COMPARISON OF ACOUSTIC AND ELECTRICAL IMAGE LOGS FROM THE COSO

Electrical and acoustic image logs collected from well 58A-10 in crystalline rock on the eastern margin of the Coso Geothermal Field, CA, reveal different populations of planar structures intersecting the borehole. Electrical image logs appear to be sensitive to variations in mineralogy, porosity, and fluid content that highlight both natural fractures and rock fabrics. These fabric elements account for about 50% of the total population of planar structures seen in the electrical image log, but locally approach 100%. This fabric is unlikely to contribute to permeability in the reservoir. Acoustic image logs reveal a similar natural fracture population, but generally image slightly fewer fractures, and do not reveal rock fabric. Both logs also record textural properties of deformed materials within fractures; these textures can be related to variations in mineralogy, alteration, or porosity using the electrical log and can be used to infer slip history. In addition, locations of high fracture density occur adjacent to major faults, but also occur as zones confined within intervals of distinct rock type. The acoustic image logs collected in Well 58A10 reveal large numbers of drilling-induced borehole wall tensile fractures and breakouts. In addition, previously un-recognized drilling-induced petal-centerline fractures are evident in images of the borehole wall from this well. These features are thought to form just ahead of the drill bit and provide additional constraint on the orientation of the minimum horizontal stress. Both breakouts and petal-centerline fractures are difficult to identify in electrical image logs because of the low percentage of the borehole wall imaged. Analysis of induced structures in acoustic image logs indicates that the minimum horizontal stress is oriented along an azimuth of 102º±18º. This orientation is consistent with the local strike of the Coso Wash normal fault that bounds the basin containing well 58A-10