Application of high-density plugging oil based drilling fluid in Well Gaotan-1 of Xinjiang Oilfield

Well Gaotan-1 is one wildcat well drilled in the Xinjiang Oilfield in 2018, and it is located at the Gaoquan east anticline of Sikeshu Sag in the thrust belt along the southern margin of the Junggar Basin. In this well, there are faults and fractured belts in the Taxihe Formation and high-pressure water layers in the Tuguluqun Formation, and Anjihaihe Formation is of extremely strong water sensitivity, so its drilling tends to suffer borehole instability, pipe sticking and frequent downhole complexities. The technical difficulties in the drilling process of oil based drilling fluid were analyzed based on the geological characteristics of Gaoquan east anticline, and the corresponding technical countermeasures were proposed. Then, the formula of the high-density plugging oil based drilling fluid was experimentally determined and its properties were experimentally analyzed and evaluated. The experimental results show that the loss of this high-density plugging oil based drilling fluid at high temperature and high pressure is 2.8 mL after 30 min, 2.9 mL after 60 min and nearly 0 after 30 min. Under the action of this drilling fluid, the expansion ratio of Anjihaihe Formation mudstone is 0.7% and its progressive cuttings recovery is 97.6%. It is resistant to 10% cuttings, 15% water, 4% cement, 15% NaCl and 3% gypsum contamination. Under the temperature of −30 ℃, its funnel viscosity is 126 s, its initial gel is 8 Pa and final gel is 16 Pa. The field application indicates that high-density plugging oil based drilling fluid can solve the drilling difficulties at the Gaoquan east anticline of Sikeshu Sag in the thrust belt along the southern margin of the Junggar Basin, and its efficiency improvement effect is remarkable with complex accident ratio dropping to 1%, drilling cycle shortening by 50% and composite drilling cost reducing by 50%. Finally, it was suggested to apply this drilling fluid in ultra-deep wells in complex strata, e.g. those with strong water sensitivity or high ground stress along the southern margin of the Junggar Basin.