| Coalbed methane(CBM) is a kind of unconventional natural gas that is generated by and stored as adsorbed gas in coal seams. Efficient development of coalbed methane can increase the energy supply security level in our country, adjust China’s energy consumption structure, and solve the safety problems in the process of ming coal, so it is full of great significance to promote the sustainable and healthy development of China’s economy and society.Coal beds have complex pore structure, variable pore scale, and the way to store gas is different from conventional reservoirs. Because of various migration mechanisms in the development of coalbed methane, including desorption, diffusion and percolation and so on, it results in that the development difficulty of CBM reservoir is far higher than that of conventional gas reservoirs, and the percolation theory of conventional gas reservoirs cannot be directly applied to guide the development of CBM reservoirs. It has made a lot of success in accumulation mechanism, completion and production technology of coalbed methane, but flow mechanism of coalbed methane in coal seams is far behind the field application, and it cannot predict the development performance correctly and optimize productivity, so it is not good for economic development.Based on the extensive investigation and research at home and abroad, combined with the indoor experiment, this paper studied pore structure, occurrence and storage condition, the law of adsorption and desorption, and multiple migration mechanisms of coal seams, set the motion equations of coalbed methane adsorption desorption, diffusion and seepage; considering different migration mechanisms of coalbed methane in matrix, this paper also built seepage physical model of different coal seams. However, in actual coal-bed methane development process, decreasing pore pressure will cause gas desorption, and it will also change the effective stress of coal seam skeletons, deform skeletons, so it results in change of physical parameters of coal seams, like porosity, permeability and so on, as a result, coalbed methane seepage process could be influenced. Using the classic triple-porosity and dual-permeability is not sufficient to describe the actual process of coalbed methane migration, at the same times, it must fully consider the dynamic coupling process of seepage field and stress field. Therefore, combining theoretical analysis and numerical simulation, considering the effects of in-situ stress on coalbed methane desorption, coal seams permeability and porosity, this paper established fluid-solid coupling numerical model of coalbed methane migration.The existence of formation water which in coal seams would bring the influence on the process of coalbed methane desorption and seepage, in addition, the construction process of the extraneous fluid contact with the coal will have great influence on coalbed methane production. Therefore, the influence on coal permeability and desorption are tested by experiments. Two phase effective permeability, wettability and the damage degree of liquid on coal permeability and desorption capacity, and such key parameters have been received. On the basis of experimental research and theoretical model, a fluid-solid coupling model of coalbed methane under the condition of liquid has been established.Through the study of this article, the main results obtained are as follows:(1)The porosity and permeability of coal which from Jincheng area are very small, the peak aperture radius are mainly distributed in between 2-6 nm. The water in the micro pore is difficult to completely discharge when coalbeds are saturated, which would affect the desorption, diffusion and seepage of coalbed methane. The Poisson’s ratio of coal is bigger than sandstone, and elasticity modulus is a little small. The coal has a strong stress sensitivity.(2)Different scales dry coal samples and coal samples under the condition of liquid are tested through independent development of coal desorption device. The experimental results show that the smaller the particle size of coal, the faster the adsorption rate. After adsorption time of coal sample last 12 hours, unit mass of coal could adsorb 15.8 mL/g ~ 34.2 mL/g methane. The adsorption of the top 10 min adsorption quantity accounts for about 40% ~ 60% of total amount of adsorption. Compared with the dry coal samples desorption rules, the largest stage of desorption quantity is when pressure fall off from 0.6MPa into 0.1MPa. The damage degree of influence on desorption of coalbed methane by different liquid could be tested by experiment. Effective stress will reduce the coalbed methane desorption capacity and rate, and the influence degree is higher than the influence of formation water. Desorption quantity and desorption rate of coal under the condition of stress and water is respectively account for 25.3% and 27.8% of those in coal samples without stress and water, which explain the reason of actual coalbed methane low production phenomenon.(3) Based on the experimental data of coalbed methane desorption, and pore structure property, it studies the inner relationship between deformation of coal seams, gas seepage and desorption laws in this paper. Based on conventional seepage equation, considering the seepage and the influence of in-situ stress on coalbed methane desorption and the effect of desorption on matrix shrinkage of coal seams, it also built multi-field fluid-solid coupling mathematical model of desorption-diffusion-seepage. According to aperture change of CBM fracture, dynamic porosity-permeability model under stress was derived and production simulation of coal seams was conducted. Using numerical calculation method to analyze pore pressure distribution of coal beds, descent velocity of pressure gradient, permeability changes and desorption rate of CBM. Results show in the development of coalbed methane, formation pressure decreases, and in-situ stress in the coal seams makes closure of pores and fractures, it results in decreasing permeability, so penetration rate of CBM in pores and fractures reduces. But with the increase of development time, due to the gas desorption, coal matrix shrinkage, the permeability around wellbore area has a certain degree of rise. Pressure gradient of coal seams is continuously dropping, but rate of descent gradually slows down, gas around wellbore desorbs at large scale, and gas far from wellbore cannot put to use, simulation results conform to actual CBM production rules.(4) After formation water and fracturing fluid flowing into micro-fractures and coal cleat, the flow path that gas desorption and seepage would be blocked. The surface active agent of fracturing fluid will also adsorb in the surface of coal, which reduce desorption and seepage ability of coalbed methane. The surface active agent of fracturing fluid will change the wettability of coal, and lead to flow back difficulties of fracturing fluid and formation water. On the basis of fluid-solid coupling mathematical model of coalbed methane migration, according to the experiment research of influence on desorption and seepage process of coalbed methane by liquid. The influence of liquid on the pressure transmission and production are analyzed through the finite element numerical simulation. |
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