Wellbore Instability Induced by the Coupling of High-pH Fluid–Shale Reaction and Fracture Surface Sliding in Shale Gas Wells: Experimental and Field Studies

Abstract

The wellbore stability in shale gas reservoir is the key factor for safe drilling and effective development of shale gas. However, the instability mechanism remains unclear in shale gas wells. In this work, the wellbore instability mechanism was studied by conducting experimental and field case studies in Longmaxi shale. A high-pH condition strengthened clay mineral hydration by increasing the absolute ζ-potential of the shale clay minerals. Alkali-soluble minerals (clay minerals, quartz, feldspar, etc.) were eroded greatly in a high-pH fluid, resulting in a large number of dissolution pores and weakening the microstructure of the shale. The high-pH fluid further reduced the difficulty of shale frictional sliding by lubricating the shale surface and weakening the asperity strength of the surface. Adjusting the pH value of drilling fluids and plugging lost circulation channels became key jobs to maintain the wellbore stability in shale gas wells. This study provides theoretical guidance for drilling fluid optimization used in shale gas wells.