Mechanisms of Borehole Wall Instability of Deep Coal Seam in Ordos Basin and Drilling Fluid Countermeasures

In response to the problem of wellbore instability in deep coal seams in the Ordos Basin, the mechanism of wellbore instability in the 8^# coal reservoir of the Benxi Formation in the Nalin 1 block was revealed from the perspectives of mineral analysis, microstructure analysis, physical-chemical properties and mechanical properties. Combined with the multiple synergy wellbore stability theory, a three-effect synergistic anti-sloughing drilling fluid technology strategy of plugging performance-inhibitive capacity-lubricity was proposed. By optimizing the key inhibitor, plugging agent and lubricant, an anti-sloughing drilling fluid system suitable for Benxi Formation in Nalin 1 block was formed. A comprehensive evaluation was conducted from the perspectives of basic performance, plugging property, inhibitory property, and pressure transmission ability. The laboratory test results show that the anti-sloughing drilling fluid formulation system had good rheological properties. The drilling fluid had API filtration rate of 2.4 mL, HTHP filtration rate of 7.5 mL, strong inhibitory ability. The percent recovery of shale cuttings was greater than 90%, 16 h linear swelling rate was less than 5%. The drilling fluid had excellent sealing performance. Plugging of 400 μm fractures with the drilling fluid renders the formation in which these fractures exist a pressure bearing capacity of 5 MPa. Field application has shown that the newly developed anti-collapse drilling fluid can effectively suppress the collapse of deep coal seams in the Nalin 1 well area, reduce the diameter expansion rate, increase the mechanical drilling rate, and prevent complex downhole accidents. It can meet the needs of coal and rock gas drilling on site and has good prospects for promotion and application.