Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 3D Dual Porosity Modeling of Tracer Transport in Palinpinon 1 Geothermal Field, Philippines Anthony E. Ciriaco 1 and Michael O’Sullivan 1 Energy Development Corporation, 38F One Corporate Center, Julia Vargas corner Meralco Avenue, Pasig City 1605 Philippines Department of Engineering Science, University of Auckland, Level 3, 70 Symonds St, Auckland 1142 New Zealand 1 ciriaco.ae@energy.com.ph Keywords: Palinpinon, 3D dual porosity, tracer ABSTRACT A small-scale three-dimensional dual-porosity model was developed to simulate the results of the 2009 multi-production/multi- injection NDS tracer test carried-out in Palinpinon I Geothermal Field, Philippines. A single porosity model was first generated using the windows-based pre- and post- processing software for TOUGH2 known as PETRASIM. Partitioning of the single porosity mesh into two computational volume elements to create a double-porosity grid was done using the program called GMINC. Calibration of the model focused on the tracer return data from the four most responsive production wells only. An acceptable match of the tracer recovery profiles for the four observation wells was obtained. Results of the simulation suggest that both the highly permeable faults and the lithologic boundaries provide pathways for transport of tracer chemicals from the injection well to the production area. 1. INTRODUCTION Tracer testing has received a significant amount of attention in the geothermal industry. In the Philippines alone, several tracer tests had already been conducted in the past investigating the cause of significant cooling in the production area brought about by the injection of brine (cooler separated fluid being injected back into the reservoir for environmental compliance and pressure support, a secondary benefit). Shook (2005) reported that more than 100 geothermal tracer tests had been carried out worldwide in the previous 40 years. In 2009, three (3) different Napthalene Di-sulfonate (NDS) tracers were injected in three (3) different injection wells in the Palinpinon 1 sector of Southern Negros Geothermal Production Field (SNGPF), more commonly known as Palinpinon I Geothermal Field. The amount of tracer recovered in the production wells was analyzed and interpreted using the programs TRINV and TRMASS in the software package ICEBOX (United Nations University Geothermal Training Programme, 1994). The latest study attempts to simulate the 2009 Palinpinon 1 tracer test results using a small scale 3D reservoir model. The objective of this endeavor is to determine the dominant flow path of fluid transport, whether it is lithologically or structurally-controlled. And since the study is still at the initial stage of investigation, the following simplifications will be imposed in the model: 1) Use the dual-porosity approach in creating the model. This is the closest numerical representation of a highly fractured hydrothermal system like the Palinpinon field. 2) Only the results from one of the three tracer tests will be used as calibration parameters. 3) The tracer recovered only from the top four wells of the selected test will be treated as observation data. These production wells with the highest tracer recovery can be treated as strongly connected to the injection well where the tracer was introduced. 2. THE PALINPINON GEOTHERMAL FIELD The Palinpinon Geothermal Field is located in Valencia, Negros Oriental, central Philippines. It was commissioned in 1983 with a total installed capacity of 192.5 MWe. The field is divided into two sectors: the Palinpinon-1 (112.5 MWe) and Palinpinon-2 (80.0 MWe). Early studies conducted in the field suggest that the main structures (Figure 1) that provide channels to the fluid flow are Lagunao Fault, Ticala Fault and its splays, and Puhagan Fault in Palinpinon 1. The Palinpinon Geothermal Field is a high temperature, liquid-dominated geothermal system with localized two-phase zones in the shallow levels of the reservoir. The conceptual model of the field is shown in Figure 2. Pressure, temperature and geochemical pre-exploitation data suggest a major south-southwest of the Puhagan area. Also, the integrated data indicates presence of two outflow zones – one towards the northeastern sector and the other one towards the western sector of the field. 3. EARLY TRACER TESTS AND ANALYSES Urbino (1986) attempted to determine the structural flow paths of injected fluid based on tracer tests conducted in 1981. The study suggested that the rapid and strong returns of the tracer injected indicated direct flowpaths between the injection and production wells provided by faults which have been observed at the surface and subsurface. Bullivant (1988) used computer simulation to analyze the results of the tracer test conducted in 1985. The computer simulator is based on a model of the fluid flow in the reservoir which can include the effects of injection wells, production wells, a single fracture and background flow. Malate (1990) conducted a modeling study to model the silica changes observed in one of the production wells which was affected by injection breakthrough. Urbino (1991) used algorithms developed in Operations Research to determine the rate and extent of communication