Geology, Temperature, Geophysics, Stress Orientations, and Natural Fracturing in the Milford Valley, UT Informed by the Drilling Results of the First Horizontal Wells at the Cape Modern Geothermal Project

Abstract

Fervo Energy (Fervo) has completed the drilling of the first deep geothermal wells at Project Cape, including a vertical observation well to 9,844 ft and four 13,000+ ft-MD horizontal wells drilled in a ‘wine rack’ pattern with 5,000 ft laterals at vertical depths of 8150-8550 ft. These new wells were drilled west of the DOE FORGE project in Milford Valley and were informed by the significant datasets generated by FORGE. New datasets collected through Fervo drilling have substantially increased knowledge of the geology, temperature, state of stress, and natural fracturing in Milford Valley, leading to new 3D basin models with an unprecedented amount of well data collected. A new detailed gravity study combined with density logs from the deep wells enabled a basin-spanning depth to granitic basement model tied to drilling results from existing wells. Smectite clay zones observed in Fervo wells combined with magnetotellurics (MT) data collected by FORGE led to the discovery of an extensive clay body which was modeled in 3D throughout the basin. DTS fiber and wireline temperature logs confirmed average temperatures of 200°C across the laterals. Image logs across the horizontal wells were used to create an integrated fracture density model across the granitic reservoir area. Image logs from vertical portions of new wells confirm a maximum horizontal stress orientation (SHmax) of NNE-SSW with consistent SHmax orientations measured across the basin. Major improvements in drilling performance were gained throughout the drilling program, demonstrating a rapid learning curve to enable fast lateral drilling through granitic basement rock.