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The Sweet Lake geopressured-geothermal well is located in a basin on the north flank of an east-west trending salt ridge that includes the Hackberry, Big Lake, and Sweet Lake structures. The south side of the basin is
bounded by a fault downthrown to the north. This fault converges eastward with a major east-west trending fault downthrown to the south. The two faults form a graben which terminates to the east by the convergence of the faults and is open to the west. Within the basin the primary geopressured-geothermal aquifer is the Miogypsinoides sand of the Camerina zone (upper Frio Formation of Oligocene-Miocene age). Within the graben, sediments dip northwest into the basin with dip angles as high as 20°. The first Miogypsinoides microfossil was picked at 14,970 ft (4,563 m) and the first good sand occurred at 15,065 ft (4,592 m). The sequence is 640 ft (195 m) thick (15,000 to 15,640 ft, 4,572 to 4,767 m), with 250 ft (75 m) of net sand. There are seven potentially productive sands within the sequence. Four diamond cores were taken. Data from Core 3, 15,389 to 15,405 ft (4,690 to 4,695 m) indicated the sands are medium to fine-grained, with 1 to 20% silt-sized material. Median grain size is 0.26 mm. Thin sections parallel and perpendicular to the core axis show the grains to be angular to subangular. X-ray analysis showed 75% quartz, 19% feldspar, 4% illite, 2% mixed-layer clay (illite/smectite), and a trace of kaolinite. SEM photographs showed cement as quartz overgrowths and clay as very fine hairs in the pore spaces. Porosity is 24% and permeability 3,600 md in Core 3, the fifth sand in sequence. Initial reservoir pressure in this perforated zone (15,387 to 15,414 ft, 4,690 to 4,698 m) was 12,060 psi (83,154 kPa). Temperature at the middle point in the sand was 299 ;F (148°C). The calculated SP salinity for the well was 50,000 to 70,000 ppm. If Rw/2 is used instead of Rw, the calculated salinities increase to 125,000 to 140,000 ppm.
Reservoir testing included drawdown and buildup tests. The well flowed a total of 240 days. Flow rates varied from 1,000 bbl/day to 34,000 bbl/day. Average surface pressure during the testing is 2,500 psi (17,238 kPa). The gas to water ratio is 25 to 28 SCF/B, of which 20 SCF/B are recovered. Additional testing of the reservoir is planned.
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