Experimental study on synergistically enhancing the wellbore stability of coal measure strata by electrical inhibition and neutral wetting

Mixed metal hydroxide(MMH) could be used to inhibit the hydration of clay mineral and coal rock,mean- while,surfactants can be used to enhance the hydrophobicity of reservoir rocks. Comprehensive evaluation on the prop- erties of water-based drilling fluid which could enhance the wellbore stability of coal measure strata in the co-exploita- tion of “three gases” was conducted. Results show that inorganic MMH-1 had a good electrical inhibitive performance. The contact angle of composite surfactants solution with coal sample is 102°,and the contact angle with shale is 91°, which shows that the composite surfactants solution could obviously enhance the hydrophobicity of coal rocks in coal measure strata. Compared with fresh water,the composite surfactants and inorganic MMH-1 can significantly retard the pore pressure transmission speed in coal rocks and shale,and decrease the permeability of rock samples up to 99% . Inorganic MMH-1 and composite surfactants have a good compatibility with water-based drilling fluid. The contact an- gles of the optimized drilling fluid with coal and shale are 77． 5° and 66. 5°,respectively. It has excellent hydration inhibition and salt resistance,good tolerance to temperature and pollutants( inorganic salts,drilling cuttings),and very low reservoir damage rate(3. 8% ) to the original coal. The achievements of this paper can provide a guideline for the design of water-based drilling fluid in the co-exploitation of “three gases” in coal measure strata.