Drilling and evaluation of Ascension #1, a geothermal exploration well on Ascension Island, South Atlantic Ocean

A deep (3126 m) geothermal exploration well (Ascension #1) was drilled on Ascension Island in the South Atlantic Ocean as the culmination of an exploration program that began in 1982. Ascension #1 encountered several geothermal fluid entries below a depth of 2400 m, and had a bottomhole temperature approaching 250°C. However, the fluid flow rate was limited. While attempting to improve production by drilling a second leg, a mechanical failure resulted in loss of the well. An analysis of the geologic controls on fluid production suggests that fracture permeability is oriented to the northeast and often associated with felsic dikes. The system may be sealed by a thick sequence of hyaloclastites that are mechanically unable to sustain open fractures. The reservoir intersected by Ascension #1 apparently lacks the permeability required for commercial fluid production.