Using Capillary Condensation and Evaporation Isotherms to Investigate Confined Fluid Phase Behavior in Shales

Abstract

The abundance of nanopores (pores with diameters between 2 and 100 nm) in shale and ultra-tight reservoirs precludes the use of common pressure-volume-temperature (PVT) analyses on reservoir fluids. The small sizes of the pores cause capillary condensation, which is a nanoconfinement-induced gas-to-liquid phase change, that can occur at pressures more than 50% below the corresponding bulk phase change of the fluid due to strong fluid-pore wall interactions. We quantify this phenomenon by measuring propane isotherms both in a synthetic nanoporous medium and a core from a shale gas reservoir. Comparison of our results in the two porous media indicates the occurrence of capillary condensation in shale rock. At the same time, we observe capillary condensation hysteresis for shale, in which the density of the fluid is significantly lighter during desorption than adsorption. This indicates structural changes to the rock matrix caused by the phase behavior of the confined fluid. We use scanning electron microscopy to corroborate our findings. These results have significant implications for determining the PVT properties, porosity, and permeability of shale and ultra-tight formations for use in reservoir modeling and production estimations.

Effect of Pore Size Distribution on Capillary Condensation in Nanoporous Media

“…In our previous work, we hypothesized that the lack of a type IV isotherm for natural materials, such as shale, is not due to the lack of capillary condensation . Rather, the shapes of shale isotherms arise from the lack of a single pore size in the porous medium .…”

“…Capillary condensation is a confinement-induced gas-to-liquid phase change that typically occurs in nanopores, which are pores with diameters between 2 and 100 nm. − It arises from the relatively strong intermolecular forces that are characteristic of nanoconfinement. , Capillary condensation has been studied for decades in model nanoporous media characterized by uniformly sized and shaped cylindrical pores, such as MCM-41 and SBA-15, as a means of elucidating the underlying physics that control fluid phase behavior in natural nanoporous media, including ultratight hydrocarbon bearing reservoir rocks ,,, and biological membranes. − …”

“…Adsorbent complexity is a spectrum. The most complex adsorbents, such as shale rock, can contain interconnected pores characterized by a variety of sizes, shapes, chemistries (i.e., wettabilities), and material strengths. , Less complex adsorbents may vary from model adsorbents in more subtle ways. For example, inkbottle and funnel-shaped pores are uniform in shape, chemistry, and material strength but contain pore elements of varying size.…”

“…Thus, no experiments have been performed on capillary condensation in size distributions of identically shaped, unconnected nanopores. The absence of experimental studies highlights a major knowledge gap in the literature . Although experimental studies do not exist for distributions of nanopores, studies have been carried out on distributions of nanopores and micropores (i.e., pores with effective diameters smaller than 2 nm) and distributions of micro-, nano-, and macropores (i.e., pores with diameters larger than 100 nm) .…”

“…The absence of experimental evidence regarding the applicability of the Superposition Principle to multimodal nanoporous systems has led to the assumption that capillary condensation and continuous pore filling do not occur in complex natural media. For example, although experiments suggest that capillary condensation may occur in shale rock, many recent studies continue to overlook this evidence. − Instead, the antiquated idea that only adsorbed and bulk gas (also known as free gas) phases can exist in the nanopores of shale gas reservoirs persists. − The rationale for this assumption is based on the fact that the shapes of isotherms measured in shale rock can deviate significantly from the IUPAC type IV isotherm. ,, In the IUPAC classification scheme, the type IV isotherm can either be completely reversible or display different types of hysteresis . Thus, identification of the condensation step and/or hysteresis loop is critical to quantifying capillary condensation phenomena, including the phase change pressure and critical points, from type IV isotherms …”