Temperature Effects on Subsurface Equipment In Steam Injection Systems

Abstract

Abstract This paper deals with the effects of temperature change on tubing, casing and associated downhole equipment as the result of injection or production of fluid at temperatures substantially greater than the surrounding formations. A key to the stress analysis of the casing is knowledge of the equilibrium temperature distribution and the effects of mechanical buckling. A method for making an approximation of the casing temperature and for analyzing the buckling criterion is presented, which leads to an analysis of combined fiber stresses, and thus of conditions under which plastic failure occurs. The effect of temperature changes on tubing is also analyzed in the light of dimensional changes, compressive stresses and bending stresses due to buckling phenomena. The elastic and plastic behavior of the tubing is approximated to allow a prediction of steel grade requirements. Recommendations based upon the study are presented to aid in the selection of casing and tubing grades and other downhole equipment for steam injection systems. Introduction The reservoir studies and lab and field experiments conducted by the industry have resulted in the varied operating techniques used in steam injection programs today. These techniques are generally tailored according to their effects on the reservoir mechanism, and consequently the sub-surface equipment associated with the completion of the well is placed into an environment of extreme temperatures. These temperatures range between 400 and 600F and are expected to rise to 700F in the near future. Such conditions present a multitude of mechanical problems associated with completion and production, and certainly are outside the areas of conventional completion techniques and equipment. One of the keys to successful equipment is the designer's knowledge of the environment in which the equipment has to perform. As a rule, it is relatively easy to define this environment based on operational field data, but in the case of steam injection systems, where operating information is kept secret, it became necessary for the equipment manufacturers to establish design parameters on the basis of theoretical studies. This paper is a report of such a study covering the effects of the temperature changes on tubing, casing and associated equipment, effects which result from injection or production of fluids at a temperature substantially greater than that of the surrounding formations. Various heat-transfer models are described and one model, based on radial radiation, is presented together with its restrictions and limitations. This heat-transfer model permits the calculation of an average casing temperature after prolonged injection or production periods, which leads to evaluation of the thermal elongation and subsequent analysis of the elastic and plastic behavior of the casing string. Various schemes leading to the reduction of casing and tubing stresses, as well as mechanical problems associated with high-temperature requirements in downhole equipment, are discussed and examples and recommendations are presented. The mechanical problems associated with steam injection operations include seal design against water vapor at temperatures up to 700F and 3,000 psi. Design techniques which lead to the solution of these seal problems are summarized in this paper, and the summary should be of assistance in the selection of equipment for operational steam injection installations. Some of the theories presented here have found their way into design and application, while others still await realization. Little correlation with field data has been possible so far, especially in the case of the heat-transfer model and casing stress analysis. EFFECT OF STEAM INJECTION ON CASING AND TUBING The injection of steam causes rapid and high-level temperature increases downhole. Since the rate of linear expansion of the tubular goods is directly proportional to the rate of change of the average temperature, it is obvious that we must expect them to elongate. The magnitude of this has been physically observed when the wellheads of steam injection wells rose above the ground. Considerable changes in stress level of these elastically instable downhole systems will occur if thermal elongations are purposely or accidentally prevented. JPT P. 93ˆ