Theoretical analysis and field application of cooling bottom hole by mud density reduction during drilling in deep shale gas well

During deep shale gas well drilling process, high bottom hole temperature (>= 150 degrees 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. A quantitative analysis was performed to assess the impact of rheological parameters, engineering parameters, thermal parameters and two cooling methods on bottom hole temperature. In addition, a density reduction and cooling scheme for drilling in the horizontal section was developed and implemented on H-29-1 and H-29-6 Well. The results indicated that the temperature model is in good agreement with the Kirbir & Hasan model and real -time measurement data with an error less than +/- 6 %. Mud density, behavior index, pump rate, weight on bit (WOB) and geothermal gradient had great influence on the bottom hole temperature (up to over 30 degrees C). 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 H29-6 Well, mud density was reduced from 2.25 g/cm 3 to 1.80 g/cm 3 , resulting in a maximum bottom hole temperature of 143 degrees C in the horizontal section, and rotary steerable tools and screw worked stably in downhole. Compared to adjacent wells on the same platform that bottom hole temperatures are all above 152 degrees C, the maximum bottom hole temperature decreased by 9 - 13 degrees C. Furthermore, in the horizontal section, the Rate of Penetration (ROP) is increased by more than 48 %, the pump pressure is reduced by 2 - 5 MPa, and the average hole enlargement rate is only 1.92 %. All findings 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.