A great number of Middle East fields have too harsh reservoir conditions (high temperature, high salinity) for conventional EOR polymers used as mobility control agents. Traditional synthetic polymers such as partially hydrolyzed polyacrylamide (HPAM) are not thermally stable. At temperatures above 70°C, acrylamide moieties hydrolyze to acrylate groups which ultimately may lead to precipitation and total loss of viscosifying power. Thermal stability can be improved by incorporating specific monomers such as ATBS or NVP. However, their polymerization reactivity can cause some compositional drift and limit their molecular weight / viscosifying power. Compared to HPAM, they will require a higher dosage and higher cost. In this study, we present thermal stability and propagation behavior of a new class of synthetic polymers with high thermal stability. In harsh conditions of Middle East brines, with salinity ranging from sea water to 220 g/L TDS, they present excellent thermal stability until temperature as high as 140°C. Adsorption and mobility reduction were evaluated through coreflood experiments using carbonate cores and Clashach sandstone cores, with permeability ranging between 100mD and 700mD. Mobility and permeability reductions indicate a good propagation in both types of rocks. The development of this new polymer is a major breakthrough to overcome the current limits of polymer EOR applications in harsh reservoir conditions. Moreover, molecular weights can be tailored from low to high molecular weights depending on reservoir permeability. Further work is needed to evaluate resistance to mechanical degradation, salt tolerance and adsorption in carbonates and sandstones.

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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.

Chemical EOR polymers are in general anionic, synthetic, acrylamide based co-and ter-polymers. Those polymers provide viscosity to water based system thanks to their high molecular weight (Mw) and the repulsions between the ionic charges. Uncoiling and entanglements are the two mechanisms linked to Mw and ionic repulsion.When a polymer is injected, different mechanisms will act on its chemical structure by chemical, mechanical, thermal, free radical modifications. The understanding of the resulting rheological properties of the modified PAM is very important since it can change the control over the mobility ratio when it propagates downhole in the reservoir. Various lab experiments can be performed to predict those changes over time. In particular, the intrinsic viscosity is a convenient way of reaching the dimension of the macromolecule in a given solvent and at a given temperature but is time consuming.In this publication, a new and efficient capillary rheology technique is presented allowing measuring the bulk solution rheology and intrinsic viscosity of polymers in various brines even at high temperature (140°C) and at a controlled and low shear rate. It is thus possible to access to the rheological properties of dilute polymer solutions under conditions that, to our knowledge, cannot be performed by viscometers commercially available. The new technique involves a specific set up including a capillary and pressure transducers. It allows monitoring the effect of various chemical, mechanical, free radical and thermal modifications on the PAM structure after various ageing times. The performances of the technique were determined in terms of accuracy, and acquisition time. Various PAM compositions and different molar masses (10 4 -10 6 g/mol) were used for calibration purposes.The technique was then applied to investigate the behavior of two industrial PAM (MϾ10 6 g/mol) used in EOR: a sodium acrylamido-tertiary-butyl sulfonate (ATBS) based PAM (SPAM), and a PAM terpolymer with ATBS, and N-vinylpyrrolidone (NVP) (PAM called SAV). Rheological properties and intrinsic viscosities were determined at low shear rates 8 to 800 s -1 , at 25, 85 and 140°C, and for different salinities (20,000 to 240,000 TDS). In addition, the mechanical stability was evaluated by injection of a polymer solution in a capillary with a low internal diameter (125m), and with a shear rate up to 1,500,000s -1 .The results obtained using the new technique provide an efficient and reliable way of selecting the proper PAM chemistry for a given reservoir condition. Ultimately, it defines the salinity, temperature and shear rate domains where the PAM, SPAM or SAV are stable. The technique can be automated, is made of transportable devices, can be used in anaerobic conditions and provides a fast way of following changes in the rheological property of very high Mw industrial PAM over time.

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