Effect of Formation Pressure on Pore Structure Evolution and Hydrocarbon Expulsion in Organic-Rich Marine Shale

Abstract

Exploring the relationship between formation pressure and shale pore evolution is helpful for the enrichment and development of marine shale gas accumulation theory. The thermal evolution experiment was carried out on the Xiamaling Formation (Pr3x) lowly matured marine shale, which has a similar sedimentary environment to the Longmaxi Formation (S1l) highly matured marine shale. Comparative experiments of open and semi-closed pyrolysis and multiple pore structure characterization techniques, including CO2 and N2 physisorption, mercury intrusion porosimetry, and field emission scanning electron microscopy, were conducted. The marine shale pore evolutionary model under formation pressure is proposed by characterizing pore evolution, and hydrocarbon expulsion and retention for shales under and without formation fluid pressures. The results show that the existence of formation pressure increases the percentage of quartz and reduces the content of clay minerals. The change in formation pressure has no obvious effect on the maturity evolution of shale samples. With the increase of formation pressure, the pore morphology of shale gradually changes from narrow slit pores to ink bottle-shaped pores. The retained hydrocarbons in shale mainly occupy the mesopore space, and the existence of formation pressure promotes hydrocarbon expulsion, especially the hydrocarbon expulsion in the mesopore. In addition, formation pressure improves pore connectivity, especially in the high-over mature stage of shale. With the increase of formation pressure, the micropore volume decreases slightly, the mesopore volume increases significantly, and the macropore volume changes have two stages.

Experimental Studies on Pore Structure and the Gas Content Evolution Mechanisms of Shale Gas Reservoirs at Different Burial Depths in the Longmaxi Formation, Southern Sichuan Basin

“…Therefore, the shale reservoirs in the Zhaotong area, which are currently shallowly buried, have well-developed interP pores and MF (Figure 5a-d). As the burial depth increases, the inorganic pores are continuously compressed, while the OM pores are preserved due to fluid overpressure [65,69,70]. Therefore, we can observe that as the burial depth increases, the inorganic pore size gradually decreases.…”

“…As the burial depth increases, the pressure of the overlying strata increases, and the inorganic pores and MF are gradually compressed or even closed, especially the InterP pores. Due to the compression resistance of minerals and the overpressure of the shale gas, the OM pores in the OM between minerals are well-preserved [65]. This results in the shale reservoirs of the Luzhou area, which have burial depths greater than 4000 m, being dominated by OM pores, the proportion of which can be up to 60%.…”

“…Due to different tectonic processes, the degrees of strata uplift are different in the studied regions, resulting in differences in the current burial depth. Due to the high degree of thermal evolution of the shale reservoirs, at the maximum burial depth, the organic matter has experienced or is currently at a peak of gas generation, and the formation is in an overpressure state [65]. As the burial depth of the formation decreases, the reduction in overlying formation pressure will disrupt the formation balance under overpressure, forming a new pressure balance, thereby affecting the development and maintenance of the pores [5].…”

“…The σ bp is influenced by the pressure of the overlying strata, resulting in differences in the characteristics of organic pores in the shale at different burial depths. Since shale gas reservoirs in the southern Sichuan Basin have experienced the maximum ancient burial depth, these shale reservoirs are mostly overpressured, and the formation pressure coefficient increases with the burial depth increase [65]. Based on the mechanism of bubble pore formation, taking the characteristics of OM pores in shale with burial depths larger than 4000 m as a reference, the changes in OM pores during the process of decreasing burial depths were analyzed.…”

“…After experiencing the maximum burial depth, the greater the degree of uplift of shale reservoirs, the lower the pressure exerted by the overlying strata. After the overpressure state is removed, this will stimulate the formation of more MF or inorganic pores [65]. Therefore, the shale reservoirs in the Zhaotong area, which are currently shallowly buried, have well-developed interP pores and MF (Figure 5a-d).…”