Fracture Permeability and Saturation Effects on the Seismic Attributes of Hydrothermally Altered Rocks from a Philippine Geothermal Field
David Carlo AUSTRIA, Philip BENSON, Annette GÖTZ, Dean Bullen
[Energy Development Corporation, Philippines]
Geophysical methods, particularly seismic attribute analysis, are seeing increased usage in studying geothermal resources in order to maximize extraction potential and minimize risks. However, to better understand deep processes it is necessary to calibrate surface seismic data to develop new models. Here, we present a series of controlled laboratory experiments where key rock physics data such as fracture network density and permeability are directly measured as a function of simulated depth. We use a suite of fresh and hydrothermally altered rocks from a Philippine geothermal field (Palinpinon or Southern Negros Geothermal Project) which are deformed using a conventional triaxial cell fitted with sensors for microseismicity, fluid pressure and fluid flow. Samples of 100mm length and 40mm diameter were prepared with small notches and a pair of offset 3mm drill holes such that once fractured, the offset drill holes form an access pathway along the newly generated fault plane. In this way, the formation and evolution of natural fracture and damage zones and resulting permeability can be directly evaluated. The sample assembly was encased in a rubber nitrile jacket to separate the sample from the confining fluid (oil) and fitted with up to 18 ports for Acoustic Emission sensors to monitor microseismicity. This setup allows an experiment to gather seismic data in different ray paths and key attributes (static and dynamic moduli) while fracture permeability develops. Our initial results reveal a significant reduction in all seismic attributes (P/S-wave velocities and elastic moduli) after fracture development, except for Poisson’s ratio, which shows the opposite trend. Further, the reduction in fracture permeability coincides with decreasing Poisson’s ratio and increasing P and S wave velocities, dynamic bulk modulus, Lame’s first coefficient and Young’s modulus. Experiments conducted at elevated temperatures (175 C) show a ‘swarm’ of Acoustic Emission events at the moment of, and shortly after, pore fluid decompression. We postulate that this is due to rapid fluid movement and phase changes from liquid to gas within the damage zone. We correlate the resulting data trends to present new links between fracture permeability and seismic data in the case of the altered andesite of the Southern Negros Geothermal Project, thus allowing for better interpretations of surface seismic models of volcanic geothermal fields in the Philippines.
|        Topic: Geophysics||Paper Number: 13010|