Fast drilling technology of ultra deep wells in the Sinian reservoirs in the Leshan - Longnüsi palaeohigh, Sichuan Basin

Abstract

The Sinian strata of Leshan - Longnüsi palaeohigh in the Sichuan Basin are of 5, 000 - 6, 000 m depth underground, where the drilling rate has been restricted by poor drillability of formations, multiple pay zones & multiple pressure systems, and other complex geological conditions. In view of this, scientific research and field trials were performed, focusing on the wellbore structure, specific design of a drill bit, and the innovative organic salt drilling fluid system, thus a fast drilling technology model was formed applicable to this region. a. The wellbore structure is optimized; compared with the conventional one, the length of the large size borehole is shortened and the operating time is reduced; additionally, the requirements for well completion and later-stage production are fulfilled. b. The PDC bit is particularly designed to increase the penetration rate (ROP) by 31 - 473%; with the rigid PDC bit, the average ROP in the Shaximiao Fm is up to 16.57 m/h, close to the effect of air drilling; with the composite drilling of the PDC bit and the equal-wall thickness screw rod with a long service life in proper well sections, the ROP is increased by 44% higher than that by the normal rotary drilling. c. With the use of organic salt and a low-solid content, the innovative fluid system has a much higher inhibitive capacity compared with the polysulfonate drilling fluid and the content of sub-micron particles is also decreased, thus the ROP is improved and the well wall is more stable than that when using the polysulfonate drilling fluid. With the above techniques, the average ROP of 6 deep wells drilled and completed in this study area in 2012 was increased from 1.69 to 2.54 m/h, and the average drilling cycle was shortened from 302 to 189 days.