| Abstract |
Ohaaki field underwent large-scale discharge testing (1967- 1972), followed by a long period of recovery (1972-1987). Production began in 1988, and continues at about 16 Mt/yr (6 Mt/yr net mass loss). During testing the deep reservoir pressures were drawn down by 1.5 MPa, subsequently recovered by 1.0 MPa, and have declined a further 2.0 MPa since production started. After testing began, the overflow rate from the Ohaaki Pool declined from a natural value of 9 l/s and ceased in early 1968. As testing continued, the water level fell by 9.5 m but slowly recovered when testing ceased. Since 1980, addition of separated bore water and sealing of the base of the pool have maintained overflow. No changes to the chemistry of the pool water occurred until introduction of the bore water. After production began, many smaller hot pools dried up and have been invaded by vegetation, but upflows of steam to most surface features still occur. New steam vents have occurred where subsidence has created tension cracks up to 5 cm wide at the ground surface. Over most of the field, shallow (1m depth) ground temperatures have not changed. However, temperatures have increased by up to 75 oC near the tension cracks, and decreased by up to 45 oC at the edges of some areas of previously thermal ground which are now at ambient temperature. Shallow groundwater levels have been unaffected by testing or production except in local areas near thermal features, where several metres of water level decline has occurred. Groundwater temperatures have not been affected by the drawdown, except near cold downflows through previously active thermal vents. Most changes to groundwater chemistry appear to be associated with leakage of separated bore water from holding ponds or temporary surface discharges. |