Alkali is an important component for alkali/surfactant/polymer technology for enhanced oil recovery (EOR). The mechanism and advantages of traditional inorganic alkali for EOR was reviewed in this paper. The rheological and dynamic properties of the combination of alkali and polymer were analyzed. The results show that the polymer solution with ethanolamine has better shear viscosity and elastic properties at room temperature. Surfactant (Alfaterra 123-8S-90) with concentration of 0.15 wt % was added into each alkali−polymer (AP) solution. No significant change was observed in rheological properties of AP solutions with and without surfactant. Emulsification tests show that ethanolamine has better performance with oil. Injectivity tests were also conducted. The results indicated that the residual resistance factor (RRF) for an ethanolamine− polymer solution is always higher at each flow rate tested, in comparison to a NaOH-based AP solution, which is beneficial for oil recovery. The interfacial tension (IFT) tests results indicated that ethanolamine has better synergy with the surfactant. Polymer adsorption using both static and dynamic measurements was conducted. Polymer solution in an ethanolamine system has lower adsorption for both measurements. The pressure comparison during core flooding experiments shows that it has higher injection pressure in ethanolamine conditions, which result in good sweep efficiency. The ethanolamine−polymer flooding showed a significant increase in oil recovery (15.33%) over NaOH−polymer flooding. After the addition of surfactant, the total recovery improves by 14.8% for ethanolamine−polymer−surfactant flooding over its inorganic counterpart. The better performance indicates that ethanolamine can become a potential alkali and can replace NaOH for EOR.
Summary Various types of ultrahigh-molar-mass polyacrylamides (HPAMs) and their copolymers and terpolymers used not only in enhanced oil recovery (EOR) but also in drilling, fracturing, water treatment, and tailing applications require an accurate description of polymer molar mass (Mw) and hydrodynamic size for their optimal design. The range of Mw for various types of available HPAMs is between 4 and 30 million g/mol and is typically determined by use of intrinsic-viscosity measurement. Molecular-weight distribution (MWD) cannot be determined because neither standard with low polydispersity index (PDI) nor gel-permeation-chromatography (GPC) or size-exclusion-chromatography (SEC) techniques exist today for such ultrahigh-molar-mass polymers. Moreover, the solution environment in underground reservoirs, characterized by high temperatures, pH values, and the presence of monovalent and divalent ions, may often lead to changes in polymer-macromolecular conformation. Current techniques, SEC, ultraviolet-visible measurements, and liquid chromatography, are not capable of accurately investigating these complex macromolecular structures for various reasons. In this paper, the asymmetrical-flow field-flow fractionation (AF4) system was used to fractionate four different ultrahigh-molecular-weight HPAM samples, varying in molar mass and commercially used for oilfield applications, in various carrier pH values ranging from 12 to 3 (pH values of 12, 7.4, and 3). The system uses field-flow fractionation (FFF), a family of analytical techniques developed specifically for separating and characterizing macromolecules, colloids, and particles. The theoretical separation range for AF4 is between 103 to 1012 g/mol. Other advantages over conventional GPC/SEC include minimum shear degradation, mild operating conditions, and no sample loss caused by adsorption. The flow system was equipped with a multiangle-light-scattering (MALS) and refractive-index (RI) detectors to measure molar mass and radius of gyration (Rg). The results show that the observed molecular weight of the polymer aggregate increased substantially as the pH value of the carrier solution decreased from 12 to 3, especially for higher-molar-mass polymers. The sample Rg showed the opposite trend, decreasing as the pH of the carrier solution changed from basic to acidic. For ultrahigh molecular HPAM at high pH, a narrower molar mass and radius distribution was observed with disaggregated molar mass and increased branching or swelling (therefore larger hydrodynamic radius). Use of this direct separation and measurement technique can improve understanding of polymer-macromolecular structure and corresponding changes in the reservoir brines.
Typically, a polymer for enhanced oil recovery (EOR) is selected on the basis of the viscosity range or average molecular weight, concentration, and brine composition, besides other reservoir properties. There is not much emphasis given on how the elasticity of polymers could enhance the oil recovery. In this study, in an effort to find a systematic approach for selecting the best polymer for water flooding, the effect of molecular weight distribution (MWD), a direct measure of a polymer's elasticity, was studied on oil recovery performance. The individual effect of the elasticity of polymers on oil recovery, breakthrough and overall recovery, and residual resistance factor (RRF) was determined by keeping the viscosity constant and varying the elasticity during secondary and tertiary recovery experiments. Within two different groups of polymers each with similar average molecular weight studied here, nearly 10% higher recovery for the highest elastic polymer was observed during secondary recovery, whereas for tertiary flood ∼6% higher recovery with ∼5 times higher RRF value was observed for the highest elastic polymer solution studied here. Results have shown that average molecular weight by itself might not be the best criterion to select the optimum polymer fluid composition for polymer flooding operations. Polymer elasticity should be weighted more than the average molecular weight, as it could correspond to higher sweep efficiency due to the stretching of polymer along the pores. Considering the polymer elasticity or MWD together with average molecular weight seems to be a better approach for achieving higher oil recovery performance at lower polymer concentrations.
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