In recent years a number of new HT/HP reservoirs have been developed in the North Sea, for example the ETAP (Eastern Trough Area Project) and Shearwater fields, which have a number of important "scale" control issues. In addition to harsh downhole conditions (T > 175°C; P • 13,000 psi), high salinity (HS) formation water chemistries indicate that the control of inorganic precipitates in these systems will be very important. Two aspects, in addition to their ability to prevent scale formation, have been demonstrated in previous studies to be of significant importance in the choice of appropriate scale inhibitors. These are: (i) inhibitor/brine compatibility and (ii) the thermal stability of the scale inhibitor.
Previous reported work has strongly indicated that amine methylene phosphonic acid based inhibitor species such as Penta-Phosphonate (DETPMP) and Hexa-Phosphonate (TETHMP) are considerably less thermally stable than polymeric species such as PVS and VS-Co species. Significant thermal stability issues have been recorded for DETPMP and TETHMP based species at temperatures < 130°C, whereas polymeric species exhibit good thermal stability when tested at temperatures up to 180°C. As a consequence the phosphonate based species have been reported as less applicable for deployment in HT reservoir systems than certain polymeric species, such as PVS and VS-Co species. However, more recent studies examining a range of different amine methylene phosphonic acid based inhibitor species have shown that certain species are thermally stable at temperatures in excess of 160°C. This thermal stability, coupled with more effective carbonate scale inhibition performance of the phosphonates, may make them more applicable for carbonate scale in certain HP/HT fields. In addition, examination of the molecular structure of the different amine methylene phosphonic acid species allows initial interpretation of the factors controlling thermal stability of such species. This paper presents an improved understanding of the factors controlling thermal stability/degradation of generically different scale inhibitor products. It also demonstrates how relatively small changes in the structure of phosphonate based inhibitor species can impart significant improvements in thermal stability and consequentially their potential application in HP/HT reservoir systems.