Surface Area and Pore-Size Distribution in Clays and Shales

Abstract

Abstract One of the biggest challenges in estimating transport and storage properties of shales is a lack of understanding of their complete pore size distributions. Shale matrix has predominantly micro (pores less than 2 nm diameter) to meso-pores (pores with 2-50 nm diameter). These small pores in shale matrix are mainly associated with clay minerals and organic matter. Thus, comprehending the controls of clays and organic matter on the pore-size distribution is critical to understanding the shale pore network. Historically mercury intrusion techniques are used for pore-size analysis of conventional reservoirs. However, for unconventional shale reservoirs, very high pressures (> 60,000 psi) would be required for mercury to access the full pore-structure which has potential pitfalls. Nitrogen gas adsorption techniques can be used to characterize materials dominated by micro and mesopores (2-50 nm). A limitation of this techniques is that it fails to measure large pores (diameter >300 nm). In this study, we have used a combination of nitrogen gas adsorption and mercury intrusion to study the spectrum of pore-sizes in shales and clays. Our results on pure clay minerals show that (i) clays have a mulit-scale pore-structure at different dimensions, (ii) Micropores are associated with montmorillonite clays but not with kaolinite, (iii) compaction results in decrease in pore volume and reduction of pore-size in the macropores of the clays while the micropores are shielded from the effect of compaction, and (iv) in natural shales, mineralogy controls the pore-size distributions for shales and presence of micropores in natural shales can be correlated with the dominance of illite-smectite type of clays in the rock.