Waterflooding Reservoirs With a Water Leg Using: the Dyn.amic Blocking Process

Abstract

Abstract In many light or moderately viscous oil (1 to 200 mPa.s) reservoirs in Alberta and Saskatchewan, a high water saturation zone of varying thickness and extent, referred to as water leg or bottom water, occurs in communication with the oil zone above. As a result, the primary production period is short, and water coning occurs early (six to 12 months) in the life of the reservoir. Later, during the secondary recovery stage, such a zone can have an adverse effect on the waterflood efficiency. This paper addresses the problem of waterflooding such reservoirs. The presence of a bottom water zone results in poor vertical sweep efficiency due to water channelling through the bottom water zone. A mathematical model was developed to account for cross fIow, and experiments were carried out in a two-dimensional model to test the theory. Many techniques have been proposed to improve waterflood performance in reservoirs under bottom water conditions. Most of these are based upon the use of chemicals to plug the bottom water zone. However, due to crossflow, part of the chemical slug often migrates into the oil zone. Thus, the injection strategy becomes an important factor for maximizing mobility control. A new displacement process, the Dynamic Blocking Process (DBP), was developed which aims at minimizing crossflow between the oil and bottom water lanes. This process is unique in that displacement and blocking processes operate simultaneously. Experimental results showed that DBP yielded approximately 15% higher oil recovery compared to a waterflood conducted after injection of a single emulsion slug into the bottom water zone. Introduction Waterflooding oil reservoirs with a communicating bottom water zone is usually inefficient due to the flood water channeling into the bottom water zone. This problem was first recognized in the early sixties when Barnes(1) suggested the use of a viscous water slug as a mobility control agent to lower the WOR. Since then, the use of various chemicals, such as polymers and emulsions, has been proposed aas mobility control and blocking agents to improve waterflood performance(2,4). However, none of these studies accounted for the crossflow between the oil and bottom water layers when planning a chemically augmented waterflood. Other studies have shown that croosflow plays a major role in waterflood performance I layered reservoir(5,14). Therefore, it is important to account for the croosflow effect in mobility control in the presence of bottom water. The main objective of his research was to develop mathematical model of crossflow to study water channelling under bottom water conditions. With this model and based on the minimization of crossflow between the oil and bottom water zones, the Dynamic Blocking Process (DBP) is proposed to improve waterflood performance under bottom water conditions. Waterflood experiments were conducted in a model to verify the theory and to study mobility control using emulsions. Derivation of the Crossflow Equation In the following, an attempt is made to develop an expression for crossflow while waterflooding a two-layer reservoir, the lower layer being a water zone.