Heavy Oil Waterflooding: Effects of Flow Rate and Oil Viscosity
Authors: Mai A. , Kantzas A. 
Published: March 2010

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. However, in many smaller, thinner reservoirs, or reservoirs at the conclusion of cold production, thermal enhanced oil recovery methods will not be economic. Waterfloods are relatively inexpensive and easy to control; therefore, they will still often be employed in high viscosity heavy oil fields. This paper presents experimental findings of waterflooding in laboratory sandpacks for two high viscosity heavy oils of 4,650 mPa.s and 11,500 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 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

Although conventional oil reserves are declining in many countries, the global energy demand is still increasing. As a result, the industry focus is now shifting towards unconventional oil resources, such as the oil sands in countries like Canada and Venezuela. The size of this resource base is immense, but the production of high viscosity crude oil carries its own unique challenges.

Heavy oil is a special class of this unconventional oil, and has viscosities ranging from 50 to 50,000 mPa.s. Heavy oil reservoirs are often found in highly porous, highly permeable, 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, however, a significant amount of oil still exists for potential secondary recovery. Many of these reservoirs are small and thin or were disturbed during primary production, making them poor candidates for expensive thermal enhanced oil recovery strategies. In times of uncertain commodity pricing, it is beneficial to examine the potential for relatively inexpensive, non-thermal oil recovery techniques.

Waterflooding is often employed, at least initially, in heavy oil reservoirs, both along with or after primary recovery in order 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 the waterflood. Specifically, proper design and maintenance of waterfloods requires comprehension of how viscous oil can be displaced by water, and how oil recovery can be optimized. This work presents the results for water injection into laboratory sandpacks containing gas-free heavy oil of varying viscosity. The responses for different waterfloods are compared in order to investigate the mechanisms by which heavy 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).