Problems and research performed
TC6 was defined for numerical modeling of an in-situ field injection experiment of a sub-horizontal fracture conducted in a borehole of 56 mm in diameter at Luleå University of Technology, Sweden, in 1992 during the early stage of this DECOVALEX I project. The Test Case addressed the role of coupled hydro-mechanical processes of water injection effects on the behavior of a sub-horizontal fracture located at a depth of 356–7 m below the ground surface, with the section of the borehole containing the fracture isolated by pressurized packers (see Fig.10a). A series of three types of injection tests were conducted: pressure pulse test, step pressure test (hydraulic jacking), and constant pressure injection test with a pressure exceeding the overburden pressure. The TC6’s objective was calibration of numerical models against the measured field test results, with targeted calibration of Young’s modulus of the host rock, initial hydraulic aperture of the tested fracture at the initial stress state, type and location of the outer boundary of the tested fracture and effective normal stress as a function of normal displacement of the tested fracture.
Five research teams participated in TC6 with different computer codes and numerical methods: KTH (supported by SKI) using the code ROCMAS of FEM, AECB (self-supporting) using the code FRACON of FEM, AECL (self-supporting) using the code MOTIF of FEM, AEA (supported by NIREX and CEC) using code NAPSAC of DFN and LTH (Lund University, Sweden, supported by SKB), using analytical solutions. Figures 10b and 10c show the results of measured and calculated well pressure variations vs. time and well flow, respectively.
Main achievements and outstanding issues
The TC6 modeling was in good agreement with the measured data, with the differences mainly caused by slight differences in parameters, such as the initial values of the fracture properties, and possibly also the effects of the fracture size estimations. TC6 showed that, for injection pressure below the virgin stress normal to the fracture plane, the coupled hydro-mechanical process of the fracture dictated the pressure-flow responses, and the key parameters were the hydraulic aperture, normal stiffness and effective normal stress of the facture.
Publications
- J. Claesson, S. Follin, G. Hellström, N.-O. Wallin. On the use of the diffusion equation in Test Case 6 of DECOVALEX. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1995; 32(5): 525-528. (Note: This paper is concerned with the DECOVALEX project’s Test Case 6, but the authors were not participants of the DECOVALEX project).
- L Rutqvist. Determination of hydraulic normal stiffness of fractures in hard rock from well testing. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1995; 32(5): 513-523.