Shale pore alteration: Potential implications for hydrocarbon extraction and CO2 storage

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

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.

Effects of salinity and shear stress on clay deformation: A molecular dynamics study

The deformation of clay minerals is an important phenomenon that is relevant to many problems, particularly those that occur in subsurface geological formations. The salinity of the formations and external shear stress applied to them are two important factors that contribute to the deformation of such porous media. To gain a deeper understanding of such phenomena, we have carried out extensive molecular dynamics simulations using the Na-montmorillonite (Na-MMT) structure as the model of clay minerals and have studied the effect of salt concentration on its swelling. As the NaCl concentration increases, so also does the basal spacing. We demonstrate the effect of the coupling between the applied shear stress and NaCl salinity on the swelling behavior of Na-MMT, namely, deformation of the interlayer space that results in swelling. According to the results, the extent of Na-MMT deformation depends on both the brine salinity and the shear rate.

Geochemical controls on CO2 interactions with deep subsurface shales: implications for geologic carbon sequestration

One of the primary drivers of global warming is the exponential increase in CO₂ emissions. According to IPCC, if the CO₂ emissions continue to increase at the current rate, global warming is likely to increase by 1.5 °C, above pre-industrial levels, between the years 2030 and 2052. Efficient and sustainable geologic CO₂ sequestration (GCS) offers one plausible solution for reducing CO₂ levels. The impermeable shale formations have traditionally served as good seals for reservoirs in which CO₂ has been injected for GCS. The rapid development of subsurface organic-rich shales for hydrocarbon recovery has opened up the possibility of utilizing these hydraulically fractured shale reservoirs as potential target reservoirs for GCS. However, to evaluate the GCS potential of different types of shales, we need to better understand the geochemical reactions at CO₂-fluid-shale interfaces and how they affect the flow and CO₂ storage permanence. In this review, we discuss the current state of knowledge on the interactions of CO₂ with shale fluids, minerals, and organic matter, and the impact of parameters such as pressure, temperature, and moisture content on these interactions. We also discuss the potential of using CO₂ as an alternate fracturing fluid, its role in enhanced shale gas recovery, and different geochemical tracers to identify whether CO₂ or brine migration occurred along a particular fluid transport pathway. Additionally, this review highlights the need for future studies to focus on determining (1) the contribution of CO₂ solubility and the impact of formation water chemistry on GCS, (2) the rates of dissolution/precipitation and sorption reactions, (3) the role of mineralogical and structural heterogeneities in shale, (4) differences in reaction mechanisms/rates between gaseous CO₂vs. brine mixed CO₂vs. supercritical CO₂, (5) the use of CO₂ as a fracturing fluid and its proppant carrying capacity and (6) the role of CO₂ in enhanced hydrocarbon recovery.

Micro- and Macroscale Consequences of Interactions between CO2 and Shale Rocks

In carbon storage activities, and in shale oil and gas extraction (SOGE) with carbon dioxide (CO2) as stimulation fluid, CO2 comes into contact with shale rock and its pore fluid. As a reactive fluid, the injected CO2 displays a large potential to modify the shale’s chemical, physical, and mechanical properties, which need to be well studied and documented. The state of the art on shale–CO2 interactions published in several review articles does not exhaust all aspects of these interactions, such as changes in the mechanical, petrophysical, or petrochemical properties of shales. This review paper presents a characterization of shale rocks and reviews their possible interaction mechanisms with different phases of CO2. The effects of these interactions on petrophysical, chemical and mechanical properties are highlighted. In addition, a novel experimental approach is presented, developed and used by our team to investigate mechanical properties by exposing shale to different saturation fluids under controlled temperatures and pressures, without modifying the test exposure conditions prior to mechanical and acoustic measurements. This paper also underlines the major knowledge gaps that need to be filled in order to improve the safety and efficiency of SOGE and CO2 storage.