Application of the Darcy-Stefan model to investigate the thawing subsidence around the wellbore in the permafrost region

Abstract

A key challenge in Exploring oil and gas resources in permafrost regions is to predict the thawing subsidence of frozen soil around the wellbore. At present, the traditional thermo-hydro-mechanic (THM) model calculates thawing subsidence without actually considering the spatial-temporal evolution of the thawed-frozen boundary. The Darcy-Stefan model is used to describe the internal mobile interface problems in the heat transfer of saturated porous media due to ice/water phase transitions. In this paper, we established the coupling model of the Darcy-Stefan model and the elastic theory of porous media, including boundary conditions of heat transfer and Darcy flow on the moving boundary. The new model was validated through a series of small-scale examples and a test well in the Arctic region, which demonstrated the predicted subsidence amount of ground surface around the wellhead was in good agreement with the field test data. Further research indicated that with the increase of the thawing zone, the accuracy of calculation for the thawing subsidence could be significantly improved by using the THM model and considering the internal moving boundary. The models and conclusions in this paper can help design more reliable wellbore in permafrost regions throughout the expected life.