, the deep coal seam gas content testing method based on pressure-maintaining coring described in the present invention has the following advantages:

[0058] (1) The present invention establishes a method for testing the gas content of deep coal seams based on pressure-maintained coring. Pressure-maintained coring can keep the core in the original formation conditions, minimize the escape of gas during the test, and achieve significant results in calculating the ratio of free gas and adsorbed gas in deep coal seams;

[0059] (2) The present invention includes a method for calculating the volume of gas in a pressure-maintaining cylinder by measuring the pressure in stages. Based on this method, an intersection diagram of the remaining gas volume and pressure in the pressure-maintaining cylinder can be obtained, thereby providing guidance for obtaining the critical desorption pressure;

[0060] (3) The present invention establishes a method for analyzing gas components of desorbed gas by pressure-maintaining coring, and performs gas component analysis on the gas collected in the gas collecting bottle. The gas components that do not belong to coalbed methane can be eliminated by pressure-maintaining coring, thereby accurately obtaining the content of coalbed methane in the collected gas;

[0061] (4) The present invention does not require sealing fluid to wrap the core, which effectively avoids the contamination of coal rock by sealing fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0063] Figure 1 This is a flow chart of a method for testing gas content in deep coal seams based on pressure-maintaining coring according to an embodiment of the present invention;

[0064] Figure 2 The specific execution process of step S10;

[0065] Figure 3 Pressure versus residual gas volume curve obtained for a stepwise depressurization. DETAILED DESCRIPTION

[0066] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0067] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0068] like Figure 1-Figure 3 As shown, the present invention is a method for testing the gas content of deep coal seams based on pressure-maintained coring. Compared with the rope coring method, the cores taken out by the pressure-maintained coring technology of the present invention can maintain the state of the original formation, can minimize the loss of oil and gas in the cores, and can directly and accurately obtain the gas content parameters of the reservoir under formation conditions. In addition, the present invention does not require the core to be wrapped with a sealing fluid, thus avoiding the contamination of the coal rock by the sealing fluid. More importantly, the staged pressure reduction method proposed in the present invention can obtain the intersection diagram of the remaining gas volume and pressure, thereby providing guidance for obtaining the critical desorption pressure. At the same time, the present invention innovatively performs gas component analysis on the gas collected in the gas collecting bottle, and the proposed formula (3) more accurately obtains the content of coalbed methane.

[0069] Specifically, the steps include:

[0070] S10, obtain the target coal seam sample by pressure-maintaining coring; connect the metering cylinder with the pressure-maintaining cylinder, open the pressure-maintaining cylinder venting valve, and record the gas volume V in the metering cylinder S1 At the same time, the gas is collected by using a gas collecting bottle through the gas collecting hole above the metering cylinder for testing gas components in the laboratory;

[0071] In a preferred embodiment of the present invention, the step of obtaining a target coal seam sample by pressure-maintaining coring comprises:

[0072] S101, coring and drilling: the drilling speed should be controlled within 1m / s, the drilling tool should be lowered smoothly, and forced drilling by reaming is not allowed until the target coal seam is encountered;

[0073] S102, ball-throwing and core cutting: a steel ball with a diameter of 50.8 mm is thrown in and pumped at the same displacement as that during coring drilling;

[0074] S103, pressure measurement after drilling: After drilling to the ground, check whether the ball valve of the pressure-maintaining cylinder is closed, and transport the pressure-maintaining cylinder to the designated area; the pressure-maintaining cylinder includes an inner cylinder and an outer cylinder, of which the part where the core enters is the inner cylinder. Separate the inner and outer cylinders and use a pressure gauge to detect the pressure inside the pressure-maintaining cylinder.

[0075] In a preferred embodiment of the present invention, the metering cylinder is filled with water and placed horizontally, and the gas volume is obtained by water displacement collection method.

[0076] In a preferred embodiment of the present invention, a mud filter barrel is connected between the pressure-maintaining barrel and the metering barrel; the mud filter barrel is filled with water, a one-way check valve is left on the top, and the one-way check valve is connected to the metering barrel through a pipeline.

