Direct Visualization of Nanoscale Salt Precipitation and Dissolution Dynamics during CO2 Injection

Abstract

CO2 injection to enhance shale oil recovery provides a win-win solution to meet the global fuel shortage and realize ultimate carbon neutrality. When shale reservoirs contain high salinity water, CO2 injection can result in salt precipitation to block the nanometer pores in the shale, causing undesirable formation damage. Understanding salt precipitation and dissolution dynamics at the nanoscale are fundamental to solving this practical challenge. In this work, we developed a shale micromodel to characterize salt precipitation and dissolution based on nanofluidic technology. By directly distinguishing different phases from 50 nm to 5 μm, we identified the salt precipitation sites and precipitation dynamics during the CO2 injection. For the salt precipitation in the nanometer network, we identified two precipitation stages. The ratio of the precipitation rates for the two stages is ~7.9 times that measured in microporous media, because of the slow water evaporation at the nanoscale. For the salt precipitation in the interconnected micrometer pores, we found that the CO2 displacement front serves as the salt particle accumulating site. The accumulated salt particles will in turn impede the CO2 flow. In addition, we also studied the salt dissolution process in the shale micromodel during water injection and found the classical dissolution theory overestimates the dissolution rate by approximately twofold. This work provides valuable pore-scale experimental insight into the salt precipitation and dissolution dynamics involved in shale formation, with the aim to promote the application of CO2 injection for shale oil recovery.

Experimental Investigation on Salt Precipitation Behavior during Carbon Geological Sequestration: Considering the Influence of Formation Boundary Solutions

“…In core-scale experiments, techniques such as nuclear magnetic resonance (NMR) and computerized tomography (CT) are commonly employed to characterize the distribution of solution and salt within pores. It has become a consensus that salt precipitation results in a decrease in permeability. ,, The growth process of salt in pores has been directly observed using microscopes and cameras, − revealing two primary forms: large crystal structures grown in solution and polycrystalline structures grown in the CO 2 phase. , He et al discovered that increasing the injection rate of CO 2 , increasing the contact angle, and enhancing the heterogeneity of the porous structure can effectively mitigate the local accumulation of salt at the inlet, thereby reducing the impact of salt precipitation on injectivity. ,, While extensive research has elucidated salt precipitation and the associated mechanisms mentioned above, it is important to note that most laboratory studies have not considered the boundary conditions related to solution replenishment. In actual underground reservoir conditions, the pores of the saline aquifers are initially saturated with a solution.…”

Research on Transformation of Connate Water to Movable Water in Water-Bearing Tight Gas Reservoirs

“…Tight gas reservoirs are currently a focal point in unconventional natural gas exploration and development, playing a pivotal role in natural gas production [1][2][3][4]. According to China's fourth oil and gas resource assessment, onshore tight gas resources in China total 21.85 × 10 12 m 3 , primarily concentrated in regions such as the Ordos Basin, Sichuan Basin, Songliao Basin, and Tarim Basin [5][6][7][8].…”