“Variable speed drainage-low casing pressure” control method of high rank CBM wells in South Qinshui Basin

In order to improve the scientificity of CBM drainage and production,take Fanzhuang block of south Qinshui basin as a study case,based on the gas-water transport process and relative permeability characteristics of No. 3 coal reservoir,the controlling mechanism and influencing factors of gas-water productive process in coal reservoir are dis- cussed,and the influencing models and main stages of various factors affecting gas-water production are analyzed by carrying out experiments on gas-water migration in different scale fracture systems. For reducing reservoir damage caused by factors affecting gas-water migration and permeability loss in each drainage stage, the “ Variable speed drainage-low casing pressure” control method is used for Qinnan block. The research shows that the gas and water products are successively passed through matrix pores,microscopic pores,macroscopic fractures and artificial fractures, during the passing period,the capillary force,effective stress,trigger pressure and the relative permeability of the four elements coupling control the process. In particular,the resistance of the capillary force increases significantly after fracturing,the increase of effective stress leads to crack closure,starting pressure makes gas output lag,and relative permeability affects the stability of flow regime. The main influencing factors of drainage and production efficiency are different in each production stages. Based on the differences above,the process of CBM well pressure reduction and gas production can be divided into four stages according to the relationship between reservoir pressure( Pc ),critical de- sorption pressure( Pde ),gas sight pressure ( Pjq ) and flow pressure ( Pjd ). When Pc < Pjd ,it needs to be drained at 0． 1 MPa / d to overcome capillary force quickly. When Pjx