Pore development and evolution mechanism during hydrocarbon generation: insights from hydrous pyrolysis simulation experiment

The pore structure of coal reservoir is the primary space of natural gas adsorption and storage, and the structural characteristics of coal in thermal evolution are crucial for the enrichment and exploration of coalbed methane. However, the current research mainly concentrates on the pore structure change process in actual geology, and there is a lack of continuous analysis of the pore structural evolution characteristics of coal samples with different maturity through hydrous pyrolysis simulation experiments. Coal samples ( R o = 0.6%) were collected from the Hedong coalfield, and hydrous simulation experiments (a total of 5 coal samples, temperatures from 250°C to 450°C, 50°C interval, 24 h duration) were carried out using a large amounts of water and low-rank bituminous coal samples (1.5 mL water : 1 g coal) in a pyrolysis closed system to investigate the evolutionary of the pore structure parameters during the hydrocarbon-generating process. The results showed that the pore volume and specific surface area of ultra-micropore consistently dominated the pore structure. For micropores and transition pores, the aromatization and side-chain breakage result in a decrease in specific surface area and pore volume. The dissolution of organic acids and the release of volatile components lead to an increase in the specific surface area and pore volume of the transitional pores. Furthermore, the intensification of coalification, the rearrangement of pore structures, causes the secondary pores to be compressed, and the transitional pores further decrease. HQ-O exhibits a rough pore surface and an uneven pore structure, while as the temperature increases, the pore surface of HQ-250-HQ-450 becomes relatively rougher and the pore structure grows more uniform. With the increase of total gas yield, the coal structure is dense, resulting in the pores reduction or closure, and pore volume and the specific surface area decrease. Additionally, water promoted the minerals dissolution and the migration of Si and Al within the coal samples, which swell and deform the coal skeleton and merge to form new pores. The geological evolution of coal under the influence of temperature, gas generation, and water-coal interaction is divided into three stages, and the pore structural evolution model is established. By studying the pore structure evolution of coal samples at different coalification stages through hydrous pyrolysis simulation provides important scientific basis and guidance for the exploration and development of CBM resources in actual geological characteristics of coal reservoirs.