Permafrost Thawing Around Producing Oil Wells

Abstract

Abstract A mathematical thermal model for predicting the time dependent temperature distribution surrounding a production well is described. The model provides for two-dimensional radially symmetrical and vertical heat conduction in the surrounding earth. Heat transfer from the produced fluids to the earth by conduction, convection and radiation is accounted for through an over-all heat transfer coefficient. The latent heat of fusion of water is included in the Model. The boundary of the melted zone at any time corresponds to the location of the 32 °F isotherm at that time. This boundary can be determined for various well completions either with or without insulation protection. Thus, the need for insulation can be determined as well as the effectiveness of various insulation schemes. Typical calculated results are presented for a well completed through 1,400 feet of permafrost with the top 700 feet of tubing insulated. INTRODUCTION THE DISCOVERY OF OIL IN ARCTIC REGIONS has introduced many new and unique problems which must be dealt with successfully before that oil can be brought to market. One anticipated difficulty is the thawing of the permafrost around the producing oil wells in these regions. This thawing is caused by the production of large volumes of hot fluids (possibly at 200 °F bottom-hole temperature) for extended periods of time. Any thawing that occurs at the cement-permafrost interface will destroy the cement bond. Thawing of the permafrost in the layers near the surface will result in sloughing of the soil around the wellbore. The ensuing mechanical stresses imposed on the casing could result in irreparable damage and necessitate premature abandonment of the well. In view of the tremendous drilling and completion expenses encountered in the Arctic, this is obviously undesirable. There are two possible approaches to solving this problem. One is to prevent thawing of the permafrost through a section thick enough to maintain a competent well completion. The other is to allow permafrost thawing and design a well completion which will be compatible with the adverse conditions resulting there from(1). For the first of these solutions there are again two possible means for accomplishing this goal. One method is to adequately insulate the production string through some portion of the permafrost zone. The second method is to refrigerate the casing string through some portion of the permafrost zone. In order to evaluate the alternatives of insulating the production string, a mathematical model giving a detailed description of the heat flow in and around the producing well is required. Previous models of wellbore heat flow(2,3) have neglected vertical heat flow in the earth surrounding the well, and the latent heat of fusion was not included in their energy balance. The use of these older models must be considered as only approximate solutions for the thawing problems treated here. A mathematical model has been developed which can predict the extent to which thawing will occur in unprotected wells and can evaluate the effectiveness of various schemes of insulating the production string.