Effect of thermal maturity on nanomechanical properties and porosity in organic rich shales (a Bakken shale case study)

Abstract

Unconventional resources (Shale Plays) have been the game changer of energy supply worldwide. The ever improving technologies and expanding of knowledge related to unconventional hydrocarbon accumulations has generated an enormous opportunity for countries with these resources towards energy self-sufficiency. For successful development of these resources, we need remote techniques that allow for spatially extensive in-situ reservoir characterization and sweet spots detection. In self-sourcing mudrocks, maturity (extent of oil generation), porosity and abundance of natural fractures are the main parameters to make them economical. Understanding the elastic properties of self-sourcing formations and their correlation with maturity can provide crucial information about amount of generated hydrocarbon, petrophysical properties and geomechanical characteristics from well logs and seismic surveys. As kerogen matures and decomposes to lighter hydrocarbons, its physical and chemical properties change. Kerogen decomposition and consequent shrinkage also change the load bearing state of the minerals and organic matter and affect pore system since early stages of maturation. Generating representative rock models to describe the behavior of organic-rich rocks is challenging due to complex textural properties of these rocks and the uncertainties in pore morphology and mechanical properties of some of the major components. Despite previous studies on the texture of organic-rich rocks in relation to their elastic properties, anisotropy and mechanical characteristics of the grains there is limited available data on the characteristics of some essential components such as clays and organic matter. Moreover, changes in properties of kerogen and clay minerals during the course of thermal maturation are poorly understood. Consequently, any rock models must rely on anecdotal or extrapolated data about various types of kerogen. The focus of this thesis was to determine underlying nano-scale changes in textural characteristics, mechanical properties and porosity evolution in organic-rich shales as these are main controlling factors of bulk properties. Textural and nano-mechanical properties of Bakken Shae samples from a wide range of thermal maturity were studied at natural state and after hydrous pyrolysis using FESEM imaging, nanoindentation and nano-DMA techniques. Laminated texture of shales at low maturity levels (caused by abundance of organo-clay mixtures) changed to increased stiff grains contact and isolated kerogen particles. We found lowering of Young's modulus of kerogen particles and organo-clay components with natural and synthetic maturation. Significant amount of bitumen is generated and retained in the source rocks due to thermal maturation or synthetic hydrous pyrolysis which fills the kerogen and clay porosity. Lack or presence of bitumen in the pore system may introduce significant discrepancy in our evaluation of porosity and pore characteristics. In this research, successive solvent extraction was performed to determine what portion of the pore space is excluded from quantification due to oil trapped in pores and to understand the storage and flow capacity of source rocks in the oil window. The PSD and SSA measured after each extraction shows revival of the pore system with successive cleaning. Most significant was the recovery of kerogen-hosted pores with removal of soluble, oil-like organic material. Using successive extractions we were able to determine the evolution of organic matter porosity through maturation which was otherwise not feasible using visual techniques or other conventional laboratory procedures.