Secondary Natural Gas Recovery: Targeted Technology Applications for Infield Reserve Growth in Fluvial Reservoirs, Stratton Field, South Texas

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1993

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Integrated evaluations of geology, geophysics, reservoir engineering, and petrophysics were conducted for mid-Oligocene-age fluvial reservoirs in Stratton field as part of this study. Located in South Texas within the Frio Fluvial-Deltaic Sandstone along the Vicksburg Fault Zone play (FR-4), Stratton field represents a mature gas field with significant opportunities for natural gas reserve appreciation. These fluvial reservoirs exhibit heterogeneity and often contain multiple compartments.

The study identifies a considerable potential for reserve appreciation, with documented opportunities for a 100 percent increase in reserves within a large contiguous area of Stratton field, despite 40 years of prior development. Remaining natural gas reserves can be accessed through recompletion of existing wells that have bypassed reservoir compartments or by drilling infield wells to target compartments not effectively drained at current well spacing.

Exploration efforts to discover new reservoirs, identify incompletely drained compartments, or tap bypassed gas zones in old fields can benefit from detailed geological studies integrating engineering, petrophysical, and geophysical methodologies. Various geophysical techniques, including 3-D surface seismic, vertical seismic profiling, amplitude versus offset, and 2-D seismic inversion, were utilized to visualize subtle changes in reservoir topology and compartment boundaries at depths as low as 6,800 ft.

The study delineates three classes of compartment sizes based on analysis of ten groups of Frio reservoirs. Forward stochastic modeling of maximum gas recovery suggests that well spacings of 340, 200, and 60 acres (or less) offer optimal gas-contact efficiency in large, medium, and small compartment size reservoirs, respectively.

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