Geothermal exploration at Southampton in the UK : a case study of a low enthalpy resource

Two geothennal wells have been drilled in or near Southampton since 1979. One is at Marchwood. just outside Southampton, and theothcrisintheccntreofthecity;theweltsare!.85 kmapart. They proved a geothermal reservoirin thc upper 25 to 40 m ofthe Triassic Sherwood Sandstone at a depth of about 1700 m. The reasons for siting the wells in Southampton are discussed and the .- nature ofthe reservoirdcscribed. The welis have been extensively tested. The Marchwood well yielded 30 Is"' for a pressure reduction of 3.7 MNm"^ after a test of 33 days, while the Western Esplanade well gave 20 Is"' for about 3.0 MNm"^. Both yielded brine with a salinity of over 100 g 1"' at a well-head temperature of between 70 and 74°C. Thc transmissivity of the reservoir is 6 m^/d (3.5 D.m) and the storage-coefficient 4 x I0~'. Computer modelling of changes in reservoir pressure suggests that near the wells there is a region of relatively high permeability but the permeability declines at distances of more than a few kilometres from Ihe wells. This could take oneof several forms including a bounded reservoir or a narrow wedge-shaped reservoir. The thermal yield from either well at an abstraction rate of 20 ls~' would be about 3 MW.