Scale inhibitor core floods in carbonate cores: the influence of pH on phosphonate-carbonate interactions

Scale inhibitor core floods were performed using outcrop Jurassic Portlandian chalk cores. The effect of pH on the phosphonate-carbonate interaction was studied by performing core floods at various fixed injected pH values (pH ~6, 4, and 2). The scale inhibitor (SI) used in these core floods was 5000 ppm active DEPMP adjusted to the application pH for that core flood. Effluent concentrations of scale inhibitor, lithium tracer, calcium and magnesium are measured, as are the effluent pH values. This thorough set of measurements makes it possible to interpret the inhibitor/carbonate interaction mechanisms quite clearly. Some carbonate dissolution is evident in all core floods which is quantified in our floods and, as expected, the degree of dissolution increases as pH decreases. The pH 2 core flood (C4) showed the highest carbonate dissolution to the extent that the fluid induced the formation of a worm hole through the core. Flood C3 carried out at pH4 showed more carbonate dissolution than in floods carried out at pH6, as well as giving high pH (~7) effluents. There is higher interaction between the injected solutions and the rock material in flood C3 (SI retention, î"=4.5 mg/g), and an increase in permeability between the pre-treatment and post-treatment stages (~13%). All floods were modelled using a well established methodology [1,2]. For the high pH floods (pH6) the SI return curves were modelled very accurately using an absorption isotherm which appears to provide an excellent description of the SI/carbonate rock interaction. For the lower pH floods (pH4 and 2), a reasonably good, but not perfect, match was obtained using a "pseudo-absorption isotherm" approach. However, a more complete description of the SI/rock interaction involving the role Ca²⁺ is required in order to accurately model all the features of the phosphonate/carbonate interaction.