Numerical and experimental study of oil transfer in laminated shale

Abstract

Lamination is ubiquitous in shale oil reservoirs and significantly influences the oil transfer process. Understanding oil transfer in laminated shale is important for shale oil reservoir development. Therefore, a mathematic model is developed to describe the processes involved in vacuum-imbibition tests. In constructing the mathematic model, viscous flow in inorganic pore, oil adsorption, and absorption in organic matter, and oil diffusion through organic matter surface and interior are considered. In addition, a crucial link between the model and physical characteristics of shale (specific surface area, clay content, and total organic carbon content) is established. The results reveal that the saturation capacities of free oil, adsorbed oil, and absorbed oil in shale depend on the porosity, specific surface area and volume of organic matter, respectively. The oil contents in the adsorption and absorption phase range from 2.2% to 8.3% and 41.8% to 51.4% of the total oil content for the three samples in this study, respectively. When the equilibrium time is used to determine the oil transfer in shale, a larger specific surface area, a smaller thickness of lamination, and smaller volume fractions of organic–clay matrix to total volume of the samples hasten oil transfer. Also, an ideal vacuum-imbibition process of oil in laminated shale is discussed. The results reveal that oil diffusion through the surface of organic matter accelerates the oil transfer in laminated shale. However, the effect of the effectivity diffusion coefficient of oil in the adsorption phase (Ds) on the oil transfer is smaller than that of the effectivity diffusion coefficient of oil in the absorption phase (Dd).