Well-formation transient heat transfer model during drilling of deep shale gas horizontal wells

At present, the service environment temperature of rotary steering tools commonly used in deep shale gas wells is 135 °C. Accurately predicting the circulating temperature during the drilling of deep shale gas horizontal wells plays an important role in scientifically extending the service life of downhole measurement tools and effectively extending the length of horizontal sections. Based on the principle of energy conservation, considering radial and axial heat transfer mechanisms among well, fluid in the drill string, drill string wall, annular fluid, and formation near the well, a well-formation transient heat transfer model was established. Solve the mathematical model with fully implicit finite difference method, and analyze the influence of various sensitive factors on the well temperature. The research shows that the annular fluid temperature is higher than the original formation temperature in the initial period of fluid circulation in shale gas horizontal wells; the increase in circulation time, fluid specific heat capacity and density, and the decrease in inlet temperature can reduce the bottom hole temperature; excessive displacement will cause heat generation because of friction, making well temperature rise; reducing the inlet temperature through the surface cooling device can offset the heat generated by the extended distance of the horizontal section, prolonging the safe drilling distance of the horizontal section. Combined with the data from Well Y101H26-1, when the inlet temperature is 35 ℃ and 65 ℃, the temperatures corresponding to the drilling depths of 5 650 and 5 250 m do not exceed the service environment temperature of the rotary steering tools, and the calculation error is within 3%. The research results can overcome the blindness and inefficiency in applying well cooling engineering technology.