Monitoring Thermal and Mechanical Stability of Enhanced Oil Recovery (EOR) Acrylamide Based Polymers (PAM) Through Intrinsic Viscosity (IV) Determination Using a New Capillary Rheology Technique
Authors: Rodriguez L. , Mejia A. , Reynaud S. , Lespes G. , Favero C. , Antignard S. , Giovannetti B. , Gaillard N. , Dupuis G. , Loriau M. , Jouenne S. , Grassl B. 
Published: March 2016

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 (104-106 g/mol) were used for calibration purposes.

The technique was then applied to investigate the behavior of two industrial PAM (M&gt;106g/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 (125μm), 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.