Heavy Oil Waterflooding: Effects of Flow Rate and Oil Viscosity

ABSTRACT

Many countries in the world contain significant heavy oil deposits. In reservoirs with viscosity over several hundred mPa?s, waterflooding is not expected to be successful due to the extremely high oil viscosity. In many smaller, thinner reservoirs or reservoirs at the conclusion of cold production, however, thermal enhanced oil recovery methods will not be economic. Waterfloods are relatively inexpensive and easy to control; therefore they will still often be employed even in high viscosity heavy oil fields. This paper presents experimental findings of waterflooding in laboratory sand packs for two high viscosity heavy oils: 4650 mPa?s and 11500 mPa?s, at varying water injection rates. The results of this work show that capillary forces, which are often neglected due to the high oil viscosity, are in fact important even in heavy oil systems. At low injection rates, water imbibition can be used to stabilize the waterflood and improve oil recovery. Waterflooding can therefore be a viable non-thermal enhanced oil recovery technology even in fields with very high oil viscosity.

Introduction

The Canadian deposits of heavy oil and bitumen are some of the largest in the world. Our conventional oil reserves are now steadily declining, while the global energy demand continues to increase, along with a higher uncertainty about foreign oil sources. As a result, the Canadian oil sands will help Canada to remain an important energy source for the world in future generations.

Heavy oil is a special class of this unconventional oil that has viscosity ranging from 50 – 50000+ mPa?s. Heavy oil reservoirs are often found in high porosity, high permeability, unconsolidated sand deposits. At reservoir conditions, the oil may contain dissolved solution gas, thus some oil can be initially recovered using the energy from heavy oil solution gas drive. At the end of primary production, a significant fraction of oil still exists for potential secondary recovery. Many of these reservoirs are small and thin or segmented, making them poor candidates for expensive thermal enhanced oil recovery strategies.

Waterflooding is often employed at least initially in these heavy oil reservoirs after primary recovery is finished. Water injection can be used to re-pressurize the reservoir and displace oil to producing wells. In these applications, it is very important to understand the forces that are present in the reservoir, and how they can be used to properly design a heavy oil waterflood. This work presents the results for water injection into laboratory sand packs containing gas-free heavy oil of varying viscosity. The responses for different waterfloods are compared in order to understand the mechanisms by which oil can be recovered by water injection.

Theory

Waterflooding of oil reservoirs is a well-recognized technique for oil recovery after primary production. In conventional oil, waterflooding theory has been well documented 1. The inherent assumption in conventional oil waterflooding theory is a similarity in viscosity between oil and water 2,3. In heavy oil applications this is not the case, thus even concepts like oil/water relative permeability do not have the same meaning in heavy oil reservoirs.

A full version of this paper is available on OnePetro Online.