[0077] Step S10 specifically includes the following steps:

[0078] S11, recording the initial parameter P0 of the pressure gauge in step S103, and calculating the core pressure holding rate B using formula (1); connecting the pressure holding cylinder to the mud filter barrel through a pipeline, and the mud filter barrel is connected to the metering cylinder through a pipeline;

[0079]

[0080] Wherein, B is the core pressure holding rate, expressed as a percentage, %;

[0081] P0 is the measured core pressure on the ground, MPa;

[0082] P d is the formation pressure, MPa;

[0083] S12, slowly open the vent valve of the pressure-maintaining cylinder, and methane carries the mud through the mud filter barrel into the metering cylinder. Use the staged pressure reduction method to record the volume V of the gas desorbed each time. S1i , and use a gas collecting bottle to collect gas; where i is the number of measurements;

[0084] S13, record the ambient temperature T for each measurement n and atmospheric pressure P n , use formula (2) to calculate the standard volume of a single measurement:

[0085]

[0086] Among them, V S1n is the volume of gas under standard conditions (temperature 0°C, pressure 101.325 kPa), cm 3 ;

[0087] n and i are the number of measurements and take the same value;

[0088] T n is the atmospheric temperature, °C;

[0089] P n is the atmospheric pressure, kPa;

[0090] S14, using a gas chromatograph to measure the gas components in the gas collection bottle in step S12, and respectively calculating the volume proportion A of the coalbed methane in each gas collection bottle n ;

[0091] S15, repeat steps S12-S14 until no bubbles are generated in the metering cylinder, and close the pressure-maintaining cylinder vent valve; free gas volume V S1 It is the sum of the gas volumes measured multiple times, as shown in formula (3):

[0092] V S1 =V S11 ×A1+V S12 ×A2+· · ·+V S1n ×A n (3).

[0093] Through the above content, the present invention establishes a method for analyzing gas components of pressure-maintained coring desorption gas, and through steps S13-S15, gas components that do not belong to coalbed methane can be eliminated, thereby accurately obtaining the content of coalbed methane in the collected gas. At the same time, the present invention performs gas component analysis on the gas collected in the gas collection bottle, and obtains the content of coalbed methane more accurately through formula (3).

[0094] In a preferred embodiment of the present invention, in step S12, the method of staged pressure reduction and gas collection includes:

[0095] S121, open the pressure-maintaining cylinder vent valve. When the gas enters the metering cylinder and the reading is 10L-15L, close the pressure-maintaining cylinder vent valve and install a pressure gauge to read the remaining pressure P in the pressure-maintaining cylinder. m , where m is the number of tests; the pressure in the cylinder and the remaining gas volume are plotted as Figure 3 The pressure corresponding to the inflection point in the figure is the critical desorption pressure.

[0096] S122, fill the gas collecting bottle with water and invert it into the water tank, making sure that the mouth of the gas collecting bottle is facing downwards and is placed below the water surface. Use a hose to connect the air hole on the metering cylinder to the gas inlet of the gas collecting bottle. After the air in the hose is exhausted, insert the end of the hose connected to the gas inlet of the gas collecting bottle into the gas collecting bottle, and let the gas flow into the gas collecting bottle through the hose. When the gas is collected to 120cm 3 ~150cm 3 Finally, unplug the hose in the sink and cover the gas inlet of the gas collection bottle with a rubber stopper, then stick a label on the outside of the bottle, turn it upside down in the box, and wait to be sent to the laboratory for gas composition analysis;

[0097] S123, repeat step S121 and step S122 until no bubbles are generated in the measuring cylinder.

[0098] The present invention uses a calculation method for measuring the gas volume in the pressure-maintaining cylinder by periodically reducing the pressure, and can obtain an intersection diagram of the remaining gas volume and pressure in the pressure-maintaining cylinder, thereby providing guidance for obtaining the critical desorption pressure.

