Laboratory studies of shale fracturing behaviors with rock-drilling fluid interactions

Key words:  rock fracturing behaviors  fracture development  chemical erosion  drilling fluid  shale

Abstract:

Drilling in shale formations has been a challenge due to wellbore instability problems. Loss of drilling fluid into the formation can cause complex interactions of rock-fluid and change the cracking behavior of shale during chemical erosion with reactive clays. In this study, high temperature immersion testing was conducted to investigate the interactions of shale rock samples with various drilling fluids in order to reveal the mechanisms of rock failure, in which the fracture development and microstructure changes of shale rocks during drilling fluid immersion were analyzed, and the fracturing behaviors and its controlling factors were investigated. The results show that strong chemical interactions between the shale samples and drilling fluids in terms of fracture development have been observed. These interactions can enhance the propagation of fracture, and the fracturing behavior is time and chemical dependent. The fracturing of shale can be significantly reduced by increasing the plugging capability and adjusting the pH value of drilling fluids. The shale fracturing induced by chemical erosion of drilling fluids can be attributed to hydration reactions and alkali effects. Optimized oil-based fluid and well designed water-based fluid are essential for wellbore stability. This study can provide valuable insights into the nature of the wellbore instability in shale drilling.