CONTROLS OF FRACTURE HOSTED PERMEABILITY IN BASIN AND RANGE GEOTHERMAL FIELDS: A CASE STUDY FROM THE THERMO GEOTHERMAL FIELD, SW UTAH

The commissioning of a new production well (73-32) at the Thermo geothermal field in SW Utah has allowed for examination of fracture-controlled permeability in the Basin and Range. Interpretations of acoustic borehole image (ABI) and pressure temperature spinner (PTS) logs collected in 73-32 allowed comparison of sets of fractures with differing orientations throughout the well's production interval. Furthermore, multiple flow tests conducted on the well enabled an assessment of the productivity of specific fractured intervals. From these data, a structural analysis of the different sets of productive fractures was carried out to investigate whether the orientations of fractures and their expected dilatational tendency explains the variability in productivity (i.e. permeability).

The Basin and Range is an area of present-day continental extension. Near Thermo, few wells with stress field data exist. However, the nearby Utah FORGE well 58-32 measured maximum and minimum horizontal stresses from borehole breakouts. The stress data 58-32 and stress values from the world stress map allowed for reasonable estimates of the local horizontal stress directions at Thermo. A maximum horizontal stress of 023° was estimated for Thermo, and ultimately guided the targeting of 73-32, which was oriented to intersect fractures with dilation tendency in this stress field. In general, faults and fractures striking ~023° in this region are thought to have the highest dilation tendency, and thus most likely to transmit fluids.

73-32 encountered highly fractured strata. Many of the fractures encountered by the wellbore were open fractures indicated by the ABI log, however not all the open fractures were productive indicated by the PTS logs and flow test data. 3 major production zones were identified in 73-32, each of which was associated with a subset of fractures identified in the ABI log. The most productive subset of fractures is oriented primarily 350°. This most productive subset of fractures is where total lost circulation occurred during drilling, the highest fracture density is observed, and the largest fracture aperture was observed. In this instance, the orientation of fractures relative to the stress field is at least secondary to other factors, including fracture density and aperture.