Polymer Stability at Harsh Temperature and Salinity Conditions

Abstract

Abstract Polymer flooding has a large potential for unlocking EOR reserves in carbonate reservoirs in the Middle East. ADNOC has developed an EOR roadmap to realize some of these reserves. A milestone on this roadmap is to identify and verify polymers that have good performance at high temperature and high salinity conditions. The reservoir conditions include temperatures in the range of 100 – 130 ºC and formation brines of more than 200 000 ppm TDS with high concentrations of divalent ions in carbonate rock. These conditions have been beyond the limitations of synthetic polymers and most biopolymers. Here we report thermal stability measurements performed at anaerobic conditions for a number of synthetic polymers at high temperature, high salinity (HTHS) conditions. A custom thermal stability chamber and flooding rig was designed and produced where storage and measurements were performed without contact with oxygen or iron. A series of synthetic polymers with different ratios of the monomers amide, acrylate, ATBS and NVP were tested for temperature stability for up to 2 years at 120 ºC. Shear viscosity was measured frequently from the start while the time between tests was increased after trends in viscosity versus temperature was established. The viscosity was determined by measuring the differential pressure over a long coiled tubing. Three different brines ranging from ~200 to ~180 000 mg/L TDS were used as solvents. The results show that polymers with a low degree of ATBS and/or NVP have poor stability at the combined conditions of high temperature and high salinity and degrade within a short time. Polymers with a high degree of ATBS showed good stability at HTHS conditions. For these polymers high salinity improved stability, probably by stabilizing polymer conformation. In this paper, we report thermal stability at HTHS conditions for a number of polymers using a custom designed thermal stability chamber and flooding rig for storage and viscosity measurement. The design gives significant improvement over other methods where samples are either removed and exposed to air during measurement or opened to inert atmosphere during measurement. Polymers stable at HTHS conditions were identified, expanding the boundaries for polymer EOR applications to carbonate reservoirs with temperatures up to 120 ºC and high salinity formation water.