Wellbore Stability Technology of Fractured Carbonate Formation Drilling Fluid in Shunbei Region

Based on the geological analysis of the carbonate rock formation in Shunbei oil and gas field, the reasons for the formation's easy wellbore instability are clarified: first, the rock is easy to be broken and the fracture develops, which is easy to have secondary fracture development, malignant leakage and even well collapse; Second, the bottom hole temperature is high, and the drilling fluid treatment agent is easy to fail at high temperature; Third, the plugging capacity of the currently used polysulfonate drilling fluid is insufficient, which can’t effectively plug formation cracks and reduce pressure transfer; Fourth, the cementation ability of the current drilling fluid is insufficient, which can’t effectively improve the compressive strength of the near-wellbore rock. To solve the above difficulties, a high temperature resistant cementing sealer AD-1 was synthesized from acrylamide, dimethyldiallyl ammonium chloride, 2-acrylamide-2-methylpropanesulfonate sodium and dopamine hydrochloride. The cementation and plugging properties of the cementing agent were evaluated. The experimental results show that the axial compressive strength of the cemented dry carbonate sand beds is up to 2.5-5.0 MPa at 180℃, and the compressive strength is increased by more than 4 times. The compressive strength of the unconsolidated wet sand beds is 0 MPa, and the compressive strength of the consolidated wet carbonate sand beds is increased to 0.2-0.5 MPa. After adding the cementing sealer, the plugging ability of the polysulfonate drilling fluid was significantly improved, and the drilling fluid could effectively plug the sand beds composed of 40-60 and 60-80 mesh carbonate debris, with the maximum pressure ≥6 MPa, and the cumulative loss in 30 minutes was about 10 mL. In addition, the viscosity of the drilling fluid will increase dramatically when the additive amount exceeds 1.0%, and the additive amount should be controlled as appropriate. Therefore, the polysulfonate drilling fluid system with AD-1 as the core has good temperature resistance, cementation and plugging properties, which can provide strong technical support for borehole stability technology in Shunbei region.