Pore Structure Analysis Using Subcritical Gas Adsorption Method

Abstract

Abstract The oil and gas industry has been tapping into tight reservoirs with low permeability and porosity. This calls for methods to enhance our understanding of tight reservoirs, such as shale. A large percentage of gas in place in shale reservoirs is in the form of adsorbed gas on the surfaces of pores. The pores of shale reservoirs range from micro-, meso- to macropores. Small pores contribute to large surface area accessible for adsorption, emphasizing the importance of studying pore structure in nanoscale. Subcritical gas adsorption isauseful methodto characterize pore structures in nanoscale in terms of quantifying pore volume, determining the presence of micropores, predicting pore geometries and the accessible surface area for adsorption. This paper discusses results obtained from running adsorption on samples of four different formations: Niobrara shale, Lokpanta oil shale, Hawaiian basaltic rock and Berea sandstone. Some of the results used to both qualitatively and quantitatively study pore structures are isotherm, pore size distribution and specific surface area. These results both affirm and debunk generalizations tied to certain formations. For instance, sandstone typically has large grains and this is confirmed by the zero micropores shown in both its isotherm and pore size distribution. On the contrary, some of the basaltic rocks are expected to have highspecific surface area due to its high clay content, yet the samples showed low specific surface area instead. This informs the lack of information available on the sample, thus, initiating more tests to understand other factors that may be causing low specific surface area. Moreover, Lokpanta oilshale does not have any external information provided, but the isotherms and pore size distribution help to intuitively grasp the image of the samples' pore structures. This paper illustrates how best to analyze and compare the experimental results. Subcritical gas adsorption makes it easy to picture pore structures at nanoscale. Understanding pore structures, specific surface area available for adsorption and micropore volume for storage purposes is crucial for applications, such as carbon capture and storage (CCS).