An Anti-Collapse Drilling Fluid with Strong Inhibitive and Plugging Capacity for Use in the Fuxian Block in Ordos Basin

Borehole wall instability was encountered in drilling the Liujiagou formation, the Shiqianfeng formation and the Shihezi formation in the Fuxian block, and the mechanisms of the instability were investigated and revealed through studies on three aspects, namely, the mineral composition, microstructure and physio-chemical property of the formations in question. These studies are aimed at solving the borehole wall sloughing problem encountered in the drilling operation in the Fuxian block in Ordos Basin, in which the shale contains high content of clays and is developed with pores and fractures resulting in shale hydration. Based on the “synergy of multiple factors” theory, a technical measure for stabilizing borehole wall, which can be described as “physio-chemically plugging/consolidating borehole wall to retard pressure transmission-strengthening the inhibition of clay hydration-supporting the borehole wall with appropriate mud weight”, was presented. A drilling fluid with strong inhibitive capacity and plugging capacity was formulated based on the optimization of each additive in and the whole composition of the drilling fluid to deal with the sloughing of the borehole wall. The drilling fluid has HTHP filtration rate of 8.4 mL, strong inhibitive and plugging capacity, and percent recovery of shale cuttings of 90% in hot rolling test. Plugging of 400 μm fractures with the drilling fluid renders the formation in which these fractures exist a pressure bearing capacity of 6 MPa. The drilling fluid also has good reservoir protection capacity. Field application shows that this new drilling fluid is able to effectively mitigate the tight hole and sloughing, reduce the rate of hole enlargement, and increase rate of penetration. Using this drilling fluid, no downhole troubles have ever happened, providing drilling fluid technical support to the safe and efficient drilling in the Fuxian block.