Abstract

Injection of polymers is beneficial for Enhanced Oil Recovery (EOR) because it improves the mobility ratio between the displaced oil and the displacing injected water. Because of that benefit, polymer flooding improves sweep and displacing efficiencies when compared to waterflooding. Due to these advantages, polymer flooding has many successful applications in sandstone reservoirs. However, polymer flooding through carbonatic rock formations is challenging because of heterogeneity, high anionic polymer retention, low matrix permeability, and hardness of the formation water. The scleroglucan is a nonionic biopolymer with the potential to overcome some of those challenges, albeit its elevated price. Thus, the objective of this work is to characterize low concentration scleroglucan solutions focusing on EOR for offshore carbonate reservoirs. The laboratory evaluation consisted of rheology, filtration, and core flooding studies, using high salinity multi-ionic brines and light mineral oil. The tests were run at 60 °C, and Indiana limestone was used as a surrogate reservoir rock. A rheological evaluation was done in a rotational rheometer aiming to select a target polymer concentration for the injection fluid. Different filtration procedures were performed using membrane filters to prepare the polymer solution for the displacement process. Core flooding studies were done to characterize the polymer solution and evaluate its oil recovery relative to waterflooding. The polymer was characterized for its retention, inaccessible pore volume, resistance factor,in-situviscosity, and permeability reduction. Rheology studies for various polymer concentrations indicated a target scleroglucan concentration of 500 ppm for the injection solution. Among the tested filtration methods, the best results were achieved when a multi-stage filtration was performed after an aging period of 24 h at 90 °C temperature. The single-phase core flooding experiment resulted in low polymer retention (20.8 μg/g), inaccessible pore volume (4.4%), and permeability reduction (between 1.7 and 2.4). The polymer solutionin-situviscosity was slightly lower and less shear-thinning than the bulk one. The tested polymer solution was able to enhance the oil recovery relative to waterflooding, even with a small reduction of the mobility ratio (38% relative reduction). The observed advantages consisted of water phase breakthrough delay (172% relative delay), oil recovery anticipation (159% and 10% relative increase at displacing fluid breakthrough and 95% water cut, respectively), ultimate oil recovery increase (6.3%), and water-oil ratio reduction (38% relative decrease at 95% water cut). Our results indicate that the usage of low concentration scleroglucan solutions is promising for EOR in offshore carbonate reservoirs. That was supported mainly by the low polymer retention, injected solution viscosity maintenance under harsh conditions, and oil recovery anticipation.

Highlights

  • Polymer flooding is an Enhanced Oil Recovery (EOR) method based on the addition of water-soluble polymers to the injection water

  • The two-phase test focused on the comparative performance of oil recovery by polymer flooding against waterflooding, as well as on the validation of Residual Resistance Factor (RRF) and in-situ viscosity estimates in the presence of oil

  • We evaluated the filterability through the Filtration Ratio (FR), equation (1), as well as by the Viscosity Loss (VL) after filtration, equation (2): Table 1

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Summary

Introduction

Polymer flooding is an Enhanced Oil Recovery (EOR) method based on the addition of water-soluble polymers to the injection water. Polymer flooding is an option to overcome heterogeneity issues, but the technique presents additional challenges associated with some carbonate reservoirs such as high reservoir temperature and high salinity/ hardness injection and formation water These characteristics lead authors to discourage the implementation of polymer floods in naturally fractured carbonates (Al-Adasani and Bai, 2010; Boekhout, 2015; Bourdarot and Ghedan, 2011; Carcoana, 1982; Chang, 1978; Kang et al, 2016; Littmann, 1988; de Melo et al, 2002; Meyer et al, 2007; Sorbie, 1991; Taber et al, 1997)

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