Evidence for a Highly Conductive Fracture after Water Fracturing in the GeneSys Project

Abstract

The GeneSys project aims to develop a low-permeability, sedimentary reservoir of the Northern German Basin for the provision of geothermal energy to heat the Federal Institute for Geosciences and Natural Resources (BGR) located at the Geo Center in Hanover, Germany. In May 2011, a massive fracturing (frac) operation was performed to create a large heat exchanger at a depth of 3,700 m. Subsequent to the frac operation, two low-rate injection tests were conducted below fracture-closure pressure. The injection phases of the two tests provide evidence for a large and highly conductive fracture, although proppants were not used during the frac operation, indicating that the fracture stays open because of self-propping effects, at least in the explored range of effective stress. Hydraulically active fracture areas of more than 0.5 km2 were calculated from an analysis of the pressure transient during the low-rate injection tests relying on matrix permeabilities determined in laboratory experiments. In principle, such areas are sufficient to yield geothermal power in the range of 2 MW (thermal performance). The hydraulically active fracture area and estimated fracture transmissibility diminished between the two low-rate injection tests and, hence, with increasing effective normal stress. The authors suppose this observation is because of a hydromechanical component of self-propping effects. Similarly, the mismatch between the shut-in observations and conventional hydraulic models indicates that hydromechanical processes affect the pressure transient by continuously decreasing the fracture aperture.