Integrated Drilling and Evaluation Solutions to Mitigate Drilling Hazards and Improve Drilling Efficiency in the Mahu Oil Field - A Case Study from Western China

Abstract Drilling of development wells in highly heterogeneous conglomerate reservoirs in the Mahu Oil field, Western China, has been challenging due to long horizontal sections, the complex lithology and unfavorable stress conditions. Most of the wells with horizontal lengths of between 1200m and 2000m are associated with frequent hole instability-related problems such as tight hole and pack-off in the build section. The horizontal conglomerate section usually experiences an extremely low rate of penetration (ROP) and severe mud losses. To overcome the drilling challenges, an integrated multi-disciplinary solution was applied with geomechanical understanding of the related drilling issues as the basis. Drilling experiences from offset wells were reviewed to identify the possible root causes of the drilling problems. A detailed geomechanical model was constructed to understand the stress and rock mechanical property characteristics of the field. These geomechanical parameters are required, in conjunction with the planned well trajectory, to define the mud weight window for the 6½″ hole section which should prevent borehole collapse and mud losses. Drilling fluid formulation has also been optimized based on rock failure mechanisms and the recommended mud weights. Additionally, an advanced rotary drilling system was used to achieve better hole shape, increase the ROP and monitor real-time drilling parameters. One typical drilling case is presented in this paper. The case is characterized by the high risk of instability associated with a few shaly intervals in the overburden build section which have been known to cause serious wellbore instability-related problems. In the reservoir, the presence of natural fractures narrows the safe mud weight window due to the high collapse pressures of shale inferred from the geomechanical analysis and the low fluid-loss gradient of the natural fracture networks. The drilling fluid formulation has to be optimized to meet the requirements of maintaining wellbore stability, preventing mud losses, increasing ROP and decreasing drilling flat time. Additionally, optimization of the drilling bit was conducted based on the variations in rock mechanical properties to increase the ROP and reduce vibration. Finally, an optimized Rotary Steerable System (RSS) Bottom Hole Assembly (BHA) was used to obtain a smooth wellbore and a high exposure rate. This integrated drilling and evaluation solution approach not only successfully reduced the drilling hazards associated with wellbore instability, it also increased the ROP significantly. With the attainment of major flat time reduction and lesser rock failures, drilling efficiency as well as borehole quality had been enhanced. All these drilling performance matrices can be further improved with additional experience to be gained from future drilling in the block. The implementation of the integrated solution culminated in a very successful case and established the best drilling record in the block.