Coal-bed gas well full life cycle mining experiment evaluation system and method

Abstract

The invention discloses a coal-bed gas well full life cycle exploitation experiment evaluation system and method, and belongs to the technical field of petroleum and natural gas engineering. Different from the metering principles of a traditional volumetric method and a flow method, the method of exhausting gas to the metering tank is adopted, pressure and temperature data are collected, and the desorption gas amount is accurately metered in combination with a state equation; the system is composed of three metering tanks with different sizes, the appropriate metering tank is selected for metering according to the gas output, and continuous switching and continuous metering can be achieved; the invention establishes a set of experimental evaluation system and method for full-life-cycle mining of the coal-bed gas well, so as to provide a scientific basis for optimizing the production process on site, reducing the mining cost, determining a reasonable drainage and mining system and improving the recovery ratio, thereby enhancing the competitiveness of the coal-bed gas industry in China, promoting the sustainable development of the industry, and improving the economic benefits of the coal-bed gas well. And the purposes of power-assisted energy structure transformation and carbon emission reduction are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and gas engineering, in particular to a coalbed methane well full life cycle production experimental evaluation system and method. BACKGROUND

[0002] As an important unconventional natural gas resource, the development efficiency of coalbed methane directly affects the realization of energy supply and greenhouse gas emission reduction targets. Coalbed methane mainly exists in the form of adsorption (80%-95% in adsorbed state) on the surface of coal matrix. Its production process is actually a depletion type mining, which gradually expands the desorption range by reducing the bottom hole flowing pressure through drainage, forming a pressure drop funnel, and causing a large amount of methane to desorb. At this stage, the drainage rate and the reservoir pressure decline rate need to be accurately matched. If the pressure drops too fast, it will cause coal reservoir sensitivity damage, and "water lock" and "gas lock" will occur, increasing the production of coal powder; if it is too slow, it will prolong the production cycle and increase the drainage cost.

[0003] As the drainage proceeds, multiple problems gradually appear. The permeability decreases, the effective stress dominates due to long-term pressure reduction, and the permeability continues to decline, resulting in rapid production decline in the later stage. Desorption hysteresis effect occurs, and the desorption in the low pressure area is not complete, with residual adsorbed gas amount of 30%-50%. Energy depletion, when the reservoir pressure is too low, the gas transport driving force is insufficient. At this time, CO2 injection huff and puff or surfactant enhanced oil recovery technology emerges as the times require.

[0004] The drainage system of coalbed methane production and gas injection energy recovery enhancement technology are mostly determined based on field engineering experience, which is high in cost, long in cycle, high in cost, and lacks effective indoor experimental simulation evaluation support. In view of this, the present application aims to establish a coalbed methane well full life cycle production experimental evaluation system and method, in order to provide scientific basis for optimizing production process, reducing production cost, determining reasonable drainage system and improving recovery efficiency, thereby enhancing the competitiveness of China's coalbed methane industry, promoting the sustainable development of the industry, and helping to realize the energy structure transformation and carbon emission reduction target. SUMMARY

[0005] In order to solve the above problems, the present application provides a coalbed methane well full life cycle production experimental evaluation system and method. The present application develops a full life cycle production experimental evaluation system and establishes a production evaluation method, which is not only suitable for coalbed methane wells, but also suitable for all gas wells with adsorption and desorption capacity, such as shale gas wells.

[0006] The present application is realized by the following technical solutions:

[0007] The invention discloses a coalbed methane well full life cycle exploitation experimental evaluation system, which comprises an injection power system, an exploitation simulation system, a pressure control drainage system and a metering system.

[0008] The invention discloses a coalbed methane well full life cycle exploitation experimental evaluation system, which comprises an injection power system, an exploitation simulation system, a pressure control drainage system and a metering system.

[0009] Step 1, sample selection, select a full-diameter coal core sample to simulate the coal seam after fracturing, crush the sample into large coal samples, fill the sample tank, inject fluid, and select fracturing flowback fluid or formation water as the injection fluid according to the actual situation on site;

[0010] Step 2, system volume calibration, according to the actual sample amount of coal, assemble the connection system, and heat it to the formation temperature of the sample, open valve A3 and valve J32, vacuumize the system until the vacuum degree is unchanged, then close the above-mentioned valves, and read the atmospheric pressure P0; first, open valve K34 and open gas booster pump 1, fill helium into the reference tank through the injection power system to 5-10 MPa, after the pressure is stable, close the injection power system, i.e. valve K34 and gas booster pump 1, record the pressure P2 at this time, open valve A3, when the sample tank pressure 25 reaches the target pressure, close the valve, after the pressure is stable, read the reference tank 30 pressure P3 and the sample tank 25 pressure P4; then vacuumize the system, put the standard block with a known volume V0 into the sample tank, and repeat the above-mentioned operation again; finally, replace the standard block with coal samples, and repeat the above-mentioned operation again; according to the law of conservation of mass combined with the state equation, the sample tank volume, the reference tank volume and the sample skeleton volume can be obtained;

[0011] Sample tank volume:

[0012] Reference tank volume:

[0013] Sample skeleton volume:

[0014] Wherein,

[0015] In the formula, P1, P'1, P"1 are the pressures of the empty system before injecting gas, i.e. the system without the standard block and the sample, the sample tank after putting in the standard block and the coal sample, respectively, MPa; P2, P'2, P"2 are the pressures of the reference tank after injecting gas, MPa; P3, P4 are the pressures of the reference tank and the sample tank in the empty system after opening the stop valve and the pressure is stable, MPa; P'3, P'4 are the pressures of the reference tank and the sample tank after putting in the standard block after opening the stop valve and the pressure is stable, MPa; P"3, P"4 are the pressures of the reference tank and the sample tank after putting in the coal sample after opening the stop valve and the pressure is stable, MPa; Z is the compressibility factor of the fluid at temperature T and corresponding pressure P, and the subscript corresponds to different pressure states, and the same below;

