Influences of controlled microwave field irradiation on occurrence space and state of shale oil: Implications for shale oil production

The exploration and production of shale oil play a vital role in optimizing energy consumption pattern and mitigating dependence on foreign energy. Given the existing shortcomings of current commercial hydraulic fracturing, research and development on alternative or assistive technology for producing shale oil are of great significance. In order to better understand the validity of shale oil production via microwave energy, the influences of microwave irradiation on pore and fracture structure of oil-bearing shale matrix and occurrence state of shale oil dominating shale oil production were investigated through multiple characterizations typically including the total organic carbon content, X-ray diffraction, scanning electron microscope, carbon dioxide adsorption, low-temperature nitrogen adsorption and desorption, mercury intrusion porosimetry, micro-computer tomography scanning, gas chromatography and low-field nuclear magnetic resonance. Results indicate that microwave irradiation alters geochemical characteristics of oil-bearing shale through decreasing its free hydrocarbon content and increasing pyrolysis rate of organic carbon. Moreover, microwave irradiation induces decomposition, formation, and transformation of inorganic minerals. Additionally, microwave irradiation increases surface area and volume of full-scale pores but decreases their average pore diameter. Meanwhile, microwave irradiation significantly increases number of full-scale pores, particularly macropores dominating free shale oil migration. As for fractures, microwave irradiation not only extends primary fractures but also creates new fractures parallel to the bedding direction due to inhomogeneous thermal stress generated among minerals with different microwave-absorbing capabilities. Accordingly, porosity and permeability of the oil-bearing shale remarkably increase. In terms of shale oil, microwave irradiation enhances its mobility by decreased viscosity and induced hydrocarbon pyrolysis. Finally, microwave irradiation could decrease content of kerogen, adsorbed shale oil, free shale oil and primary water. In conclusion, microwave energy is hopeful to achieve high-efficient and environmental-friendly production of shale oil due to its impacts on pores and fractures of reservoirs and occurrence state of shale oil.