No abstract

The development of natural fracture will lead to lost circulation, which will be likely to cause economic losses and reduce drilling efficiency. In order to solve this problem, an oil-based pressure-bearing foam gel plugging agent for fractured shale was developed. The basic components such as foaming agent, foam stabilizer, and cross-linking agent were optimized and compounded, and the basic formula of the oil-based foam gel system was 0.6 % DSFA   foaming   agent + 0.3 % BPMO   foaming   agent + 0.5 % EPDM + 1.5 % SBS + 0.05 % rigid   cross − linking   agent   DB + 0.02 % modified   Si O 2   nanoparticles . Then, the performance of the oil-based foam gel plugging agent was evaluated through core plugging experiments, and the results showed that the prepared plugging agent had good stability and plugging performance with a plugging rate of about 90%.

Lost circulation is one of the great challenges during the drilling process as it can not only increase the risk of drilling operations but also cause an increase in drilling costs, thus greatly affecting the drilling efficiency. Wellbore strengthening has been widely used to prevent lost circulation, which ultimately expands the mud density window by increasing the formation fracture pressure. This paper proposes a combination of “preventive” wellbore strengthening and “remedial” wellbore strengthening to prevent leakage and plug and stabilize wellbores by means of summarizing the characteristics of lost circulation and wellbore instability in the Hasan area. The formula of the bridging cross-linking plugging agent is determined by experiments as well slurry + 8–10% granular material + 3–5% fiber material + 2–4% elastic material + 0.5–1% cementing material. The formula of the nano-film-forming plugging drilling fluid is determined to be 3% bentonite + 0.2NaOH + 0.2% KPAM + 3% SMP + 1% PB-1 + 2% SMNA-1 + 2% lubricant SMLUB-1 + 1% modified nano-SiO 2 particles. Then, the performance evaluation of the composite system is carried out by the high-temperature and high-pressure plugging simulation evaluation device. The results show that the bridging and cross-linking plugging agent can effectively block the 1–3 mm crack, the pressure-bearing capacity is greater than 10 MPa, and the anti-liquid return capacity is greater than 4 MPa. The nano-film-forming plugging drilling fluid has lower fluid loss and better rheological properties.