[0016] Step 3, restore to the formation state, open the connection valve A3, connect the reference tank and the sample tank, heat the system to the formation temperature, vacuumize the system until the vacuum degree is unchanged, close the connection valve A3, open the connection valve L36, open the gas booster pump 1, inject methane gas into the reference tank, determine the pre-injection pressure according to the equation of state, record the pressure value of the reference tank after the pressure is stable, open the connection valve A3, and inject methane gas into the sample tank, close the connection valve A3 after reaching the target pressure, and record the pressures of the reference tank and the sample tank after the pressure is stable, and calculate the adsorbed gas amount GSE according to formula (4);

[0017]

[0018] In the formula: m is the sample mass, g; R is the universal gas constant, 8.314 J / mol / K; T is the experimental temperature, K; P2 and P1 are the initial equilibrium pressures of the reference tank (RC) and the sample tank (SC), MPa; P3 and P4 are the pressures of RC and SC after the pressure is stable, MPa; V R is the volume of RC, cm 3 ; V V is the void volume of SC after containing the sample, V V includes the pore volume in coal, cm 3 ; Z is the compressibility factor of the fluid at temperature T and corresponding pressure P, and the subscript corresponds to different pressure states, and the same below;

[0019] The total gas amount in the sample tank after stabilization is

[0020] The free space gas amount in the sample tank

[0021] The free gas amount in the coal in the sample tank

[0022] The total gas amount in the coal

[0023] wherein, Vf is the free space volume of the sample tank, i.e. the sample tank space volume excluding coal, excluding the pore volume of coal;

[0024] Given the sample tank pressure, the reference tank pre-charge pressure determination method is used to calculate x according to the following formula,

[0025]

[0026] Since x is a constant value, if any pre-charge pressure value is given, the value of Vp can be calculated

[0027] The compression factor corresponding to this pre-charge pressure value is obtained by the compression factor table to obtain the accurate value, i.e. to find the compression factor at this pre-charge pressure and temperature Given different pressures, according to the iterative calculation principle, until The corresponding pressure at this time is the pre-charge pressure;

[0028] Step 4, water injection and gas exhaust experiment, the sample tank is connected to the injection pump, high temperature and high pressure intermediate container, high temperature and high pressure sight glass, i.e. opening the connection valve I29, connection valve B7, setting the sample tank outlet pressure control system pressure, i.e. setting the back pressure valve pressure in the pressure control system; opening the connection valve E12, opening the 0.5L measuring tank 14, outlet cooling system 21, quickly filling the fluid into the sample tank, and determining the cumulative liquid injection volume V by the difference between the pump reading volume and the high pressure sight glass liquid volume reading W , reading the 0.5L measuring tank 14 pressure P 计量 , then slowly opening the connection valve F15, connecting the measuring tank and the gas-liquid separation bottle 18, weighing the liquid mass m 液 , and measuring the liquid density p 液 , therefore the liquid volume V According to the state equation, the amount of gas in the measuring tank can be calculated,

[0029]

[0030] wherein, V g is the gas volume in the measuring tank, cm 3 ; n g is the molar mass of the gas in the measuring tank, mol; M g is the gas molecular weight in the measuring tank, dimensionless; P 计量 is the pressure in the measuring tank, MPa; V 计量 is the volume of the measuring tank, cm 3 ; T 计量 is the temperature in the measuring tank, K; p g is the gas density in the measuring tank, cm 3 / g;

[0031] The volume Vw of the liquid in the sample tank can be calculated by the following formula:

[0032] Vw=Vv-V 孔 (13)

[0033] Where V 孔 is the pore volume of the coal sample, cm 3 ;

[0034] At this point, you can determine whether the previous data calculation is accurate. This shows that the previous data is measured accurately;

[0035] Step 5, drain and reduce pressure, evacuate the 1L measuring tank 10, 0.5L measuring tank 14, and 0.25L measuring tank 24 for standby use, and after the pressure in the sample tank is stable, connect the sample tank, high-temperature and high-pressure window, pressure control system and measuring tank. The initial gas volume is low, and constant pressure drainage is adopted. The pressure is gradually reduced by the pressure control system, and the pressure drop is about 1 MPa. After reaching the predetermined pressure, the sample tank and the automatic pressure control system are closed. After the pressure is stable, the sample tank pressure, the measuring tank pressure, the ambient temperature, the atmospheric pressure and other data are recorded, and then the drain valve at the bottom of the measuring tank is opened, and the gas and liquid are separated by the triangular flask. The gas sample is analyzed for gas components by the gas chromatograph 17, and the liquid is weighed by the balance; after the pressure in the sample tank is stable, connect the second measuring tank, and perform drainage and pressure reduction at the second pressure point according to the above steps. Simultaneously, the gas in the first measuring tank is emptied and vacuumized for standby use;

[0036] Pressure reduction and drainage are carried out in sequence according to a certain pressure drop amplitude until the sensitive desorption pressure is reached. Below the sensitive desorption pressure, the gas volume suddenly increases and the pressure drop recovery time becomes longer. At this time, in order to prevent the damage to the reservoir caused by effects such as "gas lock" and "water lock", the drainage system should be adjusted to quantitative drainage, that is, drainage is carried out according to the gas quantity rather than constant pressure until normal pressure is reached;

[0037] Determine the gas volume at each drainage point based on the state equation.

