Fluid injection through artificially reconstituted bio-cemented sands

Abstract

The hydraulic fracturing technology is widely applied in the fields of groundwater hydraulics, hydrogeology, geo-environmental engineering and the oil and gas industry. Bio-cemented granular media were used as a proxy for weakly-cemented and poorly-consolidated sands in fluid injection experiments. The experiments were conducted in a testing apparatus which is capable of applying true triaxial stresses and allowed for visualisation of fracture propagation. Tests were conducted with bio-treated sands across various cementation levels at three stress states. In all tests, an opening was observed whose major principal direction was parallel to the maximum horizontal stress. A cavity was also developed with associated plastic deformation which in lower cementation levels reached even the boundaries of the specimen causing extensive ‘damage’ — disaggregation at the grain scale. At higher cementation levels, the cavity was developed near the injection point and then fractures made their presence in the expected direction. The peak pressures were approximately 10–15 times higher compared to the mean stress, while for lower cementations the pressure profiles were very noisy due to continuous breakdown of cementation. The fracture response is clearly dependant on the material properties (strength, hydraulic conductivity) and on the applied stresses. The combined effects of the applied stresses of the system, the strength and stiffness of the material were seen through the calculation of brittleness index derived from the Mohr–Coulomb and the Lade–Duncan failure criteria. The peak pressures agree better with the limit pressure calculated from the cavity expansion theory rather than the hydraulic fracturing theory.