Ultrasonic Treatment Mobilizes Shale Oil through Pore-Network Modification and Adsorbed-to-Free Oil Conversion

Abstract

Shales contain abundant liquid hydrocarbon reserves and have become a focus of attention in the energy industry. However, the ultratight characteristic of shales severely constrains the mobility of oil, leading to low oil recovery in shale reservoirs. In this study, shales with different lithofacies from the Liushagang Formation in the Beibu Gulf Basin were investigated using a combination of ultrasonic technology, nuclear magnetic resonance (NMR), computed tomography (CT) scanning, organic geochemical analysis, etc. The variations in the occurrence state, molecular structure, and mobility of shale oil, as well as the pore structure, before and after ultrasonic treatments were systematically analyzed. The effects of ultrasonic treatments on shale oil mobility and the underlying microscopic mechanisms were discussed. The results show that under the influence of mechanical vibration, cavitation, and thermal effects, ultrasonic waves effectively improve the pore structure of shales and accelerate the formation of new pores and fractures. Meanwhile, ultrasonic waves also facilitate the conversion of the adsorbed oil into free oil. After the ultrasonic treatments, the ratio of free oil content to adsorbed oil content of siliceous shales, mixed shales, and argillaceous shales increased by 1.6, 1.4, and 0.7, respectively. In addition, ultrasonic treatments induce the cracking of shale oil molecules, breaking down part of the long-chain saturated hydrocarbons, alkyl aromatic hydrocarbons, and heavy asphaltic aggregates into lighter components. The synergistic effects of these microscopic mechanisms comprehensively enhance shale oil mobility, increasing the mobile oil saturation from 44.9% to 58.1% in siliceous shales, from 39.1% to 56.0% in mixed shales, and from 33.7% to 49.6% in argillaceous shales. Moreover, the enhancement effect of ultrasonic waves on oil mobility exhibits distinct response characteristics within the multiscale pores. The effect is most pronounced in transition pores (20–300 nm), followed by the micropores (1–20 nm), whereas it is relatively weak in macropores (>300 nm) and ultramicropores (<1 nm). These findings provide theoretical support for the application of ultrasonic technology in enhancing the shale oil recovery.