| Abstract |
In order to determine the magnitude of encroachment of injected fluids from the injection sinks towards the two production sectors in Mindanao Geothermal Field (MGF) namely Marbel and Sandawa, two types of naphthalene disulfonates (NDS) were used as tracer, namely 1,6-NDS and 2,6-NDS. After a year of monitoring, all of the production wells in Marbel sector except MD1D and three wells inside the Sandawa gave positive response from both tracers. The significantly affected wells based on the faster breakthrough time and the amount of tracer recovered are SK2D, APO3D and SK4B from the injected fluid coming from Matingao, while the injected fluids from Kullay are well APO1D, SP4D and SK5D. The injection fluids are being channeled mainly by the major faults identified in the geothermal field (mostly NW trending faults) and these are enumerated in the report. Modeling was done to predict the effect of injection fluids in terms of output and temperature declines in each production wells using the Icebox« software package comprising of three sub-programs: TRMASS, TRINV and TRCOOL. SK2D, APO1D and SP4D are wells identified that have significant injection fractions in their discharges that amounts to 0.41, 0.21 and 0.186 respectively. These substantial amounts of injected fluid could lead to high decline in the production temperature ranging from 13 to as high as 39oC. The timing of the temperature decline (thermal breakthrough), which is highly dependent in the matrix porosity, it could be as early as 2 years (porosity = 60 %) or as long as 15 years (porosity = 5%) in the case of SK2D. After 8 years of exploitation, the actual decline in the production temperatures of injection affected wells imply the on-set of thermal breakthrough. Thus, with the current injection scheme, further decline in the production temperature is expected in the near future. |