During Coal Reservior Drainage And Physical Simulation Of CBM Desorption At Fixed Pressures And Flow In Southern Qinshui Basin

In order to investigate the dynamic change regularities of underground waterduring coal reservoir drainage in southern Qinshui basin and the determination offollow-up CBM desorption, diffusion and flow process to CBM production,“changerate” and “variation velocity”, characterization parameters of dynamic change of ionsconcentration, were put forward to differentiate coal reservoir water systems. Throughdesorption experiments of bulk coal sample at fixed pressures, coal gas and liquidsingle phase permeability, gas-liquid two phases permeability and coal matrixdiffusion coefficient simulation, the three-level flow laws of coal reservoir werediscussed, as well as gas-water desorption and flow features in different drainagestages were revealed.In this paper, coal reservoir water systems were classified into two types, i.e.“connected” and “unconnected”, based on the characterization parameters of dynamicchange characteristics of ion concentration. In a short time, the ionic concentration“change rate” of CBM well water of the “unconnected” type was about two timeslarger than that of the “connected” one. In addition, the difference in the magnitude of“variation velocity” of ion concentration between the two water systems was2-3orders. On the basis of desorption experiments of bulk high-rank coal at step-downpressures,“desorption hysteresis” in micro-pore and transitional pore in high-rankcoal reservoir was considered to result in the fluctuations, rise and decline of threedesorption stages in each pressure-drop stage. What’s more, there exists anexponential relationship between the accumulative desorption rate and abandonmentpressure. Under an abandonment pressure0.7MPa, the accumulative desorption ofhigh-rank coal reservoir, and total desorption amount of step-down-pressuredesorption was1.46higher than that of desorption under normal pressure. Based onstatic simulation of permeability and diffusion coefficient on coal structure ofdifferent space scales, combining with dynamic simulation of coal permeability underthe comprehensive function of effective stress effect, coal matrix shrinkage effect andslippage effect, the coal macro-fracture permeability was defined as high-efficiencypermeability while the coal micro-fracture permeability was defined as low-efficiencypermeability and transitional diffusion mode dominated in all the CBM diffusionmodels. The three-level flow was pointed out to be a continuous flow in the CBMhigh-efficiency flow space, low-efficiency flow space and CBM diffusion space.Based on response characteristics of fluid in the coal reservoir drainage process, in terms of desorption in coal reservoir the drainage process was divided into four stagesi.e.（I） liquid flow,（Ⅱ₁+â…¡₂） unstable desorption,（Ⅲ） stable desorption, and （Ⅳ）attenuation desorption. Stage I: fracturing fluid and gravity water in reservoir poresoutput without CBM desorption. Stage â…¡₁: the first short peak of gas generationoccurred with the increase of free gas and water soluble gas due to the decrease ofpressure. Stage â…¡₂: the production reached a low point because of a desorption lag ofadsorbed methane and the reduction of fracture permeability under the function ofeffective stress. Stage â…¢: the adsorbed gas desorbed in the micro-pore steadily andthe production gradually increased due to the increase of fracture permeability underthe function of coal matrix shrinkage effect and there went up to the second peak ofgas-generating. Stage â…£: the CBM production gradually reduced due to the differenceof pressure and reduction of concentration difference in the coal matrix, at lastdesorption dried up. The CBM desorption is proceeding from the wellboreprogressive outward in space. Coal reservoir has experienced the four stages ofdesorption and flow process in time and coal reservoir shows the characteristics of therelay gas supply in time and space.