Clay mineralogy and unconventional hydrocarbon shale reservoirs in the USA. II. Implications of predominantly illitic clays on the physico-chemical properties of shales

Re-interpretation of R3 mixed-layer illite-smectite (I/S) as consisting only of thin (< 50 Å) illite leads to the question of whether this really matters in the context of the overall physico-chemical properties of the shales in which this clay material is found, such as in the unconventional hydrocarbon shale reservoirs of the USA. It is argued here that the distinction between regarding the clay mineralogy of these shales as partly smectitic or wholly illitic is important, particularly when one considers the interactions between charged clay surfaces and aqueous pore fluids. These interactions are broadly discussed within the framework of the double electric layer (DEL) theory which successfully accounts for a range of phenomena that are observed in colloidal sols of clay minerals dispersed in aqueous solutions of varied chemical composition. Particle interactions in these sols depend to a large extent on the thickness of the DEL. In general, this thickness is controlled by the balance between the external surface charge density of the clay particle and the electrolyte concentration of the aqueous solution, because the higher the surface charge density the thicker will be the DEL, and the more concentrated the aqueous solution the thinner will be the DEL. Although shales are not colloids, in situations where the pore size is similar to that of the thickness of the DEL the same principles that govern particle-fluid interactions in the colloidal state should still apply. It turns out that the pore size distribution in shales is probably mainly in the mesopore range (2-50 Å) and the lower part of this range certainly falls within the thickness of the DEL associated with highly charged particles. The external surface charge density of illite may be up to about 5 times greater than that of smectite and for this reason may be expected to be highly reactive, particularly when present as fine-grained high surface area particles, and to impact upon at least some of the physico-chemical properties of shale as a whole. The characteristics to be discussed include particle size, surface area, ion exchange properties, porosity, swelling and dispersion, and these, in turn, could affect various aspects of unconventional hydrocarbon reservoirs, particularly declines in permeability from initially promising values. Many other processes might be relevant such as creep behaviour of the lithology, which is heavily influenced by clay type and abundance, salt precipitation and induced downhole diagenesis. In addition, the cation exchange between the dominant clay mineral and the fluid used for hydraulic fracture is undoubtedly an issue that needs to be addressed.