CO₂-switchable nanohybrids for enhancing CO₂ flooding in tight reservoirs: From stable colloids to a relevant viscoelastic fluid

Highlights

• A CO₂-functional monolayer densely grafted silica nanoparticles is synthesized.

• Silica nanohybrids dispersion undergoes reversibly rheological transition switched by CO₂/air.

• The nanohybrids dispersion reduces CO₂ mobility and increases the CO₂ flooding efficiency in tight reservoirs.

Abstract

Conventional CO₂ EOR techniques based on microscale chemicals have limited efficiency in tight reservoirs because the micro-nanopores of these reservoirs impede their injectivity and propagation in porous media. This work elucidated a novel well-defined silica nanohybrids named DMA-NPs with the distinct CO₂ switchability for enhancing CO₂ flooding in tight reservoirs. DMA-NPs densely encapsulated with a CO₂-functional moiety of dimethylamine was synthesized by sequential surface modification and amidation reaction. The proof-to-concept for the CO₂-switchable nanohybrids was studied by TEM, TGA, SEM, ¹H NMR, FT-IR, DLS, rheological measurements and core flooding tests. The results indicated that DMA-NPs underwent reversibly physical transition, from stable colloidal particles with hydrodynamic diameter of 62 nm to a relevant viscoelastic fluid, by repeatedly bubbling CO₂ or introducing air to remove CO₂. Moreover, DMA-NPs dispersion whose viscosity was close to water, could preferentially flow through dominant porous media. When the dispersion met with the displacement front of CO₂, these stable colloidal particles self-assembled into a relevant viscoelastic fluid which reduced CO₂ mobility and diverted CO₂ into a lower permeable zone, and thus more than 30% of original oil in place bypassed by the initial CO₂ displacement was recovered.

Introduction

With the deep exploration of conventional reservoirs, hydrocarbons from unconventional reservoirs such as tight and shale formations become a significantly alternative fossil fuel. Nowadays, the goal to increase oil production of unconventional reservoirs is the topic in petroleum industry. Carbon dioxide (CO₂) enhanced oil recovery (EOR) is a promising set of technologies for improving oil production of unconventional reservoirs due to its easy flow capability and miscible displacement mechanisms [1], [2], [3], [4], [5], [6]. However, massive oilfield pilots indicated that the high mobility of CO₂ relative to crude oil and formation water results in CO₂ bypassing oil, and the heterogeneous formations make this effect even worse [7], [8], [9]. The efficiency of CO₂ EOR process could be improved if the unfavorable mobility of CO₂ relative to crude oil and formation water can be effectively reduced. Mobility and conformance control are two major methods that address chemical control of CO₂ viscous fingering. It has been proven that the gel-based conformance technique [10], [11], foam [12], [13] and polymer [14], [15] based mobility control techniques are effective chemical EOR strategies to reduce CO₂ mobility. These CO₂ EOR techniques based on microscale chemicals for the purpose of CO₂ conformance control, however, may have limited efficiency in unconventional reservoirs because the micro-nanopores of unconventional reservoirs impede their injectivity and propagation. New strategies are urgent for chemical CO₂ EOR of unconventional reservoirs.

Recently, design and development of materials with intelligent properties in response to environmental triggers has attracted a huge scientific and technological interests in polymeric composites [16], [17], [18], [19]. Among these smart materials, the CO₂-switchable materials exhibited unprecedented advantages because their stimulus is CO₂ which is environmentally friendly, inexpensive, abundant, non-toxic, and has moderate critical temperature (31.3 °C) and pressure (7.38 MPa) [20], [21], [22]. A CO₂-functional moiety such as tertiary amine, amidine, guanidine, imidazole or carboxylic acid is required for CO₂-switchable materials [23]. Amine and amidine-containing groups are the most popular CO₂-functional moieties that have been widely investigated with versatile applications reported. When amine- or amidine-containing groups in water are exposed to CO₂, bicarbonate salts are formed as illustrated in the following formula [24]:

Actually, this reaction constructs the surface charge of materials and results in their hydrophilicity in water, indicating that not only the possibility of binding more water around each nitrogen atom enlarges the thickness of water hydration, but the possibility of electrostatic repulsion by identical charges between the neighboring chains amplifies the hydrodynamic size of CO₂-switchable materials. As a result, the molecular configuration and intermolecular reconstruction of materials occur, accompanying with reversibly physicochemical performance such as surface wettability, permeability, conductivity, shape, color and viscosity, etc. Taking these points into account, the nation of developing CO₂-switchable materials for CO₂ mobility control in unconventional reservoirs may be practicable while these devices follow two points: ① materials solutions (dispersions) must have very low viscosity even close to the viscosity of pure water (0.001 Pa) for their desirable injectivity from ground to micro-nanopores of subsurface; ② when exposed to CO₂ fluid, materials solutions (dispersions) with the dramatic increase in viscosity can block the dominant channel where CO₂ is usually viscous fingering, and thus enhance macroscopic sweep efficiency of CO₂ flooding.

On the other hand, polymeric shell hybrid nanoparticles called nanohybrids, composed of a nano-core and a layer of polymeric brushes can be programmed into intelligent properties by selecting specific core particles (rigid, soft, optical and magnetic) [19], [25], [26] and polymeric brushes (environmentally compatible, biodegradable, and responsive) [27], [28]. To the best of our knowledge, the seminal papers on polymeric brushes grafting silica nanoparticles (NPs) known as silica nanohybrids were reported by Prucker and Rühe [29], [30]. Since then these nanohybrids have been extensively studied in areas such as nanocomposites, picking emulsions, sensing, catalysis, drug delivery, biotechnology and petroleum industry because NPs are fairly feasible and can be easily modified with organic materials via various methods [31]. As for silica nanohybrids for EOR processes, most of literature information focused on the conventional reservoirs which have considerable subsurface pores for these particles propagation, whereas research of responsive silica nanohybrids for unconventional reservoirs is seldom reported. Expanding this application would certainly bestow silica nanohybrids with a significant frontier.

In this paper, a novel CO₂-switchable nanohybrids for enhancing CO₂ flooding in tight sandstones was reported. Firstly, NPs with uniform size was synthesized by employing the improvement of StÖber method. 3-aminopropyltriethoxysilane immobilized NPs was carried out to produce amine groups terminated NPs. Afterwards, successive Michael addition of methyl acrylate and amidation reaction of 3-dimethylamino-propylamine were employed to synthesize tertiary dimethylamine- encapsulated NPs (DMA-NPs). The grafting density of polymeric chains onto NPs was studied by thermal gravimetric analysis (TGA) together with transmission electronic microscopy (TEM). Secondly, the transformation between stable colloidal particles and a relevant viscoelastic fluid of DMA-NPs switched by CO₂ was deeply investigated by combining Cyro-TEM, electron microscope (SEM), angle laser light scattering instrument (DLS) and rheological measurements. Finally, a small slug of DMA-NPs dispersion was employed to improve CO₂ EOR of tight sandstone cores by two-parallel cores flooding tests.