Hydraulic in situ investigations of an artificial fracture in the Falkenberg granite

Abstract

The paper describes the results of hydraulic experiments on a large fracture produced by hydraulic fracturing in the Falkenberg granite. The friction pressure losses in the fracture were investigated for laminar and turbulent flow. Other experiments examined the pressure dependency of fracture transmissivity, fracture width and fracture storage coefficient. The following results were obtained: the friction losses in the fracture in the vicinity of the borehole increase non-linearly with the flowrate for flowrates higher than a critical value of about 0.51/sec. Injecting and venting results differed only slightly in the range of flowrate investigated (0.1–71/sec) but the difference seems to increase at higher flowrates. The observed relation between friction losses and flowrate is explained by assuming a turbulent region around the boreholes, expanding with an increasing flowrate. Fracture transmissivity and fracture width changed non-linearly with increasing and decreasing fluid pressure. Fracture transmissivity increased from a few D·m (Darcy·m) at hydrostatic pressure (corresponding to 1.9 MPa effective normal stress) to some 100 D·m at frac-extension pressure (corresponding to −0.3 M Pa effective normal stress). Fracture width ranged from 0.2–0.5 mm in the different boreholes at hydrostatic pressure to 1.0–2.0 mm at frac-extension pressure. The storage coefficient was S = 10−10 m/Pa at hydrostatic pressure and S = 10−9 m/Pa near frac-extension pressure.