An Oil-Based Drilling Fluid with Varied Particle Size Plugging Agents for Use in the Borehole Collapse and Mud Loss Coexisting Wells in Hongxing Block

The Hongxing block, a main gas production area of the Jianghan oilfield in the Jiannan structure, is an important shale gas development area of Sinopec. After many years of development, the matrix stresses of the Feixianguan formation and the Changxing formation become very low, and the Wujiaping formation is highly developed with nanometer-sized fractures and fissures. When drilling with oil-based drilling fluids, mud losses and borehole wall collapse take place frequently and are difficult to deal with. To solve these problems, an oil-based drilling fluid with particles of varied sizes to form dense plugging is developed. This oil-based drilling fluid is characterized with strong plugging capacity, low activity, low viscosity and high gel strengths. It is formulated with 4% nanometer- and micrometer-sized rigid particles, 0.5% graphene flake material, 1.5% spherical gelled MPA, 1% ultra-fine mineral fibers. These materials work together to plug and seal the micrometer-sized and nanometer-sized fractures in the formations and coat the borehole wall with dense mud cakes, thereby controlling the high temperature high pressure filter loss of the drilling fluid below 3 mL. High concentration CaCl₂ solution (≥ 36%) as the water phase of the oil-based drilling fluid lowers the activity of the drilling fluid to below that of the formation waters and thus improves the inhibitive capacity of the drilling fluid. The emulsifier used has the ability to increase the gel strengths of the drilling fluid to the required level, hence effectively carrying the drilled cuttings and the sloughing out of hole. In field operation, this drilling fluid effectively mitigates the severity of mud losses and borehole wall collapse. An obvious application result is that the time spent in dealing with downhole troubles is reduced by 90.34%.