Scaled Model Studies of Heavy Oil Recovery From an Alaskan Reservoir Using Gravity-assisted Gas Injection
Authors: Bansal A. , Islam M.R. 
Published: March 2010

Gas injection is one of the oldest enhanced oil recovery techniques used for recovering light oils. Recently, there has been renewed interest of gas injection in both light and heavy oil reservoirs. This recovery technique is particularly of interest for the heavy oil reservoirs of Alaska where the application of thermal techniques is limited due to reservoir depth and the existence of permafrost. It has been pointed out that, in order to apply gas injection, gravity stabilization is required. In absence of a natural dip, such gravity stabilization may be achieved through horizontal wells placed on top of the reservoir while producing through another horizontal well placed at the bottom of the reservoir. The possibility of such recovery technique has been investigated in this study.

A series of scaled model studies has been conducted using nitrogen, propane and carbon dioxide as injection fluids. The West Sak reservoir of North Slope, Alaska is used as the prototype for the present modeling study. Results indicate that close to 65% of the oil in place may be recovered by using gas injection from the top of a heavy oil reservoir. Even though similar ultimate recovery is obtained with gravity drainage alone, the time required to reach such a recovery is extremely long. However, these experimental runs are sensitive to the rate of gas injection. Experimental results suggest that there is an optimal gas injection pressure for nitrogen. Recovery mechanism is found to be different with propane than with nitrogen. However, the oil recovery remains higher with propane for all the cases studied. The highest recovery is obtained with carbon dioxide.

In all experiments, profuse viscous fingering takes place with different degrees of severity. This is found to be in agreement with a previously published stability theory. Viscous instability is considered to be harmless as final recovery is very high for all the cases. Final recovery with gas injection is significantly higher than the one with a waterflood.

Introduction

The oil production from Prudhoe Bay started to decline in 1989 and this necessitated the development of other petroleum resources. One such oil reserve is the West Sak reservoir located in the North Slope of Alaska. It is estimated to contain up to 3.2 × 109 m3 (25 billion barrels) of heavy oil in place. The high degree of variation in the oil gravity (12 to 22 ° API) and oil viscosity (30 to 3000 mPa •s) in highly heterogeneous, unconsolidated, fine grained sands, the presence of clay minerals, low permeability, highly faulted structure and a proximity to the overlying permafrost make the West Sak Reservoir very unique. The possibility of sharing the existing Kuparuk River Unit facilities makes the development and production of West Sak reservoir a near-term possibility.

ARCO Alaska Inc. conducted a waterflood pilot in 1984–1986 and it was found that West Sak reservoir lacks any natural drive mechanism. Also the high viscosities of West Sak crude make the reservoir a target for enhanced oil recovery (EOR) processes.