Le succès d'un projet d'injection de polymères commence avec la sélection du produit adapté aux caractéristiques du réservoir telles que la qualité d'eau, la température, la perméabilité ainsi que la présence éventuelle de contaminants comme le fer, l'hydrogène sulfuré et l'oxygène. Des études de stabilité menées à long terme en laboratoire permettent de s'assurer de la bonne tenue du produit tout au long de l'injection, tandis que des expériences d'injection sur carottes sont utiles pour vérifier l'injectivité et la propagation de la solution de polymère à travers le milieu poreux. L'étape suivante concerne le design et la sélection des équipements qui vont servir à la dissolution et à l'injection de la solution dans le réservoir. Les installations de surface jouent un rôle primordial à ce stade, le but étant d'optimiser les étapes d'hydratation, de maturation et d'injection de la solution de polymère en évitant toute dégradation, soit chimique (introduction d'oxygène), soit mécanique (duses, pompes centrifuges). La dégradation du polymère présent dans l'eau de production est un autre aspect important du projet d'injection. Plusieurs études ont montré qu'il n'y avait pas d'influence du polymère sur la séparation entre le brut et l'eau de production, le polymère étant uniquement soluble dans l'eau. Une dégradation du polymère résiduel préliminaire au passage des équipements de traitement d'eau peut être mise en oeuvre afin d'abaisser la viscosité à 4 mPa.s et éviter toute difficulté dans le déroulement du processus.

While polymer flooding has proven to be an effective technique to improve oil recovery from mature reservoirs, the new objective pursued by the main stakeholders consists now in optimizing the overall costs and decrease the risks associated to the implementation of chemical enhanced oil recovery methods. This is particularly true for reservoir conditions with high temperatures (above 85°C) using injection brine with high hardness.In such conditions, using specific robust polymers is necessary to ensure stability over months during propagation in reservoirs. These polymers containing Acrylamido-Tert-Butyl-Sulfonate (ATBS) and N-Vinyl Pyrrolidone (NVP) are more expensive than regular HPAM and have to be overdosed to reach target viscosities since their molecular weights are low. In extreme cases, the project can be aborted for economic reasons. However, many fields, especially in South America, have access to very soft injection brines with a total dissolved salt lower than 3000 ppm in many cases. Using such brine is a tremendous advantage for polymer flooding since less robust polymers can be selected and target viscosity can be achieved with low dosages. This paper describes the selection and evaluation of polymers dedicated to high temperature reservoir conditions (from 85°C to 140°C) using very low salinity brine in comparison with harder brine. A series of rheological, shear and thermal stability tests have been performed to select the most appropriate polymer for each case. The impact of the chemical composition and the microstructure shows that the incorporation of NVP is not necessarily required to ensure stability over 6 months above 100°C in soft brines. Results show as well that ATBS improves shear and thermal stability in both soft and hard brine conditions. Besides, the incorporation of thermo-responsive moieties in the polymer improves viscosity properties resulting in a lower dosage even at high temperature to reach target viscosity.The objective of the study is also to demonstrate the possibility to develop innovative and cost-effective polymers for each reservoir condition. It encompasses an early and particularly close cooperation between the polymer manufacturer and the company willing to improve oil recovery from its reservoirs.

While polymer flooding has proven to be an effective technique to improve oil recovery from mature reservoirs, the new objective pursued by the main stakeholders consists now in optimizing the overall costs and decrease the risks associated to the implementation of chemical enhanced oil recovery methods. Several options do exist to tackle these challenges. Developing more costeffective polymers is one of these. Traditional partially hydrolyzed polyacrylamides (HPAM) are sensitive to brine hardness and temperature; increasing both lead to a drop in viscosity which could be detrimental to the success of the project. As a result, it is generally necessary to increase the dosage of the HPAM to reach the targeted viscosity and balance for the viscosity drop induced by polymer collapse, but always at the expense of the economics of the project. In extreme cases, the project can be aborted for economic reasons. This paper describes a new class of polymers, called “stimuli-responsive”, which has been designed to overcome the aforementioned challenges. This new family of polyacrylamide-based polymers has structurally modified to counterbalance the negative effect of salinity and temperature encountered during the transit through the reservoir. The placement, number and quantity of stimuliresponsive grafts can be finetuned to each reservoir condition to develop its full potential in the subterranean formation. This variable comes on top of other possible adjustments made to the polymer backbone including molecular weight, hydrolysis and incorporation of thermostable groups such as ATBS (acrylamide tertio butyl sulfonic acid) and/or NVP (Nvinyl pyrrolidone). A series of rheological tests has been performed to demonstrate several features of the solutions prepared from these polymers, including salinity resistance, effect of temperature and longterm stability tests. Moreover, injectivity and retention tests have been performed to evaluate the behavior of these polymers in porous medium. A reduction in the polymer dosage on the injection side and the stimuli responsive behavior of the polymer also open new opportunities to minimize the impact of the presence of said polymer in the coproduced water on surface facilities. Some preliminary experiments have been performed in these directions. The objective of the study is also to demonstrate the possibility to develop innovative and costeffective polymers for each reservoir condition. It encompasses an early and particularly close cooperation between the polymer manufacturer and the company willing to improve oil recovery from its reservoirs.

The properties of novel polymers for proppant transport in hydraulic fracturing operations are discussed. Acrylamide based associative polymers have been synthesized using various industrial production processes. Anionic polymers investigated are acrylamide (AMD) based co- and ter-polymers functionalized with monomers such as sodium acrylate (AA), sodium acrylamido-tertiary-butyl sulfonate (ATBS) and a home-made surfactant monomer. The rheological properties of the developed polymers in different brines are evaluated and compared to commercial guar gums usually used for fracturing fluids. The viscoelastic properties as well as settling time of proppant in graduated cylinder have been evaluated. The impact of oxidizing breakers and surfactants added to increase or decrease the viscosity of solutions are reported as well. The new polymers can be used in slickwater, linear gel and cross-linked hydraulic fracturing fluid. They have the ability to carry the proppant down to the target zone. With this technology, proppant can be transported and placed into the fractures with lower concentrations of product and reduce or eliminate the need for using guar gum. Fluid viscosity can be controlled (either increased or decreased) by the addition of surfactants and broken by conventional oxidizers. Laboratory data shows that new associative polymers solutions give very high viscosity at low shear rate and a strong reversible shear thinning effect. Typically, for 3000 ppm of associative polymer, settling time of proppant is 10 times longer than for the same concentration of guar gum. In gel-based fracturing jobs, transport of proppant is generally achieved using guar gum whose viscosity may be improved by adding cross-linkers such as borate or zirconium. The polymer presented in this paper could be a total or partial alternative to guar for proppant transport whose price is subjected to harvest aleas. Results show that lower polymer concentrations are required to obtain the same or even better suspending properties. Besides, traditional oxidizing breakers could be replaced by surfactant to decrease the viscosity of the fluid once the proppant has been placed in the fractures.

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