A Pore-Scale Investigation of Low-Salinity Waterflooding in Porous Media: Uniformly Wetted Systems

Abstract

The potential of low-salinity (LS) water injection as an oil recovery technique has been the source of much recent debate within the petroleum industry. Evidence from both laboratory and field-level studies has indicated significant benefits compared to conventional high-salinity (HS) waterflooding, but many conflicting results have also been reported and, to date, the underlying mechanisms remain poorly understood. In this paper, we aim to address this uncertainty by developing a novel, steady-state pore network model in which LS brine displaces oil from a HS-bearing network. The model allows systematic investigation of the crude oil/brine/rock parameter space, with the goal of identifying features that may be critical to the production of incremental oil following LS brine injection. By coupling the displacement model to a salinity-tracking tracer algorithm, and assuming that a reduction of water salinity within the pore network leads to localised wettability alteration, substantial perturbations to standard pore filling sequences are predicted. The results clearly point to two principal effects of dynamic contact angle modification at the pore scale: a “pore sequence” effect, characterised by an alteration to the distribution of displaced pore sizes, and a “sweep efficiency” effect, demonstrated by a change in the overall fraction of pores invaded. Our study indicates that any LS effect will depend on the relative (scenario-dependent) influence of each mechanism, where factors such as the initial wettability state of the system and the pore size distribution of the underlying network are found to play crucial roles. In addition, we highlight the important role played by end-point capillary pressure in determining LS efficacy.

Appendices

Appendix 1: Salinity Allocation to Newly Invaded Pores

The following procedure is used to assign salinity values to each pore newly invaded during the latest capillary pressure step:

• 1. Identify newly displaced water-filled pores without bulk connectivity to the inlet (i.e. snapped-off pores). These pores are assumed to have accumulated pure HS water, and assignment of a nonzero tracer concentration is delayed until any such pore becomes part of a spanning water cluster.

• 2. Assign a tracer concentration of one (i.e. salinity = 0) to all newly displaced inlet pores.

• 3. Beginning from the inlet, track flow paths through the spanning water cluster(s) and assign tracer concentrations to all newly displaced, or newly connected, flowing pores according to the flow-weighted average concentration of upstream neighbours.

• 4. Identify “dead-end” water clusters (i.e. pores branching from a backbone of flowing water pores but not yet flowing themselves), and non-spanning water clusters with bulk connectivity to the inlet. For each of these clusters, calculate the total cluster volume $V_\mathrm{clus}$, the total mass of tracer newly added to the cluster $M_\mathrm{new}$ (i.e. pores just displaced by LS injection), and the total mass of tracer in all previously displaced pores associated with that cluster $M_\mathrm{old}$ (i.e. pores that were displaced by HS or LS at earlier times). Use these values to calculate a new average tracer concentration to assign to all pores within the cluster, $C_\mathrm{clus} = (M_\mathrm{old} + M_\mathrm{new}) / V_\mathrm{clus}$.

Appendix 2: Salinity Updates in Flowing Water-Filled Pores

For each global saturation change following the onset of LS injection, a designated period of convective tracer transport is simulated to determine the salinity evolution within the flowing bulk water. Assuming a source concentration of one (i.e. salinity = 0) at inlet pores, and using the previously calculated elemental flow values q, tracer concentrations $C_\mathrm{old}$ are updated as follows at each timestep $\Delta t$:

• (i) Calculate the mass of tracer flowing out of each bond, $M_\mathrm{out} = q \cdot \Delta t \cdot C_\mathrm{old}$;

• (ii) Sum the appropriate $M_\mathrm{out}$ values to determine the total mass of tracer $M_\mathrm{node}$ flowing into each node;

• (iii) Assuming perfect mixing in nodes, calculate the mass of tracer $M_\mathrm{in}$ entering each outflowing bond according to $M_\mathrm{in} = q / q_\mathrm{node} \cdot M_\mathrm{node}$, where $q_\mathrm{node}$ represents the total flow into the node from all upstream bonds;

• (iv) Calculate the new tracer concentration $C_\mathrm{new}$ in each bond according to $C_\mathrm{new} = C_\mathrm{old} + {(}M_\mathrm{in} - M_\mathrm{out}{)} / V$, where V is the volume of the bond.

During the above procedure, the mass of tracer leaving any bond in a single timestep cannot be in excess of that available. Mass conservation therefore stipulates the condition $\Delta t = \mathrm{min} (V/q)$, where the minimum is taken over all flowing bonds.