(School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22 1008)

Author: Fu, born in September, 1965, male, from Hengyang County, Hunan Province, doctor, professor, doctoral supervisor, engaged in the teaching and scientific research of energy geology.

Project: National Key Basic Research and Development Plan-"973" coalbed methane project (No.:2002CB2 1 1704).

In this paper, the basic research problems of coalbed methane exploration and development in China are briefly analyzed from the aspects of coalbed methane occurrence mode, supercritical adsorption, gas content test method of low-rank coal, dynamic gas content in mining influence zone, multi-stage pressure drop and multi-stage seepage of coalbed methane, gas slippage effect of coal reservoir permeability, effective stress effect, coal matrix shrinkage effect, the relationship between water pressure and gas pressure in coal reservoir pressure, gas production defects of high-rank coal and balanced development of coalbed methane. It is pointed out that balanced development according to the characteristics of coal reservoirs with different coal ranks is an important measure to ensure the sustainable and stable development of coalbed methane exploration and development in China.

Dynamic gas content and dynamic permeability balance; Coalbed methane development

Analysis on several basic problems in coalbed methane exploration and development

Fu xuehai

(China University of Mining and Technology, Xuzhou 22 1008)

Abstract: Several basic problems in the exploration and development of coalbed methane are briefly analyzed, including the occurrence mode of coalbed methane, supercritical adsorption, gas content test method of low rank coal, dynamic gas content in mining influence zone, multistage depressurization and multistage seepage of coalbed methane, gas slippage effect of coal reservoir permeability, effective stress effect, coal matrix shrinkage effect, the relationship between gas pressure and water pressure in coal reservoir, gas problem in high rank coal and balanced development of coalbed methane. The author points out that the balanced development of coalbed methane in different coal ranks is an important measure to ensure the sustained and stable development of coalbed methane in China.

Keywords: coalbed methane; Dynamic gas content; Dynamic infiltration; balanced development

introduce

Coalbed methane reservoir is a special type of pressure adsorption deposit between solid deposit and fluid deposit. Through more than 30 years' research, the United States has established basic theoretical systems such as the generation and storage advantages of middle and low rank coal, secondary biogas accumulation, and dual-hole diversion of coal reservoirs, formed laboratory experimental testing technologies for coal reservoir properties such as porosity, permeability, adsorption, and developed drainage and depressurization technologies for coalbed methane, completion technologies suitable for reservoir properties, stimulation technologies, multi-well interference technologies, on-site testing technologies for reservoir pressure and permeability, and numerical simulation technologies for coalbed methane and hydropower. This theory has some adaptability in Canada, but its application effect in nearly 30 other countries or regions is not good, which shows that this theory has great limitations. More than 600 coalbed methane wells and 10 well groups have been built in various coal-rank coal mining areas in China, most of which have been tested for gas production. The stability of coalbed methane and water energy is poor, and it changes greatly between wells and different production stages in the same well. The relationship between coalbed methane production and well test permeability is not very consistent. Even with high permeability and low production, low permeability has high stable gas production [9]. This reality puzzles the coalbed methane workers in China and seriously interferes with the exploration, development and deployment of coalbed methane in China. How to configure reservoir parameters and drainage system to obtain stable and sustainable productivity? Different scholars or engineers and technicians have made some useful explorations on one aspect of the above problems from their respective professional scopes, but they have not grasped them as a whole. This paper puts forward some ideas on the basic research problems in the exploration and development of coalbed methane in China and discusses them with you.

1 coalbed methane occurrence mode and gas content of low rank coal

1. 1 solid solution gas problem

Coalbed methane is composed of adsorbed gas, free gas and water-soluble gas, which has been recognized by coalbed methane workers. However, the relative gas emission during coal and gas outburst is several times to nearly 100 times that of coal seam gas content, and it is an indisputable fact that even the coalbed methane and coal-formed gas in the surrounding rock in the affected area of coal seam mining can not reach such a high level. Obviously, the solid solution proposed by Elune exists objectively, even its proportion in the whole coalbed methane is much higher than the proportion of alternative solid solution 2% ~ 5% and interstitial solid solution 5% ~ 12% that Elune thinks [10]. Solid solution gas (body) may be similar to natural gas hydrate-combustible ice, which is released when coal and gas outburst. It can be seen that solid solution gas (body) is also an important occurrence mode of coalbed methane.

