Comparison of Micro/nano-indentation Results for Pottsville and Marcellus Shale

摘要

Shale rocks play an essential role in petroleum exploration and production. They can occur either as caprocks for subsurface storage in conventional reservoirs or as unconventional reservoir rocks for hydrocarbon extraction via hydraulic fracturing. The utilization of a shale rock depending on its ability to immobilize fluids: caprocks requires a low permeability and resilience to the in-situ formation of fractures; on the contrary, unconventional reservoir rocks need a significant increase of permeability by engineering hydraulic fracturing. The mechanical properties of the rock are the key factor that determines the likelihood of fracture initiating and propagating. This paper used two types of shale rock as representatives of a shale cap rock (Pottsville shale) and a source rock (Marcellus shale), to relate the mechanical properties and differences in their mineralogical composition and microstructures. Indentation tests were conducted at both micro and nanometer level on drilled rock core samples to get the mechanical properties of bulk and individual phases of these multiphase materials. Results from micro-indentation showed Pottsville shale sample had overall higher bulk mechanical properties. The difference in mechanical properties is the result of the alteration in microstructures and mineralogical composition. The mechanical properties map created from nano indentation results showed the distribution of each single phase based on differential mechanical properties. This indicated higher hard grain content in Pottsville over Marcellus shale. This paper showed the utilization of nano-indentation to provide a direct link between geochemistry and geomechanics of shale rocks. Through mechanical properties mapping, individual phase properties can be correlated with the bulk response of the rock and the volumetric proportions of each phase can be estimated. The maps could be also useful for modeling the rock behavior to predict the fracture occurrence potential, as it linked the microstructural features with their mechanical properties.