Transient Thermal Model of Drilling Fluid in Wellbore under the Effect of Permafrost Thaw during Drilling in Arctic Region

Abstract The arctic could hold about 30% of the world's undiscovered gas and 13% of the world's undiscovered oil according to an assessment by the United States Geological Survey (USGS). While, arctic oil and gas exploration is faced with various challenges such as the extremely cold polar environment, the ice scour, the permafrost. Permafrost is the perennially freezing soil (actually freezing pore water), and it can be an important geologic hazard and constraint. Permafrost is very common in arctic region, about one-half of the total land area of Canada and Russia and 85% of Alaska is underlain by permafrost. Besides, sub-bottom permafrost is also wide spread phenomenon of Arctic offshore areas. At present, the traditional thermal models of wellbore use an algebraic approximation to describe the heat transfer between wellbore and formation. Thus a new model should be established considering the effect of permafrost thaw during drilling in arctic region. As a matter of fact, the phase change of ice in permafrost needs to absorb heat from wellbore which can have an effect on the wellbore temperature. At the same time, migration of water from warm to cold regions will happen during the process of the thaw of permafrost. The migration of water will have an effect on the temperature due to the heat transfer along with the water. In addition, temperature in wellbore can be negative after long shut-in time due to the extremely cold polar environment which could lead to the blocking of wellbore. In this paper, a transient thermal model of wellbore during arctic drilling is established considering the effect of permafrost thaw in this paper. On the basis of the new model, a simulation of an arctic well is made and some conclusion are made from the case study: The temperature fields in wellbore and permafrost interact with each other. Besides, the temperature in wellbore drops to negative due to the extremely cold environment after long shut-in time, and the drilling fluid in wellbore could be frozen consequently. Long shut-in time should be avoided during drilling in arctic region. The new model established in this paper can determine more reasonably the wellbore temperature in artic permafrost region than the traditional model. A method to determine the reasonable shut-in time is given in this paper which can provide safety guidance during drilling in arctic permafrost region.