Instability mechanism of sidewall and anti-sloughing drilling fluid technique for coalbed methane well drilling

Aimed at the technological problem of sidewall instability in Qinshui basin, the fabric and physical chemical properties of mud shale and ingredient were tested by X-ray diffraction (XRD), Scanning Electron Microscope (SEM), liner swelling test and hot rolling dispersion experiments, and the instability mechanism was analyzed. According to the multivariate cooperation principle for wellbore stabilization, the high performance coating inhibitor, plugging anti-sloughing agent and activity equalizer were pertinently selected, and the anti-sloughing drilling fluid system (QSFT) for mud shale stratum and drilling fluid for coal seams (QSMZ) were formed. Evaluation results show that the QSFT system has excellent viscosity, low fluid loose, thin mud cake, plugging and bearing capacity was up to 3.0 MPa for fissures of 200 μm; QSFT and QSMZ have strong hydrate inhibition, can increase uniaxial compressive strength of rocks, permeability loss ratios of wet ingredient cores are lower than 30%, and permeability recovery ratios of dry cores are higher than 95%. They can be used as coalbed methane well drilling fluid and drilling fluid for reservoir in Qinshui basin.