Simulation of Horizontal Wells to Mitigate Miscible Solvent Gravity Override in the Virginia Hills Margin
Authors: Chugh S. , Baker R. , Cooper L. , Spence S. 
Published: March 2010

Abstract
Significant incremental recovery can be obtained when vertical sweep is improved in the mature Phase I Miscible Flood area of the Virginia Hills Field. This study advances our knowledge of the gravity override processes operating in this complex carbonate reservoir. Furthermore it shows that horizontal injection wells improve sweep efficiency thereby alleviating the impact of solvent gravity override. As a result of this simulation, a successful horizontal injection well was drilled, with an actual 18 E3m3 incremental oil recovery to date and a predicted 65 E3m3 total incremental oil. The results of this study suggest redevelopment in mature EOR areas is expected to extend the life of the miscible flood and increase overall recovery.

Introduction
Miscible recovery in the Virginia Hills Field Hydrocarbon Miscible Flood (HCMF) project is significantly limited by poor vertical sweep of the solvent within individual layers of the reservoir. A vertically continuous interval within the reef margin (30 m thick) was investigated and shown to have a large bypassed tertiaryoil target. This study demonstrated that recovery could be significantly increased through the use of horizontal solvent injection wells.
To have confidence in the results of the study it was critical to utilize geological and production history data to address the interwell extent of significant heterogeneities. This required complete integration of geological and engineering staff in all phases of the study.

Background
Historical
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 field was unitized in 1963 and a waterflood pressure maintenance scheme was implemented. In the late 1980s the patternswere 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 1,000 m3/d have fallen from peak rates of 4,400 m3/d. Field average water cut is greater than 90%. Within the EOR area, well spacing is 32 ha., approximately 400 m interwell distance.
Geological
The internal architecture of this limestone reservoir is multilayered and compartmentalized, due to several stages of carbonate accumulation. From the base up these stages are: S1 (equivalent to the regionally extensive Slave Point platform), S2A, S2, S3L, S3U, and S4, (herein called S-units) reaching a total maximum thickness of 150 m. A high energy windward margin exists along the eastern portion of the build-up with superior reservoir quality (an average range of.09 to.12 phi, and 20 to 120 mD permeability). For this study the S3L in the margin area of the build-up was investigated (Figure 1).
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.