Pore Network Modeling of Gas Transport in Source Rocks

Source rocks such as resource shale are a special subclass of petroleum reservoirs where the hydrocarbons are generated and stored in the same place. They consist of diverse minerology including clays, silt, quartz, carbonates as well as varying amount of organic material. This diversity creates a multi-scale pore network including pores down to a few nanometers, micro-cracks and fractures, which influences the fluid storage and transport in the formation. In this dissertation, initially I present an upscaling study to gas transport in nano-scale within the organic material of the source rock via pore network modeling approach. The nanoscale transport effects are linked to the reservoir scale honoring the multi-physics and multi-scale nature of the formation. The pore network model is built in accordance with three-dimensional nano-scale imaging of shale samples where, in most of the cases, the organic material is observed as a network of pores with some micro-cracks of larger size cutting through, or by the edge of, the material. The interaction between the networks of organic pores and micro-cracks could be important for natural gas production from source rock, because it can control the rates at which the fluid is transported from the organic constituents of the formation. The matrix-fracture interactions could also be influenced by the existing in-situ stresses. Understanding the transient flow behavior would eventually help us optimize production. At high pressure, gas is stored in the organic material as a compressed free gas and adsorbed gas. Its transport is driven by pressure gradient with some additional fluxes caused by the degree of confinement and the presence of an adsorbed layer which can be mobile under some conditions of high pressure gradient. A modified pressure dependent definition of a scaled up organic material permeability is obtained taking into account the previously mentioned factors and using the concept of percolation theory. This permeability can be used with the classical governing equations of flow and transport. The coupling term relating the fluid exchange between the organic material and the associated micro-cracks and fractures is derived and validated through the concept of dual porosity relating the total fluid exchange to the pressure difference at the fracture-matrix interface with some modifications to account for the captured transient effect and the pressure dependency of the gas properties. At the final part of the thesis, I present a reservoir grid-block scale application of the derived organic nanoporous matrix-fracture coupling by implementing the formulation into conventional diffusivity formulation. The results show the retardation effect on production due to presence of organic nanopores. In addition, the role the transport mechanisms in the organic material play on the production is analyzed