Microgravity Measurements at Wairakei Geothermal Field, New Zealand; a Review of 30 Years Data (1961-1991)

Abstract

Twelve repeat, precise gravity (microgravity) surveys have been made at Wairakei Field between 1961 and 1991, spanning most of the production period. Gravity changes determined from these surveys, and corrected for the effects of ground subsidence and groundwater level changes, have proved to be a powerful tool for monitoring changes which occur in the reservoir during exploitation. The results have proved helpful for economic and environmentally responsible management of the field. Gravity data show that during the initial period of production there was little recharge; fluid was mined from beneath the main production borefield and an area to the northwest. After about 5 years production, natural recharge rose to about 100% and has since remained near that value. In the early 1970s positive gravity changes, indicating net mass gain, were measured in the borefield, and are interpreted as being caused by a rise in the deep liquid level in that area. By the late 1980s these gravity increases had extended to an area south of the borefield. Gravity data, taken before and after a reinjection test in the Eastern Borefield show reinjected fluid had mainly flowed westwards towards the Western Borefield and northeastwards towards the centre of ground subsidence. The reinjection caused the deep liquid level to rise in a cone of impression about m high, which slowly subsided after the test. Analysis of the gravity and pressure data obtained during the test indicates values of about d-m for kh and 9 x for for the upper part of the reservoir.