[0038]

[0039] Where, P i Measuring tank pressure, MPa; V i Measuring tank volume, cm 3 ; Z i Pressure P i The corresponding compression factor, T i Measuring tank temperature, K;

[0040] The recovery factor at each drainage pressure point is calculated according to the following formula:

[0041]

[0042] Step 6, the desorption rate experiment is improved, according to the actual field, the drainage is carried out to the depletion pressure (for example 1MPa), can improve the CH4 gas desorption rate by injecting CO2 or fluid such as surfactant and other measures, such as reservoir stimulation, energy increase; for example, the CO2 injection huff and puff test, first connect the valve M38, the gas booster pump 1, inject CO2 into the reference tank, the injection pressure is above the reservoir pressure, the pre- injection pressure is determined by formula 9, 10, ensure that the sample tank pressure is not lower than the reservoir pressure, after the pressure is stable, record the reference tank pressure, atmospheric pressure and other data, then open the connecting valve A3 to connect the sample tank, close the valve after pressure balance, after the pressure is stable, read the reference tank and sample tank pressure, according to formula (5) ~ (8) to measure the amount of CO2 and CH4 in the sample tank;

[0043] Then repeat step 5, use gas-liquid separation bottle to collect gas sample under each drainage pressure point during drainage, in order to ensure the accuracy of gas component data, at least 2 pieces of gas sample are collected at each pressure point, and the drainage gas component concentration is measured by gas chromatograph 17, and the mixed gas volume is determined according to the following formula;

[0044]

[0045] In the formula, n' i The amount of substance of the mixed gas under standard conditions, mol; The amount of CO2 in the mixed gas, mol; The amount of CH4 in the mixed gas, mol; Z' i The compression factor of the mixed gas under standard conditions; V' i The volume of the mixed gas under standard conditions, cm 3 ; The molar concentration of CO2 in the mixed gas, %; The molar concentration of CH4 in the mixed gas, %.

[0046] Compared with the prior art, the beneficial effects of the present application are:

[0047] The present application adopts a self-developed full-diameter coal core sample tank, which can be up to 50cm long, simulates the state of the coal seam after fracturing in the field, increases the sample amount and the effective volume of coal, and reduces the influence of dead volume on data; has a temperature and pressure automatic control system, which can automatically control and collect the temperature and pressure; and adopts a visual metering window to observe the liquid outflow of the metering sample tank;

[0048] The present application is different from the traditional volume method and flow method in the metering principle, accurately meters the desorption gas amount by discharging the gas to the metering tank, collecting the pressure and temperature data, and combining the state equation; the system is composed of three metering tanks of different sizes, the appropriate metering tank is selected according to the gas outflow amount, and continuous switching for uninterrupted metering can be realized; the system increases a liquid condensation system, so that the high-temperature produced gas is quickly condensed, the gas and liquid are quickly separated, and the interference of water vapor on the gas volume metering is eliminated;

[0049] The application establishes a set of coalbed methane well full life cycle exploitation experimental evaluation system and method, so as to provide scientific basis for optimizing production process, reducing exploitation cost, determining reasonable drainage and production system and improving recovery ratio, thereby enhancing the coalbed methane industry competitiveness of China, promoting the sustainable development of the industry, and helping to realize the energy structure transformation and carbon emission reduction target. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 It is a structural schematic diagram of the evaluation system of the application;

[0051] Figure 2 It is a coalbed methane drainage and gas pressure reduction output law diagram of example one of the application;

[0052] Figure 3 It is a sample tank pressure change curve after CO2 injection of example one of the application;

[0053] Figure 4 It is a CO2 injection desorbed gas concentration change law diagram of example one of the application;

[0054] Figure 5 It is a CO2 injection desorbed gas output change law of example one of the application;

[0055] Figure 6 It is a CO2 injection desorbed pressure change curve of the whole process of example one of the application.

[0056] In the figure: 1, gas booster pump; 2, pressure sensor A; 3, connecting valve A; 4, pressure sensor B; 5, high pressure sight window; 6, pressure control system; 7, connecting valve B; 8, connecting valve C; 9, pressure sensor C; 10, 1L metering tank; 11, connecting valve D; 12, connecting valve E; 13, pressure sensor D; 14, 0.5L metering tank; 15, connecting valve F; 16, connecting valve G; 17, gas chromatograph; 18, gas-liquid separation bottle; 19, balance; 20, pressure sensor E; 21, cooling system; 22, heating constant temperature system; 23, connecting valve H; 24, 0.25L metering tank; 25, sample tank; 26, pressure sensor F; 27, injection pump; 28, piston type high pressure container; 29, connecting valve I; 30, reference tank; 31, vacuum pump; 32, connecting valve J; 33, He bottle; 34, connecting valve K; 35, CH4 bottle; 36, connecting valve L; 37, CO2 bottle; 38, connecting valve M. DETAILED DESCRIPTION

[0057] The application will be further described below in combination with the drawings:

[0058] As shown in the drawings Figure 1As shown, a coalbed methane well full life cycle exploitation experimental evaluation system, the evaluation system is composed of four blocks of injection power system, exploitation simulation system, pressure control drainage system and metering system, the injection power system includes injection pump 27, piston type high pressure container 28, gas booster pump 1, CO2 bottle 37, CH4 bottle 35, He bottle 33 and its corresponding pressure sensor (i.e. pressure sensor F26), valve and connecting pipeline; the exploitation simulation system includes reference tank 30, sample tank 25 and its corresponding pressure sensor (i.e. pressure sensor A2, pressure sensor B4), valve and connecting pipeline, the reference tank 30 is communicated with the sample tank 25; the pressure control drainage system includes high pressure window 5, pressure control system 6; the metering system includes 1L metering tank 10, 0.5L metering tank 14, cooling system 21, 0.25L metering tank 24 and corresponding pressure sensor (i.e. pressure sensor C9, pressure sensor D13, pressure sensor E20), valve and connecting pipeline. The connection relationship between the pressure sensor, valve and connecting pipeline in the above evaluation system structure is shown in the drawing, which will not be repeated here.

