Water Base Drilling Fluid Technology for Horizontal Shale Gas Drilling in Sichuan and Yunnan

Borehole wall instability has been a problem frequently encountered in horizontal shale gas drilling with water base drilling fluids. A new type of water base drilling fluid has been developed for use in shale gas drilling in Sichuan and Yunnan provinces. Higher alcohols and potassium salt of sulfonated asphalt were the main additives used in formulating the drilling fluid. The basic performances of the new drilling fluid were studied through laboratory experiments such as swelling test, hot rolling test, mechanical characteristics analysis, plugging test and contamination test. It was shown that the rates of swelling of the Longmaxi formation in Yunnan, Longmaxi formation and Wufeng formation in Sichuan were reduced to 1.23%, 0.95% and 0.98%, respectively, and the rates of shale cuttings recovery were 98.94%, 99.13% and 99.05%, respectively, indicating that the new drilling fluid had strong inhibitive capacity similar to that of oil base drilling fluids in inhibiting the shales drilled. Compared with conventional water base drilling fluids, the decreasing amplitude of the compressive strength of shales in contact with water base drilling fluids was greatly reduced. The new drilling fluid can plug shale fractures efficiently. Other advantages of the new drilling fluid included salt tolerance (5% NaCl), bentonite contamination tolerance (5%), drilled cutting contamination (20%).