Simulation of Temperature in Gas Drilling with Gradient Method

Abstract

Accurate prediction of gas temperature profile is critical for designing gas drilling operations. Traditionally the gas temperature distribution in wellbore is assumed to equal the static formation temperature where the influences of fluid flow and Joule-Thomson cooling effect have not been considered. This work presents a gradient equation method to predict the gas temperature profile in the wellbore with considerations of gas flow and Joule-Thomson local cooling at drill bit. The method applies equations of pressure, temperature, density and velocity to gas flow inside drill string and annulus. The solution of the gradient equations takes the form of the fourth order Runge-Kutta equation. Bottom hole temperature measurement from depth 700m to 2000m in a real well is found consistent with the temperature values predicted by the gradient method, both showing 20℃ ~ 30℃ temperature drop due to the Joule-Thomson cooling effect at the bit nozzles. Result of sensitivity analyses with the gradient method indicates that the temperature drops due to the Joule-Thomson cooling effect can be up to 50℃, depending on nozzle size. The steady temperature profile can be established within minutes of gas circulation. Because of the influences of gas flow and Joule-Thomson cooling, the gas temperature in the wellbore significantly deviates from the formation geothermal temperature in flowing conditions. The gas temperature in the drill string increases with the well depth and then decreases rapidly toward the bottom of hole. As the gas flows up the annulus, the gas temperature first jumps up, overshoots the formation temperature, and then gradually decreases along the geothermal gradient trend.