Analysis and Interpretation of Naphthalene Disulfonate Tracer Tests at the Mahanagdong Geothermal Field, Philippines

Abstract

To define the hydrological flow paths and rate of return/inflow of cooler peripheral waters in the Mahanagdong geothermal reservoir, three types of naphthalene disulfonate (NDS) tracers were introduced in three different wells in January 2003. 1,6-NDS was injected in Well MG-4DA to trace the movement of the Paril groundwater. 1,5-NDS and 2,6-NDS were injected in Wells MG-21D and MG-5RD, respectively, to characterize the power plant condensates and separated brine injected at the MGB3 and MGRD1 injection pads. Of the twenty wells monitored for more than nine months, twelve wells showed positive tracer responses from one or two of the three injection wells, indicating the influence of several types of peripheral waters. The tracer test data was analyzed using a multi-path model based on the one-dimensional transport equation for porous media. The principal outputs from the analysis were recovery and discharge fractions and average residence time of the fluids. Analysis results established the hydrological connection between the injection and production wells, quantified the effect of returns in terms of mass fractions, and estimated the velocity of return/inflow. A discussion of the results is presented.