Stress Induced Changes in Two-Phase Permeability

An accurate knowledge of porosity and permeability is crucial to accurate simulation of reservoir performance and enhanced reservoir management. Porosity and single-phase permeability of reservoir rocks is dependent on the stress experienced by the rock mass, which in the simplest case is given by the overburden stress minus the pore pressure. Hydrocarbon production decreases the pore pressure causing an increase in stress. This causes coupled changes in permeability and porosity, which have a significant effect on production rates. The stress and permeability changes are particularly large in the near wellbore region. The experimental programme was performed to examine the changes in two-phase permeability during deformation. This consisted of experiments where, starting from a given hydrostatic stress level, s₁ was increased until the rock failed and a shear fault was formed. In all experiments core flooding took place during the stress changes, either single-phase core flooding, oilflooding at S_wi or waterflooding at S_or. The experiments were performed on a water-wet sandstone. The deformation experiments were performed over a range of confining stress. Single phase, oilfloods and waterfloods, were performed at each stress. The stress strain behaviour, peak stress, residual sliding stress were independent of flooding type, but the variation of permeability was different depending on whether it was single or two phase flow. At low confining stress the single-phase experiments showed a smooth drop in permeability with strain, the oilflood showed first a decrease then a recovery then a steep drop at failure. The waterflood gave an intermediate behaviour between the other two. The results can be explained as an interplay between two mechanisms, compaction induced increase in tortuosity decreasing permeability and a dilatant increase in pore volume increasing permeability. At high confining stress the results showed the same effects except that the higher pressure inhibited dilatant cracking. This work has applications in determining the effect of the stress path on the permeability of the reservoir. The differences between the single and two-phase permeability are of great importance when the reservoir stress changes are likely to cause failure. The single-phase data greatly underestimates the actual permeability change.