Pore Structure Characterizations of Shale Oil Reservoirs with Heat Treatment: A Case Study from Dongying Sag, Bohai Bay Basin, China

Pengfei Zhang; Shuangfang Lu; Junqian Li; Junjie Wang; Junjian Zhang; Yajie Yin

doi:10.1021/acsomega.3c03260

Pore Structure Characterizations of Shale Oil Reservoirs with Heat Treatment: A Case Study from Dongying Sag, Bohai Bay Basin, China

ACS Omega. 2023 Jul 16;8(29):26508-26525. doi: 10.1021/acsomega.3c03260. eCollection 2023 Jul 25.

Authors

Pengfei Zhang¹, Shuangfang Lu², Junqian Li³, Junjie Wang², Junjian Zhang¹, Yajie Yin¹

Affiliations

¹ College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, Shandong, China.
² Sanya Offshore Oil & Gas Research Institute, Northeast Petroleum University, Sanya 572025, Hainan, China.
³ Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, Shandong, China.

Abstract

Heat treatment plays a significant role in determining the petrophysical properties of shale reservoirs; however, the existing studies on the evolution of pore structures are still insufficient. This study conducts a series of tests, including Rock-Eval, low-temperature nitrogen adsorption-desorption, nuclear magnetic resonance (NMR) T₂, and T₁-T₂ tests on samples from Shahejie Formation, Dongying Sag, Bohai Bay Basin. The tests aim to determine the changes in the shale pore structures under increasing heat treatments (ranging from 110 to 500 °C) and identify the factors that control pore structures. The results show that the gradual decomposition of organic matter leads to an eventual decrease in the total organic carbon (TOC) content. The decrease in TOC is more prominent when the temperature exceeds 300 °C. For shales with lower TOC contents (<2%), the Brunauer-Emmett-Teller specific surface area (BET SSA) first decreases, then increases, but eventually decreases again. However, the average pore diameter demonstrates an opposite trend when the temperature increases. In contrast, for organic-rich shales (TOC > 2%), the BET SSA increases at temperatures above 200 °C. The similarity between the D₁ values implies that the complexity and heterogeneity of shale pore surface only undergo minor changes during heat treatment. Porosity shows an increasing trend, and the higher the contents of clay minerals and organic matter in shales are, the greater the change in porosity is. The NMR T₂ spectra suggest that micropores (<0.1 μm) in shales first decrease and then increase, whereas the contents of meso- (0.1-1 μm) and macropores (>1 μm) increase, corresponding to the increase in free shale oil. Moreover, shale pore structures are primarily controlled by clay minerals and organic matter contents during heat treatments, with higher contents resulting in better pore structures. Overall, this study contributes to detailing the shale pore structure characteristics during the in situ conversion process (ICP).