As an unconventional gas resource, coalbed methane (CBM) reservoir has unique flow mechanism and production scheme compared with conventional and other unconventional gas reservoirs. The permeability of coal cleats system varies with the production of water and gas, in order to accurately forecast gas productivity, the dynamic permeability model should be investigated first before establishing gas production model of CBM wells. At present, many investigators have proposed some dynamic permeability models of coal correlating to pressure during CBM production process. However, few studies consider pore compressibility, coal matrix shrinkage, pulverized coal output and blockage in the model at the same time.
In this paper, firstly, the porosity model is established based on stress sensitivity effect and matrix shrinkage effect separately. Secondly, the variation mechanism of the porosity with pressure resulted from pulverized coal output and blockage is investigated and the corresponding correlation is built up by fitting the current experimental data and pulverized coal output data from actual CBM wells. Thirdly, coupling stress sensitivity effect, matrix shrinkage effect and pulverized coal output and blockage effect together, the united porosity model with pressure is proposed, and then on the basis of PM model, the dynamic permeability model considering these three effects is derived. Finally, the model is validated by data from literatures, then factors influencing on permeability is analyzed, and the rational dewatering schedules are suggested for CBM reservoirs with different ranks, gas contents and permeabilities.
Good agreements between permeabilities from literatures and those calculated by the proposed model are obtained, indicating that the proposed dynamic model of coal cleat system is rational and accurate to apply in the establishment process of water and gas productivity model for CBM wells. Results show that: if only considering stress sensitivity effect, the permeability of coal cleat system decreases sharply first and then smoothly with decreasing pressure; if only considering coal matrix shrinkage effect, the permeability remains constant before desorption but increases with decreasing pressure after desorption; if only considering pulverized coal output and blockage effect, the permeability remains constant at higher pressure but increases slightly because of pulverized coal output and then decreases gradually with pressure because of pulverized coal blockage; if considering these three effects simultaneously, the permeability decreases first and then might decrease or increase at the late time with decreasing pressure. At the dewatering stage, the bottom hole flowing pressure should be controlled near the critical pressure with maximum pulverized coal output, while, at gas production stage of CBM wells, the difference between average pressure and bottom hole flowing pressure should be controlled at some value with maximum permeability of coal cleat system.
Because the proposed dynamic permeability model considers three different mechanisms of fluid through coal cleats simultaneously, the permeability variation with pressure and the corresponding productivity of CBM wells can be accurately forecasted. The dewatering and production schedules for different types of CBM reservoirs can also be determined using this dynamic permeability model of coal.
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