Influence of CO<sub>2</sub> injection on the pore size distribution and petrophysical properties of tight sandstone cores using nuclear magnetic resonance

The recoverable hydrocarbon reserves of conventional oil and gas resources are very limited in China. As important alternative resources, unconventional oil and gas have become a research hotspot. Though tight reservoirs have great potential to alleviate the increasing demand, issues during the development process, such as the rapid pressure depletion, fast decline in production, low productivity, and difficulties in water injection, are usually encountered due to poor physical properties like small pore throats and strong heterogeneity of the pore structure. The CO2 flooding technique could effectively replace crude oil from micro-nanopores, which is considered as a promising way to enhance the development performance of tight oil. However, precipitation and dissolution phenomena usually occur along with the CO2 injection process into reservoirs, affecting the pore structure evolution and oil displacement efficiency. In addition, artificial and natural fractures will even make this process more complicated. This paper presents the commonly used experimental approaches for CO2 injection into tight reservoirs and summarizes the main methods for investigating the influence of CO2 injection on the pore structure of reservoir rocks. Based on this, we highlighted that more attention should be paid to the influence of fractures and their dynamic changes on the evolution of pore structure during CO2 injection and the study of the solid–liquid interactions. To establish a method that could quantitatively evaluate the full-scale evolution of pore throats after CO2 injection is necessary. Meanwhile, the interaction strength of precipitation and dissolution and their effects on pore structure also remain open. Finally, a rigorous framework that could reveal the evolution mechanism and characterize the multiscale pore structure involving multiple influencing factors is urgently warranted.

Section: Tight Reservoirsmentioning

confidence: 99%

A Minireview of the Influence of CO₂ Injection on the Pore Structure of Reservoir Rocks: Advances and Outlook

Gao

Xie²,

Cheng

et al. 2022

“…Besides, the interfacial tension between CO 2 and crude oil disappears after CO 2 reaches the miscible pressure, and in this scenario, CO 2 can enter the small pores that cannot be readily swept below the MMP, thereby improving the oil production in small pores . On the other hand, CO 2 and crude oil will undergo a multiple-contact miscible process, and the oil recovery efficiency also depends on the pore structure of reservoir rocks. − …”

Section: Introductionmentioning

confidence: 99%

Experimental Investigation of the CO₂ Flooding Characteristics of Tight Sandstone Reservoirs Using a High-Temperature and -Pressure Visualization Apparatus

Gao

Luo

Xie³

et al. 2022

The microscopic pore structure of tight sandstone reservoirs significantly impacts CO2 flooding characteristics. In this work, two types of realistic sandstone visualization models were selected based on petrophysical properties and the pore structure feature. CO2 flooding experiments under different injection pressures and volumes were carried out using the in-house high-temperature and -pressure visualization flooding system. Then, the characteristics of oil movement and residual oil distribution were quantitatively described and analyzed for two rock types. The results show that the type I model has better physical properties and a more favorable pore structure, thus a higher oil recovery than the type II model. The immiscible CO2 flooding efficiency of the type I model is up to 64.5%. On the other hand, the oil recovery of the type II model increases when the miscible pressure is reached, and the maximum oil recovery is 49.5%. In the high-pressure miscible flooding stage, two types of models have similar oil recovery increments, which are 10.7 and 10.6%, respectively. Additionally, the residual oil distribution varies with the pore structure. The type I model has a small residual oil region and thus a high oil recovery efficiency. In contrast, the residual oil saturation of the type II model is larger, and the final oil recovery decreases. Furthermore, as the injection pressure and volume increase, the residual oil saturation becomes smaller, and oil recovery of both models increases. The occurrence characteristics of residual oil are oil droplet, cluster-shaped residual oil, flake oil, and dead corner oil, and the main influencing factors are capillary force, injection pressure, and pore connectivity.

“…40 In addition, when CO 2 injection leads to rupture and slip of the preexisting fault, a new fracture nucleates and progressively propagates, 44 which increase the porosity of the rock. For example, Zhao 45 found that, when the confining pressure is not applied, the porosity of homogeneous sandstones after CO 2 exposure increases by 1.5−2.0%. However, heterogeneity, such as the bedding plane 20 and natural fracture, 17 is common in underground reservoirs.…”

Section: Introductionmentioning

confidence: 99%

Microscale Damage Induced by CO₂ Storage on the Microstructure of Sandstone Coupling Hydro–Mechanical–Chemical Processes

Nie

Zhang

et al. 2022

A good understanding of CO2–brine–rock interactions that cause microscale damage to the microstructure of sandstone, affecting pore fluid transport and reservoir stability, is important for studying storage efficiency during CO2 storage. In this study, the equipment is first designed to saturate rock samples using CO2–brine under in situ conditions and then the homogeneous and heterogeneous sandstone samples are saturated for 30 days with and without the confining pressure (20 MPa). Through scanning electron microscopy (SEM) observation and analysis, it is found that the confining pressure reduces the degree of damage in various scales from micrometer to millimeter, such as reducing the mineral dissolution, decreasing the induced swelling of the bedding planes, and causing some pores to collapse. The nuclear magnetic resonance results show that the change in porosity is mainly caused by the increase in the number of relatively small-sized (characterized in terms of fluid relaxation time T 2 < 10–3 s) pores after CO2–brine saturation. In addition, the alterations of the pore system in homogeneous and heterogeneous sandstones are different, such as a larger increase in the porosity of homogeneous sandstone (by 5.29%). In heterogeneous sandstone, the effect of mineral dissolution induced by chemical reactions on damaging the microstructure is more obvious than mineral swelling. In addition, it is found that the CO2–brine saturation decreases the fractal dimension of the sandstone by nearly 2%, increasing the pore connectivity. This study is helpful for understanding the evolution of the sandstone microstructure under geological conditions during the long-term CO2 storage process.