The history of the exploitation of the Mutnovsky geothermal field includes multi-wells flowtests 1983-1987, a build-up period 1988-1998, followed by 12 MWe pilot PP start up in 1999, with a full-scale installed capacity of 62 MWe since 2002. An initial 3D rectangular TOUGH2 numerical model (Kiryukhin, 1996) was designed ˇ§as simple as possibleˇ¨ to contain a minimum number of elements (500+) to describe existing production/injection wells and the reservoir. Such a model is referred to as a ˇ§hydraulic type modelˇ¨, which is acceptable by Russian Authorities for geothermal reserves estimation. This model covers 5„e5„e2 km3, includes 21 domains with different petrophysical properties, heat and mass recharge defined at the base layer, and recharge corresponding to known significant hot springs and steam ground areas. We decided to recalibrate the model using 1983-2006 history exploitation data using the inverse modeling capabilities of iTOUGH2-EOS1. Twenty-two wells were defined in the model, including 18 production and 4 injection wells. Double-porosity was assigned in the model elements containing slotted intervals of production/injection wells. Note, that double-porosity needs to reproduce excess enthalpies of the modeling production wells during exploitation. Monthly averaged enthalpies of five production wells during the time period 1984-2006 are used for model calibration, with the total number of observational points being 523. Fractures permeabilities and porosities of the model elements containing these five production wells, and reinjection rates of four wells were assigned as parameters to be estimated. Use of the reinjection rates as adjustable parameters was needed because of the impossibility to reproduce enthalpies of the production wells at reported reinjected rates. A sensitivity analysis shows that the most influential parameters are the reinjection rates, with estimates in range of 0-12% of reported values. Additional improvement of the model convergence was found after capillary pressure functions were assigned in two-phase model elements, containing production wells.

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