Impact of drilling fluids on friction coefficient of brittle gas shale

The stability of the horizontal long section of a wellbore is the main restriction of efficient development of shale gas. The use of oil-based drilling fluids in the initial stage of Longmaxi shale gas exploration in the Southern Sichuan Basin, China still causes wellbore collapses for reasons not well understood. Based on the geological characteristics and engineering conditions of brittle shale, a significant factor influencing shale wellbore stability-friction coefficient is proposed. Due to the developed facture networks of brittle shale in this region, frictional sliding along the fracture surface is the main source of movement within the Longmaxi Formation. Here, we determined the effects of drilling fluids on the Longmaxi Formation shale in Southern Sichuan Basin. Natural fractures slide due to compression shear slip, and the friction coefficient of the fracture surfaces is a controllable factor of shear and slip in natural fractures in shales. The typical experimental apparatus of rock friction sliding was modified to accommodate for drilling fluids and typical in-situ temperatures. Results show that the friction coefficient decreases, induced by infiltration of drilling fluids filtrate, and the friction coefficient is more sensitive to oil-based drilling fluid filtrate than water-based. The high-pH oil-based drilling fluid filtrate shortens the length of time taken by static friction stage, and smooths the time vs friction coefficient curve, which creates an easier situation for frictional sliding. High viscosity and good wettability lead to the strong lubricity of oil-based drilling fluid. The surface properties of the shale samples changed after the high alkalinity fluid immersion, such as fracture surface planarization, and floc-like particle generation, causing an obvious reduction of friction coefficient between fracture surfaces. As a result of rock strength decreasing after immersion into drilling fluids filtrate, asperities on fracture surface were prone to shear failure in the process of frictional sliding, and their failure products migrated with the fracture surface movement. This not only decreased the roughness shale surface but also acted as a friction reduction agent between the sliding surfaces. This study provides theoretical guidance for drilling fluid selection and optimization during brittle shale well drilling.