Time-Lapse (4d) Microearthquake and Precision Gravity Joint Inversion of The Darajat Geothermal Field, Indonesia

Geophysical monitoring of the Darajat geothermal field has included continuous microearthquake (MEQ) surveillance (since 2006) and precision gravity & levelling repeat surveys (since 1996). Microseismic surveillance has recorded in total about 7,500 MEQ events, most induced around condensate injection wells and during the drilling of make-up wells. Travel-time tomography inversions have been performed on the MEQ dataset and indicate consistent low Vp and Vp/Vs ratio around the central portion of the Darajat field, from where there has been steam production since 1994. This velocity depression is consistent with results from the repeat gravity surveys, a maximum gravity decrease within similar areas in the central and southern regions. These parts of the field are where the longest-term production has occurred and where there is a net mass loss in the reservoir.

Strong agreement between the observed changes mapped by the gravity and microseismic monitoring suggested the opportunity of performing a pioneering approach of time-lapse joint inversion of the both datasets to obtain mutually consistent models. The plan was implemented last year in a collaborative project between Star Energy and CGG Multiphysics. The inversions were conducted for velocity and density changes during the period 2006 until 2019 when both data are simultaneously available. For the inversion, the time dimension is divided into two (2006-2012 and 2012-2019) and also four epochs (2006-2010, 2010-2012, 2012-2016 and 2016-2019) considering temporal and spatial variation of the data. Significant variation in MEQ locations and the receiver network led to changes in ray path coverage over time that represented challenges to the 4D analysis. A series of single-domain tomographic inversions for the full period was carried out to obtain a common starting model for subsequent parallel inversions of the individual epochs. Afterwards, a sequential workflow deployed the tomographic result from an early epoch as a starting point of the later epoch data inversions, inverting jointly for both velocity and density changes. Cross-gradient constraining in this joint workflow promotes structural similarity of the property changes; and the area outside the reservoir was fixed (no change).

The joint inversion results indicate the largest density decrease in the southern Darajat and part of the central of production zone. This region coincides with the area of the maximum superheated vapor, representing the occurrence of mass draining from the reservoir matrix which potentially leads to future reservoir decline. Correlation with a discrete fracture network (DFN) model of the reservoir indicates agreement between the largest density drops and the area with bigger fracture permeability; implying that in the “sweet spot” production areas, more mass is being extracted from the matrix than in the less productive area. The signature from injection impact is more obvious in the velocity domain. The inversions showed a Vp/Vs increase (Vs decrease) near the northeastern edge-of-field injection well, suggesting increasing water saturation in deep reservoir fractures and matrix, which weakened the shear modulus in the area. No extension of the injection impact toward production wells was observed in the joint inversion results.