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Abstract: Reserve Analysis and Economic Evaluation of Wasatch/Mesaverde Reservoirs, Greater Natural Buttes Producing Area, Southeastern Uinta Basin, Utah


Previous simplistic reservoir engineering evaluations of the Greater Natural Buttes (GNB) Producing Complex have failed to recognize the full gas potential of the field. Pipeline incapacity and gathering system bottlenecks has lead to operator-imposed constraints on individual wells' production performance. Completion of an additional pipeline in 1994 provided an additional capacity of 180 MMCF/D. Individual wells' decline curves outside the immediate service area demonstrate the additional capacity relief. In addition, recompletion of PUNP zones in existing well bores has added additional deliverability, and a consequent 40% increase in Estimated Ultimate recovery. When adjustments for gas deliverability constraints and PDNP zones are included, a mean value EUR is 1.4 to 1.6 BCF/well on 80 acre spacing of the upper 1500 to 2000 feet (460 - 610 m) Wasatch Formation. Using a significant number of wells within the GNB area, a Probability vs. Reserve Size Distribution (RSD) plot is computed.

Risk analysis of the play is calibrated using standard source-generation-expulsion-migration-trap scenario. Since GNB has been described as a "basin-centered" gas deposit, the risk analysis using concepts of Continuous-type hydrocarbon deposits (Schmoker, 1995) are appropriate. Correlation of the RSD computed vs. actual shows strong positive correlation, demonstrating that conventional risking techniques can be modified to incorporate continuous-type accumulations.

The impact of corrections due to additional reserves has a dramatic impact on the economics of the existing gas play. Using standard well costs based on historical cases, Rates of Return (ROR) for the GNB area are seen to double from 13 % to 26% after Federal and State Income Taxes (AFIT), without Tight Gas Sand (TGS) tax credits.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado