Water adsorption isotherm and its effect on pore size distribution of clay minerals

Key words:  clay mineral  water adsorption isotherm  water saturation  pore size distribution  specific surface area

Abstract:

Water is ubiquitous within shale gas reservoirs, and its distribution can have a remarkable influence on pore size distribution, gas adsorption capacity and production mechanisms. In this study, montmorillonite, kaolinite and illite, as common clay minerals in shale reservoirs, were selected to carry out water vapor adsorption experiments. Meanwhile, N₂ adsorption/desorption experiments were also performed on the clay samples that were equilibrated in different relative humidity (RH) environment before the adsorption experiments. The characteristics of water adsorption were investigated, and its effect on pore size distribution of clay minerals was studied. The results indicate that the Guggenheim-Anderson-de-Boer (GAB) model can provide a good fit for the water adsorption data, and there is an adsorption mechanism transformation depending on the humidity, which can be called as "monolayer-multilayer-condensation". It also reveals that the distribution characteristics of water are different within different size of pores, which can cause a "disappearance" of fine pores in measured pore size distribution and decrease the specific surface area. In the case of montmorillonite, when water saturation approaches to 50%, water can exist in larger pores in the form of water films, and capillary water can appear in small pores, which can make the small pores (the diameter is less than 5 nm) "disappear" in the pore size distribution and reduce over 80% of the surface area contributed by the pores within 10nm. Therefore, the experiments conducted at dry conditions may overestimate the contribution of fine pores in clay minerals on shale gas content and flow capability.