Influence of net confining stress on NMR T2 distribution and two-phase relative permeability

Relative permeability stress sensitivity studies reported in literature do not have a common conclusion with regards to end point saturations and their respective relative permeabilities. Moreover, there is lack of experimental results on stress sensitivity of pore fluid distribution, which is a key parameter for the behavior of relative permeability curves. This study extended core homogeneity to include similar porosity, permeability and NMR T-2 distribution as well as examined NMR T-2 distribution behavior with stress on artificial sandstone cores and the results used in explaining relative permeability stress sensitivity under net confining stresses of 5, 15 and 25 MPa. The effect of stress on displacement efficiency was also investigated. The bulk volume irreducible (BVI), which is the fraction of the bulk volume containing irreducible water, generally increased with stress. Meanwhile the free fluid index (FFI), which is the fraction of the bilk volume responsible for fluid flow, decreased with increased stress. NMR irreducible water saturation and irreducible water saturation obtained from relative permeability experiments both increased with stress. Irreducible water saturation was strongly related to the behavior BVI stress sensitivity. Residual oil saturation increased with increase in stress as a result of reductions in FFI pore volume. End-point oil relative permeability had a significant decrement with stress while water end-point relative permeability had a negligible decrement. The significant reduction in end-point oil relative permeability was due to reductions in pore radius evident in a corresponding reduction in FFI. Displacement efficiency decreased with increased stress indicating lower oil recovery on microscopic scale but end-point mobility ratio increased as stress increased. Relative permeability stress sensitivity could be tied to changes in BVI and FFI of NMR T-2 distribution when changes in stress occur in petroleum reservoirs.