Steam Corefloods With Concurrent X-Ray Ct Imaging

Abstract

Abstract A series of steam corefloods has been performed utilizing X-ray CT imaging to monitor phase saturations during the floods. A numerical simulator was used to analyze the experimental results. The steamfloods of initially water-filled sand packs were carried out at low enough flow rates that capillary pressure was comparable in magnitude with viscous pressure drop. Numerically marching the observed pressure drops provided a significant test of the assumed steam-water capillary relationship. A capillary pressure curve of Leverett j-function form was found suitable for the simulations. The steam relative permeability curve was found by numerically matching both pressure drop and CT saturation data. The expected steam frontal behaviour was seen in the CT images, but the images also revealed unexpectedly large variations in water saturation transverse to the steam propagation direction. Although core porosity derived from the CT images showed no significant variation across the core, it was hypothesized that small variations in steam-water capillary pressure could have resulted from more subtle core packing variations. The expected type of core heterogeneity was subsequently confirmed by subjecting the core to petrographic image analysis. Numerical simulations embodying a realistic degree of core heterogeneity were able to reproduce the transverse saturation variations as a consequence of capillary crossflow. Introduction The steam displacement or ‘steam drive’ method is one of the major techniques for thermal recovery and has been intensively studied both theoretically and experimentally. Particularly important from a theoretical standpoint was the work of Miller(1) showing that the frontal propagation of steam was far more stable than that of a non-condensible gas. Although many analytic methods have been developed and used for simplified predictions(2, 3, 4), in most practical situations a numerical simulator is employed to model steam processes. A fundamental physical property required for the simulation of steam-based processes is the relative permeability of steam. Customarily, numerical models treat steam flow in the same manner as the flow of any ocher reservoir gas. As noted by Sanchez and Schechter(5) there is little direct evidence to support this assumption. Indeed, the issue of steam relative permeability has generated debate in the literature right up to the present. For example, Sanchez and Schechter(5) and Closmann and Vinegar(6) present experimental results confirming the similarity between steam and non-condensible gas relative permeabilities. Verma et al.(7) however, find that steam relative permeability is significantly higher than that of a non-condensible gas. The latter authors further argue that it is reasonable to expect such differences because of the possibility of phase transformation in the flow channels. Because of the diversity of opinion surrounding the basic issue of relative permeability, it is important to investigate steam flow using a range of tools. In particular, the technique of X-ray computed tomography (CT) provides the possibility of monitoring fluid saturations while processes are taking place in a laboratory coreflood experiment. This relatively new technique in petroleum research has already proven itself to be very useful in a wide range of experimental studies(6, 8, 9, 10).