The Impact of EOR Polymers on the Adsorption of Phosphonate Scale Inhibitors

Abstract

Abstract Enhanced oil recovery (EOR) is critical to optimally produce existing reserves with a minimised carbon footprint. However, it is essential that the EOR process does not impact ongoing production chemistry treatments. Here, the focus is placed on the interactions between EOR polymers and scale inhibitors in terms of adsorption. The adsorption of both EOR polymer and scale inhibitor is assessed using static adsorption tests to understand and analyse the competitive adsorption between the two species. The study also clarifies some features of the adsorption kinetics of the EOR polymer used in this study, HPAM (partially hydrolysed poly acrylamide). Static adsorption bottle tests were performed under conditions of fixed pH and mass/volume ratio (m/V); m = mass of substrate and V = volume of solution. In these experiments, HPAM and DETPMP were used as the polymer and as the scale inhibitor, respectively. The adsorption levels of HPAM and DETPMP were tested under a range of conditions, viz. adsorption was measured separately, in combination initially (at t = 0), and in sequential addition experiments (at t = t1). A series of experiments was carried out to construct the HPAM adsorption isotherm in order to have a better understanding of the results. All tests were conducted at 70°C as this is the typical upper temperature at which HPAM would be applied in the field and is a common mid-range temperature at which DETPMP would be used. The main experiments were conducted for 72 hours with sampling every 24 hours. For the same concentration in North Sea seawater, it was possible to observe that the adsorption was constant over time for the DETPMP (i.e. at equilibrium). However, over this period the HPAM adsorption values increased with time indicating that the system was not at equilibrium. In combination experiments, HPAM appears to have a lower adsorption in the presence of DETPMP – or is kinetically retarded by DETPMP – regardless of the order in which the chemicals are combined. In order to better understand these observations, a detailed kinetic study of HPAM was performed, which showed that adsorption equilibrium was only reached after about 14 days. This study suggests, that (i) competitive adsorption between DETPMP and HPAM can result in reduced HPAM adsorption (and possible kinetic modification), and (ii) that the adsorption of HPAM occurs kinetically slowly in terms of well production time scales. These results are amongst the first observations of this type in the literature, and they highlight the need for the industry to develop a better understanding of the competitive interactions between scale inhibitor treatments and EOR polymers. The implications of our experimental findings for field applications is highlighted in this paper.