Geochemical characterization of omani sandstone cores-analysis of change in ph and rock mineral constituents concentrations

pH sensitive polymers such as polyacrylic acid (PAA) hydrogel have recently been proposed for the oil industry as a conformance control agent. Previous studies showed that this type of microgel solutions will swell considerably as their pH increases above a critical value. Thus for an optimal condition for propagation through porous media, the micogel polymer should be in the unswelled state during injection. Therefore, understanding the geochemical properties of a reservoir is crucial to condition the reservoir before polymers can be applied for conformance control. In this paper, the geochemical behavior of Omani sandstone cores which were obtained from a reservoir candidate for conformance control were characterized by quantifying the change in the effluent pH after injecting 3wt% NaCl brine and 0.1 M HCl solution. The effect of shut-ins and injection rate on the pH of the effluent was investigated. The investigations were carried out in three 1.5 inch diameter by 2 inch long cores selected from the same formation. The measurements indicated a steady state pH value of around 2.8 between the injected acid and the rock minerals. Five pore volumes of 0.1 M HCl were required to achieve this steady state. This is because the fast reacting mineral which is the calcium was almost consumed as soon as the reaction started. The effluent pH was found to increase from the steady state value to about 3.5 (which is the equilibrium pH value) when shut-ins effect was applied for longer than one day while no change in the effluent pH was observed when shut-in period was less than one day. Large volume of acid injection decreased the increase in pH to values less than 3.5 even for very long shut-in periods. There was no effect of injection rate on the acid volume required to attain steady state. However, at steady state, the pH of the effluent was found to decrease as the injection rate decreased. From the geochemical data, we concluded that the Omani field of interest from which the tested cores were collected can be considered as a good candidate for conformance control applications using the PAA polymers.