Experimental Study on Enhancing Wellbore Stability of Coal Measures Formation with Surfactant Drilling Fluid

Abstract

The phenomenon of self-cleavage of coal and rocks leads to poor continuity and a high degree of heterogeneity of the coal-rock matrix, causing the wellbore in the process of drilling. The problem of wellbore stability in the drilling process of the coal strata has become one of the key scientific problems. To solve the problem, single-agent optimization and compounding tests of surfactants have been carried out. Based on the test results, the surfactant compounding composition has been selected, which can effectively reduce the surface tension of the drilling fluid and increase the contact angle between the drilling fluid and the coal and rock surface. To evaluate the effect of the drilling fluid wettability on wellbore stability of the coal measures strata, the expansibility test, rolling recovery test, and shale pressure transfer test have been carried out, combined with the basic performance tests of the water-based drilling fluid. The results show that the composite surfactant can effectively reduce the surface tension of the drilling fluid by 80.83% and increase the contact angle with coal by 54.17%. The recovery rate of the composite surfactant water-based drilling fluid is as high as 96.2%. The compound surfactant drilling fluid can significantly slow down the transfer rate of shale pore pressure, reduce the invasion degree of the drilling fluid to coal and rock matrix, prevent hydration erosion, and effectively enhance the stability of coal and rock sidewall.