Theoretical and field studies of coupled hydromechanical behaviour of fractured rocks—2. Field experiment and modelling

Abstract A series of water injection experiments were performed on four horizontal fractures intercepted by a borehole at the University of Lulea Rock Mechanics Laboratory site in Sweden. The fractures are located in a borehole section 80–350m in depth. Three types of injection tests were carried out: (1) constant-pressure injection tests, with pressures ranging from below to above the overburden pressure; (2) constant-flowrate injection tests; and (3) step pressure tests (hydraulic jacking test). In the former two cases, the results clearly depict the coupled effect of mechanical fracture-aperture opening due to increased hydraulic pressure, even in the pre-hydrofracturing regime. A parallel numerical study was carried out to simulate these injection tests using the computer code ROCMAS II [see Part 1, Int. J. Rocl Mech. Min. Sci. & Geomech. Abstr. 29 , 401–409 (1992)]. We were able to simulate closely the pre-hydrofracturing coupled effect in the step pressure and constant pressure test. The success in this attempt validates the soundness of the conceptualized constitute relations governing the coupled hydromechanical behaviour of rock masses. The results also point out a potential methodology of using a joint field and theoretical study of fracture hydromechanical responses to extract key characterizing parameters of the rock and fractures.