Naphthalene Disulfonate Tracer Test in Mahanagdong Geothermal Field, Leyte, Philippines

Abstract

Tracer test was conducted in Mahanagdong using three types of Naphthalene Disulfonate (NDS) tracers: 1,5-NDS, 1,6-NDS and 2,6-NDS. The objective of the test is to define hydrological flow paths of cooler peripheral and injected fluids into the production sector and their assess impact to the producing wells in terms of degree of fluid mixing and cooling effect using the TRINV/ICEBOX software (United Nations University Training Programme, 1994)).
The tracers were injected simultaneously at: 1) MG4DA, close to the source of a natural groundwater recharge; 2) MG21D, located at the northwest section of the field utilized for hot brine injection; and 3) MG5RD, located at the southeast section of the field utilized for hot brine injection. Multiple tracers analyzed in a single sample but yielding discrete results can be used in evaluating several reservoir processes occurring together in a single field.
The 1,5-NDS tracer injected at MG21D indicated fast returns of injected brine into most of the wells north of the field with average velocities of 40 to 100 m/day and tracer recoveries of 0.30 to 2.0%. The conduits of the injected brine are the Malitbog, North Mamban and Ewex faults, which are the major structures intersected by MG21D and the adjacent producing wells. On the other hand, the 2,6-NDS tracer injected at MG5RD yielded positive returns in wells located south of the production field with the Lower Mahanagdong fault as conduit. The result suggests a much slower return of injected brine (ave. velocities ~ 13-17 m/day) into the production sector and tracer mass recoveries of 3 to 7%. Consistent with the result of the geochemical evaluation wherein no thermal decline was observed even though most of the wells have already shown significant increase in fluid mineralization, low brine return velocities may allow sufficient fluid heat-up. The 1,6-NDS injected at MG4DA likewise yielded positive returns in wells located at the northern section of the field, with average velocities of 7.0 to 9.0 m/day. This basically confirmed the natural groundwater recharge from the Paril area in the west, with preferential flow towards pad MGDL, the area with the highest drawdown.
Except for groundwater recharge, cooling predictions based on ICEBOX software indicate reservoir temperature decline of 0.5 to 2.0?C and 2.0 to 8.0?C in the north and south sections of the field, respectively.