Transient Pressure Behaviour Of A Heavy Oil Reservoir Undergoing Steam Injection In The Presence Of A Bottom-Water Zone
Authors: Gomes E. , Ambastha A.K. 
Published: March 2010

Abstract
There are numerous heavy oil reservoirs in Canada which are underlain by bottom-water zones and are undergoing steam injection. Steam may be injected in the reservoir or the bottomwater zone, depending on the injectivity considerations. Due to gravity override effects, zones swept by steam in these reservoirs may have irregular shapes.
Using a new analytical solution for a multi-layer, composite reservoir model, this study investigates transient pressure responses for partially-penetrating, steam injection wells with irregularly-shaped fronts in the presence of infinite or finite bottomwater zones. Pseudosteady-state crossflow is allowed among adjacent layers to model the vertical flow of fluids. Pressure and pressure derivative responses have been studied to determine the effects of penetration ratio, crossflow parameters, mobility ratio, storativity ratio, and dimensionless front radii in different layers. Results of this study demonstrate the variety of well-test responses expected in such complex scenarios. Also, this study outlines the difficulties associated with the traditional pseudosteady-state approach to estimate the swept volume in the presence of bottom-water zone effects. However, this analytical transient-pressure model should prove helpful in automated (or automatic) type-curve matching analysis of well-test data obtained from thermal recovery operations with and/or without bottom-water zone effects.

Introduction
Steam injection process is widely used in heavy oil recovery operations. As a result of steam injection, at least two regions of different fluid properties are created and the reservoir resembles a composite reservoir. Because of gravity override effect, an inclined fluid front is created between the hot and the cold zones. Many times, heavy oil reservoirs are accompanied by a bottomwater zone. The purpose of this study is to investigate the transient pressure behaviour of a steam-stimulated heavy oil reservoir under bottom-water conditions.
A reservoir undergoing a thermal recovery process has been idealized as a composite reservoir for a long time(l-6). But most of the studies consider-piston-like movement of the fluid front, neglecting the gravity override effect. Satman(7) used the concept of a tilted (inclined) front in pressure transient analysis of a two layer, composite reservoir. He proposed that fluid front would propagate at different rates in different layers. For steam flooding, Satman and Oskay(8) considered the discontinuity boundary as a tilted front to account for the gravity override effect and modelled the reservoir as a multi-layer, composite reservoir without crossflow. They concluded that the tilted-front model is a better representation of the actual reservoir.
According to published reports on steam-drive projects(9,10), gravity override effect is a common phenomenon which results in a tilted front between the swept and the unswept zones. If gravity override effect is not taken into consideration, the predicted performance of the steam flooding project may be quite different than the actual performance. Singhal(l1) conducted some scaled physical model studies of steam-flood in a pool containing heavy oil. He presented some temperature profiles obtained from his model which showed very strong gravity override effect. Blevins et al.(9) discussed the application of steam-flooding for light oil reservoirs.