Improving Alkali Polymer Flooding Economics by Capitalizing on Polymer Solution Property Evolution at High pH

Abstract

Summary Alkali polymer (AP) flooding is a promising enhanced oil recovery (EOR) method to increase oil recovery from reactive oils. It is essential to carefully select the alkali and polymer type and concentration to optimize incremental oil recovery. In addition to the conventional laboratory tests for polymer flooding, the effects of the high pH on the polymer and its evolving properties over time need to be investigated. Consideration of near-wellbore and reservoir effects is key in designing the process. We are showing how understanding and taking advantage of the polymer performance in a high pH environment allow for cost reduction and increase in injectivity and incremental oil recovery for AP projects. The polymer performance was evaluated for AP flooding of the Matzen field (Austria). Evaluations included changes in polymer rheology during aging at high pH conditions, phase behavior tests, and single-/two-phase corefloods with aged and nonaged polymer solutions. In addition, adsorption of the aged polymer and interfacial tension (IFT) were measured. The aging was studied in anaerobic conditions at reservoir temperature and through an accelerated method at elevated temperatures. The accelerated method developed earlier for neutral pH range provides a possibility to run aging at elevated temperatures in a short time frame and transfer the data to reservoir temperature to give information on the long-term performance. The transfer takes place through a conversion factor derived from the first-order kinetics of acrylamide hydrolysis in pH 6–8. In the present work, the applicability of the accelerated method is evaluated for elevated pH by determining the degree of polymer hydrolysis over time via nuclear magnetic resonance and linking it to viscosity performance at various temperatures. The AP conditions in the Matzen AP flooding project (pH > 10) lead to an increased initial rate of polymer hydrolysis of the tested hydrolyzed polyacrylamide (HPAM) by a factor of 100 compared to hydrolysis at a neutral pH level. This resulted in a rapid increase in a polymer solution viscosity of 160% compared with initial conditions within days at a reservoir temperature of 49°C, after which the viscosity leveled off. Accelerated aging experiments at higher temperatures predict long-term stability of the increased viscosity level for several years. Single-phase injection test in a representative core confirmed the performance of the aged solution compared to a nonaged solution at the same polymer concentration. The retention of polymers is reduced in AP conditions compared with traditional neutral pH conditions, 19 vs. 48 µg/g in the static adsorption test, respectively. Two-phase coreflood tests showed increased polymer viscosity at reservoir conditions. The displacement efficiency of the aged and nonaged polymer solution was similar, confirming the potential for cost savings using lower polymer concentration. This is leading to an improved injectivity and makes use of the increased polymer viscosity down in the reservoir through hydrolysis. The current work combines multiple aspects that should be considered in the proper planning of AP projects—not only improvements in polymer viscosity performance due to water softening but also long-term effects due to increased pH. Additionally, these aspects are combined with changes in adsorption properties. The results show that the design of AP projects will benefit from the holistic approach and understanding the changes in polymer rheology with time. The costs of AP projects can be reduced owing to the lower required polymer concentrations for the same displacement efficiency and reduced retention of polymer. An efficient design of AP projects takes good injectivity of nonaged polymers and the aging of the polymer solutions in alkali into account. Overall, we aim to reduce the polymer concentration—which is a key cost driver—compared with a nonaged application. We show that for AP effects, these effects should be evaluated to improve the economics.

Optimizing Polymer Costs and Efficiency in Alkali–Polymer Oilfield Applications

“…The increase was linked to a fast change in the degree of hydrolysis providing a positive impact on viscosity in the soft brine. A more detailed description of the studies can be found in [13,14]. Even though the field temperature in Matzen is 49 • C, the aging was also carried out at 60 • C and 70 • C to accelerate the aging.…”

“…For the optimum utility factor, aging of AP slugs is performed to optimize the project economics. This study is a continuation of our previous research [13,14], with an emphasis on AP flooding in reservoir sand packs and reservoir conditions (live oil).…”

“…Thermal Aging of Polymer Solutions: Polymer solutions for thermal aging studies were prepared in a glove box under nitrogen atmosphere to ensure an anaerobic environment reflecting field conditions. The method is described more in detail in [13,14]. The aging was conducted in various concentrations.…”

“…Interfacial tension (IFT Upper Phase/Middle Phase) at the fluids' interface was calculated using the volume ratio between the upper phase (UP) and middle phase (MP) using the approach adopted by Liu et al [20] and Hincapie et al [9]. Interfacial tension data were reported in [13,14]; here, we compare results.…”

Enhancing Polymer Flooding Efficiency in a Sandstone Oilfield in Kazakhstan through Silica Nanoparticle-Based Control of Polymer Adsorption

“…30−32 It may also alter the wettability of the rock surface and enhance the oil recovery. 33 On the basis of the study of Nurmi et al, 34 the presence of alkali reduces the adsorption of HPAM on the Berea sandstone. The outcome of the static adsorption test without additional sodium carbonate was 31 μg/g, which was reduced to 24 μg/g by adding alkali.…”