[0059] The evaluation method based on the coalbed methane well full life cycle exploitation experimental evaluation system, specifically includes the following evaluation steps:

[0060] Step 1, sample selection, select the coalbed after fracturing to fill the sample tank, inject fluid, according to the actual site, select fracturing flowback fluid or formation water as the injection fluid;

[0061] Step 2, system volume calibration, according to the actual sample amount of coal, assemble the connection system, and heat it to the formation temperature of the sample, open the valve connection valve A3, connection valve J32, vacuumize the system until the vacuum degree is unchanged, then close the above-mentioned valves, read the atmospheric pressure P0; first open the connection valve K34, open the gas booster pump 1, fill the helium into the reference tank through the injection power system, add to 5-10MPa, after the pressure is stable, close the injection power system, i.e. connection valve K34, close the gas booster pump 1, record the pressure P2 at this time, open the connection valve A3, after the sample tank 30 pressure reaches the target pressure, close the valve, after the pressure is stable, read the reference tank 30 pressure P3, sample tank 25 pressure P4; then vacuumize the system, put the standard block with known volume V0 into the sample tank, repeat the above operation again; finally, replace the standard block with coal sample, repeat the above operation again; according to the law of conservation of mass combined with the equation of state, the sample tank volume, reference tank volume and sample skeleton volume can be obtained;

[0062] Sample tank volume:

[0063] Reference tank volume:

[0064] Sample skeleton volume:

[0065] wherein,

[0066] In the formula, P1, P'1, P"1 are the pressures of the system before injecting gas, i.e. the system without adding standard blocks and samples, after adding standard blocks and after adding coal samples, MPa; these values can be considered as 0 after vacuumizing; P2, P'2, P"2 are the reference tank pressures of the system after injecting gas, after adding standard blocks and after adding coal samples, MPa; P3, P4 are the reference tank and sample tank pressures of the system after opening the stop valve and after the pressure is stable, MPa; P'3, P'4 are the reference tank and sample tank pressures after opening the stop valve and after the pressure is stable, MPa; P"3, P"4 are the reference tank and sample tank pressures after opening the stop valve and after the pressure is stable, MPa; Z is the compressibility factor of the fluid at temperature T and corresponding pressure P, the subscript corresponding to different pressure states, the same below;

[0067] Step 3, restore to the formation state, open the connection valve A3, connect the reference tank and the sample tank, warm up the system to the formation temperature, vacuumize the system until the vacuum degree is unchanged, close the connection valve A3, open the connection valve L36, open the gas booster pump 1, inject methane gas into the reference tank, determine the pre-injection pressure according to the equation of state, after the pressure is stable, record the reference tank pressure value, open the connection valve A3, introduce methane gas into the sample tank, close the connection valve A3 after reaching the target pressure, after the pressure is stable, record the reference tank and sample tank pressures respectively, calculate the adsorbed gas amount GSE according to formula (4);

[0068]

[0069] In the formula: m is the sample mass, g; R is the universal gas constant, 8.314 J / mol / K; T is the experimental temperature, K; P2 and P1 are the initial equilibrium pressures of the reference tank (RC) and the sample tank (SC), MPa; P3 and P4 are the pressures of RC and SC corresponding to the adsorption equilibrium state, MPa; V R is the RC reference tank volume, cm 3 ; V V is the SC void volume after containing the sample, V V including the coal pore volume, cm 3 ; Z is the compressibility factor of the fluid at temperature T and corresponding pressure P, the subscript corresponding to different pressure states, the same below;

[0070] The total gas amount of the sample tank after stabilization is

[0071] The sample tank free space gas amount

[0072] Free gas in coal

[0073] Total gas in coal

[0074] wherein, is the free space volume of the sample jar, i.e. the sample jar space volume excluding coal, excluding the pore volume of the coal;

[0075] Given the sample jar pressure, the reference jar pre-charge pressure determination method is used to calculate x according to the following formula,

[0076]

[0077] Since x is a constant value, if any pre-charge pressure value is given, can be calculated

[0078] The compression factor corresponding to this pre-charge pressure value is obtained by the compression factor table to obtain the accurate value, i.e. find the compression factor at this pre-charge pressure and temperature Given different pressures, according to the iterative calculation principle, until At this time, the corresponding pressure is the pre-charge pressure;

[0079] Step 4, water injection and gas removal experiment, the sample jar is connected to the injection pump, high temperature and high pressure intermediate container, high temperature and high pressure sight glass, i.e. opening the connection valve I29, connection valve B7, setting the sample jar outlet pressure control system pressure, i.e. setting the back pressure valve pressure in the pressure control system; opening the connection valve E12, opening the 0.5L metering tank 14 outlet cooling system 21, quickly filling the sample jar with fluid, and determining the cumulative liquid injection volume V by the difference between the pump reading volume and the high pressure sight glass liquid volume reading W , reading the 0.5L metering tank 14 pressure P 计量 , then slowly opening the connection valve F15, connecting the metering tank and the gas-liquid separation bottle 18, weighing the liquid mass m 液 , and measuring the liquid density p 液 , so the liquid volume can be obtained According to the state equation, the amount of gas in the metering tank can be calculated,

[0080]

