Investigation on the Mechanism of Wellbore Instability of Continental Shale in Ordos Basin, China

During the drilling process, the shale in Ordos Basin has a serious wellbore destabilization problem, which causes great economic loss. For the purpose of studying the mechanism of well wall destabilization in the Chang 7 shale, the characteristics of the Chang 7 shale formation itself and the impact of drilling fluid on the formation were analyzed. Reservoir characteristics were studied by porosity and permeability tests, whole-rock analysis and scanning electron microscopy. The experimental findings revealed that the Chang 7 section’s land-phase shale of the Chang 7 section is a tight reservoir with a clay mineral content of up to 44%, which may lead to water-shale interactions. The results of scanning electron microscopy experiments show that microfractures are more developed in the Chang 7 terrestrial shale. Laminae and microfractures create a path for drilling fluid to into the reservoir and encourage its contact with the shale, which could cause wellbore instability. At the same time, higher demands are placed on the type of inhibition and plugging of drilling fluids. Triaxial compression tests and Brazilian splitting tests were carried out on the shale after soaking for various amounts of time in order to study the impact of field drilling fluid on the shale formation in the Chang 7 section. Based on the test results, the safe density window and collapse period of the formation were calculated, and the performance of the field drilling fluid was assessed and improvement measures were suggested. According to the experimental findings, the interaction between shale and drilling fluid causes both the compressive and tensile strengths of shale immersed in drilling fluid to decrease significantly. Tensile strength is even reduced by 80% after 48 h of interaction with drilling fluid, which is the result of the interaction between shale and drilling fluid. At the same time, we can also know that drilling fluid filtration loss reduction needs to be enhanced. After the drilling fluid’s activity, the formation’s cohesiveness and internal friction angle also lowered to varying degrees. The safety density window of the formation also becomes narrow rapidly after drilling fluid action, which is also not conducive to wellbore stability. By calculating the collapse period of the formation after drilling fluid action, it can be found that the formation will face the possibility of collapse after two days of shale and drilling fluid action. The collapse period after the action of drilling fluid in the field is lower than the field action time, which cannot meet the field needs. From the experimental results, in addition to the possible wellbore instability characteristics of the formation itself, the current drilling fluid has poor filtration reduction effect, poor sealing of micro-nano holes, and general inhibition of hydration of clay minerals. According to the destabilization mechanism of Chang 7 borehole, the filter loss reducing agent, plugging agent and corrosion inhibitor are preferably selected. Apart from effectively reduces the drilling cost, it also serves as a guide for the secure development of comparable formations.