The impact of organic solvent oil extraction on shale pore system-Key Issues in Shale Pore Evaluation

The premise of porosity measurement with convention-al methods is to expose all the pores in the shale, which requires oil extraction to remove the retained oil in the shale. Shale oil reservoirs are characterized by oil-bearing, tight (low porosity and permeability), rich in organic matter, and clay swelling. The interactions between the solvents and shale minerals, organic matter, and pores have significant impacts on the pore system. Some researchers tried to evaluate porosity after removing all organic matter from the shale with solvents. Kuila et al.^[1] used sodium hypochlorite to extract organic matter from five shales. The results showed that the pore volume increased within certain pore size ranges and decreased within others, which could be attributed to the decrease of organic pores after extraction. Similarly, DiStefano et al.^[2] used solvent extraction to remove the organic matter from Eagle Ford shale samples. The results pointed out that the porosity did not always increase with the amount of extraction. This study used different organic solvents to conduct the solvent extraction on clay-rich and carbonate-rich shales. Nuclear magnetic resonance, nitrogen adsorption, and gas measurement were applied to reveal the changes of the solvent extraction efficiency, porosity, and the pore size distributions to investigate the impact of solvent extraction on shale pore system.

The samples used in this study were taken from the fourth member of the Shahejie Formation in the Jiyang Depression of the Bohai Bay Basin and the first member of the Qingshankou Formation in the Gulong Depression of the Songliao Basin.  Sample A is a carbonate-rich, low-maturity shale and Sample B is a clay-rich, high-maturity shale.

During the organic solvent extraction, the changes in shale pore systems may result from the combination of the following three mechanisms: (1) The removal of internal fluids (oil and water) and soluble organic matter in shale is the primary reason for the increase of shale pores. (2) During the organic solvent extraction, interactions between shale components and the organic solvents, including clay swelling, extraction of bitumen, and the dissolution of minerals, change in the original pore structure. (3) The kerogen would expand when contact with organic solvents, which will fill partial pores. The expansion coefficient depends on the maturity of the kerogen and the solubility of the organic solvent.