A Reservoir Monitoring And Modelling Using Repeat Microgravity Measurements At Ohaaki Geothermal Field

Abstract

We continue to repeat microgravity measurements which were begun in 2010 and examine the influence on the reservoir caused by the production and the reinjection at the Ohaaki geothermal field. Repeat microgravity measurements were conducted at Ohaaki geothermal field, New Zealand, in December 2010, February 2012 and September 2012. The measurements were conducted at 26 benchmarks using a SCINTREX CG-3+ gravimeter. After free-air correction and precipitation correction, the maximum gravity increase was +38 μgal at the north side of the eastern production area (BR26) and the maximum gravity decrease was -60 μgal at the northwestern area of the field (H305) from December 2010 to February 2012 (Period 1). The maximum gravity increase was +15 μgal at the northeastern area of the field (A69) and the maximum gravity decrease was -36 μgal at the north side of the western production area (H335) from February 2012 to September 2012 (Period 2). To estimate the mass change in the reservoir, a density change model was created and we calculated the gravity changes from this density change model. We assumed that the absolute density change in the reservoir was 166 kg/m3, which was caused by phase transition between steam and hot water. The depth of density change was between -200 m asl and -260 m asl. As a result of the calculation, gravity changes were +39 μgal at BR26 and -57 μgal at H305 during Period 1 and +14 μgal at A69 and -35 μgal at H335 during Period 2. These results can represent the results of the gravity measurements.