Porosity and Pore-Size Distribution of Geomaterials from X-ray CT Scans

Abstract

Determination of transport properties of geomaterials is an important issue in many fields of engineering analysis and design. For example, in petroleum engineering, permeability of an oil reservoir may be crucial in establishing its viability for exploitation whilst prevention of leakage from underground storage facilities for oils and gas, nuclear waste as well as CO2 crucially depends on its long term values. Permeability is an illusive parameter which is difficult to obtain not only in field situations but also in controlled laboratory environment. Its determination is further complicated by the fact that values are needed for low permeability porous media such as clays/rocks at various degrees of saturation and at elevated temperatures which makes physical experiments not only expensive but also difficult and time-consuming. Permeability is indirectly related to the porosity, pore-size distribution and pore-architecture. A well-known way to obtain this information is through mercury porosimetery but this procedure has safety issues associated with it. Moreover, it is not an easy experiment to conduct. In this paper, we demonstrate the use of micro X-ray CT scanning technique to obtain porosity and its variation in clays. Since the resolution of micro CT equipment is not high enough to be able to observe specific pores in clays, an experimental programme to correlate porosity with data from scanning was undertaken. It consisted of consolidating specimens made from a mixture of kaolinite and bentonite in an oedometer, unloading them and obtaining 32mm samples from various locations, scanning them as well as determining void ratio of these specimens using standard laboratory procedures. It is observed that the Average CT Number (ACTN) for the specimens correlated well with the porosity (void ratio) whilst spatial variation of CT numbers seems to indicate the capability of scanners to capture pore size distribution. This indicates the possibility of computing permeability of low porosity media through CT imaging.