| Title | Monitoring Microseismicity during Well Stimulation at the Salak Geothermal Field, Indonesia |
|---|---|
| Authors | Djoko Anityo S. Wibowo, Gregg A. Nordquist, Jim Stimac and Aquardi Suminar |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | microseismicity, MEQ, stimulation, injectivity index, permeability |
| Abstract | Awi 18-1 well was completed as an injection well west of the proven Salak (a.k.a. Awibengkok) geothermal field on November 27th, 2006. It was expected permeability in this well would come from existing fracture systems with common NE and NNE trends. The well encountered low permeability with an injectivity index (II) of 0.45 kph/psi, but was considered a good stimulation candidate. This was based on a XRMI log which showed an abundance of sealed and partially open fractures with less common open fractures. Long-term high pressure stimulation was started on May 8th, 2007 and continued through Aug 26th, 2008. Microseismic monitoring was conducted during this injection stimulation to map the distribution of microearthquakes (MEQs) induced by the stimulation. The microseismic monitoring showed a strong correlation between Awi 18-1 injection stimulation and occurrence of microseismic activity. Before injection stimulation began there were no MEQs measured near the well. Immediately following the startup of injection stimulation and through the entire period of stimulation, clusters of MEQ activity became evident. During periods of relatively low injection rates and low well head pressure (WHP), only a few MEQ events were induced. The majority of the MEQs were induced during high rates of injection and when pressures were above the fracture opening pressure (estimated at 600 to 650 psi). There were no MEQs induced that had a magnitude large enough to be felt at the surface. MEQ events induced during this stimulation were distributed around Awi 18-1 along NE, NNE and E-W trends. These trends tend to follow existing fault trends inside the Cianten Caldera. Initially, the MEQs were located near the wellbore, but with time events expanded laterally and moved deeper, eventually extending to depths of more than 5 km bsl. This expansion of the area and volume of the MEQ clusters had a good correlation with the improvement of permeability in the well. The MEQ clusters extend to the interpreted eastern margin of the Cianten Caldera, but probably do not cross it. This suggests a possible structural boundary between the Cianten Caldera and Salak reservoir. With the uncertainty of the velocity model used, this boundary may not be located precisely. |