Abstract
Following an earlier paper (SPE 37274, 1997),1 we have continued to investigate the thermal stability of scale inhibitors for application in high pressure/high temperature (HP/HT) reservoirs. HP/HT reservoirs are currently under development in the North Sea and are characterised by high salinity brines (TDS > 300,000 ppm) high temperatures (T > 175°C) and high pressures (12,000–15,000 psi, ~ 800–1050 bar). At present produced brines are treated with scale inhibitors to prevent mineral scale deposition within tubulars and topside equipment. However, the conventional polymer and phosphonate scale inhibitors currently being used may not be appropriate for application in the harsh HP/HT reservoirs.
In this work, results are presented from static thermal stability tests conducted on conventional polymer and phosphonate scale inhibitors including phosphino-polycarboxylate (PPCA), polyvinyl sulphonate (PVS), penta-phosphonate (DETPMP) and hexa-phosphonate (TETHMP). The stability of the scale inhibitors has been considered at temperatures ranging from 120°C to 200°C and other factors which may affect thermal stability have also been investigated, including solution pH and the presence of oxygen radicals, iron and common cations (Na+, Ca2+, Mg2+) in the inhibitor solution. The results from this work demonstrate how factors such as pH and brine composition control thermal stability. Furthermore, the controlling influence of certain factors, such as solution pH, affect different species differently. This work assists in the selection of scale inhibitors for use in HP/HT applications.
