CN111271038A - Novel coalbed methane yield increasing method for low-permeability coal body - Google Patents
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- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 claims abstract description 68
- 239000011780 sodium chloride Substances 0.000 claims abstract description 34
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- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
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Abstract
The invention provides a novel coalbed methane yield increasing method for low-permeability coal bodies, which comprises the steps of cutting a raw coal sample into a cubic shape, airing, and soaking in a JFC solution; placing the coal body in a rigid triaxial pressure chamber of a high-voltage electric pulse permeability-increasing experimental platform to simulate the formation pressure; injecting NaCl solution into the rigid triaxial pressure chamber by using a pressure pump; and (3) switching on a switch on a high-voltage pulse power supply, carrying out high-voltage electric pulse discharge loading on the electrode in a NaCl solution, and finishing the high-voltage electric pulse loading after repeated discharge for multiple times. The invention is based on the technology of fracturing coal bed by liquid medium hydrostatic pressure high-voltage electric pulse discharge, utilizes the triple effects of improving the hydrophilicity of coal body by JFC solution and improving the conductivity by NaCl solution to form a systematized coal bed permeability increasing and coal bed gas yield increasing method, provides a safer and more effective technical means for coal mine gas drainage and coal bed gas exploitation, and can increase the exploitation efficiency and the extraction amount of coal bed gas.
Description
Technical Field
The invention relates to a novel coal bed gas yield increasing method for low-permeability coal bodies, and belongs to the technical field of coal bed gas exploitation.
Background
The resources of coal bed gas accumulated in China are rich, but the utilization rate and the extraction efficiency of the coal bed are low due to low permeability of the coal bed, so that the resource is greatly wasted, and the problem of how to improve the extraction efficiency of low-permeability coal bodies is still a big problem in the industry at present.
The traditional hydraulic fracturing technology has lower peak pressure, and if a coal mine is exploited to a deeper position, the equipment volume is enlarged, the structure is complex, the water injection pressure is high, high-pressure sealing is difficult, and the success rate is reduced due to the fact that the hydrostatic pressure is simply increased; for a soft coal seam, the hydraulic fracturing method can easily cause that a propping agent is embedded into a coal body crack and does not continue to develop, and even the crack is closed. The gas high-pressure water in the hydraulic fracturing method is difficult to discharge after entering, the mining rate is not high, and the gas high-pressure water is only suitable for coal bodies with unobvious bedding development, good connectivity and high permeability, and has poor effect on low-permeability coal bodies. Meanwhile, most of the gas exists in the coal bed in an adsorption state, the traditional hydraulic fracturing technology has little influence on gas adsorption, and therefore the method is very limited for improving the extraction effect of the coal bed gas.
The high-voltage electric pulse technology is mainly applied to oil exploitation, and the application and research of the coal industry are few, so the research on the action mechanism of the high-voltage electric pulse technology is not deep. The action mechanism of the electric pulse is mainly focused on the heat effect of the electric pulse, the research on internal deformation, crack development and the like is less, and particularly, a great blank still exists in the aspect of the energy loss rule of the high-voltage electric pulse. At present, only the application of high-voltage electric pulse technology on the ground is proposed, and the application mode of the technology in the underground is not clear.
The static blasting technology is a novel crushing technology gradually developed in recent years and is often applied to coal bed permeability increasing tests in complex environments under coal mines. The arrangement of the static blasting powder holes is particularly critical to the static blasting effect, but at present, the static blasting powder holes are determined by the practical experience of the field, and the theoretical basis and the foundation are lacked. In addition, the formation and development conditions of blasting cracks are not systematically researched, and a more ideal coal body cracking effect cannot be obtained.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a novel coalbed methane yield increasing method for low-permeability coal bodies, which is based on a technology of fracturing a coalbed by hydrostatic pressure and high-pressure electric pulse discharge in a liquid state and utilizes a JFC solution to improve the hydrophilicity and H of the coal bodies2O and CH4And under the triple auxiliary effects of the competitive adsorption relationship of molecules and the improvement of the conductivity of the NaCl solution, the coal bed gas exploitation efficiency and the extraction efficiency of the low-permeability coal body are further improved.
