A New Thermally Stable Synthetic Polymer for Harsh Conditions of Middle East Reservoirs: Part II. NMR and Size Exclusion Chromatography to Assess Chemical and Structural Changes During Thermal Stability Tests

Abstract

Most Middle East fieds present harsh reservoir conditions (high temperature, high salinity, low permeability carbonates) for polymers used as EOR mobility control agents. Traditional synthetic polymers such as partially hydrolyzed polyacrylamide (HPAM) are not thermally stable. At temperatures above 60°C, acrylamide moieties hydrolyze to sodium acrylate which ultimately leads to precipitation and total viscosity loss. Thermal stability can be improved by incorporating monomers such as ATBS or NVP. In a previous paper, we reported the development of terpolymers where incorporation of NVP was shown to provide improved stability up to 120°C. Unfortunately, NVP increases the cost of the polymer and limits its molecular weight. Additionally, NVP also causes drifts in the polymers composition, thereby impairing injectivity in low permeability carbonate rocks. The price of the final product, to achieve a given viscosity, is approximately 3 times higher compared to conventional HPAM polymers and 2 to 2.5 times higher than SPAM polymers (sulfonated polyacrylamide). More recently, we reported the synthesis of NVP-free polymers incorporating different mol precentages of ATBS. The ATBS containing polymers are cheaper than the NVP polymers and enable dosage reductions of up to 50%, to obtain the same viscosity. Additionally, they outperformed the NVP polymers in terms of injectivity and thermal stability, as well as pushed the stability limits from 105-110°C up to 130°C and 140°C in brines withTDS of 230 g/L and 100 g/L respectively. In this study, we present new data for viscosity and thermal stability of NVP-free polymers optimized in terms of process and molecular weight. In particular, the thermal stability study was completed with NMR spectroscopy and Size Exclusion Chromatography (SEC) analysis to obtain information on the evolution of the chemistry and the molecular weight distribution of the polymers during long-term aging. NMR and SEC analysis reveal that the reduction of the viscosity during aging is due to an evolution of the polymer chemistry (conversion of acrylamide and ATBS units in acrylates) as well as chain scission. The incorporation of ATBS, into the polymer backbone, appears to slow down hydrolysis and limits the viscosity loss. There was no modification of the chemistry observed for the polymer having the highest level of ATBS and any viscosity loss observed is directly related to a decrease in molecular weight. The optimization of the NVP-free polymers redues the dosage by one third, making them very attractive from an economic standpoint. Both NMR and SEC techniques, have been shown to be efficient tools to understand the mechanism involved in viscosity changes for polymer solutions during long-term thermal aging.

Impact of Mechanical Degradation on Polymer Injectivity in Porous Media

“…Shift in onset of shear thickening has been an indication of reduction of molecular weight distribution [9,23,25]. It is generally difficult to quantify the reduction of MWD due to the difficulties in determining MW of high molecular weight polymers by methods such as size exclusion chromatography (SEC) and asymmetric flow field-flow fractionation (AF4) [49,50,51,52]. To characterize the reduction by core floods would usually require a large number of core floods.…”

Polymer Injectivity: Investigation of Mechanical Degradation of EOR Polymers Using In-Situ Rheology

“…Hence, degraded polymer solution has lower polydispersity. This concept was also confirmed by size exclusion chromatography (SEC) [59] and asymmetric flow field-flow fractionation (AF4) [30,[60][61][62]. Reduction of Mw or change in MWD is the reason of observed reduction in screen factor [12,63] that correlates with reduction of resistance factor as well.…”

Polymer Injectivity: Investigation of Mechanical Degradation of Enhanced Oil Recovery Polymers Using In-Situ Rheology

“…Hence, degraded polymer solution has a lower polydispersity. This concept was also confirmed by size exclusion chromatography (SEC) [59] and asymmetric flow field-flow fractionation (AF4) [30,[60][61][62]. Reduction of Mw or change in MWD is the reason for the observed reduction in screen factor [12,63] that correlates with reduction of resistance factor as well.…”