ABSTRACT

Geologically based infill drilling or recompletion programs, developed from detailed geologic analyses that identify isolated or poorly drained reservoir compartments, are the key to recovering additional gas resources in mature Gulf Coast fields. The additional natural gas resource that can be recovered by strategic infill drilling or recompletion in a reservoir can be estimated initially by subtracting gas volumes derived using the material balance (pressure decline) method from volumes derived using a volumetric method based on geologic analysis. This resource potential represents remaining gas that is not in communication with existing wells.

Frio reservoirs in mature, nonassociated gas plays located downdip from the Vicksburg fault zone in south Texas are characterized by multiple, vertically stacked sandstones. The Brooks reservoir in La Gloria field lies in a coarse-grained fluvial system that contains dip elongate channel sandstone belts 1 to 2 mi wide. Within these belts are six or more vertically stacked channel fill, point-bar, and crevasse-splay sandstone stringers that are separated vertically and laterally by thin floodplain mudstones.

Gas in place in the Brooks reservoir was calculated volumetrically by planimetering sandstone isopach maps of two sandstone stringers. This volume was compared with a gas in place estimate made from the commonly used, reservoir-wide pressure decline plot. Incremental gas recovery calculated for the geologically heterogeneous Brooks reservoir is 23% of post-cycling gas in place, or 40 Bcf at 92% recovery.

In contrast with the heterogeneous Brooks reservoir in La Gloria field, the I-92 reservoir in Julian North field is composed of relatively homogeneous deltaic deposits. The I-92 reservoir contains thin (5 to 15 ft) sandstone bodies that commonly contain no interbedded mudstones. These sandstone bodies are continuous along depositional strike and represent wave reworked channel-mouth bar and delta-front facies.

Although the target gas volume for both the I-92 and Brooks reservoirs is 23% of initial gas in place, the percentage of reservoir volume contacted is greater in the I-92 than in the Brooks reservoir, despite lower well density in the I-92. Whereas existing wells may adequately drain the relatively homogeneous I-92 reservoir, compartmentalization has resulted in significant amounts of gas remaining in the Brooks reservoir. This gas can be efficiently targeted through a geologically based infill development or selective recompletion program.