Gas Reserve Growth Potential on a Play Basis as Predicted by Infield Completion Recoveries, Frio Formation, South Texas

Abstract

ABSTRACT Infield well completion histories between 1940 and 1988 were examined in Frio Formation nonassociated gas plays in South Texas to determine gas reserve growth potential. In this study, funded by the Gas Research Institute, infield completions at <640-acre spacing were analyzed for geometry and pressure continuity. Four types of infield completions were identified: completions in gas-cycled reservoirs (type 1), completions reported to be in the same reservoir but interpreted not to be so, based on geologic analysis (type 2), completions in a single reservoir with evidence of pressure communication (type 3), and completions in a single reservoir with the infill completion pressure significantly above reservoir pressure in nearby wells (type 4). Additional types identified in a larger study including Vicksburg, Wilcox, and Miocene strata are not included in this report. In gas-cycled reservoirs (type 1, 37 percent of the 241 Frio samples studies), additional production may come from infield completions, but cycled gas is difficult to distinguish from new reserve additions. Geologic analysis suggests that type 2 infield completions (18 percent of the Frio samples), interpreted to be in two or more zones, represent new pool additions. Type 2 completions are most abundant in progradational electric log facies characteristic of proximal deltaic, delta flank, and fluvial-deltaic plays located in structurally complex, less permeable lower Frio strata. Infield completions in types 3 and 4 are in a single stratigraphic interval containing partially or totally isolated compartments. Type 3 completions contact gas in pressure communication with existing wells and do not represent reserve growth, whereas type 4 completions represent within-reservoir reserve growth at pressures significantly above pressures in nearby wells. Completions at a single reservoir level are most abundant in aggradational log facies in the upper Frio fluvial and fluvial-deltaic plays. The infield drilling examples in this study reveal the complex nature of production and completion histories and stratigraphic and structural relations in South Texas Frio strata. Statistically based reserve growth predictions should take this complexity into account. Infield recompletions made using improved cased-hole logging analysis, reservoir-scale facies analysis, downhole pressure testing, and seismic interpretation will continue to identify economic gas targets in existing fields.