Microstructural and petrophysical characterization of Muderong Shale: application to top seal risking

Analysis of the Muderong Shale from the Carnarvon Basin suggests the shale is dominated by interstratified illite–smectite with a high percentage of illite interlayers. Capillary pressure measurements indicate that gas columns of c. 250 m could be sealed by such shale, although the choice of drying method used does influence the accuracy of this calculation. Freeze drying yielded the most consistent threshold pressure results, whereas air drying and vacuum drying showed a greater range of values. Similar calculations in regard to carbon dioxide sequestration indicate column heights of between 550 m and 750 m could be retained. Column height variation is primarily dependent on the contact angle of supercritical carbon dioxide with shale. Microstructurally, the shale is clay supported, exhibiting differential compaction of clays around more rigid grains and containing numerous high aspect ratio discontinuous fractures. These fractures do not affect the capillary properties of the shale, even when injection is fracture-parallel, suggesting they are unlikely to influence reservoir-scale fluid-flow properties. Comparison of the Muderong Shale laboratory data with hydrocarbon column heights from Carnarvon Basin discoveries indicate that top seal failure by capillary breakthrough is unlikely given the maximum lengths of hydrocarbon columns encountered to date. Potential for top seal failure is more likely to be influenced by formation integrity, pore pressure and in situ stress conditions.