Abstract

Low salinity water flooding (LSWF) is a cost-effective EOR technique. However, the underlining mechanism of LSWF EOR is still a matter of debate due to the complexity of the Crude Oil/Brine/Rock (COBR) system. The objective of this paper is to study the mechanisms of LSWF EOR processes by means of geochemical modeling.

A reactive transport geochemical modelling approach with a geochemical simulator PHREEQC (pH-REdox-EQuilibrium in C programing language) is applied in this paper to handle complex interplay of different geochemical effects such as ion exchange, rock mineral dissolution, flow and transport during LSWF EOR processes.

This paper proposes and demonstrates a geochemical mechanism for LSWF EOR, i.e., spreading of mineral dissolution waves driven by cation exchange process.

The sandstone reservoirs often contain clays and carbonate cements. Clay minerals are present in some carbonate reservoirs. LSWF disturbs the geochemical equilibrium between connate brine and rock minerals in the reservoirs and results in carbonate dissolution. This study shows that without the presence of clay minerals, carbonate mineral dissolution can only occurs locally close to the inlet of core plug or injection well in the field during LSWF and then new equilibrium between brine and rock minerals is reached. Because of the presence of clay minerals and the nature of cation exchange process, clay surface even more preferentially absorbs divalent cations over mono valiant cations during LSWF. This cation exchange process softens the injected low salinity brine and propagates the softened low salinity brine and therefore carbonate mineral dissolution waves from injection well to production well in the field. The spreading of carbonate mineral dissolution waves and the resulting local pH rise have several field wide consequences: (1) The carbonate mineral dissolution and the resulting local pH increase can occur field wide. (2) Mineral dissolution not only releases oil attached to the mineral surfaces but also creates more fresh water-wet rock surfaces and improves oil recovery of LSWF. (3) Rising pH makes both rock surfaces and acid components of crude oil more negatively charged. The subsequent electrostatic interactions help release of polar components from rock surfaces, and increase the water wetness of the rock surface and therefore improve oil recovery.

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