Abstract

Horizontal water injection wells were used to initiate a waterflood in the Valhalla Boundary Lake "M" pool. A simulation model of the pool indicated that vertical injection wells would not provide adequate injectivity to match withdrawals. Horizontal wells were proposed, drilled and used to repressure the reservoir to bubble point, achieve target injection rates and maximize production from the reservoir. The targets for the horizontal injectors and the associated waterflood have been surpassed to date.

Introduction

This paper is a case study of the Valhalla Boundary Lake "M" pool and the successful use of horizontal wells to increase water injectivity, oil production rates and cash flow within a waterflood project. The pool was discovered in January, 1991 with the drilling of a well at 8-32-76-10 W6M. The pool was delineated by 1994 with the drilling of six additional oil wells (Fig. 1). The reservoir was identified as an excellent waterflood candidate early in its life. To model the waterflood a simulation study was initiated. This simulation indicated that pool productivity would be restricted in a vertical injector scenario. Horizontal injection wells were simulated and found to supply sufficient injectivity to support the full production capability of the reservoir. Two horizontal wells were drilled into the pool in 1994 and injection was initiated that fall.

Stratigraphy And Sedimentology

The Boundary Lake Zone Member of the Charlie Lake Formation was deposited in late Triassic time across a broad, gently dipping sabkha coastline similar to the present day Persian Gulf. The member consists of a basal anhydrite overlain by porous limestones and dolomitic limestones that comprise the reservoir facies in the Valhalla area. They are unconformably overlain by siltstones and mudstones of the upper Charlie Lake Formation. Local erosional relief on the unconformity can exceed 10 m in the immediate area.

Stratigraphic interpretations show that erosion at the end of Boundary Lake time resulted in the presentation of the Boundary Lake Zone member where paleostructural lows were present. Within these paleo-lows, porosity was preserved and hydrocarbon traps were developed leading to oil accumulations such as the Valhalla Boundary Lake "M" pool. The top seal to the reservoir is provided by the overlying siltstones/mudstones of the upper Charlie Lake Formation. The bottom and lateral seal is provided by tight limestones and anhydrites of the Boundary Lake Zone. The result is an areally restricted, closed reservoir system with definitive pool boundaries.

Reservoir Description. The productive Boundary Lake Zone consists of a vertically upward succession of lithofacies. The lithofacies grade from porous, dolomitized algal stromatolitic limestones at the base, through non reservoir peloidal skeletal argillaceous limestones in the middle, to porous intertidal algal limestones at the top. All three lithofacies correlate easily between wells suggesting excellent lateral continuity across this 600 ha pool, except where erosion has truncated them at the pool's perimeter. Porosity and permeability are best developed in the intertidal algal limestone lithofacies at the top of the Boundary Lake Zone which contains most of the pore volume. Porosity and permeability average 15% and 22 md, respectively, in up to 3 m of pay. The pay zone is dominated by pinpoint vugular and intercrystalline porosity.

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