Rheological Evaluation of HPAM fluids for EOR Applications

The interest in techniques for enhanced oil recovery has grown due to the need to improve oil productivity reducing water injection and production. Although waterflooding is the most widely used method to recover oil, this process becomes less effective when the mobility ratio is unfavorable and the displacement efficiency is low. Furthermore, great volumes of injected and produced water lead to a premature economic limitation of the production wells and the field itself. In this context, the polymer flooding appears as a good alternative method to reduce water production. The addition of polymer increases fluids viscosity and, therefore, a more favorable mobility ratio is achieved. This improvement on mobility ratio leads to a better swept efficiency, so, the water breakthrough is postponed and also the oil production is anticipated. On the other hand, there are concerns about the effectiveness of the polymer when formation water with highsalinity and hardness are considered. The main objective of this work is to analyze the procedures for tailoring a polymer fluid from a given synthetic formation water composition aiming to reach a target viscosity to further displacement experiments. Moreover, the polymer solutions properties, such as, overlapping concentration, molecular weight, salt influence and ionic strength are evaluated under rheological analysis. As a result, the target viscosity for a specific reservoir condition and polymer were determined. Additionally, it was possible to identify two concentration regions, a dilute and a semi-dilute, and the overlapping concentration (C*) as a function of the shear rate. It was found that all salts added to the synthetic formation water composition negatively affect the apparent viscosity of the polymer fluids. Also, this effect is more pronounced when salts with divalent ions content are considered. Index Term-Polymer solution, rheology, polymer flooding, enhanced oil recovery (EOR) INTRODUCTION Recently, techniques to enhance oil recovery (EOR) have received great attention from the oil industry. This activity has been stimulated by oil price, uncertainties of supply, depletion of known reserves and low efficiency associated to traditional recovery methods (RAMIREZ, 1987). Therefore, oil demand will be satisfied not only by new explorations but also by the improvement of the recovery of reservoirs already known. There are several techniques to increase oil recovery (thermal, chemical, miscible, microbial), and the key to assess the potential use of each technique is based on a clear understanding of the physical phenomena and the mechanisms involved. It is important to highlight that any technique depends on specific conditions. Therefore, its application must be evaluated case by case. The polymer flooding is a chemical method to enhance oil recovery. Water-soluble polymers are added to the displacing fluid to be injected in order to increase its viscosity. Although residual oil saturation remains essentially the same, the addition of polymer lowers the total water volume needed to achieve this residual saturation due to a better mobility control which improves the sweep efficiency (RAMIREZ, 1987, SORBIE, 1991). Several polymers have been considered for EOR application. However, the polymers often studied and applied in chemical EOR methods are Xanthan gum (XG) and hydrolyzed polyacrylamide (HPAM) (NEEDHAM & DOE, 1987; LAKE, 1989; SORBIE, 1991, DU & GUAN, 2004). Xanthan gum is known by its excellent viscosifying properties, even for high-salinity brines. It also presents high resistance to shear degradation, making it easier to handle and inject (NEEDHAM & DOE, 1987). Despite the benefits, the Xanthan gum is less available and also more susceptible to biodegradation, and safety concerns about biocide addition, hamper its use (OSTERLOH & LAW, 1998). HPAM is the polymer most often used for EOR applications especially because of its availability and relatively low cost comparing to polysaccharides. It is less susceptible to biodegradation, has good viscosifying properties, and it also reduces water permeability unlike the Xanthan gum (LAKE, 1989; OSTERLOH & LAW; 1998; MOREL, et al., 2007; ABIDIN, PUSPASSARI & NUGROHO, 2012). Therefore, the HPAM was chosen as the focus on this research. POLYACRYLAMIDE Polyacrylamide is a synthetic water-soluble polymer. It usually consists of copolymers of various ratios of acrylic acid and acrylamide (CAENN & CHILLINGAR, 1996). The most widely used form of the HPAM for recovery methods is the partially hydrolyzed (LAKE, 1989; SORBIE 1991; SHENG, 2011). The HPAM is a polyelectrolyte with negative charges on the carboxylate groups (FIGURE 1), which implies strong interaction between the polymer chains and any cation present in the solvent, especially for higher hydrolysis degrees (NASR-EL-DIN, HAWKINS & GREEN, 1991). In commercial products, the hydrolysis degree ranges from 15 to 35%. In EOR applications, the general hydrolysis degree is close to 25% (MELO, et al., 2002). Zeynali, Rabii & Baharvand (2004) and Choi (2008) concluded that the higher the hydrolysis level, the higher is the viscosity until values around 30 to 40%, after which the viscosity decreases. According to Sheng (2011), this reduction on viscosity for hydrolysis degree up to 40% is the result of the severe International Journal of Engineering & Technology IJET-IJENS Vol:14 No:03 36 146603-2828-IJET-IJENS © June 2014 IJENS I J E N S compression and distortion of the flexible chains of the polymer. Fig. 1. Molecular structure of a partially hydrolyzed polyacrylamide (LITTMAN, 1988). Several relations are used to designate the viscosity of a polymer solution ( ), such as, polymer solution viscosity at very low shear ( ) and limiting value at high shear ( ), solvent viscosity ( ), relative viscosity ( ), specific viscosity ( ), reduced viscosity ( ), inherent viscosity ( ) and intrinsic viscosity [ . The intrinsic viscosity is the limit of the inherent viscosity when polymer concentration tends to zero. According to Sorbie (1991), the intrinsic viscosity can be used to estimate the molecular weight of the polymer by Mark-Houwink equation (Eq. 1):