Prediction model of drilling wellbore temperature considering bit heat generation and variation of mud thermophysical parameters

Downhole tools are at risk of high-temperature failure during drilling operations in deep and ultra-deep wells. To solve this problem, researchers have developed temperature models for various drilling conditions to determine the wellbore temperature distribution. Unfortunately, few researchers have studied downhole tool temperatures compared to wellbore fluid temperatures. To the best of our knowledge, very few models have taken into account both the bit heat generated and the axial heat conductivity in the drill pipe to accurately predict downhole operating tool temperatures. In this study, a transient temperature model coupled with mud-drillpipe-casing-cement-formation is developed. Polycrystalline diamond compact (PDC) bit frictional heat generation equations and fitted curves of drilling fluid thermophysical parameters with temperature and density are considered in the model. Then, the effects of mud systems, density, flow rate, inlet temperature, rotary speed, and weight on bit (WOB) on the wellbore temperature and downhole tool temperature are discussed, based on the example of a well in Sichuan, China. The results of this study are an important guide for downhole tool selection and drilling parameter design. The model developed in this study has been used to guide the field operation of several oil wells in Sichuan and Qinghai.