A New Method for Fast Screening of Long Term Thermal Stability of Water-Soluble Polymers For Reservoir Conformance Control

Abstract

Abstract This paper presents a new convenient screening method to study the long-term thermal stability of water soluble polymers in the absence of oxygen. The polymer products are used for improved sweep efficiency by reducing the mobility ratio between injected and displaced fluids. The use of polymers as conformance control agents at low temperature and low salinity is quite common and many polymers are commercially available for this purpose. However, many reservoirs have extremely harsh conditions such as high temperature and high salinity, which are well outside the limits of many water-soluble polymer applications due to polymer degradation. There is a need for polymers that will remain stable in reservoirs with high temperature, e.g. 90 °C (194 °F) or higher and high salnity. Therefore, long-term thermal stability is critically important to such polymers used for conformance control to improve sweep efficiency. Due to the large number of polymers requiring testing and the very low level of oxygen in the reservoirs, a novel method for screening long-term thermal stability of water soluble polymers at ultra-low oxygen levels and in a time-effective manner is necessary. The thermal stability of the polymers was evaluated in terms of viscosity of their solutions in various brines containing Na+, Mg2+ and Ca2+ cations. Viscosity measurements have been performed for a matrix of conditions of temperature and salinity. These include temperature of 90 and 120 ºC, and salinities ranging from low salinity case (TDS <3000 mg/L), to synthetic sea water (TDS ~ 35,000 mg/L), to a so-called high salinity case (TDS about 180,000 mg/L). Results show that some products at dilute concentration can create substantial initial viscosity even at temperatures as high as 90 ºC. Not surprisingly, the viscosity performance is generally much better in the low salinity brines than the higher salt brines. The better products attain a target viscosity (10~20 cp at a shear rate of several sec-1 at 90 ºC) with polymer concentrations of less than 3,000 ppm. The best product for efficiency in creating solution viscosity differs depending on the temperature and the brine salinity. The second part of the study has been to create solutions at near zero oxygen concentrations of candidate polymer solutions in sealed ampoules. Furthermore, this near zero oxygen condition is accomplished without adding oxygen scavengers to the polymer solutions. This allows the measurement of the inherent thermal stability of the different polymers without having possibly confounding results due to the effect of any other chemical. These sealed ampoule polymer sample solutions are placed in an oven held at the test temperature (focus on 90 ºC, and some samples tested at 120 ºC) to determine any loss of solution viscosity with time. After a significant effort to fine-tune the experimental procedures, we have made such samples that reliably have a dissolved oxygen concentration of less than 10 ppb, and usually less than 5 ppb without oxygen scavengers. Results to date (some samples aged for 17 months or more) indicate polymer viscosity stability is possible for an extended period of time for some products when aged at 90 ºC. These data also demonstrate that the stability of the polymers included in this study is better in the low salinity brine than the synthetic sea water, or high salt brine. Among the many polymers screened, detailed results are presented for a representative suite of 4 different polymers. Of these, only Polymer 3 and possibly Polymer 1 demonstrate stable viscosity at 90 ºC in sea salt or high salt brine for several months or more. In the low salt brine at 90 ºC, Polymers 3 and 1 were still more stable than Polymers 2 and 4.