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Current displacing agents for enhanced oil recovery (EOR), i.e., water, gas, foam, and common nanofluid, suffer plenty of problems in ultralow-permeability reservoirs, with a limited oil recovery. A novel agent, amphiphilic graphene oxide (H−GO), is confirmed to be effective in EOR under such unconventional conditions. We present the fundamental mechanisms for the improved performance by H−GO. Specifically, the effectiveness of H−GO is experimentally verified from multiple perspectives, including emulsifying ability, interfacial activity, and wettability alteration action. Experimental observations show that H−GO has excellent emulsifying ability that the amphiphilic Janus nanosheet is inclined to move onto the oil−water interface. Pickering emulsion formed by H−GO and oil, with an average droplet diameter of only 2.66 μm, is considerably stable, which can be maintained for at least 30 days. Besides, H−GO reduces the oil−water interfacial tension to 0.493 mN/m, 1 order of magnitude lower than that between the common nanoparticle and oil. H−GO shows better wettability alteration efficiency compared to a common nanoparticle. Consistently, micromodel tests show that H−GO gives a promising sweep and displacement efficiency to oil. As a result of these advantages of H−GO, the oil on the rock surface can be separated and form the emulsion or oil zone easily, leading to the incremental recovery of 10.83% using only H−GO (1 mg/mL) in an ultralowpermeability core. The study provides a novel approach for EOR in ultralow-permeability reservoirs. The high performance, convenient operation, and environmental protection make H−GO flooding have great potential for oilfield practice.

Permeability mainly governs fluid flow through hydrate‐bearing sediments, and its theoretical models play a primary role in the efficiency prediction of gas recovery from hydrate reservoirs by using numerical simulators. Most of these numerical simulators rely on empirical or semiempirical permeability models largely due to the lack of suitable parameters that well quantify the evolution of pore structures. In this study, X‐ray computed tomography scans are conducted on methane hydrate‐bearing sands, followed by extractions of the maximal diameter and fractal dimensions for the pore space occupied by fluids. These extracted parameters, including area and volume pore‐size fractal dimensions, tortuosity fractal dimension, and the area maximal pore diameter, are further extended to develop fractal theory‐based models for predictions of the hydraulic tortuosity and the saturated water permeability in hydrate‐bearing sands. Results show that the pore space occupied by fluids within hydrate‐bearing sands is inherently fractal. The area pore‐size fractal dimension decreases but the tortuosity fractal dimension changes little with increasing hydrate saturation, and the sum of these two fractal dimensions can be used to predict the volume pore‐size fractal dimension. In addition, the area maximal pore diameter decreases with increasing hydrate saturation, and a semiempirical model is proposed. The fractal theory‐based permeability reduction model agrees well with available experimental data, and it can capture the essential physics of saturated water permeability reduction in hydrate‐bearing sediments during hydrate formation.

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Clear understandings of the dissociation behaviors and characteristics of a gas hydrate are essential for the exploitation of hydrate-bearing reservoirs. To study the dissociation characteristics of a gas hydrate in porous media, measurements using the low-field nuclear magnetic resonance (LFNMR) method on three different sand samples are conducted in this work. The changes of pore radius and phase saturation during the dissociation are determined based on the transverse relaxation time (T 2) spectra. The fractal dimension is employed to depict the distributions of pore spaces occupied by water in the media, which shows a decreasing function with the decreasing hydrate saturation. Predictions of water permeability during the hydrate dissociation are comparably conducted using a fractal model and the widely used Schlumberger-Doll Research model. The fractal model performs better in showing the influences of pore-structure factors on the water permeability. The changes of specific reaction surface areas during the dissociation are estimated and preliminarily analyzed based on the results of LFNMR measurements. Results in this work may provide valuable insights for the related researches including the mechanical, acoustic, and electrical properties of the hydrate-bearing sediments.