In recent years, a number of new HT/HP reservoirs have been developed in the North Sea. The Shearwater reservoir is a gas condensate producer located in Block 22/30 in the North Sea. This reservoir has very harsh downhole conditions as follows: T ~ 370°F, P > 13,000 psi, brine TDS > 250,000 mg/l and is therefore classed as a HP/HT/HS reservoir. Significant downhole carbonate scale ([Ca2+] ~ 20,000 ppm; [Bicarb] ~ 560 ppm) is expected in the wellbore and near wellbore formation area initially at low GWR (GWR < 0.05) with the main mechanism being evaporation from the low volume brine phase into the high volume gas phase. As a result, downhole squeeze scale inhibitor treatments will be required at a very early stage of production. In a previous paper (SPE 60217), presented at the 2000 Scale Symposium, we presented results from a detailed screening study, based largely on bulk experiments (thermal stability, compatibility and performance), to select appropriate chemicals for potential application in this reservoir. Five products were selected which covered a range of generic chemical types including thermally stable phosphonates (HMDP), sulphonated polymers (PVS, S-Co) and phosphorous containing co-polymers (P-SCo).
Further work involving a series of high temperature core flooding experiments conducted at 374°F (190°C) have now been completed on the various pre-selected products. In addition to monitoring inhibitor return profiles, the performance of the released inhibitor species was also examined, with some surprising results. The results from these tests have significant implications on the selection of products for downhole application under such harsh conditions. Results are presented in the following areas:
Comparative inhibitor retention and return characteristics
Comparative field application modelling using SQUEEZE V software
Implications of adsorption on the thermal stability of the pre-selected products.
Results in this study demonstrate that the rock surface interaction of certain pre-selected chemical species in core flood experiments can alter their performance (as a result of thermal degradation), which was not recorded in the initial static thermal stability tests. This is not in accordance with previously held opinions that adsorption onto rock substrates increases the thermal stability of scale inhibitor products.