High-molar mass acrylamide-co-diacetoneacrylamide graft copolymers as viscosity enhancer for polymer flooding oil recovery

Highlights

• Acrylamide/diacetoneacrylamide based graft copolymers were synthesized in three steps.

• Very high molar mass and improved copolymer homogeneity was targeted.

• Reaction engineering was applied: semicontinuous and inverse-emulsion polymerization.

• High molar mass thermoassociating copolymer (TAP) was obtained.

• TAP has efficient viscosifying properties at decreased content and harsh conditions.

Abstract

One of the most widely applied enhanced oil recovery processes is the polymer flooding, in which aqueous solution of polymer viscosifier is introduced in oil reservoirs to increase the recuperation of the remaining oil. From the current challenges of this process, it can be referred to a high cost of materials regarding their substantially required amount and the low impact on the mobility ratio during the process due to the reduction of solution viscosity at high temperatures and high salinity environments. The purpose of this study is to investigate the concept of acrylamide-based thermosassociating copolymer (TAP), with a specific morphology and chemistry (hydrophilic main backbone made of polyacrylamide with grafted amide functionalized pending chains) as viscosity enhancer at harsh conditions of high temperature and salinity. For that aim, a specific TAP microstructure was targeted (very high molar mass linear polymer chains with improved copolymer homogeneity). It is achieved in this study throughout applying the reaction engineering approach, such as synthesis in semi-batch mode or/and in heterogeneous dispersed media. As a result, the synthesized TAP presented excellent behavior as viscosity enhancer especially under high temperature and salinity conditions with improved performance in comparison to TAP synthesized by a conventional solution polymerization approach and to actual commercial high molar mass acrylamide-based polymer.

Introduction

Polymer flooding is one of the chemical enhanced oil recovery (EOR) processes for the oil production, in which water-soluble polymers are used to modify the rheological properties of the displacing fluid, improve the water-to-oil mobility ratio, and thus enhance the displacement efficiency. Such a technique has been recognized as the most efficient and most widely applied EOR method for mature oil fields, such as Daqing and Shengli in China [1,2]. Primarily because of its low cost, polymer flooding has been performed more than any other types of EOR processes. The collapse of the oil price in the mid-1980s caused that polymer EOR developing projects virtually disappeared, giving way to a variation of the processes based on polymer gels. Returning the oil price, attention to polymer EOR has risen again in relation to its good achievements in the Chinese Daqing Field. Polymer processes have historically recovered about 5% of the original oil in place and taken about 1 lbm of polymer to produce an incremental barrel [3].

Partially hydrolyzed polyacrylamide (HPAM) is the most commonly used polymer to alter the viscosity of the pushing water in the polymer flooding EOR process in order to recover immobile, trapped oil, and improve the sweep efficiency [4]. However, in conditions of high salinity (TDS) (above 30,000 mg L⁻¹) and high temperature (>60 °C) characteristic for many oil reservoirs, side amide groups of HPAM extensively hydrolyzed, which decreases the viscosifying effect [5]. Besides, the electrostatic repulsion between the HPAM chains becomes screened by the presence of cations in the high salinity waters, resulting in collapsing of the polymer coils, decreasing the hydrodynamic volume and finally lowering the solution viscosity [6]. Under such harsh condition, the efficiency of the HPAM aqueous solution becomes weak and it is required to use higher polymer concentrations to improve the performance, leading to increase the project costs.

To develop a resistant polymer to apply in the high temperature and salinity environment, a number of researches have tried to alter the chemical structure of polyacrylamide [[7], [8], [9], [10]]. To overcome the HPAM constraints, Hourdet et al. [[11], [12], [13], [14], [15], [16], [17], [18], [19]] proposed the concept of thermoassociating polymer (TAP). Aqueous solution of such TAP is characterized by temperature and salinity dependent viscosity that is reversibly increased upon heating and/or increasing salinity. Afterwards, Wang et al. [[20], [21], [22]] synthesized a novel TAP for the EOR process. Recently, pilot commercial TAP product was developed by Beijing Hengju Polymer Co., Ltd [23,24]. The success of these TAP copolymers is based on precise control of the chemical structure via the grafting method, and control of the length and the molar mass distribution of the backbone, as well as the placement of grafts along the main chain. The grafted side chains onto the hydro-soluble main chain are usually thermosensitive and introduce a property of lower critical solution temperature (LCST). Surpassing this temperature, water becomes a poor solvent for the thermosensitive grafts, resulting in formation of self-aggregated hydrophobic microdomains, forming a network by physical junction and a macroscopic viscosity enhancement.

