Fracture Attribute and Topology Characteristics of a Geothermal Reservoir: Southern Negros, Philippines

The characterization of fracture networks using attribute and topology analyses has not been widely applied to geothermal resources in order to better understand and predict secondary porosity, permeability, and fluid flow characteristics. In this study, fracture length, aperture, intensity, and topology data were acquired from remotely sensed images and surface exposures from the Cuernos de Negros region, and compared with well cores and thin sections from the underlying active geothermal reservoir: the Southern Negros Geothermal Field (SNGF), west central Philippines. We show that the fracture attributes for the analogue and reservoir are best described by a power law distribution of fracture length and aperture intensity across six to eight orders of magnitude. This characterization of outcrop and borehole fractures validates the use of surface exposures as analogues for the SNGF reservoir rocks at depth. An observed change in the scaling exponent in the 100 to 500 m lengthscales suggests that regional to sub-regional fracture systems scale differently to those at the mesoscale and macroscale, which may be a strata-bound effect or a sampling issue. Topology analyses show a dominance of Y-nodes and doubly connected branches, which indicates a high degree of fracture connectivity that is important for effective fluid flow.