[0099] S20, wrap the pressure-maintaining cylinder with a heating blanket and heat the heating blanket to 100°C for 1 hour, repeat step S10 until the pressure in the pressure-maintaining cylinder is completely released, close the vent valve, and obtain the adsorbed gas volume V S2 The purpose of wrapping the pressure-maintaining cylinder with a heating blanket and heating it to 100°C for 1 hour is to quickly desorb the gas adsorbed by the coal rock in the pressure-maintaining cylinder, thereby reducing gas loss.

[0100] S30, open the pressure-maintaining cylinder, take out all the coal and rock samples in the cylinder and weigh them to obtain the total mass M of the coal and rock, put the coal and rock samples into the desorption tank for desorption, until the average desorption amount of each gram of sample is less than 0.05 ml / d within one week, and obtain the total volume of gas in the desorption tank V S3 ; Specifically include the following steps:

[0101] S31, after the pressure in the pressure-maintaining cylinder is completely released, the instrument cabin at the bottom of the pressure-maintaining cylinder is opened to take out the core inside;

[0102] S32, select all coal and rock samples from the core, wipe off the surface mud, take photos and quickly load them into the desorption tank, record the weight of the desorption tank before and after loading, and obtain the weight M of the coal and rock of this coring;

[0103] S33, continue desorption until the average desorption amount per gram of sample is less than 0.05 ml / d in one week, obtain the total volume of gas in the desorption tank and calibrate it to the standard condition using formula (2) to obtain the volume V S3 .

[0104] S40, the desorption tank is opened, a coal rock sample with a mass of m is taken out and crushed, and the residual gas volume V is measured. S4; Specifically include the following steps:

[0105] S41, after the coal sample in the desorption tank is desorbed, the desorption tank is opened, a sample with a mass of m is quickly weighed, and then loaded into a sealed ball mill tank;

[0106] S42, grind the sample to a particle size of 2 cm to 3 cm, then perform sealed residual gas measurement, measure the residual gas volume and calibrate it to V under standard conditions S4 .

[0107] Considering that the plunger diameter obtained by pressure coring is larger than that obtained by conventional coring, in a preferred embodiment of the present invention, a coal rock sample with a mass of m is taken out and crushed, wherein m ranges from 500 g to 700 g.

[0108] S50, combined with the weight of the core, calculate the free gas content G of the coalbed methane f , adsorbed gas content G a And the total gas content G, calculated using the following formula:

[0109]

[0110] G=G f +G a (6)

[0111] In the above formula, M is the weight of the coal sample actually obtained in the pressure-maintaining cylinder and the desorption tank, g;

[0112] m is the weight of the coal sample taken for residual gas measurement, g;

[0113] V S1 is the volume of gas in the pressure-maintaining cylinder when not heated, cm 3 ;

[0114] V S2 is the volume of gas desorbed by heating in the pressure-maintaining cylinder, cm 3 ;

[0115] V S3 is the volume of gas desorbed in the desorption tank, cm 3 ;

[0116] V S4 is the volume of gas desorbed during residual gas measurement, cm 3 ;

[0117] G f is the free gas content, cm 3 / g;

[0118] G a is the adsorbed gas content, cm 3 / g;

[0119] G is the total gas content, cm 3 / g.

[0120] The present invention is based on the pressure-maintaining coring technology of deep coal seams, which can not only maintain airtightness but also keep the extracted coal and rock samples at the original formation pressure, greatly avoiding the loss of methane gas in traditional coring methods. At the same time, combined with gas component testing, it can accurately and effectively evaluate the free gas ratio and the total gas content of the coal seam.

[0121] Embodiment 1

[0122] (1) Obtain target coal seam samples by using pressure-maintaining coring method.

[0123] ① Record the initial pressure P0 in the pressure-maintaining tube and calculate the core pressure-maintaining rate B using formula (1);

[0124]

[0125] Wherein, B is the core pressure holding rate, expressed as a percentage, %;

[0126] P0 is the measured core pressure on the ground, MPa;

[0127] P d is the formation pressure, MPa.

[0128] ② The pressure-maintaining cylinder is connected to the mud filter barrel through a pipeline, and the mud filter barrel is connected to the metering cylinder through a pipeline.

