Late Developments in the Seeligson Zone 20B-07 Enriched - Gas - Drive Project
Authors: Grinstead W.C. , Barfield E.C. 
Published: December 2007

Abstract
An enriched-gas-drive project has been in progress in the Seeligson Zone 20B–07 field over four years. It has been established that recovery approaches 100 per cent in portions of the reservoir actually swept by enriched gas. On the other hand, breakthrough volumetric sweep efficiency is low, on the order of 23 per cent, with the result that recovery at breakthrough is less than expected. Low volumetric sweep efficiency appears to be the result of a combination of causes, including adverse mobility ratio and sand heterogeneity. It is too early in the life of the project to present a final evaluation of the recovery potential of the enriched-gas process. For this reason operations will be continued with no major changes until definite conclusions concerning ultimate performance can be made. It is anticipated that at sometime prior to blowdown the project will be modified in an effort to increase ultimate recovery by improving sweep efficiency.

Introduction
The relatively new concept of miscible displacement has offered the petroleum industry a method of appreciably increasing its reserve of recoverable oil. As is frequently the case with new techniques or processes, the basic principle is apparently sound; however, the most effective method of application has not yet been demonstrated in actual field operations. Experimental work in the laboratory, under simulated reservoir conditions, has indicated that essentially 100 percent recovery can be achieved from that portion of an oil sand swept by a miscible fluid. In recent years, there has been a concerted effort within the industry to develop an effective method of utilizing the basic concept. Several variations of the process have been proposed. These include:high-pressure gas injection,LPG-slug injection,simultaneous injection of water and enriched gas, andenriched-gas injection.
Although all these techniques differ in application, they share the same goal-to significantly increase ultimate oil recovery through miscible displacement. Enriched-gas drive involves the injection of gas enriched at the surface with intermediate-molecular-weight components such as propane. On contact, these intermediates condense into the reservoir oil. The oil swells, its viscosity is lowered and phase boundaries disappear to be replaced by a transition zone in which fluid properties grade from those of the native oil to those of the injected gas. In the region where miscibility is achieved, capillary and interfacial tension forces are not believed to be present. In their absence there is no tendency for oil to remain as residual saturation, and 100 per cent recovery from connected pores is possible. The enriched-gas process is applicable to a wide range of crude oils and to reservoirs in which the operating pressure ranges from about 1,500 to 3,000 psi. Although the results of early laboratory testing of the enriched-gas- drive process were encouraging, it was recognized that selected field tests to substantiate these results would be required. In addition, field tests were required to investigate the effects of various other conditions, the aggregate of which could not be adequately simulated in the laboratory. Among these conditions are the following:VISCOUS FINGERING--Viscous fingering of enriched gas through the reservoir oil is a result of the adverse mobility ratio between the displaced and displacing fluid. In the absence of capillary forces, such intrusions by the displacing fluid are quite unstable and can be expected to grow rapidly.SAND HETEROGENEITY--This condition could lead both to channeling through various sand members and to the formation of viscous fingers.STRUCTURAL DIP--In low-dip reservoirs producing at practical rates, gravity forces are expected to be ineffective in mitigating the growth of viscous fingers.RESERVOIR GEOMETRY AND WELL CONFIGURATION--It is desirable to determine to what extent these two factors can be utilized to offset the inherent restrictions imposed by the other conditions.
JPT
P. 261^