Abstract

P>We analysed the response of the underground penetrated by borehole Basel 1 to two pumping sequences, a pre-stimulation test (flow rate up to 10 L min-1) followed by a step-up stimulation (starting from 10 L min-1 up to 3380 L min-1), performed to generate a heat exchanger at about 5 km depth. Throughout the test sequences an increase in flow rate resulted in an increase in injection pressure. Yet, sudden pressure drops (less than or similar to 0.1 MPa) as well as a continuous decrease in pressure occurred at constant flow rates above 100 L min-1. Analysis of the pre-stimulation record strongly suggests the existence of a vertical fracture prior to stimulation. We focused on two contrasting scenarios in fracture height and normalized fracture conductivity out of the range of possible geometrical characteristics of this pre-existent fracture as constrained by numerical modelling. Independent of chosen scenario, the initially modest pressure increase can only be modelled by hydraulic properties that change with fluid-pressure. The spatio-temporal characteristics of the pressure distributions differ significantly for the two scenarios; the 'formation linear flow'-like pattern found for the scenario with the larger fracture height of 100 m matches geometrically better with the oblate seismic cloud than the 'radial flow'-like pattern found for the smaller fracture height of 10 m. It is problematic to explain the drops in pressure observed at constant flow rate by lateral or vertical extension of the pre-existent vertical fracture in classical tensile hydrofracturing mode considering the persistent increase in pressure with every step-up in flow rate throughout the entire stimulation and the lack of reproducible and distinct instantaneous shut-in pressures. We suggest shearing and wing-extension of the pre-existent fracture(s) as an alternative mechanism since this mechanism (a) is consistent with the absence of a distinct breakdown in pressure, (b) can be initiated at fluid pressures below the least principal stress and (c) can explain the pronounced change in hydraulic properties with increasing injection pressure as well as the observed rapid small pressure drops at constant flow rate.