Microrobot-Assisted Mobilization of Residual Gas in Tight Sandstones: A Microfluidic Proof-Of-Concept Toward Pore-Scale Enhanced Recovery

Abstract

Natural gas is a clean and efficient low-carbon fossil energy source that plays a pivotal role in the global energy transition. Compared to coal and petroleum, its combustion produces significantly less CO2, NOx, and particulate matter, making it a critical bridge fuel toward a low-carbon future. As nations adjust their energy consumption structures, natural gas is projected to command an increasingly prominent share in primary energy consumption, gradually forming a "tripod" pattern alongside petroleum and new energy sources. However, as major gas fields mature, production faces mounting challenges. Tight sandstone gas reservoirs, characterized by abundant reserves, represent a core target for future production growth. These reservoirs typically exhibit low porosity, poor permeability, strong heterogeneity, and high gas-water interfacial tension, causing substantial volumes of natural gas to remain trapped within micro- to nano-scale pores, unrecoverable by conventional displacement methods. This results in persistently low recovery rates. Therefore, effectively enhancing displacement efficiency within complex pore networks has become a critical scientific and engineering challenge. Microrobotics offers a novel solution to these problems. Microrobots—artificial devices operating at micrometer to nanometer scales—possess exceptional advantages: minimal size, flexible locomotion, and precise controllability, enabling autonomous or controlled movement within complex microenvironments. During natural gas depletion scenarios, their potential functions manifest in three aspects: (1) Overcoming gas-water interfacial tension constraints to actively extract residual gas trapped in micro/nano pores; (2) Transporting through highly heterogeneous and poorly connected pore networks to transfer gas to more accessible locations; and (3) Accelerating the mobilization of trapped gas to increase production rates. These capabilities provide new insights for improving displacement efficiency and ultimate recovery in tight gas reservoirs, opening a novel research avenue beyond traditional stimulation techniques. This study presents a proof-of-concept investigation on magnetically-driven microrobots for mobilizing trapped gas. We developed spherical Fe3O4 magnetic nanoparticles (200–500 nm) via a solvothermal method and demonstrated their self-assembly into chain-like clusters under an external gradient magnetic field. Using a microfluidic platform that mimics pore-throat structures of natural rock cores, we experimentally validated that these magnetic microrobot clusters can effectively mobilize and transport gas bubbles under magnetic control. The results show that bubble displacement efficiency is positively correlated with magnetic field strength, microrobot size, and particle concentration. Fe3O4-based microrobots exhibit superior magnetic responsiveness compared to Ni-based counterparts, achieving faster and more stable bubble mobilization. This work establishes a foundational framework for pore-scale enhanced gas recovery through microrobotic intervention.