One of the most common methods of preventing downhole and topside mineral scale formation in oilfields is through the use of chemical scale inhibitors. Several aspects of the brine composition may affect the performance of the various scale inhibitors used in oilfield applications and, in this paper, we will focus on the role of calcium ion concentration. The calcium concentration in a particular reservoir and in the inhibitor slug, often determines the extent to which the inhibitor species is retained in the near well bore area, i.e. on its adsorption or precipitation behaviour. What is less well understood is the effect of calcium on the inhibition process itself. Common ion effects are well known. However, for penta-phosphonate inhibitor species (e.g. DETPMP), significant improvements in inhibition efficiency have been reported with increasing calcium concentration in solution. In this paper, we expand significantly on such observations. The effect of calcium cp tion concentration is examined for a wide range of generically different inhibitor species including penta-phosphonate, hexaphosphonate phosphino-poly carboxylate, polyvinyl sulphonate and sulphonated polyacrylate co-polymers. The results indicate clearly how different inhibitor species are affected quite differently by increased [Ca2+] and how this relates to the calcium affinity of the inhibitors active functional groups. The results were obtained by comparing the barium sulphate inhibition efficiency of various species in mixtures of a low/medium scaling (Brent type) formation brine and sea water and also in a more severe scaling (Forties type) formation brine/sea water mixture. Barium sulphate inhibition efficiencies were examined by static inhibition efficiency tests with residence times ranging from 30 minutes to 24 hours.
Phosphonate inhibitors are shown to be poor inhibitors at very low [Ca2+], indicating that their effectiveness is controlled by the formation of Ca2+/phosphonate inhibitor complexes as has been discussed by previous workers. On the other hand, polymeric polycarboxylate inhibitors are shown to be effective even at very low [Ca2+], indicating that the formation of multiple bonds between the polymer and the crystal surface allows for stronger adsorption and thereby inhibition. However, for the phosphonate based species it appears that strong ionic bonds involving calcium cation bridging is required.
As [Ca2+], in the mixed brine is increased the magnitude of the effect (i.e. calcium enhanced inhibition) is related to the functional groups present on the inhibitor in the order phosphonate >acrylate >sulphonate.
Phosphonate inhibitor species are very dramatically affected whereas sulphonated inhibitor species are less affected. This can be accounted for in terms of the calcium affinity of the different inhibitor functional groups in a similar way to comparative adsorption and inhibitor/brine compatibility effects. For the polycarboxylate inhibitor species examined in this work, a clear maximum in inhibition efficiency is observed with increasing calcium concentration. This is explained, from related experiments, in terms of complexation (incompatibility) and differences in the modes of adsorption at the scale surface.
The most important property which any oilfield scale inhibitor must possess is that it has the ability to prevent/inhibit crystal growth at threshold (i.e. sub-stoichiometric) concentration levels. Many workers have studied the mechanism of threshold scale inhibition. In describing the threshold effect, it is generally regarded that the inhibitor molecules adsorb at the active growth sites, which may be crystal defects, thus preventing further crystal growth by interference with the growth process. Coupled with this, the morphology, tendency to agglomerate and the potential of the electric double layer (the zeta potential) of the growing nucleons are also altered by the adsorption of inhibitor molecules at the growth sites.
The overall result of adding a scale inhibitor is a reduction in the tendency for crystallisation and the subsequent formation of scale, by two pathways:
Nucleation Inhibition: Disruption of the thermodynamic stability of the growing nucleons, (for homogeneous crystallisation).