Characterizing microstructural evolutions in low-mature lacustrine shale: A comparative experimental study of conventional heat, microwave, and water-saturated microwave stimulations

Most of the shale oil reservoirs in China are of medium to low maturity. The effective exploitation of hydrocarbon resources in such formations is technically challenging. This study explores the impact of various thermal stimulation methods on low-maturity shale oil reservoirs, focusing on conventional heating, microwave heating, and water-saturated microwave heating methods. A suite of techniques, including small angle neutron scattering (SANS), mercury injection capillary pressure (MICP), and field emission-scanning electron microscopy (FE-SEM) after wood's metal injection were used to investigate the evolution of microstructures under different thermal stimulation. This integrated approach allows for a comprehensive characterization of reservoir connectivity and permeability across multiple scales, from visually observable centimetre and millimetre scales down to the nanoscale. The response of shale to thermal stimulation is significantly influenced by its mineral composition, with shales rich in absorbent minerals like pyrite undergoing selective heating when exposed to microwaves, leading to new pores and fracture formations. The use of CM-SANS has revealed substantial untapped hydrocarbon potential within the reservoir, emphasizing the importance of thermal stimulation in enhancing production. Furthermore, integrating microwave irradiation with hydraulic fracturing effectively increases the number of pores, micro-fractures, and natural fracture openings, mitigates the hydration effects of hydraulic fracturing, and increases permeability by three orders of magnitude. The results indicated that microwave combined with hydraulic fracturing positively impacts the production from low-maturity reservoirs and provides the theoretical basis for in-situ thermal recovery of shale oil.