Break up of connected non-wetting phase during CO2-brine and N2-water drainage core floods

Abstract:

We present evidence of a transition from connected to unconnected non-wetting phase flow during drainage in CO₂-brine and N₂-water core floods. Connected non-wetting phase flow is controlled by heterogeneity in the pore space, with non-wetting phase pathways developing in regions of lower capillary entry pressure. During unconnected non-wetting phase flow, pore space heterogeneity has no impact on fluid flow paths and relative permeability is controlled by fluid properties such as interfacial tension. The transition is observed through a shift in relative permeability curves, maps of steady state saturation and fluid arrangement during relative permeability measurements, and pore scale observations. The transition can be achieved either by modifying the pressure, temperature and salinity conditions of a core flood to increase the wetting phase viscosity, or by increasing the wetting or non-wetting phase flow rate. We suggest the viscous pressure in the wetting phase has a strong impact on the flow behaviour and fluid arrangement during multiphase flow, even at conditions where the flow is traditionally considered to be capillary dominated.

Figure 1. Wetting and non-wetting phase capillary number [1] plotted for 7 CO₂-brine and one N₂-water core floods at temperatures and pressures of 38-91°C and 10.3-20.7 MPa and brine molalities of 0-5 mol kg^-1. Saturation maps at a steady state saturation of S_w = 56 % are shown for connected and disconnected non-wetting phase flow. Non-wetting phase relative permeability changes from low endpoint at high irreducible water saturation during connected flow to a high end point relative permeability and low irreducible water saturation during disconnected non-wetting phase flow.

[1] Datta, S. S., J.-B. Dupin, and D. A. Weitz. "Fluid breakup during simultaneous two-phase flow through a three-dimensional porous medium." Physics of Fluids (1994-present) 26.6 (2014).