Reservoir Properties at Wairakei Determined from Gravity Changes During the 1988 -1989 Reinjection Test

Abstract

In this paper we examine gravity measurements taken in conjunction with the 1988 - 1989 reinjection trial in the Eastern Borefield at Wairakei. We derive a ëline sourceí model for the change in depth of the deep liquid level which gives a reasonable match to the gravity data. The permeability-depth for this model is 9.9Darcy-metres (D-m),and the storativity of is consistent with the assumption that the reservoir contains a free water surface. Abetter result is obtained with an anisotropic reservoir model, in this case the principal khís are 18.2D-m and 5.4D-m respectively. The direction of largest kh is found to be at an azimuth of We have computed the transient behaviour of the gravity field using these models, and find that the gravity changes will remain detectable for 3 to 4 years after the cessation of injection. Values of kh and storativity have been obtained fkom pressure transients at WK53 and These are similar to the ëgravityí values, but i t has not been possible to further reconcile the two sets of measurements. In addition a TOUGH simulation based on the homogeneous model is used to check our results and obtain a more realistic picture of the reinjection process, providing a basis for future work.