Propagation of mineral dissolution waves driven by cation exchange in low salinity waterflooding

Abstract

Low salinity waterflooding (LSWF) is an emerging improved/enhanced oil recovery (IOR/EOR) technique with an applicability in both sandstone and carbonate reservoirs. The underlying recovery mechanism of LSWF is still not completely understood due to the multiscale complex interactions occurring in crude oil/brine/rock (COBR) systems. There are several proposed mechanisms, where mineral dissolution is the most prominent among them. The objective of this paper is to study the brine-rock interactions, and understand the propagation of mineral dissolution waves in LSWF processes through theoretical analysis and geochemical modeling. A reactive transport geochemical modeling using PHREEQC (pH-REdox-EQuilibrium in C programing language) simulator has been carried out to understand the interplay of different geochemical effects such as multivalent cation exchange, and mineral dissolution during flow and transport in LSWF. The presence of clays and carbonate cements is most common in sandstone reservoirs, while some carbonate reservoirs also contain clay minerals. Therefore, a core scale geochemical model containing clay and carbonate minerals as active minerals for brine-rock interactions is chosen as a representative analog for most sandstone reservoirs and some of the carbonate reservoirs in this study. The results show that LSWF disturbs the geochemical equilibrium between the formation brine and reservoir rock minerals thereby causing the carbonate mineral dissolution. It is also inferred that without the cation exchange effect of clay minerals, carbonate mineral dissolution effects are local and they occur only close to the inlet of a core plug sample during LSWF. The new equilibrium between brine and rock minerals is then established rapidly. In the presence of clay minerals and cation exchange effect, the clay surfaces were found to preferentially adsorb divalent cations over monovalent cations during LSWF. Such cation exchange process softens the injected low salinity brine to maintain the brine under saturation with respect to carbonate minerals, and subsequently propagates carbonate mineral dissolution waves across the full length of the core sample. The novelty of this work is that it proposes and demonstrates a new geochemical based mechanism for LSWF, i.e., the propagation of mineral dissolution waves driven by the cation exchange process in the reservoir. The identified mechanism has several practical implications in the field: (1) carbonate mineral dissolution and the associated local pH increase can extend from local to reservoir scale, (2) the resulting mineral dissolution effectively releases the oil attached to the mineral surfaces, and (3) increasing the pH modifies the surface charges of both the rock and crude oil to be more negatively charged. The resultant electrostatic repulsion between the two interfaces would increase the water-wetness of rock surfaces to achieve higher improved oil recovery in LSWF.