The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs

The effect of shale composition and fabric upon pore structure and CH4 sorption is investigated for potential shale gas reservoirs in the Western Canadian Sedimentary Basin (WCSB). Devonian–Mississippian (D–M) and Jurassic shales have complex, heterogeneous pore volume distributions as identified by low pressure CO2 and N2 sorption, and high pressure Hg porosimetry. Thermally mature D–M shales (1.6–2.5% VRo) have Dubinin–Radushkevich (D–R) CO2 micropore volumes ranging between 0.3 and 1.2 cc/100 g and N2 BET surface areas of 5–31 m2/g. Jurassic shales, which are invariably of lower thermal maturity ranging from 0.9 to 1.3% VRo, than D–M shales have smaller D–R CO2 micropore volumes and N2 BET surface areas, typically in the range of 0.23–0.63 cc/100 g (CO2) and 1–9 m2/g (N2).High pressure CH4 isotherms on dried and moisture equilibrated shales show a general increase of gas sorption with total organic carbon (TOC) content. Methane sorption in D–M shales increases with increasing TOC and micropore volume, indicating that microporosity associated with the organic fraction is a primary control upon CH4 sorption. Sorption capacities for Jurassic shales, however, can be in part unrelated to micropore volume. The large sorbed gas capacities of organic-rich Jurassic shales, independent of surface area, imply a portion of CH4 is stored by solution in matrix bituminite. Solute CH4 is not an important contributor to gas storage in D–M shales. Structural transformation of D–M organic matter has occurred during thermal diagenesis creating and/or opening up microporosity onto which gas can sorb. As such, D–M shales sorb more CH4 per weight percent (wt%) TOC than Jurassic shales.Inorganic material influences modal pore size, total porosity and sorption characteristics of shales. Clay minerals are capable of sorbing gas to their internal structure, the amount of which is dependent on clay-type. Illite and montmorillonite have CO2 micropore volumes of 0.78 and 0.79 cc/100 g, N2 BET surface areas of 25 and 30 m2/g, and sorb 2.9 and 2.1 cc/g of CH4, respectively (dry basis) – a reflection of microporosity between irregular surfaces of clay platelets, and possibly related to the size of the clay crystals themselves. Mercury porosimetry analyses show that total porosities are larger in clay-rich shales compared to silica-rich shales due to open porosity associated with the aluminosilicate fraction. Clay-rich sediments (low Si/Al ratios) have unimodal pore size distributions <10 nm and average total porosities of 5.6%. Siliceous/quartz-rich shales (high Si/Al) exhibit no micro- or mesopores using Hg analyses and total porosities average 1%, analogous to chert.