[0081] wherein, V g is the gas volume in the metering tank, cm 3 ; n g is the molar mass of the gas in the metering tank, mol; M g is the gas molecular weight in the metering tank, dimensionless; P 计量 is the pressure in the metering tank, MPa; V 计量Volume of the measuring tank, cm 3 ; T 计量 Temperature in the measuring tank, K; p g Gas density in the measuring tank, cm 3 / g;

[0082] The volume Vw of the liquid in the sample tank can be calculated by the following formula,

[0083] Vw=Vv-V 孔 (13)

[0084] In the formula, V 孔 is the pore volume of the coal sample, cm 3 ;

[0085] At this time, it can be judged whether the previous data calculation is accurate. If it indicates that the previous data measurement is accurate;

[0086] Step 5, drainage pressure reduction, 1L measuring tank 10, 0.5L measuring tank 14, 0.25L measuring tank 24 are vacuumized for standby, after the pressure of the sample tank is stable, the sample tank, high temperature and high pressure window, pressure control system and measuring tank are connected, the initial gas volume is low, the constant pressure drainage is used, the pressure is gradually reduced through the pressure control system, the pressure drop is about 1MPa, after reaching the predetermined pressure, the sample tank and the automatic pressure control system are closed, after the pressure is stable, the sample tank pressure, measuring tank pressure, ambient temperature, atmospheric pressure and other data are recorded, then the drainage valve at the bottom of the measuring tank is opened, the gas sample is separated through the triangular bottle, the gas sample is analyzed by the gas chromatograph 17, and the liquid is weighed by the balance; after the pressure of the sample tank is stable, the second measuring tank is connected, and the second pressure point is drained and depressurized according to the above steps, and the first measuring tank is simultaneously emptied and vacuumized for standby;

[0087] The pressure reduction and drainage are sequentially performed according to a certain pressure drop amplitude, until the sensitive desorption pressure is reached, the gas volume suddenly increases below the sensitive desorption pressure, and the pressure drop recovery time becomes longer, at this time, in order to prevent the damage of "gas lock" and "water lock" to the reservoir, the drainage system should be adjusted to quantitative drainage, that is, the drainage is performed according to the gas quantity, rather than constant pressure, until the normal pressure;

[0088] According to the state equation, the gas volume of each drainage point is determined,

[0089]

[0090] In the formula, P i is the pressure of the measuring tank, MPa; V i is the volume of the measuring tank, cm 3 ; Z i is the corresponding compression factor, T i is the pressure P i is the temperature of the measuring tank, K;

[0091] The recovery of each drainage pressure point is calculated according to the following formula:

[0092]

[0093] Step 6, desorption rate experiment, according to the actual situation, drainage to the depletion pressure, the depletion pressure can be set to 1 MPa, can be improved by injecting CO2 or surfactant and other fluids to transform the reservoir, energy and other measures to improve the desorption rate of CH4 gas; Taking the CO2 injection huff and puff test as an example, first connect the valve M38, gas booster pump 1, inject CO2 into the reference tank, the injection pressure is above the reservoir pressure, determine the pre- injection pressure by formula 9, 10, ensure that the sample tank pressure is not lower than the reservoir pressure, after the pressure is stable, record the reference tank pressure, atmospheric pressure and other data, then open the connecting valve A3 to connect the sample tank, close the valve after pressure balance, after the pressure is stable, read the reference tank and sample tank pressure, according to formula (5) ~ (8) to measure the CO2, methane gas volume in the sample tank;

[0094] Then repeat step 5, use gas-liquid separation bottle to collect gas sample under each drainage pressure point during drainage, to ensure the accuracy of gas component data, collect at least 2 gas samples at each pressure point, and measure the drainage gas component concentration by gas chromatograph 17, determine the mixed gas volume according to the following formula;

[0095]

[0096] In the formula, n' i The amount of substance of the mixed gas under standard conditions, mol; The amount of CO2 in the mixed gas, mol; The amount of CH4 in the mixed gas, mol; Z' i The compression factor of the mixed gas under standard conditions; V' i The volume of the mixed gas under standard conditions, cm 3 ; The molar concentration of CO2 in the mixed gas, %; The molar concentration of CH4 in the mixed gas, %.

[0097] Example 1

[0098] Taking the main seam No. 15 coal of M well in a block of Qinshui Basin coal seam gas as an example, several full-diameter coal cores are selected, which represent the fracture network formed after fracturing in the field. The coal seam depth is 1786 m, the reservoir temperature is 45 ℃, the reservoir pressure is 16.5 MPa, the coal sample mass is 487.27 g, the bulk density is 1.385 g / cm 3 , and the skeleton density is 1.590 g / cm 3, porosity 12.997%, coal seam under-saturated, mainly adsorbed gas, according to the isothermal adsorption experiment results, the sensitive pressure of a large amount of desorption of coal seam gas is 2.87 MPa. In order to better simulate the actual site, the well site fracturing flowback fluid is selected, which is mainly fracturing fluid containing formation water, and the salinity thereof is 2814.7 mg / L, and the water type is calcium chloride type. After the sample is selected, the steps are as follows.