Supercritical adsorption of 1.2

Under the condition of equilibrium water, the adsorption of methane by coal presents a "two-stage" evolution model, that is, Langmuir volume first increases with the increase of coal rank, and then decreases with the increase of coal rank, and its inflection point (that is, the maximum point) is about 3.5% ~ 4.5% of the maximum reflectance of vitrinite, which fluctuates greatly due to the influence of coal and rock components in lignite and low-rank bituminous coal stages [16544]

Under the formation conditions, there is supercritical adsorption of methane in coal seam. However, only when the methane pressure (air pressure) in the coal seam exceeds 5. 18MPa (table 1), the supercritical fluid really appears. In fact, there are fewer mines in China where the measured gas pressure exceeds this pressure. However, for the in-situ coal reservoir in a closed system, the water pressure in the reservoir is equal to the air pressure. As long as the buried depth of coal seam exceeds 600 meters, methane in coal seam may become supercritical fluid.

Figure 1 Liquefaction interval of carbon dioxide and ethane under normal temperature and pressure gradient

For methane and nitrogen, the reservoir temperature at any depth is higher than the critical temperature, and it will not liquefy regardless of the pressure; For carbon dioxide, when the reservoir temperature is lower than 3 1. 1℃ (table 1), for ethane, when the reservoir temperature is lower than 32.4℃ (table 1) and the reservoir pressure (air pressure) is higher than the liquefaction pressure, they can exist in liquid form. According to the normal geothermal gradient of 3℃/ 100m, the normal reservoir pressure gradient of 0.98MPa/ 100m, the constant temperature zone depth of 20m, the temperature 10℃, the buried depth of about 400m, the reservoir temperature of about 22℃ and the reservoir pressure of 3.9MPa, all of which are lower than the critical temperature and pressure. When the buried depth reaches 800m, the reservoir temperature is about 34℃, which is higher than the critical temperature, and carbon dioxide and ethane are still gaseous. However, when the pressure of carbon dioxide is greater than 7.38MPa and the pressure of ethane is greater than 4.98MPa, carbon dioxide and ethane may become supercritical fluids. Only when the reservoir temperature is lower than the critical temperature and the reservoir pressure is higher than the liquefaction pressure in the range of 400 ~ 800 m in a local well interval (closed system), carbon dioxide and ethane may exist in liquid form (Figure 1).

Table 1 Concise physical properties of coalbed methane components [12]

* Obtained by isothermal adsorption experiment of carbon dioxide at 30℃.

For coalbed methane, which is mainly methane and contains carbon dioxide, nitrogen and ethane, its supercritical state and liquefaction temperature and pressure conditions are one of the next issues worthy of attention.

1.3 gas content test of low rank coal

The field measurement of coal seam gas content in China is mostly based on MT-77-84 desorption method standard, which has good adaptability to middle and high rank coal. However, the measured gas content of low rank coal distributed in northeast and northwest China is obviously low. Because of the development of pores and fractures in low-rank coal, the coring process desorbs rapidly under the condition of formation temperature, and the desorption speed slows down when the temperature drops to the ground, and some of them even do not desorb, so the lost gas in desorption calculation will also be lost. Lignite is not involved in the evaluation of coalbed methane resources conducted by China General Bureau of Coalfield Geology 1995- 1998, and other units and individuals mostly calculate the gas content of lignite according to the isothermal adsorption experiment of lignite equilibrium water, so as to calculate the resources. Therefore, the difference of coalbed methane resources in low rank coal reservoirs is the fundamental reason for calculating the difference of coalbed methane resources by various units and individuals in China.

Based on the water content, pore and fracture characteristics, temperature and pressure conditions of low-rank coal, the adsorption gas, water-soluble gas and free gas are simulated respectively, and it is one of the next research directions in China to determine the gas content of low-rank coal.

