| Title | Precision Gravity Modeling and Interpretation at the Salak Geothermal Field, Indonesia |
|---|---|
| Authors | Gregg A. Nordquist, Jorge Acuña and Jim Stimac |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Salak, Precision Gravity, Simulation, Numerical Model |
| Abstract | Eight (8) precision gravity surveys have been conducted over the Awibengkok geothermal field. The base line survey was conducted in 1994 with the latest repeat occurring in 2008. These gravity data are used to provide constraints for calibrating the numerical model’s net depletion and for constraining reservoir mass replacement from re-injection and natural inflow from aquifers outside the production zone. For the analysis the observed gravity trends and changes are compared to the simulated gravity. The simulated gravity is calculated by associating the gravity change at the surface with predicted density changes in the numerical model’s rectangular blocks in the subsurface. A comparison of the observed gravity changes versus the simulated gravity change at Awibengkok show a mismatch in the Northeast sector of the field. Analysis of the data shows that the mismatch is a result of increased mass that is not accounted for in the numerical model. Simple 2 ½ D gravity models show that over time the area of increased mass is occurring at a constant rate and in a consistent location. Using reasonable porosities these models suggest a minimum rate of influx of about 110 kph from an external aquifer could account for the observed mass changes. The gravity models are consistent with increasing saturation in a dacitic unit located above the reservoir. An alternative model is that shallow recharge is also occurring into reservoir with a portion of the gravity changes resulting from re-saturation of the matrix in the steam cap area or pressure support requiring less support from the matrix. Geochemical trends lend some support for this latter interpretation. |