Fracture-extension pressures appreciably higher than the least principal earth-stress have been observed in hydraulic fractures formed in a pair of 3 km (9600 ft) deep boreholes drilled near the Valles Caldera in northern New Mexico. Pressurization of open wellbores in rock containing preexisting fractures may open these fractures, instead of creating new fractures at right angles to the least principal stress. The pressure necessary to flow into these fractures may be appreciably higher than the least principal stress. Upon sand-propping one such preexisting fracture, a lower fracture extension pressure was observed. A second fracture in a parallel wellbore 92 m (300 ft) away, at the same depth of 2 km (6500 ft) exhibited the lower fracture extension pressure without propping, but with about 90° difference in fracture direction. Fractures created through perforations at a depth of 3 km (9600 ft) not only exhibited breakdown pressures upon initial pressurization, but sometimes even higher "breakdown" pressures upon repressurization. These phenomena may be of interest in the interpretation of earth stress measurements made by hydraulic fracturing.
Two adjacent wells, identified as GT-2 and EE-1, were drilled on Fenton Hill, about three kilometers southwest of the Valles Caldera ring fault in northern New Mexico, in the course of the Los Alamos Scientific Laboratory Hot Dry Rock Geothermal Energy experiment. The wells entered Precambrian granitic rock at a depth of 730 m. Hydraulic fracture experiments were conducted in both wells at similar intermediate depths, about 1993 m in GT-2 and 1963 m in EE-1. The horizontal distance between the wells at these depths was only 90 m, so it was expected that the fracture direction in the two wells would be the same and that the pressure required to extend the fractures would be similar.