Evolution of the plastic zone near a microfracture: a numerical Simulation and its implications on in situ stress measurement

The instantaneous shut-in pressure has been used to estimate the far-field in situ minimum principal stress during microfracture testing. A plastic zone can be induced near the fracture. Because of the early plastic (irreversible) deformation induced near the fracture, the irreversible deformation near the fracture surface reduces the fracture pressure and generates a discrepancy between the far-field minimum stress and the fracture closure pressure, which has been identified as the minimum in situ stress in the past. In this paper, a finite-element numerical model is used to investigate this discrepancy due to the plastic deformation near a fracture. It is concluded that a plastic zone can be generated near a hydraulic fracture in poorly consolidated media. The fracture closure pressure can be much smaller than the minimum in situ stress due to the irreversible deformation generated near the fracture during the active fracturing period. Thus, one cannot use the conventional hydraulic-fracturing technique to interpret the minifracture tests in a poorly consolidated medium such as oil sand. Key words : microfracturing, in situ stress measurements, oil sands, plasticity, numerical simulation.