Abstract

Significant incremental recovery can be obtained when vertical sweep is improved in the mature Virginia Hills Field Horizontal Hydrocarbon Miscible Flood project. This study advances our knowledge of gravity override processes operating in this complex carbonate reservoir. Furthermore, it shows that horizontal injection wells can improve sweep efficiency thereby alleviating the impact of solvent gravity override. Based on the results of this study, re-development in mature EOR area is expected to extend the life of the miscible flood and increase overall recovery.

Introduction

Miscible recovery in the Virginia Hills Field Horizontal Hydrocarbon Miscible Flood (HCMF) project is significantly limited by poor vertical sweep of the solvent. In the vertically continuous areas of the reef margin this results in a large bypassed tertiary oil target. This study demonstrated that recovery can be significantly increased through the use of horizontal solvent injection wells.

To have confidence in the results of the study it was critical to use the geological and production history data in combination to determine the interwell extent of heterogeneities. This required complete integration of geological and engineering staff in all phases of the study.

Background

The Virginia Hills Beaverhill Lake A Pool was discovered in 1957. The field (Figure 1) produces oil from a Mid-Upper Devonian limestone, stromatoporoid build-up. The internal architecture of the reservoir is multi-layered and compartmentalized due to several stages of carbonate accumulation which are displaced relative to one another. From the base up these stages are: S1 (equivalent to the regionally extensive Slave Point platform}, S2A, S2, S3L, S3U, and S4, reaching a maximum thickness of ∼150m.

The distribution of present day reservoir is controlled by deposition. Reservoir quality rock is dominated by primary interparticle matrix porosity. The prevailing wind direction from the northeast created a high energy windward margin along the eastern portion of the present day buildup where reservoir development today is of superior quality. The lagoon area to the west is of streaky, poorer quality reservoir.

The field was unitized in 1963 and a waterflood pressure maintenance scheme was implemented. In the late 1980s the patterns were normalized to inverted nine spots and in 1989 a tertiary horizontal hydrocarbon miscible flood project was initiated in the south half of the Pool.

The original oil in place (OOIP) for the pool has been calculated at 59.6 E6m3 by the Operator. Cumulative oil production to the end of July 1996 was 22.9 E6m3, or 38.4 % of the OOIP. Current oil rates of 1000 m3/d have fallen from peak rates of 4400 m3/d. Field average water cut is greater than 90 %. Within the EOR area, well spacing is 32 hectares, approximately 400m apart.

Solvent Gravity Override

Solvent gravity override (Figure 2) occurs when there is sufficient time for injected solvent to segregate prior to its withdrawal due to gravity differences and therefore a smaller miscible oil target is swept. Segregation of solvent occurs within a short distance from the wellbore (100–200 m).

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