In order to achieve the purpose, the invention adopts the technical scheme that:
a novel coalbed methane yield increasing method for low-permeability coal bodies comprises the following steps:
(1) cutting a raw coal sample into cubic test pieces by a cutting machine, airing, and then placing the coal test pieces in a non-ionic surfactant solution for soaking treatment;
(2) after soaking, taking out the coal body test piece, and punching a hole in the middle of the test piece;
(3) placing the coal body on a high-voltage electric pulse anti-reflection experiment platform for operation, and firstly connecting and preparing high-voltage electric pulse equipment;
(4) placing a coal sample test piece into a rigid triaxial pressure chamber to simulate the formation pressure;
(5) injecting NaCl solution into the rigid triaxial pressure chamber through a NaCl solution injection port by using a pressure pump;
(6) and setting a charging voltage on a high-voltage pulse power supply, switching on a switch, carrying out high-voltage electric pulse discharge loading on the electrode in a NaCl solution, and stopping discharging after the test piece is repeatedly discharged for 20 times to finish the high-voltage electric pulse loading.
The cube test block in the step (1) has the size of 300mm multiplied by 300 mm; the nonionic surfactant is a JFC solution; the JFC solution has the mass concentration of 0.32 percent and the soaking time of 24 hours.
And (3) charging pulse equipment, wherein the input voltage of the high-voltage pulse discharge power supply is 220V/50Hz, the voltage can reach 5 kV-15 kV after boosting, the power is 1kW, the control box is operated to perform charging work, and the charging is finished after the preset voltage is reached.
The method for simulating the formation of the stratum in the step (4) comprises the following steps: applying 3MPa hydrostatic pressure through a hydraulic pressure pipeline, carrying out graded alternate loading on a coal sample to a set value by confining pressure formed by an ultrathin jack and axial pressure formed by a hollow jack, keeping the pressure unchanged all the time, and simulating the formation pressure; the vertical axial pressure of the simulated formation pressure is 7.28MPa, and the horizontal axial pressure is 8.66 MPa; the rigid triaxial cell may also be a large true triaxial press.
The concentration of the NaCl solution in the step (5) is 2.4 mol/L;
and (6) setting the charging voltage to be 9-13 kV.
The invention has the beneficial effects that:
1. the nonionic surfactant JFC solution is applied to coal bed gas exploitation and gas drainage, and the hydrophilicity improvement effect of the nonionic surfactant JFC solution on the surface of a coal bed is utilized to lay a foundation for competitive adsorption of water molecules entering coal and methane molecules. Because the coal body has stronger hydrophobicity, if nonionic surfactant JFC molecules with polar hydrophilic groups are added into water, the cohesion of liquid can be reduced, the liquid can be easily spread on the surface of a coal bed and in pore cracks in the coal body, and a certain adsorption layer is formed, so that the surface of the coal bed with lower energy is converted into high energy, and the affinity with water is increased. The coal contains 5-16% of water, after soaking, the water content is obviously improved, and the existence of the water can generate great influence on the occurrence and the migration of methane. When methane molecules in the coal body act on the surface of the coal bed with adsorbed water molecules, the adsorption energy of the methane molecules is remarkably reduced, the adsorption balance distance is increased, the methane molecules are forced to reach an unstable state by the water molecules, so that the water molecules enter the coal body along primary pore cracks of the coal body, and the methane molecules are desorbed and reach a free state.
2. According to the invention, the NaCl solution is used for improving the conductivity of the solution, the injection energy of discharge is improved, the maximum value of current, instantaneous power and the efficiency of converting electric energy into mechanical energy are improved, more sufficient fracturing of coal and rock masses in the loading process are facilitated, the free gas molecules are discharged from the coal mass through cracks generated by high-voltage electric pulse loading, and the extraction efficiency of the coal bed gas is further improved.
3. The invention can clarify the action mechanism of JFC solution on the surface of coal bed and H2The competitive adsorption condition of O molecules and methane molecules is analyzed by the conductivity contrast of NaCl solution and water, and NaCl solution is used for replacing water solution to carry out high-voltage electric pulse discharge loading, so that the improvement of the conductivity can be used for improving the utilization rate of the total energy, and the rule of influence on the generation, development and extension of cracks can be determined.
4. The method can further improve the effect of high-voltage electric pulse discharge, perfect the theoretical system of the water fracturing weakening permeability increasing technology, form an efficient, complete and systematic method for increasing the permeability of the coal bed and increasing the yield of the coal bed gas, provide a safer and more effective technical means for gas drainage and exploitation of the coal bed gas, reduce the safety accidents of the coal mine, and increase the exploitation efficiency and the extraction amount of the coal bed gas.
