Spending most of 2020 in quarantine taught many of us the effects that confined space can have on humans. It changes us because people like to be free, and being indoors or in confined spaces can affect behaviors, performance, and routines. Similar effects are noted in physical systems where fluids like water, gases, or oils behave differently when they are confined in narrow spaces. However, the size of these confinements (which could be closed or like hallways) is significantly smaller, on the order of nanometersǂ (1 nm = 10-9 m, one-billionth of a meter). As we found ourselves talking to the walls during the quarantine, these fluids can communicate or interact with the walls of the confinements. These interactions depend upon the nature of walls and fluids, and can significantly alter the behavior of the fluids, leading them to act anomalously. We can find these confinements everywhere around us, from the Earth’s subsurface to water filtration/desalination plants. In the case of water, it loves some walls and hates others, a phenomenon called hydrophilicity and hydrophobicity, respectively (Figure 1). This can lead to anomalous behavior of fluids, especially when a mixture of oil and water is present within such confinements. Understanding this anomaly in the behavior of fluids is of importance from both fundamental and applied points of view.
Among the biggest challenges of the future water–energy landscape is the availability of fresh water, and the competition between drinking water and water for resource recovery. We find the confined spaces naturally in Earth’s subsurface, where oil and gas are trapped. It is important to understand why there is this competition in the first place. Shale gas, which is responsible for 70% of the US natural gas supply, is extracted directly from subsurface nanopores using hydraulic fracturing approaches. During the extraction, hydraulic fracturing fluids, containing water and other additives, are injected at high pressures into the shale rocks to access the nanoscale pores. As a result, the hydrocarbons (oil and gas) in shale are pushed out, and often the water/fracturing fluid is confined and trapped in these pores, resulting in a huge loss of water each year, which is unsustainable.
Understanding why hydraulic fracturing fluids are retained by the rocks can provide us the answer to this water loss and help us develop technologies that can prevent it from happening. This journey begins with understanding the confinement effect that exists in the nanometer-sized shale confinements. We know that fluid molecules behave differently in confined spaces, such as the disruption in the hydrogen-bond network in the case of confined water (Figure 2), which results in a shift from normal behavior. Moreover, the interaction with the hydrophilic walls of confinement can also hinder the recovery of water injected during the hydraulic fracturing processes. With an increasing energy demand, the inevitable challenges to the future energy–water landscape will benefit from a better understanding of fluids in confinements. Scientists at several Energy Frontier Research Centers (EFRCs) study the effects of these nanometer-sized confinements on different properties of fluids such as thermodynamics (freezing and melting points), transport, reactivity, organization, and catalytic activity*.
At the Multi-scale Fluid–Solid Interactions in Architected and Natural Materials (MUSE) EFRC, researchers have studied the freezing behavior in Earth’s subsurface by studying water behavior in silica nanopores of different geometries and sizes [1,2]. They found that as the size of pores decreases (increasing the confinement effect), the melting point of water is shifted to a lower temperature (< 273 K) compared to the melting point of bulk water (273 K). They also reported that in these confined spaces, water freezes in a layer-by-layer manner, with ice growth proceeding from the center of the pore towards the pore wall.
In another study [3], the structure of ice confined in the nanometer-sized pores of silica (4 nm, 6 nm, & 8 nm) was investigated using a combined experimental and computer simulation approach, and it was found that the pore size has a direct effect on the type of ice formed. In smaller pores (4 nm), ice crystals arrange to form a cubic structure, while in relatively larger pores (6 nm & 8 nm), ice crystals organize as hexagonal ice. In subsurface environments, under high pressure, ice crystals can form and trap gases within these crystals. Therefore, understanding the type of ice structures in subsurface-related environments and being able to probe these structures is significant.
Going beyond water in these systems is also important since the hydrocarbons in shales are largely housed within nanometer-sized confinements, and there is limited understanding of their interactions in these rocks; investigating these oils and gases is also important. The study of methane (natural gas) confined in different-sized pores of silica [4] has shown that the gas molecules organize in these confinements as core-shell structures, with the shell thickness increasing as the pressure of methane in the pores increases. These results indicate that in tight reservoirs the gases are preferentially adsorbed on the pore walls and may require a significant amount of hydraulic fluid and pressure to release such gases. Schematics summarizing the above-explained phenomenon are presented in Figure 3.
These findings provide new insights into the effects of confinements on the behavior of fluids in subsurface reservoirs and can help to develop revised protocols for processes such as hydraulic fracturing to improve the energy–water landscape to meet future energy needs. These results are also significant to understanding the fundamental interactions of water in different biological and natural systems.
ǂ If you take a marble and consider its diameter to be 1 nanometer, then the diameter of the Earth would be about 1 meter.
* See the article "A Nanospace for Enhanced Catalysis" for further details on the effect of confinement on catalytic activity.
