Key technology and practice of horizontal drilling for marine deep shale gas in southern Sichuan Basin

The southern Sichuan Basin has abundant marine deep shale gas resources, which is a critical replacement for the current and future shale gas development. However, drilling of deep shale gas faces numerous challenges, such as inadequate thermal tolerance of steering instruments and drilling fluids, high difficulties in well trajectory control and clean-up of wells and tools, low penetration rate, and high risks of downhole complex issues, due to the geologic characteristics of deep shale gas (numerous sequences, high formation temperature, high formation pressure, and thin high-quality reservoirs). Given this, we managed to develop the key technology for horizontal drilling of deep shale gas through multiple efforts. The 165ºC-tolerant rotary steerable system was selected to reduce runs and facilitate time-efficient drilling, and moreover, the drilling fluid surface cooling system was deployed to ensure the normal functioning of downhole tools. The PDC bit with efficient rock-breaking was developed to raise the penetration rate. The managed pressure drilling was adopted to reduce the drilling fluid density. The dynamic weighting device of cuttings and degradable cutting-laden fibers were developed for efficient well clean-up. At last, the geology-engineering-integrated steering mode was introduced to accurately identify the lithology and stratigraphy of formations at the bit. The field application shows the drilling cycle of a well in the L203 well district is shortened from 199 d to 82.6 d; that of the Z201 well district, from 239.5 d to 118 d; the encounter ratio with the platinum target averages 92.8%. The proposed key technology for the deep shale gas horizontal drilling provides guidance and demonstration for future ROP-enhancing and efficiency-improvement of deep shale gas drilling.