Drawings
FIG. 1 NaCl solution density vs. conductivity;
FIG. 2 is a schematic structural diagram of a coal high-voltage electric pulse permeability increasing experiment table;
in fig. 2: 1. the device comprises a coaxial transmission cable, 2 discharge electrodes, 3 a thick pipeline connecting flange, 4 a thin transmission pipeline, 5 a rigid triaxial pressure chamber, 6 a hydraulic pressure pipeline, 7, 8 and 9 ultrathin jacks, 10 hollow jacks, 11 a coal sample, 12 a tail end flange and 13 a NaCl solution injection port.
Detailed Description
The following examples further illustrate the embodiments of the present invention in detail.
Example 1 correlation theory for novel coalbed methane stimulation of low permeability coal bodies
(1) Relationship between NaCl solution density and conductivity
Conductivity represents the ability of an aqueous solution to conduct current, the magnitude of which indirectly reflects the total amount of soluble salts in the water and also reflects the total amount of minerals in the water. The conductivity of common distilled water, tap water and industrial wastewater is shown in table 1, the relationship between the NaCl solution density and the conductivity is shown in fig. 1, and the comparison shows that the NaCl solution has higher conductivity than common industrial water, and the conductivity of the solution can be obviously improved after a proper amount of NaCl is added into the water.
TABLE 1 conductivity of distilled water, tap water, and industrial wastewater
| Item | Distilled water | Tap water | Industrial waste water |
| Conductivity (μ s/cm) | 10 | 500～5000 | 10000 |
According to the theory of energy balance, certain energy is consumed during crack propagation, and the method is mainly used for forming a new surface and plastic deformation before fracture, a certain plastic deformation is generated before fracture of a test piece, so certain plastic deformation work, namely Up, is consumed, a new surface is formed after crack propagation, partial energy is consumed, the surface energy required by the unit area of the new surface is Г, the upper surface and the lower surface need 2 Г, and the total energy required by the unit area of crack propagation is R2 Г + Up.
Therefore, if the crack is completely propagated, a certain amount of energy needs to be supplied to the experimental apparatus and the sample. If the energy provided by the experimental device is G when the crack expands per unit area, the crack will expand only when G is larger than or equal to R. Under the same charging voltage, the energy is constant, and the increase of the conductivity of the NaCl solution can increase the injection energy of a plasma channel stage during high-voltage discharge, can generate higher current maximum value and instantaneous power, and improves the electric energy injected into the channel. Therefore, the addition of the NaCl solution can enable the experimental device to provide larger energy G, which is beneficial to crack propagation.
(2) Soaking coal body with JFC solution
Soaking coal in JFC solution to increase hydrophilicity of coal, and displacing CH with water molecule4Molecule, thereby increasing CH in the free state4A molecule. The NaCl solution can improve the conductivity, thereby increasing the injection efficiency and the total energy of electric energy, and under the double auxiliary action, the coal body is fractured by utilizing the medium-high voltage electric pulse discharge technology in the liquid, so that the CH dissociated in the primary pore fractures of the coal body is further dissociated4The molecules discharge coal along cracks after high-voltage electric pulse fracturing to form a set of complete and efficient coal bed gas extraction method, and the coal bed gas yield and extraction efficiency of the low-permeability coal can be greatly improved.
Example 2 novel coalbed methane yield increasing method for low-permeability coal body
The coal body is soaked in the JFC solution and the high-voltage electric pulse loading technology is carried out by using the NaCl solution, and the related operation is carried out on a coal body high-voltage electric pulse anti-reflection experiment platform (figure 2).
Cutting a raw coal sample into cubic test blocks of about 300mm multiplied by 300mm by a cutting machine, soaking the coal sample test blocks in JFC solution with the concentration of 0.32% for 24 hours after air drying, taking out the coal sample test blocks after soaking, and punching holes in the middle of the test blocks.
Connecting and preparing high-voltage electric pulse equipment on a coal body high-voltage electric pulse permeability-increasing experimental platform: connecting a pressure-bearing pipeline, and connecting the discharge chamber with a water shock wave transmission pipeline through a flange (see figure 2); installing an electrode, putting the discharge electrode into a discharge chamber, screwing the electrode through an end nut, and pressing the electrode tightly by a pressing cover; charging pulse equipment, wherein the input voltage of a high-voltage pulse discharge power supply is 220V/50Hz, the voltage can reach 5 kV-15 kV after boosting, the power is 1kW, a charging button of a control box is pressed to perform charging work, after the preset voltage is reached, a charging stopping button is pressed, and the charging is finished after the preset voltage is reached.
