Investigation of permeability decline due to coupled precipitation/dissolution mechanism in carbonate rocks during low salinity co-water injection

Low salinity water injection is a promising water-based enhanced oil recovery (EOR) method for carbonate rocks. However, permeability impairment resulted from competition between mineral scale precipitation and rock dissolution is not well understood due to complex injection water/formation brine/rock interactions. In this paper, simultaneous effects of rock dissolution and mineral scale precipitation in low saline co-water injection on permeability performance were investigated. A dynamic flow of water with varying ion composition considering scale precipitation/rock dissolution coupling with geochemical reactions (using PHREEQC) was developed. The proposed model was validated with outlet ion concentration of single phase coreflood data. Then, permeability decline curves were obtained for each mixing ratio of formation brine and diluted sea water. The results show that in case of co-water injection, mineral scale precipitation is the dominant mechanism in early times of flooding and can be comparable with rock dissolution quantitatively in longer time. This result is in contrast with results of sequensive injection available in literature due to lack of sufficient mixing zone between injected water and formation brine and small pore volume of injection. In addition there are three different regions in permeability curve of co-water injection which is reflecting the competition between mineral scale precipitation and rock dissolution mechanisms. Moreover, as the share of formation brine increased more than 0.5, mineral precipitation became dominant and permeability curve decreased monotonically. The model can be used for distinguishing share of different mechanisms of permeability variation during low salinity waterflooding. (C) 2020 The Author(s). Published by Elsevier Ltd.