Bypassed gas zones can be identified and developed by integrating advanced geologic methods and petrophysical techniques into field development strategies. These reservoirs may provide incremental gas reserve additions at relatively low cost to the operator. Seeligson field was studied as part of a Gas Research Institute-sponsored program designed to develop, test, and verity technologies and methodologies for maximizing the recovery of gas from conventional reservoirs in mature fields.
Detailed geologic evaluation of middle Frio reservoirs in Seeligson field reveals a stratigraphic framework composed of multiple, stacked fluvial channel-fill and splay deposits interstratified with floodplain mudstones. Channel-fill deposits are 10 to 40 ft (3 to 12 m) thick and approximately 2,500 ft (750 m) wide. Splay deposits are up to 20 ft (6 m) thick proximal to channels and extend several thousand feet away from the channel. Channel-fill and crevasse-splay sandstones are reservoir fades; levee and floodplain mudstones are thought to be barriers to flow, separating individual reservoirs vertically and laterally.
Recently developed cased-hole log evaluation techniques tested in Seeligson field demonstrate their superiority for locating bypassed gas. Based on geological and spatial considerations, 14 key wells within a 4 mi2 (10 km2) area of the field were selected for cased-hole petrophysical analyses. Pulsed-neutron and gamma-ray logs were recorded in these wells. After preliminary analyses, five wells were selected for full waveform acoustic logging. Porosity was calculated from the acoustic log and compared to pulsed-neutron porosity to determine gas effect. The logs were corrected for shaliness and combined with sigma (capture cross section) data to calculate water saturation. Long-normal resistivity curves from electric logs were digitized and integrated with the newly acquired data to calculate original open-hole water saturation. Plotting and comparing the logs graphically enabled gas-productive sands to be identified.
Five successful recompletions were made as a result of this study. By estimating reserves from current and initial production, using net present value of the gas, and dividing by the cost of the project, a highly favorable return on investment is estimated.