[0129] ③Open the vent valve of the pressure-maintaining cylinder. When the gas enters the measuring cylinder and the reading is 10L~15L, close the vent valve of the pressure-maintaining cylinder and install the pressure gauge to read the remaining pressure P in the cylinder. m , where m is the number of tests.

[0130] ④ Fill the gas collection bottle with water and place it in the sink. Use a hose to connect the air hole on the metering cylinder to the air inlet of the gas collection bottle. After the air in the hose is exhausted, insert the gas collection bottle and let the gas flow into the bottle through the hose. 3 ~150cm 3 Finally, unplug the hose in the sink and cover it with a rubber stopper. Then put a label on the outside of the bottle and put it upside down in the box, waiting to be sent to the laboratory for gas composition analysis.

[0131] ⑤Record the ambient temperature T for each measurement n and atmospheric pressure P n , use formula (2) to calculate the standard volume of a single measurement:

[0132]

[0133] Among them, n and i are the number of measurements and take the same value;

[0134] T n is the atmospheric temperature, °C;

[0135] P n is the atmospheric pressure, kPa.

[0136] ⑥ Use a gas chromatograph to measure the gas components in the gas collection bottle in step S12, and calculate the volume proportion A of the coalbed methane in each gas collection bottle. n .

[0137] ⑦ Repeat steps ③ to ⑥ until no bubbles are generated in the metering cylinder, and close the pressure-maintaining cylinder vent valve; free gas volume V S1 It is the sum of the gas volumes measured multiple times, as shown in formula (3):

[0138] V S1 =V S11 ×A1+V S12 ×A2+· · ·+V S1n ×A n (3)

[0139] (2) Wrap the pressure-maintaining cylinder with a heating blanket and heat the heating blanket to 100°C for 1 hour, repeat steps ③ to ⑥ in (1) until the pressure in the pressure-maintaining cylinder is completely released, wrap the pressure-maintaining cylinder with a heating blanket and heat the heating blanket to 100°C for 1 hour, close the vent valve, and obtain the adsorbed gas volume V S2 .

[0140] (3) Open the pressure-maintaining cylinder, take out all the coal and rock samples in the cylinder, and weigh them to obtain the total mass M of the coal and rock. Put the coal and rock samples into the desorption tank for desorption until the average desorption amount per gram of sample is less than 0.05 ml / d within one week, and obtain the total volume of gas in the desorption tank V. S3 .

[0141] ① After the pressure in the pressure-maintaining cylinder is completely released, open the instrument cabin at the bottom of the pressure-maintaining cylinder and take out the core inside;

[0142] ② Select all coal and rock samples from the core, wipe off the mud on the surface, take photos and quickly put them into the desorption tank, record the weight of the desorption tank before and after loading, and obtain the weight M of the coal and rock in this coring;

[0143] ③ Continue desorption until the average desorption amount per gram of sample is less than 0.05 ml / d within one week, obtain the total volume of gas in the desorption tank and calibrate it to the standard condition using formula (2), and obtain the volume V S3 ;

[0144] (4) Open the desorption tank, take out the coal rock sample with a mass of m, crush it and measure the residual gas volume VS4 .

[0145] ①Open the desorption tank, quickly weigh 500g to 700g of sample, and put it into a sealed ball mill;

[0146] ② Grind the sample to a particle size of 2cm to 3cm for sealed residual gas measurement, measure the total gas volume and calibrate it to standard conditions to obtain V S4 .

[0147] (5) Based on the weight of the core, the free gas content, adsorbed gas content and total gas content of the coalbed methane are calculated using the following formula.

[0148]

[0149] G=G f +G a (6)

[0150] In the above formula, M is the weight of the coal sample actually obtained in the pressure-maintaining cylinder and the desorption tank, g;

[0151] m is the weight of the coal sample taken for residual gas measurement, g;

[0152] V S1 is the volume of gas in the pressure-maintaining cylinder when not heated, cm 3 ;

[0153] V S2 is the volume of gas desorbed by heating in the pressure-maintaining cylinder, cm 3 ;

[0154] V S3 is the volume of gas desorbed in the desorption tank, cm 3 ;

[0155] V S4 is the volume of gas desorbed during residual gas measurement, cm 3 ;

[0156] G f is the free gas content, cm 3 / g;

[0157] G a is the adsorbed gas content, cm 3 / g;

[0158] G is the total gas content, cm 3 / g.