After the preparation work is finished, the test equipment is connected for testing. A high-voltage pulse power supply is connected with a discharge electrode (2) by using a coaxial transmission cable (1), a pressure-bearing pipeline is connected with a thin transfer pipeline (4) through a bolt and a rigid sealing ring on a thick pipeline connecting flange (3), a coal sample test piece is placed in a rigid triaxial pressure chamber (5), a tail end flange (12) is screwed down, 3MPa hydrostatic pressure is applied through a hydraulic pressure pipeline (6) and is unchanged, meanwhile, confining pressure formed by ultrathin jacks (7-9) and axial pressure formed by hollow jacks (10) are used for carrying out graded alternate loading on a coal sample (11) to a set value, the pressure is kept unchanged all the time, and formation pressure (vertical axial pressure 7.28MPa and horizontal axial pressure 8.66MPa) is simulated. The pressure pump injects NaCl solution with the concentration of 2.4mol/L into the rigid triaxial pressure chamber through an NaCl solution injection port (13). And setting a charging voltage on a high-voltage pulse power supply to be 11kV, then switching on a switch, carrying out high-voltage electric pulse discharge loading on the electrode in a NaCl solution, and stopping discharge after the test piece is repeatedly discharged for 20 times according to the industrial field discharge times of the high-voltage discharge fractured coal bed to complete the high-voltage electric pulse loading. Then, the signal acquisition system in fig. 2 is used for acquiring and analyzing experimental data, and researching the shock wave waveform, pressure, coal body cracking effect and the like generated by the discharge of the high-voltage electric pulse in water.
The infiltration effect of the JFC solution on coal dust is measured through a coal dust infiltration test and a forward osmosis test. The coal dust sample is filled into a glass tube with scales, the glass tube is fixed on a test bed, the bottle mouth is blocked, the test bed is knocked until the coal dust is tamped, a prepared JFC solution with the concentration of 0.32% is added by a pipette, the time required for the solution to permeate into the coal dust is recorded every 1cm, and then the obtained experimental data are subjected to fitting analysis by using mathematical software origin7.0, wherein the shorter the time is, the better the infiltration effect is. The result of an analysis test shows that the JFC solution effectively improves the wettability and the hydrophilicity of a coal seam, and the wettability can be improved by about 75% by comparing the JFC solution with a coal dust sample soaked by water. Because, at this time, H in the solution2The O molecules enter the primary pore fractures in the coal block in large quantity and are in contact with CH4Formation of molecules to compete adsorption, H2After the O molecules complete adsorption occupation, the gas molecules originally adsorbed on the surface of the coal body are desorbed and are converted into a free state from an adsorption state.
The nonionic surfactant is not dissociated when dissolved in water, so that the nonionic surfactant has high stability and good compatibility with the ionic surfactant. The commonly used nonionic surfactants in various industrial fields comprise JFC, PEG4000, PEG2000, peregal O-10 and the like, through coal dust infiltration test and forward infiltration test researches, the wetting performance of other nonionic surfactants is respectively improved by 50-65% compared with that of pure water, and the wetting performance of JFC solution with the concentration of 0.32% can be improved to about 75%, so that the wetting effect of JFC on coal in the surfactant is particularly outstanding.
The coal sample soaked by the JFC solution and the raw coal sample are respectively placed on a high-voltage electric pulse anti-reflection experiment platform for loading, experimental comparison shows that the cumulative yield increase rate of the coal bed gas of the coal body which is not soaked is about 10%, the water content of the coal sample is obviously improved after the coal sample is soaked and improved by the JFC solution, and the cumulative yield increase rate of the coal bed gas can reach 26.3% and is increased by about 2.5 times. The JFC solution soaked coal sample can promote the desorption of the coal bed gas and effectively improve the accumulated yield of the coal bed gas.