1.4 dynamic gas bearing problem in mining affected area

The influence area of coal mining is a favorable part for surface coalbed methane development or underground gas drainage. The development of coal mine roadway and coal production have changed the in-situ stress field and fluid pressure field of coal seam, and broken the dynamic equilibrium relationship among free gas, adsorbed gas and water-soluble gas in coal seam. Due to the pressure relief of coal seam, cracks are opened or new cracks are formed in the affected area of coal mining, and due to mine ventilation, there is a continuous methane concentration difference between the affected area of coal mining and the exposed coal wall, which greatly enhances the permeability and diffusion performance of coal seam, desorbs coalbed methane, and diffuses, permeates or turbulences to roadway or working face under the action of concentration gradient and pressure gradient. With the continuous advancement of roadway and coal mining face, the gas content in coal seam in the affected area of mining presents dynamic change characteristics.

The coal mining influence zone can be divided into coal seam mining influence zone (horizontal mining influence zone), adjacent mining influence zone (vertical mining influence zone) and coal resource residual zone [13]. The mining influence zone of this coal seam can be further divided into the driving roadway and the mining influence zone of the coal mining face. The dynamic gas content of coal seam in mining affected area is related to the exposure time of coal wall (advancing speed of coal mining or tunneling face) and the distance from exposed coal wall. The velocity, flow direction and gas pressure of coalbed methane at any point change with time, which is an unstable flow field and it is difficult to find its analytical solution. Only numerical simulation methods can be used, such as finite element method, gas pressure continuous measurement method, gas emission method and gas emission efficiency method. , can you estimate about [13]?

2 Multi-stage pressure drop and multi-stage seepage of coalbed methane

Coal reservoir is a three-phase medium system composed of gas, water, coal matrix blocks and other substances. Among them, gas components have various phases, namely, free gas (gaseous), adsorbed gas (quasi-liquid), absorbed gas (solid solution) and water-soluble gas (dissolved); There are also many forms of water components, namely, free water in cracks, macropores, combined water in micro-cracks, micropores and aromatic layer defects, and chemical water combined with minerals in coal; The coal matrix block is composed of coal, rocks and minerals. Under certain pressure, temperature, electricity and magnetic field, the components of each phase are in dynamic equilibrium. During the development of coalbed methane under the action of drainage depressurization or external field interference, there are a series of physical and chemical interactions among three-phase media, and its reservoir physical properties also change accordingly. It is difficult to simulate the real physical properties of coal reservoirs by single-phase experimental research.

Coal reservoir system is a ternary structure system composed of macro-fractures, micro-fractures and pores [1 1]. In the process of depressurization development of coalbed methane drainage, the pressure drop of each structural system is different, and there are three levels of pressure drop objectively, and there are also three levels of seepage field in coalbed methane-water migration, namely macro-fracture system (including fracturing fractures)-coalbed methane laminar flow field, micro-fracture system-coalbed methane seepage field, coal matrix block (pore) system-coalbed methane diffusion field [14]. Diffusion includes whole diffusion, knudsen diffusion and surface diffusion, and there are also Darcy linear seepage and nonlinear seepage in seepage. The above three links are indispensable for the development of coalbed methane, and gas and aquatic products can be controlled by the flow field with the slowest seepage. Most of the previous studies ignored the diffusion of gas, and the seepage equation only considered the first two links. The numerical simulation of gas production and water production energy is far from the actual situation, which overemphasizes the study of macro-fracture, that is, well test permeability, and ignores the test and analysis of experimental permeability and diffusion coefficient of coal and rock mass. Therefore, the multi-stage coupling problem of desorption-diffusion-seepage-turbulence matching with the pore and fracture structure system of coal reservoir and the coalbed methane productivity simulation software matching with the pore and fracture structure system of coal reservoir are one of the basic research directions of coalbed methane exploration and development in the next step.

3 the relationship between water pressure and air pressure in reservoir pressure

The fluid pressure of coal reservoir is composed of water pressure and air pressure. The pressure of coal reservoirs in the United States is mainly water pressure, and the energy of gas and aquatic products is stable and continuous. The pressure composition of coal reservoirs in China is complex, and air pressure accounts for a large proportion. In different stages of pressure drop, coalbed methane and water energy are different, and they change by leaps and stages under the overall attenuation trend [15].

Hydrodynamic potential is the most active factor of coalbed methane enrichment and development, and it is the direct reflection and main contributor of reservoir pressure or formation energy. The incompressibility of water supports the fracture, and hydrodynamic force is the maintainer of coal reservoir permeability. The middle and high rank coal seams in China are relatively water-resisting layers, and the water elasticity of the coal seam itself is low, while the gas elasticity is high [16].