[0159] The method of the present invention can effectively reduce the loss of deep coal seam gas, and at the same time can obtain the adsorbed gas and free gas content of the coal seam, and provide a reference for obtaining the critical desorption pressure of the coal seam. Table 1 shows the test results of the samples using the above method.

[0160] Table 1 Test results of gas content in deep coal seams

[0161]

[0162]

[0163] It can be seen from Table 1 that the free gas content of the coal rock sample is 1.65 cm 3 / g, desorption gas content 5.50cm 3 / g and residual gas content of 0.46cm 3 / g, the total gas content is 7.61cm 3 / g. Free gas accounts for 21.98% of the total gas content.

[0164] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for testing gas content in deep coal seams based on pressure-maintaining coring, characterized in that: The following steps are involved: S10, obtaining a target coal seam sample by pressure-maintaining coring; Connect the metering cylinder to the pressure-maintaining cylinder, open the pressure-maintaining cylinder vent valve, and record the gas volume V in the metering cylinder. S1 At the same time, the gas is collected by the gas collecting bottle through the gas collecting hole above the metering cylinder; S20, wrap the pressure-maintaining cylinder with a heating blanket and heat the heating blanket to 100°C for 1 hour, repeat step S10 until the pressure in the pressure-maintaining cylinder is completely released, close the vent valve, and obtain the adsorbed gas volume V S2 ; S30, open the pressure-maintaining cylinder, take out all the coal and rock samples in the cylinder and weigh them to obtain the total mass M of the coal and rock, put the coal and rock samples into the desorption tank for desorption, until the daily average desorption amount of each gram of sample is less than 0.05 ml / d within one week, and obtain the total volume V of the gas in the desorption tank S3 ; S40, the desorption tank is opened, a coal rock sample with a mass of m is taken out and crushed, and the residual gas volume V is measured. S4 ; S50, based on the weight of the core taken, calculate the free gas content, adsorbed gas content and total gas content of the coalbed methane.

2. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 1, characterized in that: In step S10, the step of obtaining the target coal seam sample by pressure-maintaining coring includes: S101, coring and drilling: the drilling speed should be controlled within 1m / s, the drilling tool should be lowered smoothly, and forced drilling by reaming is not allowed until the target coal seam is encountered; S102, ball-throwing and core cutting: a steel ball with a diameter of 50.8 mm is thrown in and pumped at the same displacement as that during coring drilling; S103, pressure measurement after drilling: After drilling to the ground, check whether the ball valve of the pressure-maintaining cylinder is closed, and transport the pressure-maintaining cylinder to the designated area; the pressure-maintaining cylinder includes an inner cylinder and an outer cylinder, of which the part where the core enters is the inner cylinder. Separate the inner and outer cylinders and use a pressure gauge to detect the pressure inside the pressure-maintaining cylinder.

3. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 2, characterized in that: In step S10, the metering cylinder is filled with water and placed horizontally, and the gas volume is obtained by the water drainage and gas collection method.

4. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 3, characterized in that: In step S10, a mud filter barrel is connected between the pressure-maintaining barrel and the metering barrel; the mud filter barrel is filled with water, and a one-way check valve is left on the top, and the one-way check valve is connected to the metering barrel through a pipeline.

5. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 4, characterized in that: Step S10 includes: S11, recording the initial parameter P0 of the pressure gauge in step S103, and calculating the core pressure holding rate B using formula (1); connecting the pressure holding cylinder to the mud filter barrel through a pipeline, and the mud filter barrel is connected to the metering cylinder through a pipeline; Wherein, B is the core pressure holding rate, expressed as a percentage, %; P0 is the measured core pressure on the ground, MPa; P d is the formation pressure, MPa; S12, slowly open the vent valve of the pressure-maintaining cylinder, and methane carries the mud through the mud filter barrel into the metering cylinder. Use the staged pressure reduction method to record the volume V of the gas desorbed each time. S1i , and use a gas collecting bottle to collect gas; where i is the number of measurements; S13, record the ambient temperature T for each measurement n and atmospheric pressure P n , use formula (2) to calculate the standard volume of a single measurement: Among them, V S1n is the gas volume under standard conditions, cm 3 ; n and i are the number of measurements and take the same value; T n is the atmospheric temperature, °C; P n is the atmospheric pressure, kPa; S14, using a gas chromatograph to measure the gas components in the gas collection bottle in step S12, and respectively calculating the volume proportion A of the coalbed methane in each gas collection bottle n ; S15, repeat steps S12-S14 until no bubbles are generated in the metering cylinder, and close the pressure-maintaining cylinder vent valve; free gas volume V S1 It is the sum of the gas volumes measured multiple times, as shown in formula (3): V S1 =V S11 ×A1+V S12 ×A2+· · ·+V S1n ×A n (3)。 6. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 5, characterized in that: In step S12, the method of staged pressure reduction and gas collection includes: S121, open the vent valve of the pressure-maintaining cylinder. When the gas enters the metering cylinder and the reading is 10L-15L, close the vent valve of the pressure-maintaining cylinder, observe the pressure gauge, and read the remaining pressure P in the pressure-maintaining cylinder. m , where m is the number of tests; S122, fill the gas collecting bottle with water and invert it into the water tank, making sure that the mouth of the gas collecting bottle is facing downwards and is placed below the water surface. Use a hose to connect the air hole on the metering cylinder to the gas inlet of the gas collecting bottle. After the air in the hose is exhausted, insert the end of the hose connected to the gas inlet of the gas collecting bottle into the gas collecting bottle, and let the gas flow into the gas collecting bottle through the hose. When the gas is collected to 120cm 3 ~150cm 3 Finally, unplug the hose in the sink and cover the gas inlet of the gas collection bottle with a rubber stopper, then stick a label on the outside of the bottle, turn it upside down in the box, and wait to be sent to the laboratory for gas composition analysis; S123, repeat step S121 and step S122 until no bubbles are generated in the measuring cylinder.

7. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 5, characterized in that: Step S30 includes: S31, after the pressure in the pressure-maintaining cylinder is completely released, the instrument cabin at the bottom of the pressure-maintaining cylinder is opened to take out the core inside; S32, select all coal and rock samples from the core, wipe off the surface mud, take photos and quickly load them into the desorption tank, record the weight of the desorption tank before and after loading, and obtain the weight M of the coal and rock of this coring; S33, continue desorption until the daily average desorption amount of each gram of sample is less than 0.05 ml / d within one week, obtain the total volume of gas in the desorption tank and calibrate it to the standard condition using formula (2) to obtain the volume V S3 .

8. The method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 5 is characterized in that: Step S40 includes: S41, after the coal sample in the desorption tank is desorbed, the desorption tank is opened, a sample with a mass of m is quickly weighed, and then loaded into a sealed ball mill tank; S42, grind the sample to a particle size of 2 cm to 3 cm, then perform sealed residual gas measurement, measure the residual gas volume and calibrate it to V under standard conditions S4 .

9. A method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 8, characterized in that: The range of the sample mass m weighed in step S41 is 500 g to 700 g.

10. The method for testing gas content in deep coal seams based on pressure-maintaining coring according to claim 1, characterized in that: In step S50, the free gas content, the adsorbed gas content and the total gas content are calculated using the following formula: G=G f +G a (6) In the above formula, M is the weight of the coal sample actually obtained in the pressure-maintaining cylinder and the desorption tank, g; m is the weight of the coal sample taken for residual gas measurement, g; V S1 is the volume of gas in the pressure-maintaining cylinder when not heated, cm 3 ; V S2 is the volume of gas desorbed by heating in the pressure-maintaining cylinder, cm 3 ; V S3 is the volume of gas desorbed in the desorption tank, cm 3 ; V S4 is the volume of gas desorbed during residual gas measurement, cm 3 ; G f is the free gas content, cm 3 / g; G a is the adsorbed gas content, cm 3 / g; G is the total gas content, cm 3 / g.

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