Damage mechanism and protection measures of a coalbed methane reservoir in the Zhengzhuang block

•Strong water lock and serious stress sensitivity were discovered in NO.3 coal seam.•Serious stress sensitivity of NO.3 coal seam attributes to its low mechanical strength and fractured reservoir.•Permeability damage of coal core can be lowered by improving inhibition of operation liquid and modification wettability of coal surface.•Optimized working fluids lower permeability damage of coal core and promote desorption of methane from coal surface.

The Zhengzhuang block is an important development area in Qinshui basin in Shanxi Province, China. In this block, the amount of gas produced in many wells is not consistent with that predicted according to data concerning reservoir properties, even after fracturing (i.e., with clean fracturing fluid) or the implementation of horizontal boreholes through the reservoir. To investigate the damage mechanism, mineral composition and microstructures were analyzed using X-ray diffraction, scanning electron microscopy and thin section analysis; pore parameters were determined by a mercury intrusion test; hydration properties were tested by a dispersion and expansion experiment; the contact angle of water on reservoir cores was measured to analyze the wettability; and the sensitivities of NO. 3 coal cores were analyzed according to core flow experiments and an isothermal adsorption test. Based on the test results, the factors causing damage to the coalbed methane (CBM) reservoir in the NO. 3 coal seam included serious water blocking and stress sensitivity, followed by velocity sensitivity, water sensitivity, alkali sensitivity and acid sensitivity. To eliminate water blocking, an agent to adjust core wettability was chosen. Moreover, a new cheap inhibitor was provided to reduce core water sensitivity. Using the selected wettability adjustment agent, the inhibitor and other necessary additives, drilling fluid and fracturing liquid systems were optimized, causing considerably less damage to CBM reservoir cores, both in the wet and dry states, according to the results of core permeability tests. Compared with surface water and drilling mud applied during drilling in the gas field of the Zhengzhuang block, the optimized drilling fluid and fracturing liquid can promote methane desorption by reducing the Langmuir volume and increasing the Langmuir pressure of coal, thereby increasing gas production and recoverability of the CBM reservoir.