| Title | FORWARD AND INVERSE MODELLING OF GEOTHERMAL MICROSEISMICITY USING TOUGH2 COUPLED WITH AN EARTHQUAKE SIMULATOR |
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| Authors | J.M. Rivera, D. Dempsey |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | microseismicity, reservoir modelling, injection, inverse modeling, TOUGH2 |
| Abstract | Reservoir modelling is undertaken to represent the physical state of the reservoir in order to estimate its current condition and to predict future responses. Prior to use of the reservoir model for forecasting, it is common to calibrate natural state and production models using temperature and pressure data gathered from downhole surveys. In addition to these data, microseismicity presents a further opportunity to calibrate reservoir parameters, in particular the permeability of active faults that serve as major fluid pathways. Microearthquakes (MEQs) occur in areas where brine produced from the production wells is reinjected. The injection causes fluid pressure to build-up in the area, which decreases the rock yield strength and promotes failure: a small earthquake. These events may occur on active faults that are also major fluid pathways in the field. The location, migration, and number of MEQs provide information about pressure change and the nature of fluid flow through the reservoir. Many fields these days are equipped with instruments to detect and locate MEQs. The objective of this project is to integrate MEQ data into the reservoir model development workflow so as to assist model calibration and reservoir characterization. In this project, a simple reservoir model is created to represent an area into which fluid is injected. A forward run using the TOUGH2 reservoir simulator is conducted to estimate pressure changes due to injection into a single well for specified reservoir and fault parameters. Pressure change on the fault is used to compute an average seismicity rate as well as individual MEQ locations and times. Sensitivity analysis has been conducted to understand how model parameters affect the amount of seismicity generated, and the manner in which it travels along the fault. The coupling between reservoir pressure evolution and synthetic microseismicity provides the physical link necessary to use field MEQ data for calibration. In particular, we will use the seismicity migration rate to estimate permeability of the reservoir and faults. The synthetic study presented here is a proof-of-concept before application of the approach to an actual geothermal MEQ dataset. |