Research on the Mechanism of Wellbore Instability of Oil Shale in Raoyang Sag and Water-Based Anti-sloughing Drilling Fluid

The shale oil resources are abundant in the lower subsegment of Es1 in Raoyang sag, but the wellbore collapse and sticking occur in several horizontal wells during drilling process, which seriously restricts the exploration and development of this area. The effects of rock mineral composition, microstructure features, physical and chemical properties and rock mechanical properties on wellbore stability are analyzed by means of X-ray whole-rock analysis, scanning electron microscope, hydration experiments and rock mechanical experiments. The research shows that the clay minerals of oil shale in the lower subsegment of Es1 are mainly composed of illite/smectite mixed layers which have weak expansion characteristics and will induce micro-fractures when exposed to water. Due to micro-fracture development, spontaneous imbibition will induce fracture expansion and hydraulic fracturing of bedding seams by capillary action; The bedding seams are developed and the bedding surfaces and vertical surfaces are anisotropic in mechanics. In the drilling process of horizontal wells, the strength of oil shale decreases which eventually results in instability and collapse of wellbore because of the existence of weak bedding surfaces. Therefore, the techniques of water-based anti-sloughing drilling fluid are put forward, that is, “double plugging, hydration inhibiting, wellbore solidifying, oleophobic and hydrophobic, reasonable density”. The optimized formula of high-performance water-based drilling fluid is constructed. The evaluation results show that this system has better rheological property, plugging property, inhibition and lubricity. The research results provide an effective theoretical and technical support for efficient exploration and development of oil shale in this area.