Lab evaluation on formation damage of No.3 coalbed methane reservoir by drilling fluids in Qinshui basin

It is virtually important for high and stable yield of CBM to research the damage mechanism of No. 3 CBM reservoir by drilling fluids in Qinshui basin. Using clear water, bentonite-polymer drilling fluid and fuzzy-ball based drilling fluid separately to contaminate 20 core plugs which were got from the drilling site of No.3 CBM in Duanshi coal mine, the permeability of core plugs was measured and compared. It is concluded that the permeability recovery rate of core plugs after contaminating with the three solutions were 70.88%, 84.22% and 49.26% separately. Through laboratory tests and observing leak off of core plugs, the main factors of damage mechanism by drilling fluids were analyzed to be solid phase of drilling fluid, migration of coal fine and incompatibility between some surfactant in drilling fluid and the formation. The paper suggests that we should take non-solid coal fine migration controllable drilling fluid, properly use some treating agent to plug formation to decrease the contacting chance of filtration and formation, and optimize surfactant to avoid the formation permeability decrease by the incompatibility between coal bed and surfactant.