In addition, the conductivity of the solution is improved to a great extent by adding a proper amount of NaCl into the water, the injection energy of discharge is improved, the maximum value of current and the efficiency of converting electric energy into mechanical energy are improved, and the coal rock mass fracturing process is more sufficient. Firstly, performing a high-voltage electric pulse hydraulic fracturing test on an original coal sample, then performing the high-voltage electric pulse hydraulic fracturing test after adding a NaCl solution, sampling after the test is completed, performing CT scanning, and then performing quantitative analysis on the CT scanning result on the number, fracture rate, width and the like of fractures by using Pore and Crack Analysis System (PCAS), wherein through experimental comparison, geometric morphological parameters of the coal sample are improved to different degrees along with the addition of NaCl. The average value of the crack rate of the coal body test piece subjected to the high-voltage pulse discharge in water is 1.04-1.7%, and the crack width is mostly concentrated between 0.78-1.39 mm. The average value of the fracture rate of the test piece subjected to high-voltage pulse discharge after NaCl is added can reach 2.17-2.69%, and the fracture rate is increased by about 2 times; the crack width is also mostly concentrated between 1.70-2.42 mm, and is increased by about 1.5 times compared with the crack width of the original coal sample, and meanwhile, the number of cracks is correspondingly increased with the addition of NaCl through data analysis, and is increased to dozens of cracks from about ten cracks. Therefore, the geometric form parameters of the coal sample obtained after the high-voltage electric pulse discharge in the NaCl solution and the high-voltage electric pulse discharge in water are obviously improved, namely, the number of the cracks is increased, the width is increased, the crack rate is increased, and the cracking effect is more obvious. Free gas molecules can be discharged out of the coal body through the cracks, and the extraction efficiency of the coal bed gas is further improved.
Claims (7)
1. A novel coalbed methane yield increasing method for low-permeability coal bodies is characterized by comprising the following steps:
(1) cutting a raw coal sample into cubic test pieces by a cutting machine, airing, and then placing the coal test pieces in a non-ionic surfactant solution for soaking treatment;
(2) after soaking, taking out the coal body test piece, and punching a hole in the middle of the test piece;
(3) placing the coal body on a high-voltage electric pulse anti-reflection experiment platform for operation, and firstly connecting and preparing high-voltage electric pulse equipment;
(4) placing a coal sample test piece into a rigid triaxial pressure chamber to simulate the formation pressure;
(5) injecting NaCl solution into the rigid triaxial pressure chamber through a NaCl solution injection port by using a pressure pump;
(6) and setting a charging voltage on a high-voltage pulse power supply, switching on a switch, carrying out high-voltage electric pulse discharge loading on the electrode in a NaCl solution, and stopping discharging after the test piece is repeatedly discharged for 20 times to finish the high-voltage electric pulse loading.
2. The method of claim 1, wherein the cube of step (1) is 300mm x 300 mm.
3. The method of claim 1, wherein the nonionic surfactant of step (1) is a JFC solution; the JFC solution has the mass concentration of 0.32 percent and the soaking time of 24 hours.
4. The method for increasing the yield of the coal bed gas of the low-permeability coal body as claimed in claim 1, wherein the preparation work in the step (3) comprises charging by pulse equipment, the input voltage in a high-voltage pulse discharging power supply is 220V/50Hz, the voltage can reach 5 kV-15 kV after boosting, the power is 1kW, the control box is operated to perform charging work, and the charging is completed after the preset voltage is reached.
5. The method for increasing the yield of the coalbed methane of the low-permeability coal body in the claim 1, wherein the method for simulating the formation pressure in the step (4) is as follows: applying 3MPa hydrostatic pressure through a hydraulic pressure pipeline, carrying out graded alternate loading on a coal sample to a set value by confining pressure formed by an ultrathin jack and axial pressure formed by a hollow jack, keeping the pressure unchanged all the time, and simulating the formation pressure, wherein the vertical axial pressure of the simulated formation pressure is 7.28MPa, and the horizontal axial pressure is 8.66 MPa.
6. The method for increasing the yield of coal bed gas in a coal body with low permeability according to claim 1, wherein the concentration of the NaCl solution in the step (5) is 2.4 mol/L.
7. The method for increasing the yield of the coalbed methane of the low-permeability coal body according to claim 1, wherein the charging voltage set in the step (6) is 9kV to 13 kV.
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| CN112065350A (en) * | 2020-08-05 | 2020-12-11 | 河南理工大学 | Hydraulic fracturing fracture propagation mechanism and fracturing after-effect test system and method |
| CN112268818A (en) * | 2020-11-11 | 2021-01-26 | 中国科学院地质与地球物理研究所 | Rock true triaxial controllable shock wave fracturing test system and method |
| CN112943172A (en) * | 2021-03-31 | 2021-06-11 | 内蒙古科技大学 | Method for fracturing coal and rock mass by high-voltage electric pulse hydraulic pressure and guide crack opening device |
| CN113390906A (en) * | 2021-06-11 | 2021-09-14 | 内蒙古科技大学 | Evaluation method for crack permeability increasing effect of low-permeability coal rock mass under microscopic scale |
| CN114062143A (en) * | 2021-11-30 | 2022-02-18 | 重庆大学 | High-voltage electric pulse in-situ permeability-increasing gas-bearing reservoir two-phase seepage test device |
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