Abstract
The article addresses the problem of numerical simulation of the salty water properties after the dissolution of hard reservoir salt following water flooding. A new method is being proposed for the simulation of reservoir salt dissolution within the isothermal models of black oil and compositional model of hydrocarbon filtration, based on fully implicit numerical scheme.
As with water flooding of hydrocarbon fields the injected water and the reservoir water differ in salinity, there is a need to simulate mixing waters of different salinity since such key water properties as density and viscosity depend greatly on the concentration of the dissolved salt. An additional source of salinity in the water phase comes from the hard reservoir salt, its dissolution affecting not only water properties but also the overall reservoir characteristics such as porosity and absolute permeability due to the increased pore volume open to fluid filtration.
The newly proposed method for the simulation of reservoir salt dissolution draws significantly on the ideas formulated in paper [2]. For a more specific description of the change in the pore volume in the process of reservoir salt dissolution, the filtration scheme is amplified with the mass conservation equation for reservoir salt. Most hydrodynamic simulators modeling salty water properties rely on a numerical scheme with splitting where salt concentration at each new time step is found after solving a set of equations consisting of the components' mass conservation equation and the volume conservation equation, calculated for the salt concentration at the previous time step. By the virtue of the fact that water density depends greatly on the concentration of dissolved salt, using the splitting scheme results in the failure of the volume conservation equation following recomputation of salt concentration at the new time step. In this paper, the discretization of the filtration system and the reservoir salt mass conservation equation are proposed to be tackled with the fully implicit scheme devoid of the above drawback. Within numerical experiments with test models as well as actual reservoir models, the proposed fully implicit method of simulation of reservoir salt dissolution has proven its high stability and efficiency allowing computation with large time steps.
Not infrequently, in simulating salty water properties the effect of reservoir salt is being ignored despite its significant effect for the reservoir porosity and absolute permeability. In this paper, the newly proposed method adds importantly to the base model of salt water tracing, while its fully implicit realization allows for stability and high numerical efficiency in computation.
