Abstract With the breakthrough in shale oil exploration in the Fengcheng formation, this unconventional resource has become the main potential contributor of successive hydrocarbon for the Mahu, Xinjiang Oil Field. However, this ultra-hard and highly heterogeneous shale formation, with CCS ranging from 190 to 300 MPa, low ROP, short drilling footage, frequent concentric or grooving drill bit wear, and a long drilling cycle during drilling the third interval of horizontal wells, posed a great challenge to the cost-effective development of this newly tapped resource. The integrated methodology has been developed to address the issues posed by this unique geological challenge. Three-dimensional drillability prediction for pre-drill formation has been achieved through the innovative development of an unsupervised neural network clustering method for drillability prediction, based on modelling that combines seismic, logging and real-time resistivity data. This method enables both ROP and formation drillability to be predicted along the wellbore trajectory, helping to specify customized optimization solutions for the trajectory and drill string design of vertical, curved and horizontal sections. Indoor drilling parameter sensitivity tests based on sampled rock cores indicated that the WOB threshold for the Fengcheng formation should exceed 100 kN, with increasing WOB serving as the primary stimulus for enhancing ROP compared to RPM. Inverted bottom hole assembly designs, including a modified down-hole motor, RSS and HWDP, which are applicable for adopting a high WOB, have been adopted to increase ROP during horizontal drilling. Furthermore, highly efficient drill bits have also been developed and customized based on drillability prediction results along the wellbore trajectory. These bits include both PDC and impregnated diamond bits with high impact and wear resistance characteristics. The vertical and horizontal distribution characteristics of the drillability of the Fengcheng formation could be clarified based on the developed prediction method, which categorized the sections into four groups based on actual resistivity and drilling ROP data. Curved sections in the trajectory have been deliberately designed to avoid ultra-hard formations. Drill string designs, including a 5-blade PDC bit + motor + MWD, a 6-blade PDC bit + RSS + motor, and an impregnated diamond bit + steerable turbine + LWD/MWD, have been tailored for sections with varying drillability. Using high WOB with the proposed optimized drill string designs increased ROP by 20–50%, with WOB increasing from 80–120 kN to 160–200 kN. Furthermore, customized bit design criteria were formulated for vertical, curved and horizontal sections, with the drilling footage increased by 12% to 200% and the ROP by more than 20% respectively. Improved drilling performance was observed continuously during four rounds of horizontal wells with the implementation of this developed integrated methodology. The average drilling cycle was shortened from 119 days to 77 days with average ROP being increased from 4.4m/h to 7.2m/h. By summarizing the challenge, methodology and achievement during horizontal well drilling in this ultra-hard and highly heterogeneous shale formation, this paper provides a valuable reference for the cost-effective exploration and development of the similar unconventional hydrocarbon resource.
Zhang et al. (Mon,) studied this question.