[0099] 1. System volume calibration, according to the method of step 2, high-purity helium (He) is used for calibration, atmospheric pressure data is read, and pressure data at each stage is recorded (as shown in Table 1), and the reference tank, sample tank, sample skeleton volume, coal sample and tank body related volume in the system are calibrated respectively (as shown in Table 2);

[0100] 2. Return to the formation state, according to the method of step 3, the sample tank stable pressure is given in advance as 16.5 MPa, the coal sample pre-charge pressure is determined according to formula 9 as 24.84 MPa, the coal sample is fully saturated with CH4, after the pressure of the sample tank is stabilized, the temperature and pressure data at each stage are recorded (as shown in Table 1), and the total gas amount of the sample tank, the free space gas amount of the sample tank, the free gas amount in the coal in the sample tank, and the total gas amount in the coal are calculated respectively according to the corresponding formula (as shown in Table 2);

[0101] 3. Water injection and gas discharge experiment, according to the method of step 4, the fracturing flowback fluid of well M is quickly filled into the sample tank at 16.5 MPa, the pump reading and the high-pressure window liquid volume are recorded, so the liquid volume Vw pumped into the sample tank can be obtained as 188.6486 cm 3 , the discharged gas amount is measured by using a measuring tank, the pressure of the measuring tank is recorded, and the amount of gas in the measuring tank Vg is obtained according to formula 11 and formula 12 as 188.774 cm 3 , according to the VSCfree obtained in the above steps, VSCfree=188.7835 cm 3 , Vg≈VW≈VSfree, so the previous data measurement is accurate;

[0102] 4. Drainage and pressure reduction, according to step 5, different volumes of measuring tanks are selected in turn to measure the produced gas, and the 0.25 L and 0.5 L measuring tanks are mainly used in the early stage when the gas amount is low and the gas is mainly produced by constant pressure drainage; after reaching the sensitive desorption pressure, a large amount of gas is desorbed, and the gas is mainly produced by constant volume drainage, and the 0.5 L and 1 L measuring tanks are mainly used in this stage; at the same time, the room temperature, atmospheric pressure, experimental pressure and drainage weight at each point are recorded, and the gas amount, recovery rate and liquid flowback rate are measured according to formula 14 and formula 15, and the variation graphs of the gas amount, recovery rate and liquid flowback rate with desorption pressure and time are drawn;

[0103] The gas recovery rate of coalbed methane is first small and then large, indicating that the gas desorption is first slow and then fast. The initial pressure drop is large and the gas production is low. The recovery rate is only 3% when the pressure is reduced to 10 MPa. In the middle and late stages, the gas is rapidly desorbed, and the recovery rate is 40% when the pressure is reduced to 2 MPa. Thereafter, the gas production rapidly increases. It can be known that the actual sensitive desorption pressure of the coal seam of the well is 2 MPa, which is slightly lower than the 2.87 MPa calculated by the isothermal adsorption experiment. It is speculated that it is caused by the water in the coal seam. The presence of a large amount of water hinders the production of coalbed methane, resulting in a decrease in the actual sensitive desorption pressure. The pressure drop recovery time is long below 2 MPa. According to the field experience, the coalbed methane depletion pressure is 1 MPa. The final recovery rate is 69.1% when the pressure is reduced to 1.09 MPa. The liquid flowback rate presents a rule of first fast and then slow. The liquid output accounts for 60% of the total liquid output in the initial stage (corresponding to a gas desorption rate of 10%). In the later stage, it gradually stabilizes. The flowback rate is finally 18.5% (the coalbed methane drainage and gas pressure production rule is shown in Figure 2

[0104] 5. CO2 injection to improve gas recovery rate experiment. According to the actual situation on site, the injection pressure is slightly higher than the formation pressure. The pressure in the sample tank after adsorption equilibrium is 16 MPa. According to formula 9, the pre-injection pressure is about 30 MPa. According to the method of step 6, the pressure is increased to 30 MPa after water injection and gas discharge. The pressure change of the sample tank (the pressure change curve in the sample tank after CO2 injection is shown in Figure 3 ) is recorded. With the adsorption of CO2 by coal, the pressure in the sample tank first decreases greatly, and then tends to be stable after 300 h. The final stable pressure is 15.55 MPa.

[0105] Table 1 System calibration data collection table

[0106]

[0107]

[0108] Table 2 System calibration result table

[0109]

[0110] After the pressure in the sample tank is stable, the amount of gas in the sample tank can be measured according to the gas state equation. The calculation results are shown in Table 3. It can be known that the CO2 injection amount is 1.43 times the pore volume of coal, and the volume ratio of CO2 to CH4 is 5.22.

[0111] Table 3 Amount of gas in the sample tank after CO2 injection

[0112] Injection pressure MPa 50.0000 CO2 pre-CH4 gas amount (mol) 0.278646007 Sample pot pressure (MPa) 30.0281 [CO2 before CH4 gas amount (ml - standard condition)] 6241.670566 Experimental temperature °C 45.0000 CO2 gas amount (mol) 1.454032156 <![CDATA[50MPa下CO2的密度(kg / m 3 )]]> 976.8014 [Note CO2 gas amount (ml - standard state)] 32570.32029 50 injection volume of CO2 at 50 MPa (cm3 / g 3 )]]> 65.5000 System total (CO2 + CH4, mol) 1.732678163 50 MPa compressibility factor of CO2 0.8515 System total (CO2 + CH4, ml - standard conditions) 38811.99085

[0113] ​The pressure is reduced and the gas is drained, the gas amount is measured and the gas sample is collected, and then the pressure is continuously reduced and desorbed until the dry pressure is 1 MPa, and the gas amount of each component at each pressure point is calculated according to the formulae 16-18. The results show that the CO2 injection gas concentration "butterfly type" change rule, that is, with the desorption, the CH4 concentration first increases and then decreases, the CO2 concentration first decreases and then increases, the inflection point is 6.3 MPa (the CO2 injection desorption gas concentration change rule is shown in Figure 4 ), indicating the change of the gas occurrence state, that is, the CO2 adsorption gas begins to desorb, and the CH4 adsorption gas desorbs to the maximum and then begins to decrease. After 35 days of drainage and production, the methane recovery rate is increased by 27.8%, and the CO2 recovery rate is 36.3%, most of the CO2 is sealed in the coal, which also indicates that the CO2 displacement CH4 effect is good, and the coal rock has strong CO2 sealing capacity. The liquid flowback rate is only increased by 1.26%, and the increase amplitude is low (the CO2 injection desorption gas production change rule is shown in Figure 5 ).

