Mojo Posted September 1, 2020 Share Posted September 1, 2020 Dear Brian and Jia, I have been working with GWB react module for the past few months and I just wanted to let you know that the program does not report the right porosity values when the system gets a little bit complex. For example, when we include evaporation of water into the program for a simple system (water + quartz), as water evaporates during simulation time, porosity decreases! The way porosity is calculated in GWB seems to be wrong. Porosity should be calculated from: phi= pore volume/ bulk volume= (bulk volume- mineral volume)/bulk volume In a porous media, the bulk volume a rock/porous media is always constant, but in GWB it changes! Even when we have reactions, the bulk volume should remain constant. What should change is the mineral volume. For example, when a reaction led to the dissolution of minerals, the mineral volume decreases while the pore volume increases. Therefore, the bulk will remain constant. This is the real physic of porous media. As such, porosity will evolve. Now, for the case of evaporation that I mentioned, since GWB assume pore volume = fluid volume, as soon as evaporation starts, porosity decrease which is wrong and misleading. In reality, the porosity of rock will remain unchanged by evaporation of its pore fluid unless the evaporation of formation fluid led to the precipitation of minerals and an increase in mineral volume. Note that for the simple example that I mentioned, you will see mineral volume will remain unchanged. Another example is the temperature drop. If you make a simple model (water+ quartz) and drop the temperature, a reduction in the volume of fluid would occur which is expected. However, since GWB assumes fluid volume is equal to pore volume, it gives a reduction in system porosity which again is wrong. If there is no reaction or change in mineral volume, porosity should remain constant. Another example is the dissolution of CO2 in brine. If we have a sliding CO2 fugacity in a system without any reactions. We will see fluid volume increase linearly over time as CO2 dissolve in brine and therefore porosity increases! This is wrong if mineral volume remains unchanged. Do you have any feedback on this? Please let me know since it is a major problem that can mislead a lot of users and gives wrong data. This is of great importance for users using GWB for porous media applications such as scaling issues in reservoirs. I look forward to hearing from you. Kind Regards Mojtaba Quote Link to comment Share on other sites More sharing options...
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