| Title | Micro-seismicity at the Miravalles Geothermal Field, Costa Rica (1994-2009): A Tool to Confirm the Real Extent of the Reservoir |
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| Authors | Paul Moya and Waldo Taylor |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Micro-seismicity, micro-seismicity distribution, fault systems, reservoir extent and boundaries, Miravalles geothermal field, Costa Rica |
| Abstract | The micro-seismicity at the Miravalles geothermal field has been analyzed for the period from 1994 to the present. The study period is divided into four phases that coincide with the phases of development of the geothermal field: 1) January 1994 to May 1998; 2) June 1998 to February 2000; 3) March 2000 to December 2003; and 4) January 2004 to April 2009. Before the commissioning of Unit 1, the micro-seismicity did not have a defined pattern related to geological structures or the extent of the geothermal reservoir. Micro-seismicity was low during the first stage of field operation, while in the second stage, after a seismic swarm in October 1997, there was a 16-month period of seismic quiescence. Since the year 2000 (third phase) the micro-seismicity has been increasing again, and its main characteristic is that it appears in groups of tens of events with short recurrence intervals (less than one hour), at depths of 2 km, with a coda magnitude (Mc) lower than 2.0; this happens especially when the total mass production rate is less than 4 X 106 ton per month. During the last four years, the micro-seismicity has increased and is more closely related to the N-S and NE-SW-trending fault systems. This has allowed the reservoir extent and shape to be identified clearly. The production-injection relationship is not that clear yet. Seismic monitoring is being employed at the Las Pailas geothermal field, where new geothermal development is taking place, in order to develop an idea of the possible boundaries of that reservoir. This new field is different from the Miravalles geothermal field, and there is a closer relationship between fluid injection and micro-seismicity. This methodology can help to better understand the extent of the reservoir once production begins. The analysis of the micro-seismicity and its use in defining reservoir boundaries is described in the following sections. |