Basically, two types of PEO- or NIPAM-based thermosensitive functionalities were introduced into main polymer backbones via the grafting technique. Within the first type, either poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO) or PEO/PPO copolymer were grafted via a coupling reaction into the main chains of a low molar mass hydro-soluble parent polymer, such as poly(acrylic acid) (PAA) [11,17], PAM [14], copolymer of AA and 2-acrylamido-2-methyl propanefulfonic acid (AMPS) [12], terpolymer of AM, AMPS, and N-ethyl vinyl acrylamide [13,14], carboxymethylcellulose, aliginate, and carboxylated dextran [19], or PPO-based macromonomers were copolymerized with acrylamide [25,26]. Secondly, N-isopropylacrylamide (NIPAM)-based amino-end functionalized macromonomer [15] or oligomer of NIPAM and other comonomers such as AM, AMPS, or butyl methacrylate [16] were grafted onto low molar mass PAA backbone through a two-step process. Such TAP polymers applied in the EOR process through reservoirs at high salinity and temperature will not overcome some common problems of traditional polymers such as PAM, since they have a low viscosity at the beginning of the process (when injected into the well). On the other hand, due to the type of the viscosity enhancement mechanism in the aqueous phase, these polymers are more effective than other polymers used for similar aims.

Moreover, the preparation of thermoviscosifying water-soluble polymers with such a ‘‘grafting onto’’ procedure using PEO- and NIPAM-based thermosensitive stickers presented several disadvantages. Beside that the presence of carboxylic groups in polymeric precursor is required to couple the amino-terminated groups in most of the cases [[11], [12], [13], [14], [15],18,19], the grafting process always has to be done at low polymer concentration to avoid the formation of gels resulting in molar mass reduction. For example, for polymer loading in the reaction system below 4%, the molar mass of PAA was generally less than half million [14]. Furthermore, use of relatively expensive materials for the coupling reaction, including NIPAM, dicyclohexylcarbodiimide and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), and certain non-organic salts such as K₂CO₃ are required to reduce the critical temperature of the PEO-based polymers [12,14,18,19]. Finally, molar mass of the most synthesized polymers is lower than commercial ones, which generally cannot provide a corresponding increase in the viscosity at a required concentration and a higher polymer concentration is needed to achieve the targeted viscosity increase. These limitations may impede large-scale manufacturing of the polymers and acceptance of petroleum engineers.

Wang et al. [[20], [21], [22]] tried to overcome the above limitations. They increased the molar mass of TVPs and finely tuned the LCST by copolymerization of acrylamide with a newly developed thermosensitive comonomer (MPAD) based on N-(1,1-dimethyl-3- oxobutyl)-acrylamide (DAAM) [[20], [21], [22]]. They provided the rheological behavior investigation and preliminary core flooding results under simulated high temperature and salinity oil reservoirs conditions, demonstrating the efficiency of the new TAP polymers that had recovery factor of 13.5% versus only 2.1% for the HPAM. However, beside the excellent performance reported, the main drawback of the new TAP polymers is their medium molar mass polymer chains that finally may compromise the aging behavior in the reservoirs. Moreover, the synthesis strategy used in these studies (solution batch polymerization) [[20], [21], [22]] resulted in low functional monomer incorporation in the copolymer that decrease the viscosifying performance. In such conditions, the final copolymer composition that is determined by reactivity ratios of both monomers was not controlled, thus, low incorporation of functional monomer into the AM-based backbone was obtained. Up to the best knowledge of the authors, there is no report so far concerning an approach for synthesizing a TAP with higher molar mass and better control of the copolymer composition.

Therefore, in the present work by use of polymerization reaction engineering strategies, high molar mass DAAM-modified TAPs with improved copolymer composition were targeted. The TAP was synthesized in three steps throughout the synthesis of telomere, macromonomer, and TAP copolymer. In the telomere synthesis, semi-batch mode was applied by feeding one of the components of redox pair initiator system to keep low the radicals concentration in the solution. In such conditions mutual radical termination is decreased and possibility to obtain higher molar masses increased. In TAP synthesis, inverse miniemulsion polymerization strategy was applied. This heterogeneous colloidal system allows polymerization of monomer aqueous solution dispersed in the solvent continuous media, providing few advantages for the TAP synthesis. In such conditions, the monomer droplets containing high concentration of monomer and low concentration of growing radicals are the main polymerization loci. These conditions are well suited to grow high kinetic chain lengths polymers at relatively high polymerization rates, taking advantage of the radical compartmentalization effect. On the other hand, the heterogeneous system, where the both comonomers are distributed between the aqueous and organic phases, may contribute importantly to the copolymer composition and final incorporation of the functional macromonomer into the PAM backbone leading toward improved performance as rheological modifiers. Finally, the performance of the synthetic copolymers as a viscosity enhancer was compared with the same TAP synthesized in solution and with the latest AM-based commercial polymer, presenting significantly improved enhancing behavior especially at high temperature, salt concentrations and shear rate with decreased amount of polymer, which from an industrial point of view seems to be very promising.