Analysis of Three Methods to Reduce Bottom Hole Temperature While Drilling in Deep Shale Gas Well: Circulation, Ground Cooling, and Mud Density Reduction

Abstract

Abstract During deep shale gas well drilling process, high bottom hole temperature (≥150°C) often causes the conventional rotary steerable tool and logging while drilling (LWD) to malfunction. Therefore, it is essential to reduce the bottom hole temperature for safe and efficient drilling. In this paper, a simplified wellbore temperature model for deep shale gas horizontal well was established based on thermodynamic theory, comprehensively accounting for both mechanical and hydraulic heat source. According to the temperature model, a comparative and analytical investigation is conducted on the applicability range of three cooling methods: circulation, ground cooling, and density reduction. Building upon these research findings, a density reduction scheme is proposed and subsequently implemented on the H29 platform in Luzhou block for field application and cooling effect evaluation. The results indicated that the temperature model is in good agreement with real-time temperature data with an error less than ± 5 %. Variations in mud density also lead to changes in rheological parameters, consequently impacting bottom hole temperature. If the horizontal section is shorter than 1000 m, reducing the inlet temperature can effectively reduce the bottom hole temperature. But once the horizontal section exceeds 1500 m, reducing mud density is much more effective. With field application on H-29-1 and H-29-6, mud density was reduced from 2.25 g/cm3 to 1.80 g/cm3, resulting in a maximum bottom hole temperature of 143°C in the horizontal section, and rotary steerable tools and screw worked stably in downhole. Compared to the other wells that bottom hole temperatures are all above 152 °C (up to 155.7 °C), the maximum bottom hole temperature decreased by 9-14°C. Furthermore, in the horizontal section, the pure drilling time is significantly reduced, no overflow, sticking and other accidents occur, and the Rate of Penetration (ROP) is increased by more than 45 %, the pump pressure is reduced by 2-5MPa. All phenomena show that lower mud density can not only effectively reduce the bottom hole temperature, but also improve the drilling efficiency, which has positive significance for the development of deep shale gas.