Abstract

This paper describes a compositional simulation study of the Waterton Sheet III pool, with an initial raw gas in place of almost 100 billion cubic meters (BCM), and more than 33 years of production history. The pool is extensively fractured, especially along the crest of the structure, and contains a near-critical, rich gas condensate with a compositional gradient.

Until recently, reserve estimates were made using conventional P/z vs. cumulative gas plots, based on the assumption of tank-like behavior. Observed differences in individual well pressures, taken from buildup tests, were relatively small, and assumed to be largely due to measurement error. The tank material balance approach neglected large variations in permeability derived from well tests, other heterogeneities incorporated in the geological model, and the variation in hydrocart, on composition with depth. As a result, it was hypothesized that hydrocarbon storage was significantly underestimated.

Results from a companion reservoir characterization study were used to construct a full-field 3D, dual porosity, equation-of-state (EOS) simulation model. The model was tuned to match individual well pressure and deliverability declines over time. Reasonable matches of producing condensate-gas ratios (CGR) were also obtained for most wells. Sensitivity runs highlighted the degree to which close scrutiny of all available data over the long producing history could be used to reduce uncertainties.

The results of the simulation study provided an initial-gas-in-place estimate of approximately 100 billion cubic meters, which is roughly 20% higher than previously calculated using a conventional P/z plot. Further efforts will be directed towards enhanced drainage of the Sheet III pool.

Introduction

The Waterton field is located in the southwestern corner of Alberta at the front ranges of the Rocky Mountains and the foothills disturbed belt. The field consists of a westward dipping thrust sheet of Mississippian and Devonian carbonates with hydrocarbons trapped along the leading edge (see Figure 1).

The Sheet III pool initially contained approximately 100 billion cubic meters of a near-critical retrograde condensate fluid. The sheet is 675 m thick on average and has more than 1500 m of vertical relief. Extensive fracturing has allowed for prolific production rates of up to 1.6 million m3/d despite poor reservoir matrix quality of approximately 3-4% porosity and 0.05 md permeability. Twenty two of the twenty three producing wells are clustered on the crest of the structure and are predominantly located in the southern canyons due to restrictions in surface access arising from the rough terrain. As a result of pressure depletion, from an initial pressure of 33,000 kPa to about 5,000 kPa, a large amount of condensate has dropped out and remains in the reservoir. Several downhole recompletions in the late 1970's resulted in significant, but highly variable amounts of incremental condensate production. A downhole recompletion, using a sliding-sleeve assembly in WAT-36 resulted in a pronounced condensate-gas ratio increase and almost 36,000 m3 of additional condensate over a two-year period.

Previous assessments of dynamic reservoir performance were largely based on highly simplified geological models.

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