Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Dual Porosity Models of a Two-phase Geothermal Reservoir Jaime Jemuel C. Austria, Jr. 1, 2 , Michael J. O’Sullivan 2 1 Energy Development Corporation, 38/F One Corporate Centre Building, Julia Vargas, Pasig 1605, Philippines 2 Department of Engineering Science, University of Auckland, Auckland 1010 New Zealand austria.jjc@energy.com.ph; m.osullivan@auckland.ac.nz Keywords: Geothermal, reservoir simulation, dual porosity, MINC, Mt. Apo geothermal production field ABSTRACT The research reported here is part of a general study aimed at determining when dual porosity models should be preferred ahead of single porosity models for modeling geothermal systems. The Mt. Apo geothermal reservoir, in Mindanao, Philippines, was simulated using both single and dual porosity models and inverse modeling was used to estimate permeabilities and porosities. The Mt. Apo system was selected as a test case because it consists of low to moderate permeability fractured rock and some of the wells produce high enthalpy fluid. Both of these factors make it likely that a dual porosity model may be useful. The forward simulations were carried out with AUTOUGH2 (Yeh et al., 2012), a modified version of TOUGH2 (Pruess, 1991) while the inverse problem of determining the best-fit parameters for the dual natural-state and production history model calibration was solved using PEST (Doherty, 2010). The model was calibrated using steady-state temperatures and pressure data, and monthly average monthly enthalpy data for a period of 16.2 years. The results were compared for a single porosity model and various dual porosity models with the aim of determining whether or not one type of model clearly fitted the data better than the others. A dual porosity model gives the best match to the measured production enthalpies. 1. INTRODUCTION The main objective of this work is to determine how the results from single and dual porosity models compare for simulations of the production history of a two-phase geothermal reservoir such as the Mt. Apo geothermal reservoir in the Philippines. The fractured nature of the reservoir, the presence of a steam zone in the natural-state, the occurrence of wells that intersect the steam zone and discharge high enthalpy fluid makes the Mt Apo geothermal reservoir a good test case for modeling with the dual porosity approach. The Mt. Apo geothermal field is located inside the 7.01-km 2 Mt. Apo geothermal reservation area in the south-eastern part of the island of Mindanao in the Philippines (See Figure 1.) Mt. Apo at 2,954 masl is the highest peak in the Philippines. The main features of the Mt. Apo geothermal reservoir are the Sandawa Collapse, the Marbel Corridor, and the Matingao segment. The Mt. Apo geothermal reservoir is characterized by very high reservoir temperatures (>300C) and neutral chloride production fluid (Trazona et al., 2002). The main reservoir of the Mt. Apo geothermal system is controlled by fractures, with the upwelling fluid flowing horizontally to the northwest through the northwest-southeast trending faults in the Marbel Corridor which serve as paths for fluid flow (Esberto and Sarmiento, 1999). To the west of Marbel is the Matingao sector which is characterized by lower temperature fluids (<220C). Recharge comes from nearby areas of higher elevation, driven by the topography. Meteoric water derived from abundant rainfall and descending cold water provide deep recharge to the reservoir (Esberto et al., 1998). Figure 1: Location of Mt. Apo geothermal field, modified after Emoricha et al. (2002) In the natural state the vertical distribution of pressure in the Mt. Apo geothermal reservoir is approximately liquid-hydrostatic (~8.2 kPa/m). When the wells with liquid-hydrostatic pressure profile were first discharged, the production enthalpy of some of them was very high, with some of the wells discharging almost dry steam. This phenomenon was attributed to the presence of a shallow steam zone beneath the Sandawa Collapse and extending above the outflow toward the Marbel Corridor in the undisturbed