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Abstract

This paper presents results of a comprehensive study to improve our understanding of High Mobility Ratio Water Flood (HMRWF) and to improve its performance prediction. Published data on heavy oil water injection field projects are limited. Several successful HMRWF projects have been reported, and they show significant oil recovery at high water cut. However, the range of reported recovery is large – waterflood recoveries of ~ 1-2% to 20% OOIP have been reported for similar reservoirs. Higher viscosities result in lower recovery

Mechanistic study using fine-scale simulations show that viscosity (or mobility) ratio primarily controls oil recovery response, and that the recovery is lower at higher viscosity ratios. Further, viscous fingers dominate high viscosity ratio floods, and mobile water can significantly reduce recovery. Field-scale simulation results indicate that heterogeneity plays a more important role for a HMRWF (than conventional waterfloods). The amount of primary production prior to the start of the waterflood has a larger effect on incremental oil recovery for high mobility ratio floods. Further, highly-correlated, thin, thief zones reduce recovery of HMRWF more severely, and rock wettability (relative permeabilties) strongly influences oil recovery. These results indicate that accurate viscosity and relative permeability measurements are essential for a reliable performance prediction.

Introduction

Waterflood has been conducted in many high viscosity reservoirs in the past, and several water injection projects in high viscosity reservoirs are currently ongoing and planned around the world.1-18 However, published data on High Mobility Ratio Water Flood (HMRWF) performance is limited and the results are sometimes conflicting. Further, it has been postulated that some of the recovery mechanisms might be different.20 It is apparent from the literature that our understanding of HMRWF performance is inadequate. 

This paper presents results of a comprehensive study to provide improved insight into mechanisms governing HMRWF and to help improve performance prediction. The specific objectives were to: (1) evaluate published field data, (2) determine key parameters that govern the process using analytical methods and fine-scale mechanistic numerical models, and (3) quantify effects of reservoir heterogeneity on HMRWF field performance. Accordingly, the paper is divided into four parts. The first presents a review of published field data. Next, definitions of mobility ratio are examined and a preferred definition is proposed. This is followed by a fine-scaled mechanistic modeling of HMRWF to identify key parameters. Finally, field-scale simulations are conducted to delineate key differences between HMRWF and conventional water flood.

Results of this study can be used to improve forecast and interpretation of HMRWF field performance. Furthermore, the paper identifies key parameters that govern process performance, which should result in improved project design.

Field Data

Published and other available data were reviewed critically to ascertain field performance of heavy oil projects under water injection. Although several1-20 published papers discuss heavy oil field projects, only a few (Refs. 1, 3-5, 7, 8, 12, 13, and 18) have some production information. Furthermore, only two of the published papers (Refs.1 and 3) have sufficient details on actual performance. The published data are from Western Canada, California, and other parts of the world. They include offshore and onshore fields, both thick and thin reservoirs, horizontal and vertical wells, and a variety of viscosity ratio. However, a majority are from Saskatchewan, Canada, where water injection was initiated in the 1970’s-80’s, and employ vertical wells.