Studies of Pressures Generated Upon Refreezing of Thawed Permafrost Around a Wellbore

Abstract

Studies of heat transfer and of the mechanical properties of permafrost and an analysis of stresses around refreezing permafrost and an analysis of stresses around refreezing wellbores have been combined to develop a computer program that calculates refreezing pressures. Calculated values agree well with values determined in a large-scale field test. Introduction Since the discovery of oil at Prudhoe Bay in 1968, much new technology has been developed for dealing with oil production in arctic regions. Part of this new technology has been the development of successful methods for drilling and completing wells through permafrost. Several previously published papers permafrost. Several previously published papers have dealt with the thermal aspects of drilling or producing warm oil through frozen soil, and in those producing warm oil through frozen soil, and in those papers it has been pointed out that some thawing papers it has been pointed out that some thawing around the wellbore is generally expected. If a well is allowed to refreeze, casing damage may result from two mechanisms. First, fluids confined within pipes and allowed to freeze may cause pressure to rise, damaging casing. Second, refreezing of thawed permafrost or fluids external to the casing may develop permafrost or fluids external to the casing may develop pressures high enough to cause casing damage. This pressures high enough to cause casing damage. This paper considers only pressures resulting from paper considers only pressures resulting from refreezing of permafrost and fluids external to the casing.As ice melts, its volume decreases by about 9 percent. If the volume decreases within the earth, there percent. If the volume decreases within the earth, there is a tendency for pressure within the liquid phase to be low. This tendency is offset by movement of fluid into the thawed region. We envision five possible sources of liquid that could flow into thawed regions:drilling fluid filtrate,water from below the permafrost,water from near the surface,brine permafrost,water from near the surface,brine moving laterally through permeable material into the thawed region, andliquid within the thawed region that becomes rearranged by gravity flow to resaturate deep thawed regions. If a thawed region is saturated with water and then allowed to refreeze, the system volume increases as water is converted into ice. Since permafrost refreezes most rapidly near the surface, excess water may be trapped when deeper thawed regions refreeze. Pressures rise, thus forcing liquid water to flow away Pressures rise, thus forcing liquid water to flow away through permeable material; if this is not possible, pressures rise until the excess volume can be pressures rise until the excess volume can be accommodated in some other manner. To insure the use of casing with the proper collapse strength, a number of studies of refreezing pressures have been undertaken.Large-scale field tests have been conducted in full-size wellbores penetrating the permafrost at Prudhoe Bay Prudhoe BayThe mechanical properties of permafrost have been studied.Pressures generated during refreezing have been measured in laboratory models.Theoretical methods for calculating stresses and pressures have been developed. In the remainder of this paper we shall report the results of those studies. We shall show a favorable comparison between experimentally measured pressures and calculated pressures. Finally, we shall give calculated pressures for some field cases of interest. Field Tests Two full-size wellbores were drilled and completed through the permafrost at Prudhoe Bay. JPT P. 1159