In situ permeability and fluid pressure measurements at ∼2 km depth in the Cajon Pass research well



Two in situ bulk permeability tests were conducted in the metamorphic basement rocks in the Cajon Pass research well. In one interval, from 1829 m to 1905 m, we measured an effective permeability of 0.5 × 10−18 m² (0.5 × 10−6 darcy). Over an interval from 1829 m to 2115 m, we measured an average permeability of 1.67 × 10−18 m² (1.67 × 10−6 darcy). In addition, fluid pressures 5% over hydrostatic were determined through analysis of pressure build-up in the larger test interval. The in situ permeability measured is one to three orders of magnitude greater than the permeability measured in core samples; a difference we attribute to the presence of fluid conducting fractures. A large fracture at 2076 m with an aperture of 26.6 cm appears to be a zone of localized high permeability. The permeability of this fracture is as high as 1.8 × 10−16 m² (0.18 × 10−3 darcy). In light of the low permeability and fluid pressure measured, the transport of significant amounts of frictionally generated heat by hydraulic conduction is extremely unlikely.