[0114] The desorption pressure change curve of the whole CO2 injection and huff and puff process is shown in Figure 6 , and the pressure drop recovery curve shows that the pressure is quickly recovered in the initial desorption stage, the pressure fluctuation is small, and the gas production is low, and in this stage, the free gas release is mainly, and after 8 MPa, the pressure drop recovery curve begins to be long, and the gas production begins to increase, indicating that the adsorbed gas begins to be released in large amount.

[0115] In summary, the above is only a preferred embodiment of the present application, and is not used to limit the scope of the present application, and any equivalent changes and modifications made according to the shape, structure, features and spirit of the present application within the scope of the present application are included in the scope of the present application.

Claims

1. A coalbed methane well full life cycle mining experimental evaluation system, characterized by: The evaluation system consists of an injection power system, a mining simulation system, a pressure control and drainage system and a metering system. The injection power system includes an injection pump (27), a piston high-pressure container (28), a gas booster pump (1), a CO2 bottle (37), a CH4 bottle (35), a He bottle (33) and corresponding pressure sensors, valves and connecting pipelines); the mining simulation system includes a reference tank (30), a sample tank (25) and corresponding pressure sensors, valves and connecting pipelines; the pressure control and drainage system includes a high-pressure window (5) and a pressure control system (6); the metering system includes a 1L metering tank (10), a 0.5L metering tank (14), a cooling system (21), a 0.25L metering tank (24) and corresponding pressure sensors, valves and connecting pipelines.

