Investigation of the controls of fluid flow through shale and their relation to its mechanical properties

Understanding mechanisms of fluid flow through shale is very important as these sedimentary rock act as caprock, key source of unconventional hydrocarbon, seal for geological CO2 storage and radioactive waste disposal sites. Four mechanisms of fluid flow were identified; matrix flow by single or multiphase flow, flow through faults and fractures and flow through preferential pathways induced by high pressure fluid. Knowledge gaps associated with understanding the controls of these mechanisms were identified in this thesis. A series of experimental and simulation work was conducted to fill these knowledge gaps. Shale samples were collected from different location with a wide range of petrophysical, mechanical and mineralogical properties. Multiphase flow and sealing capacity assessment requires knowledge of threshold pressure of shale, which is challenging using standard methods due to the stress sensitive of shale. Using Mercury Porosimetry Under Confining Stress (MPUCS) instrument, it was proven experimentally that shale would act as effective seal and would not leak by multiphase flow through the undeformed matrix under in situ conditions. The radioactive waste management industry argues that leakage mainly occurs via flow along pathways formed by high gas pressures (pathway dilation). However, there is no micro mechanical model to describe formation and propagation of these pathways. Pathway dilation in clay-rich sediments was investigated by injecting melted Field’s metal into synthetic shale sample. Results suggest that compaction plays a key role in formation and propagation of these pathways. It is quite important to understand failure mechanics of shale in order to be able to argue the existence of conductive faults and fracture and their capability to re-seal. Anticipation of formation and closure of faults and fractures require knowledge of mechanical properties such as the apparent preconsolidation pressure, which is difficult to obtain for shale. A new simple technique was developed to measure the preconsolidation pressure under hydrostatic condition using MIP instrument. Micro-indentation was proposed to measure elastic properties as it is difficult to obtain core plugs that are sufficient long for tri-axial testing. Faults and fracture are often argued to be conduits to fluid flow across shale. However, these features could close and re-seal but knowledge of controls for fracture closure is still elusive. Controls of fracture closure in shale were investigated by conducting a series of flow experiments through artificial fracture using a set of different shale samples. It was shown that porosity, clay content and stress state controls fracture closure. It was suggested that fractures in soft shale with high porosity and clay content samples have potential to close and reseal under in situ condition whereas stiff shale will have their fracture open under same conditions. Finite element analysis (FEA) was performed to simulate the same fracture closure experiment, which also provided results that agree with the suggestions made from experimental work.