The effect of carbon dioxide on carbonate reservoir rock minerals of an Iranian oil reservoir

Increasing the concentration of greenhouse gases in the atmosphere has led to an increase in averagetemperature of the Earth surface and carbon dioxide has received more attention due to its highconcentration in the atmosphere. CO2 injection into oil fields in order to CO2 storage and enhance oilrecovery is one method of reducing the concentration of carbon dioxide in the atmosphere. Carbonic acidwill be produced after injecting carbon dioxide into reservoirs by dissolving carbon dioxide in brine.Then, Hydrogen ions will be generated from the dissociation of carbonic acid and the pH of the brine willreduce. Acidic brine leads to fluid-rock interactions (e.g. minerals dissolution and precipitation) andfinally the rock porosity and permeability will change. Therefore, the static experiments were conductedat atmospheric pressure and 70˚C in order to investigate CO2-fluid-rock interactions. CO2-rich deionizedwater reacted with carbonate rock in various time intervals of 7, 14, 21, 30 and 60 days. The rockporosity and weight and the solution pH were measured after each period of time. In addition, the calciumconcentration in solution was measured by EDTA titration. Rock mineral composition was determined bypreparing a thin section and petrographic studies before experiments. According to the results, the initialrock porosity was 16.18% and calcite was the main mineral in the rock that dissolved with CO2 injection. In addition, in the mentioned time periods, the rock weight losses were 0.067, 0.110, 0.069, 0.072 and 0.069 grams and the rock porosity increases were 20.83%, 21.64%, 20.99%, 20.73% and 20.58%respectively.

Keywords:

CO2 storage , enhance oil recovery , CO2-fluid-rock interactions , porosity , carbonate rock