Drainage Controlling Theory And Application In Coalbed Methane Well Based On “Three Fields” Coupling

After more than 30 years of development of coalbed methane(CBM) recovery in ground in China, a set of theories and techniques in aspect of geological evaluation, drilling and completion and hydraulic fracturing, etc have fundamentally been established in accordance with specific geological conditions. However, the research on the theories of optimized drainage particularly on the coupling relationship among pressure field, stress field and fracture field and its mechanism on controlling the production of CBM well in the process of drainage will still be necessary to comprehensively and systematically study. Through some theoretical analyses, a calculation model for obtaining the physical dimensions of hydraulic fractures and the flow conductivity from the curve of hydraulic fracturing was established, according to which a ternary-fracture geometric model about “matrix pores, natural fractures, hydraulic fractures” of coal reservoirs further was built. On the basis of the coupling relationship among “pressure field, stress field, fracture field" in the process of CBM drainage, a mathematical model of the CBM drainage respectively including the dynamic permeability model, the governing equation of coal deformation and the governing flowing equation of CBM and water was completed. According to laboratory experiments on testing the permeability of coal cores, starting pressure gradient and flow conductivity of hydraulic fractures, the low-speed non-linear seepage for coalbed methane and water in natural fractures and the linear seepage in hydraulic fractures were revealed, so that the migration and production theories on desorption, diffusion and seepage of coalbed methane were improved. Meanwhile, the sensitivity of stress and velocity in the recovery process and its impact on the damage to hydraulic fractures were investigated. The Comsol Multiphysics coupling software was applied for establishing the governing mathematical model for CBM drainage based on the coupling relationship of three fields, and the simulation results were then compared with the actual measure drainage data of Panzhuang and Southern Shizhuang, Qinshui Basin, by which the accuracy of this theoretical model was further confirmed. The general method and procedure for making the working system of CBM drainage based on the “three-field” theory were given and the standard for establishing drainage decompression rate according to the difference of reservoir pressure and critical desorption pressure was put forward in order to make sure that the drainage time is no less than three months before the gaseous production and the time of liquid level dropping below coal seams is no less than 9 to 10 months. For low-permeability coal seams with buried depth more than 700 m, the negative effect of permeability caused by the effective stress in the drainage process is more apparent, the water supply for coal seams is less and the mutation of flowing pressure in well bottom is so easily caused that the vicious circle of pressure surge and permeability decline were formed, therefore the application and utilization of the bottom flowing pressure meter and intelligent drainage equipment is the prerequisite of the precise control to CBM drainage. The three-field coupling numerical model for CBM wells and its calculation has been well applied to make the working system of CBM drainage, the reservoir parameter fitting and the well spacing optimization.