Modelling and upscaling of shallow compaction in basins
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Heterogeneous fine-grained sediments at shallow burial (< 1000m) below the seafloor can often experience large strain of mechanical compaction and variable degrees of overpressure in their pore space as a result of disequilibrium dissipation of pore fluid. Shallow overpressure can pose significant risks to economics and safety of hydrocarbon production and may impact on hydrocarbon generation deep in a basin and hydrocarbon migration to traps during basin evolution. However, when basin modelling ignores the heterogeneity of sediments, large strain deformation and fluid flow conditions at smaller length- and/or time-scales than those at basin scales, it can lead to incorrect prediction of sediment compaction, and hence the mass of the sediment column, the magnitude of pore pressure and its distribution at shallow burials, and consequently can impact on the simulation of basin evolution. In this thesis, the necessity of considering large-strain consolidation in modelling shallow compaction is demonstrated, and a one-dimensional large-strain numerical simulator, based on one of Gibson’s consolidation models and suitable for basin modelling, is developed and verified. An analytical upscaling technique is also developed for determining the effective compressible parameters and permeability for horizontally layered systems of certain compaction characteristics. They are used subsequently to analyse parametrically the compaction behaviours of the layered systems and to calculate effective coefficients for the systems, with results showing that fine-scale simulation is required when considering the effect of fluid-structure interaction. However, the large strain model over-predicts the pressure of the Ursa region, Gulf of Mexico, based on information from the Integrated Ocean Drilling Program (IODP). An analysis indicates that horizontal fluid flow, or lateral motion of mass transport processes, may explain the over prediction. The limitation of a 1D model is further discussed thereafter both in fluid flow and mechanical deformation. With strong applicability and fundamentality, the Modified Cam Clay model is adopted in 2D research, and related verification is provided. Modified Cam Clay can show elastic and elastic-plastic properties in basin evolution. Heterogeneous Modified Cam Clay materials can be upscaled to a homogenous anisotropic elastic material in elastic deformation and a homogenous Modified Cam Clay material in elastic-plastic deformation, however, the upscaled parameters vary with the effective stress. The value of the upscaling is demonstrated by modelling the evolution of a simplified North Africa basin model.