Summary Polymers have been successfully deployed in the oil and gas industry in various field implementations, including mobility control in waterflood, flow divergence, and well conformance control. However, lab and field applications of polymer injections often encounter polymer-induced formation damage related to pore-throat clogging from polymer entrapments, leading to permeability reduction. This phenomenon manifests as a loss of injectivity, which can diminish the recovery performance. The polymer interaction mechanisms with porous rocks are not fully understood. In this work, we use microfluidics to assess formation clogging induced by polymer flood. Microfluidic techniques offer convenient tools to observe polymer flow behavior and transport mechanisms through porous media. The microfluidic chips were designed to mimic the pore-size distribution of oil-bearing conventional reservoir rocks, with pore throats ranging from 1 to 10 µm. The proposed fabrication techniques enabled us to transfer the design onto a silicon wafer substrate through photolithography. The constructed microfluidic chip, conceptually known as “reservoir-on-a-chip,” served as a 2D flow proxy. With this technique, we overcome the inherent complexity of the 3D aspects of porous rocks to study the transport mechanisms occurring at the pore scale. We performed various experiments to assess some mechanisms of polymer-rock interaction related to the sizes of the polymer molecules and pore throats. The polymer flow behavior was compared to that of the waterflood baseline. Our observations showed that prolonged injection of polymer solutions could clog pore throats of sizes larger than the measured mean polymer-coil size, which is consistent with lab and field observations. This finding highlights a limitation in some polymer screening workflows in the industry that suggest selecting the candidate polymers based solely on their molecular size and the size distribution of the rock pore throats. This work emphasizes the need for careful core-flood experiments to assess polymer entrapment mechanisms and their implication on short- and long-terminjectivity.

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Polymers have been used effectively in the Oil & Gas Industry for a variety of field applications, such as enhanced oil recovery (EOR), well conformance, mobility control, and others. Polymer intermolecular interactions with the porous rock, in particular, formation clogging and the associated alterations to permeability, is a common problem in the industry. In this work, fluorescent polymers and single-molecule imaging are presented for the first time to assess the dynamic interaction and transport behavior of polymer molecules utilizing a microfluidic device. Pore-scale simulations are performed to replicate the experimental observations. The microfluidic chip, also known as a "Reservoir-on-a-Chip" functions as a 2D surrogate to evaluate the flow processes that take place at the pore-scale. The pore-throat sizes of an oil-bearing reservoir rock, which range from 2 to 10 nm, are taken into consideration while designing the microfluidic chip. Using soft lithography, we created the micromodel from polydimethylsiloxane (PDMS). The conventional use of tracers to monitor polymers has a restriction due to the tendency of polymer and tracer molecules to segregate. For the first time, we develop a novel microscopy method to observe the dynamic behavior of polymer pore-clogging and unclogging processes. We provide direct dynamic observations of polymer molecules during their transport within the aqueous phase and their clustering and accumulations. Pore-scale simulations were carried out to simulate the phenomena using a finite-element simulation tool. The simulations revealed a decline in flow conductivity over time within the flow channels that experienced polymer accumulation and retention, which is consistent with the experimental observation of polymer retention. The performed single-phase flow simulations allowed us to assess the flow behavior of the tagged polymer molecules within the aqueous phase. Additionally, both experimental observation and numerical simulations are used to evaluate the retention mechanisms that emerge during flow and how they affect apparent permeability. This work provides new insights to assessing the mechanisms of polymer retention in porous media.

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Microfluidics is an emerging technology that has gained attention by the industry for its capabilities to investigate and visualize fundamental recovery mechanisms at the pore scale in a microdevice, mimicking, to some extent, the actual rock pore-network. While current technologies are capable of building micromodels that are either water-wet or oil-wet, a technique to achieve a representative mixed-wet property is still unreached. In this work, we introduce a novel surface coating capability using thin film deposition to fabricate surfaces with selective wettability, oil-wet and water-wet, an effort to mimic actual mixed-wet rock. This unique approach enables the generation of hydrophobic surfaces in selected regions by altering the hydrophilic surface property of silicon substrate at the microscale. A selective wettability control mask and Perfluorodecyltrichlorosilane (FDTS) hydrophobic coating using molecular vapor deposition (MVD) were used for surface wetting properties alteration. Surface measurements, including contact angle measurements, X-ray photoelectron spectroscopy (XPS), and Transmission Electron Spectroscopy (TEM) imagining, were performed to evaluate the thin-film composition and morphology. By altering the wetting state of the substrate by the coated film, a selective mixed wettability surface was achieved. This technique has the potential to be utilized in microfluidic device developments. Tuning the wetting state of the substrate to mimic the mixed-wet characteristics of reservoir rocks, such as carbonates and shales, can enhance our understanding of complex fluid behaviors in porous media and provide a crucial contribution to many subsurface petroleum engineering applications such as enhanced oil recovery (EOR) and CO2 storage.