Abstract
Abstract The aim of this paper is to investigate an application of nanofluid-slug preflush to enhance well injectivity while improving the sweep efficiency by fines migration-assisted mobility control in multi-layer heterogeneous reservoirs during low salinity water flooding. An axisymmetric radial flow model and fraction flow analysis are applied to interpret the performance of nanofluid-slug and the following low salinity water injection in a layered heterogeneous flow system. The interplay among nanoparticles, fines and rocks is described by a physical-chemical reaction model. The improvement of mobility control is characterized as the ratio of displacement fronts' advancing velocity along each layer. The improved well injectivity by nanofluid preflush is presented as an explicit formulation of injectivity index. This paper also introduces a graphic workflow to optimize nanofluid treatment and injected water salinity for nanofluid-fines-assisted low salinity water flooding under arbitrary initial and injection conditions. The results indicate: 1) Compared with conventional water flooding, the alteration of water salinity can help to achieve uniform water flooding profile within each heterogeneous layer and then improve sweep efficiency before water breakthrough; 2) The nanofluid preflush prior to water injection can effectively control fines migration in the vicinity of injection wells to improve well injectivity, but it cannot control fines migration in reservoirs that realizes mobility control by decreasing water-phase permeability in the higher permeable layers; and 3) there does exist an optimal nanofluid concentration and slug size to offset decline of permeability near wells and then improve water injectivity. The outcomes of analytical model are validated by both numerical simulations. This paper has the following novel points: 1) the model provides physical insights to examine nanofluid utilization to improve well injectivity and enhance oil recovery; 2) The induced mobility-control by fines migration during low salinity water flooding is confirmed as an effective method to improve sweep efficiency in heterogeneous reservoirs.
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