Optimal Selection of PDC Bit Cutters for Soft‐Hard Interbedded Strata in Tight Gas Reservoirs in Central Sichuan

ABSTRACT
The second drilling section in tight gas reservoirs of central Sichuan is characterised by alternating soft and hard formations, leading to significant variations in lithology, rock mechanical properties and drillability. These geological complexities frequently result in PDC bit tooth chipping and ring cutting, severely limiting the drilling speed in the upper strata. To address these challenges and achieve rapid and efficient drilling in the soft‐hard interbedded formations of the upper sections in tight gas reservoirs in central Sichuan, a dynamic simulation model for PDC cutter rock breaking was established based on the engineering geological characteristics of the second drilling section. The rock breaking patterns of eight commonly used types of PDC cutters were systematically evaluated. The selection of cutters was guided by the stress distribution, maximum penetration stress and continuous chip stress variation range of the cutters. A PDC bit suitable for soft‐hard interbedded formations was developed and field applications were conducted. The main findings are as follows: The rock breaking process by the cutter is divided into two stages, cutter penetration into the formation and continuous rock breaking. The more concentrated the stress on the centre of the cutter, the greater the penetration stress and the smaller the continuous chip stress variation range, the stronger the aggressiveness and vibration resistance of the cutter. The new ridge‐shaped cutter, ridge‐shaped cutter and concave cutter are recommended for the soft‐hard interbedded layers. An optimised PDC bit with a three‐level composite cutting technology of new ridge‐shaped cutter + cylindrical cutter + cone‐shaped cutter was developed, and field application resulted in one trip for the second drilling, saving four trips compared to the same platform wells, with a drilling speed increase of 38.49%, demonstrating significant speed improvement. The research findings provide theoretical and technical support for the speed enhancement in the upper soft‐hard interbedded strata of tight gas reservoirs in central Sichuan.