Storage-seepage Mechanism And Development Features Of Deep Coalbed Methane Reservoir

There is abundant coalbed methane（CBM）resource in the deep areas of Chinese sedimentary basins and about 22.5×10¹² m³ of reserves are in the burial depth ranging from 1000 m to 2000 m.However,the overall development situation of deep CBM is unsatisfactory.Due to the low porosity and permeability,strong stress sensitivity,weak shear resistance and great fracturing difficulties of coal reservoirs,the CBM wells generally have low productivity which can hardly form industrial gas flow.Focusing on the key factors restricting the high productivity and efficient development of deep CBM,this research took the coal seams of Baode,Linxing,Liulin,Shilou,Linfen and Hancheng blocks in eastern Ordos basin as the objects and took the coalification degree,in-situ stress,reservoir pressure and temperature as the clues.Studies have been carried out from the aspects of storage space development,gas-containing and adsorption behaviors,porosity/permeability and mechanical properties,and production dynamics of deep CBM.（1）The coal metamorphism degree fundamentally controls the pore system development.With the increase of coalification degree,the coal wettability weakens,the capillary pressure curve of centrifugation changes from gentle to steep,the connectivity between pores and throats weakens,the sorting degree of pores increases,the amount of macro and mesopores decreases yet that of micropores increases,the fractal dimension increases,and the pore genesis transients from primary cavity pores and breccia pores to gas holes and particle pores.（2）As the dominant role of pressure or temperature changes from shallow to deep environments,the maximum gas content of coal reservoirs first increases and then decreases in the vertical direction.Under the high pressure and temperature conditions,the coalbed methane is in a supercritical state.Its absolute adsorption amount is larger than the excess adsorption amount.By combining the adsorption potential theory with supercritical parameters,a specific adsorption characteristic curve of adsorption space and adsorption potential can be obtained,which is not related to pressure and temperature,and the mathematical model of limiting adsorption amount was established.Based on the adsorption thermodynamics and adsorption kinetic principles,the response of surface free energy,isosteric heat of adsorption and adsorption activation energy to pressure and temperature were analyzed for the coal-methane adsorption system.（3）The in-situ stress makes the porosity and permeability poor,the mechanical strength large and the shear resistance weak through controlling the crack closing degree and the coal body structure of coal reservoirs.Geological connections exist between the coal porosity/permeability and mechanical properties,which means the porosity and permeability can be calculated according to the strain of rocks and have a slow decreasing rate under the influence of the varying pore volume compressibility associated with changing stress.Moreover,there is a negative exponential correlation between the coal permeability and effective stress.The physical effect of temperature on the permeability is also in a way of thermal stress essentially.（4）The deep CBM development is featured in long single-water drainage time,slow increasing rate of gas yield,weak production stability and poor working continuity,with the average daily gas yield of single well less than 1000 m³/d.It is suggested that the allogenic water should be avoided in the CBM drilling and drainage process;the matching relationship between different layers should be coordinated when conducting commingled production;the working system of gas wells should be kept continuous,stable and slow.