Perpustakaan Digital ITB

The most important thing in time-lapse microgravity measurement using relative gravimeters is in an acquisition step. As we know very well, measurement using relative gravimeters in oil / gas / geothermal fields has less accuracy compared to laboratory measurements. We have to set a high regulation of quality control of our time-lapse microgravity data acquisitions in order to get accurate response from subsurface sources. Values of time-lapse microgravity anomaly are relatively small, in ?Gal order. Therefore, the time-lapse microgravity data acquisition should be set up for this order of anomaly target. The first acquisition of time-lapse microgravity data becomes our baseline map. So for the next acquisition, we have to do exactly the same things as we did in the first acquisition. In this paper, we describe the several steps of quality control for the 2011 microgravity data acquisition using a relative gravimeter (Scintrex CG-5) over the Wayang Windu geothermal field, West Java, Indonesia. These steps involve consistency of gravimeters, tidal correction using base observation, drift correction (less than 35 ?Gal), repeatability check in the same station with different loops (less than 10 ?Gal), derive our time-lapse maps using 10 ?Gal contour interval, and considering different levels of gravimeter tripods. Finally, we present the application of 2008-2011 time-lapse microgravity in order to understand geothermal reservoir behavior. After going through the calculation of time-lapse microgravity anomaly, we apply several methods of gravity modeling (estimating the distribution of density contrast changes, estimating the apparent saturation, 2.5D forward gravity modeling, and 3D inverse gravity modeling) to draw our interpretation. These results give a good agreement with the previous and others studies. Time-lapse microgravity results led us to a more comprehensive understanding of geothermal reservoir behavior relating to injection and production processes, fault delineation and its behavior to fluid movement (complemented with the other study results).