Simulation of oil expulsion by 1-D and 2-D basin modelling — saturation threshold and relative permeabilities of source rocks

Abstract

In order for oil to accumulate in economic quantities, it first has to be generated and expelled from source rocks in sufficient quantities. In spite of long term efforts, the mechanism of oil expulsion from the source rocks is not completely understood. For modelling expulsion, the adoption of pressure-driven multiple-phase fluid flow governed by Darcy’s law is widely accepted. However, relative permeabilities for fine-grained source rocks, which is an essential parameter for this model, are very difficult to specify. The conventional reservoir-rock curve is obliged to be used for the modelling. Simplification of the relative permeability model is generally used for one-dimensional basin modelling. The 1-D model also requires substantial optimization of the key parameter, saturation threshold. Laboratory measurement of relative permeabilities for fine-grained rocks is very difficult, therefore, we carefully interpreted the analysed laboratory data on various sandstones to establish a relationship between relative permeability curves and pore geometry parameters, with a view to extrapolate the relationships developed in sandstones to fine-grained source rocks. It was found that the relative permeability curves, or irreducible water saturation are controlled by two factors ; grain size and clay content. Since the total surface area of the pore system becomes greater as the grain size decreases, we consider surface water adsorbed on the grain surface as a part of irreducible water. Since clay contains micro-porosity which cannot be displaced by oil due to high capillary pressure, we regard this to also play a role. Prediction of relative permeability curves for fine-grained rocks by both processes results in the curve with high irreducible water saturation. The new relative permeability curves were tested by both one-dimensional and two-dimensional basin modelling. Test results indicate that the new curves can reproduce expulsion efficiency, locations of accumulations and leaking through cap rock, which is consistent with actual observations.

Oil and gas assessment of the Kuqa Depression of Tarim Basin in western China by simple fluid flow models of primary and secondary migrations of hydrocarbons

“…(4) is a general formula to calculate the liquid expulsion coefficient before and after compaction, which is the principle behind the purecompaction method for determining liquid expulsion. Two empirical formulas for describing two-phase (oil-water) in source rock were presented by Okui et al (1998):…”

“…Okui et al (1998) presented the 2-phase (oil and water) relative permeability curves as a function of absolute permeability in source rocks (see Figure 11 in their paper), by a view to extrapolate the measured curves in various sandstones to fine-grained source rocks. They tested the figure by both 1-D and 2-D basin modeling, and the results were consistent with actual observations.…”

“…Hunt, 1990;Berg and Gangi, 1999), the fluid flow model related to Darcy's Law and relative permeabilities (e.g. Nakayama, 1987;Özkaya, 1991;Okui and Waples, 1993;Okui et al, 1998), and the fluid migration is related to sedimentary compaction (e.g. Magara, 1978;Zimmerman et al, 1997) and, furthermore, that oil expulsion is related to compaction (e.g.…”

Physical properties of the shallow sediments in late Pleistocene formations, Ursa Basin, Gulf of Mexico, and their implications for generation and preservation of shallow overpressures

“…Fully coupled and integrated basin simulators provide information on pore pressures, porosities, temperatures and fluid flow patterns through time. We conducted two-dimensional simulation using SIGMA-2D (Okui et al, 1996(Okui et al, , 1998 (Bryant et al, 1975). The black line was used as the modelled porosity-vertical permeability relationship for hemipelagic clays.…”

“…The initial temperatures were set to be those obtained from the previous time step for existing grids, and seawater temperature for the newly added grids. Boundary conditions for the heat flow in SIGMA-2D are specified temperatures along the upper boundary, a specified heat flux along the bottom, and no flux along side boundaries (Okui et al, 1996(Okui et al, , 1998. At the upper surface of the model, temperature and pressure were modelled as seafloor temperature and seafloor pressure.…”