How to Waterflood Reservoirs With a Water Leg
Authors: Yeung K. , Ali S.M. Farouq 
Published: March 2010

Waterflooding oil reservoirs with an underlying water zone is often problematic because the injected water can channel into the water zone, Yet such reservoirs constitute a large resource in Alberta and Saskatchewan.

This research was directed toward reducing water mobility in the bottom water zone for more efficient waterflooding. Three novel approaches were tested: the Emulsion Slug Process (ESP), the Alternating Water-emulsion (AWE) Process, and the Dynamic Blocking Procedure (DBP). All three methods result in an improvement in the vertical sweep efficiency and involve the injection of polymer or emulsion, with water. In each case, there are two modes: blocking and displacing. The differences among these processes lie in the application of the two modes. In ESP, the blocking and displacing modes are used only once with the blocking agent injected first, followed by a waterflood. In the AWE process, the two modes are performed alternately. While in DBP, the two modes are performed simultaneously.

Experiments conducted in this study showed that by using a suitable mobility control agent in DBP, a noticeable increase in both oil production rate and ultimate oil recovery was realized. The opposite may occur for the wrong choice either of the mobility control agent, or of the approach. The experimental results are presented, with a discussion of the roles of mobility control and blocking agents.

Introduction

Waterflooding oil reservoirs with a water leg (i.e., a communicating bottom water zone) are 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 as mobility control and blocking agents to improve waterflood performance(2–4). However, none of these studies accounted for crossflow between the oil and bottom water layers when planning a chemical augmented water-flood. Other studies have shown that cross flow plays a major role in waterflood performance in layered reservoirs(5–10).Thus, a systematic way of utilizing a chemical or an emulsion as a blocking agent under bottom water conditions accounting for crossflow is required for efficient displacement of oil. In this paper, the Emulsion Slug Process (ESP), the alternating Water-emulsion (AWE) Process, and the Dynamic Blocking Procedure (DBP) are discussed.

Emulsion Slug Process (ESP)

The main objective of the three processes proposed in this paper is to try to isolate the bottom water zone when displacing oil by water. An emulsion is used as a blocking agent to increase the flow resistance of the bottom water zone. In the Emulsion Slug Process, the blocking mode is performed once at the beginning, followed by a waterflood. Under bottom water conditions, the flow resistance of the oil zone is much higher than that of the bottom water zone, i.e., Ro &gt; &gt; Rw where Ro and Rw are the flow resistances of the oil zone and water leg respectively; thus, the WOR will be high at the beginning of the flood.