The impact of heterogeneity induced mixing between the injected low salinity water and the high salinity connate water is investigated in secondary low salinity waterflooding (LSW). Although LSW has proved to be a promising EOR process in laboratory experiments and field trials, its efficiency can be reduced due to mixing with in situ higher salinity connate water. This mixing is caused by molecular diffusion and dispersion and it has been suggested that it may be exacerbated by heterogeneity.
We have investigated the impact of 1) conformance caused by layering in the sandstone and 2) mixing caused by water-saturated shale layers adjacent to the reservoir sand on recovery. The study used a commercial reservoir simulator in which low salinity flooding is modelled by varying relative permeability as a function of salinity. All simulations used very fine grids so that physical (rather than numerical) diffusion and dispersion dominated the displacement. In single phase flow, it was found that physical dispersion can only be modelled for when the grid Peclet number is less than 60, for higher values numerical dispersion dominates over physical values. This threshold Peclet number varies with the number of grid blocks, e.g. the optimum number of grid blocks was found to be 10 for (Pe <12) compared with 1000 grid blocks (Pe = 60).
The transverse dispersion number (NTD) originally proposed by Lake and Hirasaki (1981) is shown to be a very robust way to measure the impact of mixing on the performance of LSW. Generally, for any (NTD > 1), diffusion dominates the flow and thus the efficiency of LSW is reduced. In layered high net to gross reservoirs mixing can actually improve recovery from low salinity water flooding and we find that the impact of molecular diffusion increases as the thickness of the higher permeability layer decreases. In lower net-to-gross reservoirs containing thin, shaly zones filled with higher salinity connate water molecular diffusion reduces the effectiveness of LS. This performance reduction is more noticeable as the thickness of the sand layers between the shales decreases.