Fractures in geological formations, which are commonly encountered during oil and gas exploration, pose significant challenges by facilitating fluid loss and reducing wellbore stability. Polymer gels hold significant promise as lost circulation materials due to their excellent deformability under pressure and self-adaptability. However, conventional gels face certain limitations, such as poor retention of gel-forming suspensions in fractures, low structural strength, and instability at high temperatures. These issues result in low pressure-bearing capacity and, consequently, reduced effectiveness in sealing formation fractures. This study presents the synthesis of a supramolecular polymer gel (SPG) composed of acrylamide (AM), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), divinylbenzene (DVB), polyvinyl alcohol (PVA), TEMPO-oxidized cellulose nanofibers (CNFs), and laponite via in situ radical polymerization. Due to the synergistic effects of hydrogen bonding and electrostatic interactions, the supramolecular polymer gel-forming suspension exhibits high retention capacity in fractures. The as-synthesized gels demonstrate remarkable temperature resistance and high mechanical strength, attributed to covalent bonding, multiple hydrogen bonding, and electrostatic interactions, along with the high aspect ratio and modulus of CNFs and laponite. This study broadens the application scope of CNFs, laponite, and SPGs, offering a novel approach for the efficient development of unconventional oil and gas resources.

With the depletion of shallow and mid-depth oil and gas resources, exploration and development have increasingly shifted toward deep and ultra-deep reservoirs. However, high bottom-hole temperatures and pressures, along with complex geological conditions, pose significant challenges. Additionally, growing environmental regulations restrict the use of conventional petroleum-derived chemicals such as polyacrylamide, sulfonic acid resins, and sulfonic acid asphalt. In recent years, lignin has demonstrated significant potential in petroleum exploration and development due to their sustainability, biodegradability, and excellent thermal, chemical, mechanical, and rheological properties. This article reviews the structure, classification, preparation, and modification methods of lignin, followed by a systematic discussion of its applications in oilfield operations. In drilling fluids, lignin and its derivatives serve as rheological regulators, fluid loss control agents, and shale inhibitors, contributing to enhanced cuttings transport and wellbore stability. In fracturing fluids, they function as thickeners and breaker agents, improving fracturing efficiency while protecting the reservoir. In enhanced oil recovery, lignin-based polymers act as surfactants and profile control agents, reducing interfacial tension between water and rock surfaces and increasing the effective permeability of sandstone reservoirs. Furthermore, in oilfield wastewater treatment, lignin-based materials effectively remove oil-water mixtures, heavy metal cations, and solid particles through filtration, adsorption, and flocculation.

Unconventional oil and gas reservoirs have become a new focus of energy development due to their wide distribution and abundant reserves. However, the exploitation of these reservoirs is often accompanied by varying temperatures, which impose higher requirements for novel material, equipment, and technology. Recently, phase change microcapsules have been attracting increasing attention in oilfield applications, because they can absorb or release considerable latent heat during the phase change process, enabling stable temperature control. Herein, the current status and future development trend of phase change microcapsules in oilfield applications are reviewed. The classification of phase change materials, including solid-solid, solid-liquid, solid-gas, and liquid-gas phase change materials, is introduced, with an emphasis on their advantages and disadvantages. Then, the microencapsulation methods for phase change materials are presented. Next, the critical thermophysical properties of phase change microcapsules relevant to oilfield applications, including melting and freezing points, latent heat capacity, thermal conductivity, and cycling stability, are discussed. Subsequently, the specific applications of phase change microcapsules in oilfields, including temperature regulation of drilling fluid, thermal management of cement paste, thermal protection of drilling equipment, and thermal insulation of submarine oil and gas pipelines, are thoroughly overviewed. Finally, the critical challenges and future perspectives are outlined. This review highlights the critical role of phase change microcapsules in advancing thermal management solutions for the efficient development of oil and gas from high- and low-temperature reservoirs, guiding future research and development efforts.