The role of calcium and magnesium in the mechanism of the scale inhibitor retention in carbonate systems

Oilfield scale has long been recognized as one of the major problems of the oil production industry with the most common scales being calcium carbonate (CaCO3) and barium sulphate (BaSO4). The most common remedy is to treat the formation with scale inhibitor chemicals. The phosphonate scale inhibitor DETPMP is known to be amongst the strongest adsorbing scale inhibitors onto carbonate substrates. This paper presents the results of corefloods using long (12") carbonate cores where less than 1 pore volume of DETPMP scale inhibitor is injected at the inlet and then back produced. In these floods, the core is never fully saturated and these floods are referred to as "contained" core floods. Very extensive data on SI, cation (Ca, Mg and Li) and pH effluents are collected. This study showed that the higher the concentration of SI and lower the pH, the more calcium dissolution is observed (from the [Ca2+] effluents); In all treatments there is a decrease in the [Mg2+] effluent corresponding directly to the increase in calcium; The effluent cation results in the long corefloods strongly support the view that both magnesium and calcium are binding quite strongly to the DETPMP scale inhibitor. These observations lead us to a number of conclusions on the factors that must be included in a full carbonate model. In particular, our experimental results - along with some simple modelling - greatly clarify the role of both calcium and magnesium in the mechanism of the scale inhibitor retention in carbonate systems.