Quantification of Injection Fluids Effects to Mindanao Geothermal Production Field Productivity Through a Series of Tracer Tests, Philippines

Abstract

Tracer tests using napthtalene disulfonates were conducted in Mindanao Geothermal Production Field, Philippines in 2003 and 2006 to describe the extent of the re-injection fluids encroachment to the production sector and to quantify its effects to well-to-well performance and over-all field productivity. Brine fluids injected since late 1999 in Matingao and Kullay injection sinks were already observed in the Marbel production sector. Modeling was done to predict the effects of RI 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. Majority of the production wells drilled in this sector indicated positive tracer response with % tracer recovery ranging from 2% to as high as 40%. Well SK2D, APO3D, APO1D, and SP4D are wells identified that have significant RI fractions in their discharges that amounts to 0.41, 0.45, 0.21 and 0.186, respectively. These substantial amounts of injected fluid could lead to significant decrease in production temperature ranging from 13oC to as high as 39oC. The timing of the temperature decline (thermal breakthrough) is highly dependent in the matrix porosity which could be as early as 2 years (porosity = 60 %) or as long as 15 years (porosity = 5 %) in the case of SK2D. The declines in production temperatures were translated to loss in steam flow in each wells and the steam availability was recalculated to determine the impact. The re-injection fluids are being channelled mainly by the major faults identified in the geothermal field (mostly NW trending faults). The steam availability of the field has an average decline rate of 0.8 kg/month which is solely attributed to reservoir cooling. Changes in the reservoir management were implemented following the integrated information obtained from the tracer tests and geochemical monitoring of production wells resulting to thermal recovery of the Marbel production sector and sustained overall Mindanao field productivity.