Techniques to Evaluate Resevoir Performance in a Tight Gas Field: The Wamsutter Experience

Frederick E. Bakun, Christopher N. Cecil, Howard C. Gee, Kenneth A. Haley, Elizabeth R. Ellis, and Sara L. McConkey. BP America Inc, P.O. Box 3092, Houston, TX 77253-3092, phone: 281-366-0188, [email protected]

The Greater Wamsutter Development Area (Wamsutter) is a giant tight gas accumulation in south central Wyoming, half way between the Rawlins and Rock Springs Uplifts. The field produces primarily from the Almond Formation, a Late Cretaceous collection of stratigraphic traps with ultra-low permeability. Wells are massively hydraulically fractured in 2-3 stages placing 150-250 thousand pounds of proppant per stage. Initial flow rates range from 700 – 1200 mscfd and quickly decline in a generally hyperbolic fashion.

Performance evaluation in Wamsutter focuses on three key issues: rate forecasting, ultimate recovery prediction, and hydraulic fracture evaluation. While these issues are not unique to Wamsutter, the fact that reservoir permeabilities are a few micro-darcies significantly complicates traditional analysis techniques. Further, wells can be in transient flow for years or even decades.

Several techniques are currently used to evaluate the Wamsutter field including, decline curve analysis, Agarwal-Gardner Type Curve matching, single and multi-well simulation and analog benchmarking. Examples of each technique will be shared along with observed limitations and how integrating all of the above listed techniques can avoid potential pitfalls.