Microbial fluid-rock interactions in chalk samples and salinity factor in divalent ca²⁺ ions release for microbial enhanced oil recovery purposes

In this study core laboratory experiments were performed on chalk samples from Danish sector of the North Sea to study microbial fluid-rock interactions with carbonate rock and to evaluate the dissolution of rock matrix (CaC03). The microbial media consisted of a strain of Clostridium tyrobutyricum with reinforced Clostridia media and molasses as carbon and energy sources. Results showed that the average concentration of Ca2+ ions after microbial fluid interactions with chalk samples in media of salinity 40-100g/l increased from initial average concentration of 203 mg/1 at the start of the experiment to 1178 mg/1 in 28 days. All the bacteria media irrespective of their salinity condition experienced a decrease in measured pH from average of 7.0 to 6.0 in the first week; this however started to increase again from second week with release of calcium ions indicating a correlation between the pH and calcium ions released. 3-D surface plot for the range of salinity measured revealed delineation of the measured salinity into two groups with a boundary between 70 - 80 g/1. The highest Ca + ion concentration was measured at the highest salinities by the end of experiment which means that the dissolution rate of chalk increased with increase in salinity. This could be due to the lower values of pH as an indication of that higher salt concentration facilitates more acid production by bacteria.