Major additions to future gas reserves will depend partly on development of resources that are presently unconventional. Recently, coproduction of gas and water has been proposed as a method to increase ultimate gas recovery from reservoirs that have begun to water out or were abandoned because of high water production. The enhanced recovery method involves producing large volumes of water and reducing reservoir pressure to the point where dispersed gas, trapped in the water-invaded zone, is mobilized and flows to the well bore. Research involving the coproduction method was funded by the Gas Research Institute.

Integration of geological and engineering studies indicates that the Port Arthur field in Jefferson County, Texas, is a prime area for research to test the coproduction techniques. Prior to abandonment, the field produced from lower Hackberry (Oligocene) sandstones at depths less than 12,000 ft (3,600 m). The field contains 11 reservoirs with a maximum net sandstone thickness of over 450 ft (135 m). Individual sandstones are 10 to 120 ft (3 to 36 m) thick, but gas columns are usually less than 20 ft (6 m) thick. The reservoirs are relatively narrow and elongate and are oriented nearly perpendicular to the local structure, a simple roll-over anticline. These reservoirs, interpreted as submarine channel and fan deposits, are internally heterogeneous owing to the vertical imbrication and dip-alignment of the sand lenses. Optimum reservoir facies occur as thick, massive sandstones with uniformly high porosities and permeabilities that average 28% and 60 md, respectively. Reservoir quality is more variable and diminishes somewhat away from the channel axes in the more thinly bedded sandstones with intercalated shales. Upward decreases in porosity and permeability characterize the zones of gas accumulation.

The "C" sandstone was selected for detailed investigation because of high abandonment pressure, excellent reservoir quality, high productivity, and good lateral continuity. Reservoir simulation studies suggest that 5.5 bcf of gas and some condensate could be recovered from the "C" sandstone under natural flow conditions by producing slightly more than 9 million bbl of water over a 5-year period. Brine disposal would be accomplished by injection into thick Miocene sandstones that occur at shallow depths. If successful, the recovery efficiency of the "C" sandstone would be increased from 42% (primary) to 53% of the original gas in place.

An economic analysis of the enhanced recovery project indicates a favorable payout under prevailing gas prices. Project economics are even more attractive if gas prices increase or if gas production from other abandoned reservoirs is commingled with production from the "C" sandstone.

End_of_Article - Last_Page 475------------