Impact of clay mineralogy on geomechanics of shale caprocks

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Conference Proceeding

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Shale cap rocks are nature's best hydraulic barrier geomaterials. They are effective seals for underground hydrocarbon bearing formations as well as CO storage formations in carbon capture and storage (CCS) projects. The sealing properties of shale rocks are directly related to its minerals and the internal arrangement of clay and non-clay minerals. This is known as the microstructure. Since shales are predominantly composed of clay minerals, the type and amount of clay minerals contained within the rock are the key factors of its sealing properties. The goal of this study is to gain a better understanding of how different types of clays behave in a typical CO storage reservoir condition. Clay minerals have layered structures which often carry negative surface charges. The combination of large reactive surface areas and charge bring complexity in terms of their reactivity to fluids. Therefore, even the same type of clays can have different properties depending on their depositional environment, which was influenced by different fluid properties (pH, T, P, salinity). The same is also true of the exposure of clay-rich rocks to reactive fluids during geologic times as well as under subsurface engineering conditions (nuclear waste storage, injection of waste water and fracking fluids in oil&gas, and carbon sequestration). For this study, artificial shale rock samples were designed using purified natural minerals in different ratios. These samples allowed us to observe the impact of the mineralogical composition on mechanical properties and obtain a systematically quantified comparison between samples. Indentation tests were conducted to evaluate changes in mechanical properties as a result of composition alteration, while Scanning Electron Microscopy (SEM) was used to probe any changes to microstructures. Observations in this study indicate: a) high clay content shale formation has better sealing properties because its mechanically more stable and has lower permeability due to the low porosity; b) increasing the salinity of pore fluid can decrease the thickness of double layers of clays, causing an increase in permeability because of the increase of effective porosity. Salinity has an additional weaker effect on the mechanical properties because as the sample dries, salt crystallized within the sample and creates an internal expansion force resulting in integrity failure. 2 2

Publication Source (Journal or Book title)

51st US Rock Mechanics / Geomechanics Symposium 2017

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