2. The evaluation method of a coalbed methane well full life cycle mining experimental evaluation system according to claim 1 is characterized by: The steps include: Step 1: Sample selection: select a full-diameter coal core sample to simulate the coal seam after fracturing, break it into large coal samples, fill the sample tank, and inject fluid. Depending on the actual situation on site, select fracturing return fluid or formation water as the injection fluid; Step 2: System volume calibration. According to the actual sample volume of coal, assemble the connection system and heat it to the formation temperature where the sample is located. Open the valve connection valve A (3) and the connection valve J (32), and evacuate the system until the vacuum degree does not change. Then close the above valves and read the atmospheric pressure P0. First, open the connection valve K (34) and the gas booster pump (1). Fill the reference tank with helium through the injection power system to 5-10 MPa. After the pressure stabilizes, close the injection power system, that is, the connection valve K (34), and close the gas booster pump (1). The booster pump (1) is turned on, and the pressure P2 at this time is recorded. The connecting valve A (3) is opened. After the pressure of the sample tank (25) reaches the target pressure, the valve is closed. After the pressure stabilizes, the pressure P3 of the reference tank (30) and the pressure P4 of the sample tank (25) are read. Then, after the system is evacuated, a standard block with a known volume V0 is placed in the sample tank, and the above operation is repeated. Finally, the standard block is replaced with a coal sample, and the above operation is repeated. According to the law of conservation of matter and the equation of state, the volume of the sample tank, the volume of the reference tank and the volume of the sample skeleton can be obtained. Sample tank volume: Reference tank volume: Sample skeleton volume: in, Wherein, P1, P'1, and P"1 are the pressures of the empty system before gas injection, i.e., the system without standard blocks and samples, and the sample tank after the standard blocks and coal samples are placed, respectively. These values ​​can be considered to be 0 after vacuuming, in MPa; P2, P'2, and P"2 are the pressures of the reference tank after gas injection, after the standard blocks and coal samples are placed, respectively. P3 and P4 are the pressures of the reference tank and sample tank in the empty system after the shut-off valve is opened and the pressure is stable, in MPa; P'3 and P'4 are the pressures of the reference tank and sample tank after the shut-off valve is opened and the pressure is stable and the standard blocks are placed, in MPa; P"3 and P"4 are the pressures of the reference tank and sample tank after the shut-off valve is opened and the pressure is stable and the coal samples are placed, in MPa; Z is the compressibility factor of the fluid at temperature T, corresponding to pressure P. Its subscripts correspond to different pressure states, the same below; Step 3, restore to the formation state, open the connecting valve A (3), connect the reference tank and the sample tank, heat the system to the formation temperature, evacuate the system until the vacuum degree does not change, close the connecting valve A (3), open the connecting valve L (36), turn on the gas booster pump (1), inject methane gas into the reference tank, determine the pre-injection pressure according to the state equation, record the reference tank pressure value after the pressure stabilizes, open the connecting valve A (3), and pass methane gas into the sample tank. Close the connecting valve A (3) after the target pressure is reached. After the pressure stabilizes, record the reference tank and sample tank pressures respectively, and calculate the adsorbed gas amount GSE according to formula (4); Where: m is the sample mass, g; R is the universal gas constant, 8.314 J / mol / K; T is the experimental temperature, K; P2 and P1 are the initial equilibrium pressures of the reference tank (RC) and the sample tank (SC), respectively, in MPa; P3 and P4 are the pressures of RC and SC after pressure stabilization, respectively, in MPa; V R is the volume of the RC reference tank, cm 3 ; V V is the void volume of the SC after the sample is placed, V V Contains the pore volume in the coal, cm 3 ; Z is the compressibility factor of the fluid at temperature T and pressure P, and its subscripts correspond to different pressure states; The total gas volume of the sample tank after stabilization is Free space volume of sample tank The amount of free gas in the coal in the sample tank n SC游离 =n 总 -n SC自由 -GSE (7) Total gas volume in coal n 煤总 =n SC游离 +GSE (8) Where V SC自由 is the free space volume of the sample tank, that is, the volume of the sample tank space outside the coal, excluding the pore volume in the coal; After the sample tank pressure is given, refer to the tank pre-filling pressure determination method and calculate x according to the following formula: Since x is a constant, if any pre-injection pressure value is given, it can be calculated The compression factor corresponding to the pre-injection pressure value can be accurately obtained through the compression factor table, that is, the compression factor Z at this pre-injection pressure and temperature can be found. 2查表 , given different pressures, according to the iterative calculation principle, until Z 2查表 =Z 2计算 , the corresponding pressure at this time is the pre-injection pressure; Step 4: Water injection and exhaust experiment: connect the sample tank to the injection pump, the high-temperature and high-pressure intermediate container, and the high-temperature and high-pressure window, that is, open the connecting valve I (29) and the connecting valve B (7), set the pressure of the sample tank outlet pressure control system, that is, set the back pressure valve pressure in the pressure control system; open the connecting valve E (12), open the 0.5L metering tank (14) and the outlet cooling system (21), quickly fill the sample tank with fluid, and determine the cumulative injection volume V by the difference between the pump reading volume and the high-pressure window liquid volume reading. W , read the pressure P of the 0.5L metering tank (14) 计量 Then slowly open the connecting valve F (15), connect the metering tank with the gas-liquid separation bottle (18), and weigh the liquid mass m 液 , and measure the liquid density ρ 液 , so the volume of the liquid can be obtained According to the state equation, the amount of gas in the metering tank can be calculated. Where, V g Gas volume in the measuring tank, cm 3 ;n g Molar mass of gas in the metering tank, mol; M g is the molecular weight of the gas in the measuring tank, dimensionless; P 计量 is the pressure in the measuring tank, MPa; V 计量 is the volume of the measuring tank, cm 3 ;T 计量 Temperature in the metering tank, K; ρ g Gas density in the metering tank, cm 3 / g; The volume Vw of the liquid in the sample tank can be calculated by the following formula: Vw=Vv-V 孔 (13) Where V 孔 is the pore volume of the coal sample, cm 3 ; At this point, we can judge whether the previous data calculation is accurate. If V g =V W =V S自由 This shows that the previous data is measured accurately; Step 5, draining and reducing pressure, evacuating the 1L metering tank (10), 0.5L metering tank (14), and 0.25L metering tank (24) for standby use, after the pressure of the sample tank is stable, connecting the sample tank, high temperature and high pressure window, pressure control system and metering tank, the initial gas volume is low, adopting constant pressure drainage, step-by-step pressure reduction through the pressure control system, after reaching the predetermined pressure, closing the sample tank and the automatic pressure control system, after the pressure is stable, recording the sample tank pressure, metering tank pressure, ambient temperature, atmospheric pressure and other data, then opening the drain valve at the bottom of the metering tank, separating gas and liquid through a triangular flask, performing gas component analysis of the gas sample through a gas chromatograph (17), and weighing the liquid through a balance; after the pressure of the sample tank is stable, connecting the second metering tank, performing drainage and pressure reduction at the second pressure point according to the above steps, and simultaneously draining and vacuumizing the gas in the first metering tank for standby use; Pressure reduction and drainage are carried out in sequence according to a certain pressure drop amplitude until the sensitive desorption pressure is reached. Below the sensitive desorption pressure, the gas volume suddenly increases and the pressure drop recovery time becomes longer. At this time, in order to prevent the damage to the reservoir caused by effects such as "gas lock" and "water lock", the drainage system should be adjusted to quantitative drainage, that is, drainage is carried out according to the gas quantity rather than constant pressure until the pressure reaches normal; Determine the gas volume at each drainage point based on the state equation. Where, P i Measuring tank pressure, MPa; V i Measuring tank volume, cm 3 ; Z i Pressure P i The corresponding compression factor, T i Measuring tank temperature, K; The desorption rate at each drainage pressure point is calculated according to the following formula: Step 6, improve the desorption rate experiment, according to the actual situation on site, the exhaustion pressure (for example, 1MPa) can be reduced by injecting fluids such as CO2 or surfactants to transform the reservoir, increase energy and other measures to improve the CH4 gas desorption rate; taking the CO2 injection and huff-and-puff test as an example, first connect valve M (38) and gas booster pump (1) to inject CO2 into the reference tank. The injection pressure is above the reservoir pressure. The pre-injection pressure is determined by formulas (9) and (10) to ensure that the sample tank pressure is not lower than the reservoir pressure. After the pressure stabilizes, record the reference tank pressure, atmospheric pressure and other data, open the connecting valve A (3) to connect the sample tank, close this valve after the pressure is balanced, and after the pressure stabilizes, read the reference tank and sample tank pressures, and measure the CO2 and methane gas amounts in the sample tank according to formulas (5) to (8); Then repeat step 5, and use a gas-liquid separation bottle to collect gas samples at each drainage pressure point during the drainage process. To ensure the accuracy of the gas component data, at least two gas samples are collected at each pressure point, and the concentration of the drainage gas components is measured by a gas chromatograph (17). The gas volume of the mixed gas is determined according to the following formula: Where n' i The amount of substance in the mixed gas at standard conditions, mol; The amount of CO2 in the mixed gas, mol; The amount of CH4 in the mixed gas, mol; Z' i Compression factor of the mixture under standard conditions; V' i Mixed gas standard volume, cm 3 ; CO2 molar concentration in the mixed gas, %; CH4 molar concentration in the mixed gas, %.

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