Hydrological flow and thermal interference modelling in the Mahanagdong geothermal field, Philippines, using four types of naphthalene disulfonate tracer

Abstract

The Mahanagdong geothermal field has been supplying steam since 1997 to its two power plants with a total installed capacity of 180 MWe. The field employs 100% brine reinjection and tracer testing has been used as one of the main tools in optimizing the reinjection strategy. A tracer test using four types of naphthalene disulfonate tracer was conducted in June 2011. Three tracer types were injected into three reinjection wells in the northern injection sink to evaluate the effects of brine returns, while a fourth type of tracer was injected into MG-4DA, a well located in the western part of the field, shut throughout the tracer test duration, to trace the inflow of groundwater to the production sector. More than 900 samples were collected within a year of monitoring. Tracer test results indicate that under the conditions during the tracer monitoring, reinjection in the northern injection sink did not cause a significant return of reinjected brine as to have any significant effect on the production sector. However, tracer analysis results for groundwater downflow through well MG-4DA yielded disquieting results. Thermal interference modelling indicated that for the pessimistic scenario of a 10 kg/s downflow rate in well MG- 4DA, a maximum temperature decline of 1.4°C was predicted after 20 years.

The predicted decline in temperature of the well affected by groundwater inflow was then compared with the actual cooling observed. Data on measured cooling from 2003 to date indicated that actual temperature decline of all the affected wells was higher than the predicted temperature decline, even after 20 years. This finding suggested that an even greater inflow of groundwater – greater than the downflow observed in well MG-4DA – exists and must, therefore, be estimated. Groundwater inflow modelling was conducted and results indicated that the rate of groundwater inflow to the production sector is estimated to be around 200 – 300 kg/s. Calculation of the average groundwater velocity indicated that the downflow observed in well MG- 4DA could most likely be just a small part of a much greater groundwater inflow towards the production sector. Management interventions with regard to reinjection strategies, pre-emptive groundwater encroachment, long-term field development, as well as developing a more detailed 3D numerical model are presented.