The American well test method for testing the pressure and permeability of coal reservoirs with single-phase water flow as the medium will definitely have great defects when applied to coal reservoirs with gas saturation in China, that is to say, the pressure and permeability of coal reservoirs in China obtained by the American well test method are not accurate, and the gas saturation, critical desorption pressure and theoretical recovery calculated from reservoir pressure, gas content and isothermal adsorption curve are also not accurate.

The author thinks that the water pressure of coal reservoir in closed system is equal to air pressure, and the reservoir pressure of coal reservoir in open system is equal to the sum of water pressure and air pressure. The composition and conduction of coal reservoir pressure and the relationship between gas and water in coal reservoir control the desorption, diffusion and seepage characteristics of coalbed methane, which is a key scientific problem to be solved urgently in coalbed methane development at present.

4 coal reservoir dynamic permeability problem

In the process of dehydration and depressurization, with the desorption, diffusion and discharge of water and methane, there are effective stress effect, coal matrix shrinkage effect and gas slippage effect in coal reservoir permeability, and the comprehensive effect of the three effects makes the coal reservoir permeability change dynamically [1 1].

4. 1 effective stress effect

Effective stress is the main controlling factor of crack width change. The increase of effective stress will close the cracks and reduce the absolute permeability of coal. The lower the permeability, the greater the relative change, and some decrease by two or three orders of magnitude. In the process of dewatering and depressurizing to develop coalbed methane, with the discharge of water and gas, the fluid pressure of coal reservoir gradually decreases, the effective stress gradually increases, and the permeability of coal reservoir shows a dynamic change process of rapid decrease and slow decrease [1 1].

4.2 Coal Matrix Shrinkage Effect

The expansion or contraction of coal matrix caused by gas adsorption or desorption can be described by Langerhans model. Using CO2 as the medium, the authors have carried out over-adsorption expansion experiments on cylindrical coal samples with different coal ranks (each point is only balanced at 12h). The results show that the shrinkage coefficient of coal matrix decreases with the increase of coal rank [1 1]. During the development of coalbed methane, when the reservoir pressure drops below the critical desorption pressure, coalbed methane begins to desorb and the coal matrix shrinks. Due to the lateral constraint of coal reservoir, the shrinkage of coal matrix will not cause the overall horizontal strain of coal reservoir, but only local lateral strain along the fracture, which will open the original fracture of coal reservoir, increase the fracture width and gradually increase the permeability, and the increase of middle coal is greater than that of high coal [1 1

4.3 gas slippage effect

In coal, a porous medium, because the average free path of gas molecules is in an order of magnitude with the fluid channel, gas molecules interact (collide) with the wall surface in the channel, which leads to gas molecules sliding along the channel wall surface. This sliding phenomenon caused by the interaction between gas molecules and solids makes the gas velocity and coal permeability increase, and with the decrease of reservoir pressure, it first increases slowly and then increases rapidly at low pressure.

5 Gas production defects of high rank coal reservoirs

The permeability of high rank coal reservoir is sensitive to stress, and the stress permeability decays quickly. High adsorption, micropore and self-sealing effect are obvious; High rank coal has high bound water saturation and low relative permeability; After many tectonic movements, its repeated pressurization and decompression greatly damaged the permeability; The shrinkage of coal matrix is weak, so it is difficult to improve its permeability in the process of coalbed methane development [17].

Firstly, micro-fractures are not developed in high-rank coal reservoirs. Most of the high-rank coal reservoirs have experienced strong tectonic movement, and the coal seams are broken coal, broken speckled coal and mylonite coal.

Secondly, the stress permeability of high rank coal reservoirs decays rapidly. The permeability experiment of constant fluid pressure and increasing confining pressure shows that the permeability of high rank coal and rock decreases exponentially with the increase of confining pressure, and the attenuation coefficient is much larger than that of middle rank coal and rock. Because the in-situ stress gradient (usually about1.6 MPa1000 m in China) is larger than the reservoir pressure gradient (normal pressure gradient is 0.98 MPa/1000 m), with the increase of coal seam buried depth, the effective stress of coal reservoir increases and the permeability of coal reservoir decreases.

