Multiphase CFD modelling of water evaporation and salt precipitation in micro-pores

The precipitation of salt in porous reservoir rocks is an impairment to gas production, particularly in mature fields. Mitigation is typically achieved with regular water washes which dissolve the deposited salt and transport it in the water phase. However, since the process of salt precipitation is intrinsically linked to pressure drop, its onset often occurs in the porous rock around the wellbore, which makes it challenging to model. A method is sought to predict the amount and location of salt precipitation and the subsequent effect on transport properties, the permeability and capillary pressure curve. This prediction can be used to model the onset and speed of production decline as well as mitigation strategies. In this study, a transient multiphase Computational Fluid Dynamics (CFD) model (based on the Volume Of Fluid method) has been used to calculate the transport of salt in pores filled with a compressible gas and incompressible fluid. Using a review of the important physical processes of multiphase gas/water flow through porous media and their dependence on the presence of dissolved salt, pressure and temperature, models for water evaporation, salt precipitation and surface tension have been added to the CFD code FLUENT and tested at conditions similar to those seen in gas reservoirs. The amount of salt precipitated is tracked, providing some information of the rate at which pores will block and result in flow blockage. It has been found that CFD can capture multiphase capillary flow at micro-scale with water evaporation and including salt transport. The applicability of these CFD models is to give input to a larger scale (macro-scale) model describing the near well bore region. This macro model will be validated using field data and can be used to optimize production strategies.