Borehole Strengthening Technology with Oil-Based Drilling Fluid in the Western Sichuan Basin

Wellbore collapse, lost circulation and sticking frequently happen in deep formations when drilling in the western Sichuan Basin. When drilling with oil-based drilling fluids, it is difficult for a thick filter cake to form. Plus, the low density of the filter cake leads to high pressure penetration ability at the wellbore. To solve this problem, based on the mechanism of drilling fluid film forming, a wellbore protection agent with the compact film, CQ-NFF, was synthesized from styrene, acrylate and other raw materials using the core-shell structural design, which is suitable for the middle layer with a temperature no more than 150 °C in deep well drilling in the area. According to the analysis results of the geological characteristics and engineering characteristics of high-risk formations and the d₉₀ rule, an inner-filling plugging agent was formed from ultra-fine grains of calcium carbonate, elastic graphite, high strength resin and highly dispersed fiber. The film-forming mechanism and morphology of drilling fluid were analyzed by scanning electron microscope (SEM). Film forming effects were evaluated in filtration tests under high temperature and high pressure along with water and oil loss tests during drilling penetration at high temperature. By means of the sand bed filtration experiments under high temperature and high pressure, the plugging capacity of the filling material was evaluated. Experimental results show that CQ-NFF can form a compact film in water-in-oil emulsion and oil-based drilling fluid, and the film can be adsorbed on the surface of the filter cake effectively. The pressure resisting and plugging capacity of the film can reach 2.0 MPa. After the inner-filling plugging agent was added to the oil-based drilling fluid, the pressure bearing capacity of the inner filter cake was increased to over 3.5 MPa. The borehole strengthening technology with oil-based drilling fluid, treating chemicals as the core, was applied in Well Shuangtan 6 and Well Zhongjiang 2. The results show a remarkable application effect that the downhole downtime due to lost circulation in deep formations, wellbore collapse or drill pipe sticking were greatly reduced.