High Temperature Naptha to Replace Steam in the SAGD Process

Abstract

Abstract World-wide experience indicates that thermal methods are the only effective methods for the in-situ recovery of heavy oils. Furthermore, the steady progress within horizontal well technology increases the applicability of thermal methods in conjunction with horizontal wells. The steam-assisted gravity drainage (SAGD) process is the prominent example of the synergism of horizontal wells with steam injection. The major portion of the dollar per barrel operational cost for steam injection is currently attributed to the generation of steam and water handling requirements. In the future, more stringent environmental regulation might further prohibit the use of water for the recovery of heavy oils. Naptha is present at the site as a diluent for pumping and pipe line transport of the produced heavy oil. The injection of naptha vapour instead of water vapour combines an effective thermal process with the diluent mechanism of the naptha. We present the results of simple analytical calculations, demonstrating the applicability of naptha-assisted gravity drainage (NAGD) compared to the SAGD process. Numerical simulations have been performed on a two-dimensional cross section perpendicular to the horizontal well pair. We discuss in detail the reservoir mechanisms present during NAGD. For the conditions studied naptha circulation rates are some 14 times the bitumen production rate. Evaluation of the process shows that it will be economic with naptha recycling and by minimising the naptha remaining in the reservoir. Introduction Heavy oil and bitumen resources can more than double the earth's hydrocarbon reserves. Unfortunately, most heavy oil and bitumen accumulations cannot be produced economically with current in situ recovery technology. Canada owns some 40 per cent of these unrecoverable resources. The emergence of horizontal wells allowed a larger reservoir exposure, which increases productivity, accelerates recovery, and reduces coning tendencies. For example, horizontal wells are used, at present, for the primary recovery of heavy oil and bitumen in Canada. The combination of thermal methods and horizontal wells for heavy oil recovery increases the potential for more economic operations. In the SAGD process, heat is injected in the form of saturated steam through a horizontal injector. The injector is located above and more or less parallel to a horizontal producer. The distance between the two is dictated by early-time communication considerations. The SAGD process has proven successful at the UTF site in Alberta, Canada. As shown at the UTF site, an optimised process demands that the wells are regulated in such a way as to let the drainage mechanism operate freely. Hence, fluids need to be produced when they arrive at the producer, and injection rates need to satisfy the heat demand of an expanding steam zone. At the same time, a liquid level has to be established above the producer to limit the production of steam. Butler has given a basic description of the SAGD process. P. 475