A Comprehensive Study of Fracture Patterns and Densities in The Geysers Geothermal Reservoir Using Microearthquake Shear-Wave Splitting Tomography

Abstract

In this project we developed a method for using seismic S-wave data to map the patterns and densities of sub-surface fractures in the NW Geysers Geothermal Field. 1) This project adds to both the general methods needed to characterize the geothermal production fractures that supply steam for power generation and to the specific knowledge of these in the Geysers area. 2) By locating zones of high fracture density it will be possible to reduce the cost of geothermal power development with the targeting of high production geothermal wells.3) The results of the project having been transferred to both US based and international geothermal research and exploration agencies and concerns by several published papers and meeting presentations, and through the distribution of the data handling and other software codes we developed.Our project extended over a 3 year time period. It included extensive compilation of data from existing and past seismic networks surrounding the Geysers Geothermal area. The study focused on identifying, measuring, modeling, and interpreting the effects of fractures on seismic S-waves at this site, specifically the North West Geysers. Computer codes to handle and process such observations were developed. The last year of our project included a DOE guided, scientific and collaborative exchange with Dr. Silas Simiyu of the Kenya Electric Power Generating Company, Inc., the national power company of the Government of Kenya.As part of our collaboration with industry and other exploration groups, we provided a training course on S-wave splitting tomography. The notes from this course are included in this Final Report, as are compressed versions of the publications resulting from from our work.A further benefit to both the geothermal research community and geothermal industry resulted from our collaboration with KenGen: after hearing our presentation of our results on this project at the World Geothermal Congress in Japan, we were approached by the World Bank and United Nations Environmental Program to begin a similar project in the East African Rift Region. Comparison of the Accomplishments with Goals and ObjectivesFor this project our original Goals and Objectives were:1. S-wave splitting identification and processing of microearthquake recordings from geothermal exploration and production areas. Application of this identification and processing approach to the NW Geysers 2. Determining the crack orientations around the seismic networks, past and present, in the NW Geysers.3. Complete a 3-D inversion for the crack density distribution in the NW Geysers.4. Develop further the computational methods for forward modeling of anisotropic wave propagation.5. Develop a software package that can handle all of the steps above.The Accomplishments in each of these areas of our project were:1. We both tested the S-wave splitting analysis approaches of the University of Edinburgh group (S. Crampin, PI) and developed our own method. The new methods are summarized in Lou et al. (1997), a copy of which is attached t...