Third, the permeability of high rank coal is low. The relative permeability shows that the irreducible water saturation of high rank coal is large, ranging from 7 1.3% to 84.82%, and the areas of single-phase water flow and gas-water two-phase seepage are narrow. During gas-water two-phase seepage, the sum of the maximum relative permeability of gas phase and the maximum relative permeability of water phase of high rank coal is between 25.4% and 40.78%, with an average of 33.2%, that is, the sum of the effective permeability of gas phase and water phase is about 1/3 of its Kjeldahl permeability. The gas phase permeability of high rank coal under water is only 15.7% ~ 22. 1% of its kjeldahl permeability, with an average value of 18.2%, that is, the effective gas phase permeability of high rank coal is not1/5 [/kloc-] under the condition of multiphase medium.

In the process of depressurization and exploitation of coalbed methane, the fluid flows along the area with good permeability, which makes the finger fluid bypass the displacement phase in a large area and form an "island" of displacement phase. The irreducible water saturation of high rank coal is large, that is, there are many such "isolated islands", which make it difficult to reduce the pressure by drainage and desorb coalbed methane, and most coalbed methane is left behind. However, because its adsorption time is only 1 ~ 9 days, it can reach the peak of gas production quickly (several months later), resulting in the "bottleneck" phenomenon of high resources and low productivity [17].

Fourthly, the permeability improvement ability of high rank coal reservoirs is weak. The adsorption/expansion experiment of coal and rock mass in multiphase medium shows that the adsorption capacity of high rank coal is the largest, and the expansion capacity is lower than that of middle rank coal. On the contrary, the adsorption/expansion and desorption/compression of coal are mutually reversible processes, that is, in the process of coalbed methane development, the shrinkage capacity of high-rank coal is weak. The numerical simulation results show that the positive influence of coal matrix shrinkage on permeability is lower than the negative influence of effective stress on permeability, and the permeability of high-rank coal reservoirs gradually decreases during coalbed methane drainage.

It is an important research direction of CBM exploration and development in the next stage to carry out methane adsorption expansion experiments of coal pillar samples with different coal rank (adsorption equilibrium time is as long as several months) and test gas-water flow and diffusion capacity under different pressure drop, different pore and fracture structures.

6 Balanced development of coalbed methane

Coal reservoir is composed of porous fracture structure, and there are multi-stage pressure drop and multi-stage diffusion/seepage field when coalbed methane is discharged. Because of the idea of quick success and instant benefit in the early stage, the drainage of coalbed methane wells often breaks the gas-water seepage balance of coal reservoirs, and fails to properly handle the relationship among casing pressure, liquid level decline and bottom hole pressure. Due to the excessive increase of gas-water energy, it is bound to accelerate the consumption of energy in the original reservoir and shorten the production duration. Therefore, in the production stage of gas test, according to different reservoir physical conditions, shut-in pressure measurement is carried out, Horner curve of pressure recovery is drawn, and the slope of pressure recovery curve is calculated. Then, according to the average daily production before shut-in pressure measurement, it is converted into volume flow in the reservoir, and combined with reservoir coefficient and compressibility, the gas flow coefficient and effective gas permeability in the reservoir under the realistic conditions of gas well are estimated, so as to determine the equilibrium productivity of the reservoir [18]. According to the historical analysis of productivity of Well TL007 in Qinnan and Well Tiefa DT3, the balanced productivity of Well TL007 in Qinnan is about 2000m3/t, and that of Well Tiefa DT3 is about 3000m3/t [9]. Therefore, in the formulation of drainage and production work, it is one of the problems that should be paid attention to in the next step of coalbed methane exploration and development to continuously adjust casing pressure, liquid level drop and bottom hole pressure, maintain the balanced development of gas and water energy and increase the service life of borehole.

7 conclusion

The development of coalbed methane in China is currently in the start-up stage of commercial production. Temperature and pressure conditions of coal-bed methane supercritical state and liquefaction, gas content test method of low-rank coal, dynamic gas content in mining affected area, dynamic permeability of drainage and depressurization development, pressure composition and conduction of coal reservoir, relationship between gas and water medium in coal reservoir, multi-stage coupling theory of desorption-diffusion-seepage-turbulence matching with pore fracture structure system of coal reservoir, drilling, completion and stimulation technology matching with coal reservoir characteristics.

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