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.

Summary: A key challenge in polymer-flood forecasting is the prediction of polymer stability far from the injector. Degradation may result from various mechanical-degradation events in surface facilities and at the wellbore interface, as well as possible oxidative degradation caused by the presence of oxygen and reduced transition metals. All these steps must be closely examined to minimize degradation and ensure propagation of a viscous polymer solution. In this paper, polymer solutions are pushed toward degradation rates that would be unacceptable for enhanced-oil-recovery applications to better understand the underlying physics. Multistep degradation events are induced in various geometries, such as capillaries, blenders, and porous media. For the geometries and range of polymer and salt concentrations investigated, degradation (as defined here) approaches an asymptotic value as the number of degrading events increases. An empirical normalization method is proposed, allowing superimposition of curves of viscosity loss vs. time across multiple possible geometries. The normalization procedure is applied to predict the extent of degradation during a field injection in which near-wellbore degradation occurs after degradation in surface facilities. We predict that degradation in the porous medium reaches a stable value after passing through approximately 6 mm of rock. Finally, degradation is proposed as a tool to probe the molecular-weight distribution and to narrow the polydispersity of polymers, which can be used for maximizing both viscosifying power and injectivity simultaneously.

Whereas on-shore polymer injection may be qualified as a mature EOR technique, considering the hundreds of operations that have been conducted all over the world, only one polymer pilot has been implemented offshore, and none in deep offshore conditions. A very thorough feasibility study of polymer injection has been made on the Dalia field in Angola, a typical deep-offshore high permeability (>1D as an average) sandstone reservoir containing medium viscosity oil (3 to 7cP under reservoir conditions). The study has demonstrated that high molecular weight hydrolyzed polyacrylamides could be used under a wide range of salinities covering sea water and a mixture of sea water and produced water. Additional recoveries in the range of 3 to 7 % can be expected in this particular context of large well-spacing development of a medium viscosity field. Powder polymer supply is achievable for deep offshore fields either with a specific bulk carrier or using standard international containers to transport big bags (750 kg). Although the on-deck option is simpler, even in the case where no room is left on the existing FPSO, marine options can be found to safely process the polymer on a barge connected to the FPSO (depending on the sea conditions), but the on-deck option is more simple. The need for an injectivity pilot is compulsory to demonstrate the operability of the facilities, and the injectivity of the polymer solution. A single well test has been designed and is planned fall 2008 in Luanda on one of the well of the Camelia reservoir. A skid dedicated to the injectivity test has been designed, assembled and constructed to prepare a mother solution of polymer from powder. Tested in France (no injection), the skid has been shipped to Luanda for installation on the FPSO during summer 2008. Future of the project will depend on the injectivity tests results and on-going studies on a phased approach.

Conventional polymers used in enhanced oil recovery (EOR) are acrylamide-based copolymers of very high molecular weight. Their viscosity in aqueous solution depends on various physicochemical parameters such as monomer composition, concentration, average molecular weight, polydispersity, salinity level and ionic composition, temperature, etc. Moreover, solutions are non-Newtonian; they exhibit low-shear Newtonian plateau viscosity at a low-shear rate followed by a shear thinning region at a higher shear rate. In the absence of a predictive model, for any new polymer grade or lot, any new or slightly varying field condition, it is necessary to perform a whole set of viscosity measurements at varying concentrations, which is tedious, time-consuming, and not valuable. Flow curves (viscosity vs shear rate) were measured on a great number of polymer solutions in various physicochemical conditions (variation of the polymer microstructure, monomer composition, molecular weight, brine salinity, and temperature). The flow curves in dilute nonentangled, semidilute nonentangled, and semidilute entangled regimes were modeled by only two adjustable parameters: the intrinsic viscosity [η] and the relaxation time in the dilute regime λd. The zero-shear viscosity η0 (more specifically, the specific viscosity ηsp) and the power law index n obey master curves that are solely functions of the overlap parameter C[η]. The relaxation time λ depends on C[η] and the relaxation time in the dilute regime λd. All these results are consistent with predictions for a neutral polymer in a good solvent. By using these master curves, intrinsic viscosity of any polymer/brine system can be easily obtained at various temperatures from a single measurement in the semidilute regime in which viscosity is higher than water, and classic rheometers are very sensitive. The whole flow curve η(γ˙) can be predicted at any concentration, temperature, and molecular weight. For any unknown polymer/brine system, the determination of λd enables us to determine the viscosimetric average molecular weight M of the polymer. Finally, by using the additive property of the intrinsic viscosity of binary solutions, a method is proposed to evaluate the molecular weight of field samples. Polymer physics is today considered well described and well known. However, the beauty and the usefulness of this physics have been partly ignored by the EOR community up to now. This study gives a methodology to predict the viscosifying behavior and the molecular weight of any acrylamide-based copolymer/brine system. By attributing the molecular weight rather than a viscosity value, on-site and lab quality control will be greatly improved.

Summary: Hydrolysis of polyacrylamide (PAM) -based polymers is rapid and extensive under the alkaline conditions typical of alkaline/ surfactant-polymer (ASP) flooding. Even at room temperature, significant hydrolysis occurs within 1 to 2 months in the presence of sodium carbonate. While this implies that polymers used in ASP floods will rapidly become susceptible to precipitation with divalent cations, in most cases the alkali present will be the most sensitive component to precipitation, so this may be a moot point. Also, autoretarding kinetics under alkaline conditions limit hydrolysis at 100°C, whereas complete hydrolysis occurs under neutral conditions. Furthermore, in-situ hydrolysis of initially unhydrolyzed PAM is proposed as a promising strategy for ASP floods because the injectivity of the unhyrolyzed PAM will be greater than that of hydrolyzed PAM (HPAM) because of its lower initial viscosity. The lower initial viscosity is not a disadvantage because once it has been hydrolyzed in-situ, its viscosity will increase.