The geological characterisation and permeability evaluation of fractures in the Southern Negros Geothermal Field, Negros Island, Philippines

Abstract

Fluid flow pathway characterisation is critical to geothermal exploration and exploitation. In fractured geothermal reservoirs, it requires a good understanding of the structural evolution, fault distribution and fluid flow properties. A dominantly fieldwork-based approach has been used to evaluate the potential fracture permeability characteristics of a typical high-temperature geothermal reservoir in the Southern Negros Geothermal Field, Philippines. This is a liquid-dominated resource hosted in the andesitic Quaternary Cuernos de Negros Volcano, Negros Island. Fieldwork reveals two main fracture groups based on fault rock characteristics, alteration type, relative age of deformation, and associated thermal manifestation, with the younger fractures mainly related to the development of the current geothermal system. Fault kinematics and cross-cutting relationships suggest they have been formed during three fracturing events under changing stress fields since perhaps the Pliocene. This deformation history was influenced by the development of the Cuernos de Negros Volcano and the northward propagation of the Yupisan Fault. A combined slip and dilation tendency analysis of the mapped faults indicates that NW-SE structures should be particularly promising drilling targets under the inferred current stress regime. Frequency versus length and aperture plots of fractures across six to eight orders of magnitude can be described by a power law distribution, with a change in scaling exponent in the 100 to 500 m length-scales. An evaluation of the topology of the fracture branches shows the dominance of Y-nodes that are mostly doubly connected suggesting good connectivity and permeability within the fracture networks. The self-similarity of the fracture size distribution of the outcrop and borehole datasets suggests that the fracture network mapped in the surface are good analogues for the fracture system at the reservoir depth. Finally, a semi-quantitative permeability analysis of representative rock samples of the reservoir indicates that fractures contribute >90% of the borehole permeability in some of the Southern Negros Geothermal Field wells. The results obtained in this study suggest that field-based multiscale datasets can be globally applied during the different phases of exploration, development, and exploitation to better characterise fracture systems in geothermal reservoirs.