Numerical models of petroleum migration via buoyancy‐driven porosity waves in viscously deformable sediments

Abstract

The apparent ability of petroleum to migrate rapidly through low permeability sediments in sedimentary basins has led to many questions about the manner of its transport. One possible explanation is suggested from observations of the compaction of viscously deformable porous media. These systems have been found in some cases to give rise to regions of locally elevated liquid fraction, in the form of fluid‐filled porosity waves that can ascend at rates much greater than that of the background flow. Previous research on the phenomenon has focused on its implications for magma transport, but recognition of the fact that the compaction of viscous porous media can take place in sedimentary basins has suggested the possibility that porosity waves could similarly be important for hydrocarbon transport. The purpose of the present study was to test this hypothesis by quantifying the transport that would occur as the result of porosity waves initiated during the conversion of kerogen to petroleum. A one‐dimensional numerical model was constructed solving equations for the mechanics of viscous compaction and for the kinetics of reactions describing the formation of petroleum from kerogen. The results showed that porosity waves would develop readily in viscously deformable regions of sedimentary basins, but would not necessarily provide enhanced transport over that of the background flow regime. In order for the waves to achieve this enhanced transport, they must develop high amplitudes, i.e. high porosities relative to the background porous medium. To achieve the high wave amplitudes, the background porosity must be very low in absolute terms. In addition, high kerogen contents are needed in the source layer, and the source layer needs to be buried rapidly to a high temperature region of the oil window. Considerable uncertainty exists as to the value of the matrix shear viscosity of sediments in basins. However, the wave volumetric transport capacity was not found to be significantly altered as a result of variations in the value of this parameter. The physical form of the waves was strongly altered by the matrix shear viscosity, with higher values leading to lower amplitudes and generation frequency, but higher wavelengths. Thus the waves become less recognizable physically at higher values of the matrix shear viscosity. As the waves ascend to higher stratigraphic levels, where the porosity is higher, they gradually lose their physical definition and become absorbed into the background.

How contact metamorphism can trigger global climate changes: Modeling gas generation around igneous sills in sedimentary basins

“…The extra porosity generated from devolatilization can cause viscous compaction and initiation of porositywaves, which provides an effective transport mechanism to the atmosphere (e.g. Connolly, 1997;Connolly and Podladchikov, 1998;Appold and Nunn, 2002).…”

Potential of porosity waves for methane transport in the Eugene Island field of the Gulf of Mexico basin

“…fluidefilled porosity) where rates of hydrocarbon generation were high. In contrast to elastic porous media where porosity waves ascend along pressure gradients, in viscous porous media porosity waves ascend due to buoyancy at rates Appold and Nunn (2002) calculated to be up to about 1 mm per year. Revil and Cathles (2002) derived an analytical solution for porosity wave velocity from differential equations for fluid mass conservation in elastic porous media.…”

“…The very rapid migration rates of porosity waves become possible if permeability increases sharply in response to increases in pore fluid pressure and porosity. Appold and Nunn (2002) investigated the capability of porosity waves to transport oil in viscous sediments by building a oneedimensional numerical model that simulated the burial of an organic rich sedimentary layer and the conversion of kerogen to oil. Their results showed porosity waves to form as zones of elevated liquid fraction (i.e.…”

The Fate of Liquids Trapped During the Earth's Inner Core Growth

“…Compaction of a solid matrix has been extensively studied in the geosciences literature for processes such as magma chamber solidification (McKenzie, 1984(McKenzie, , 2011, magma extractions (e.g., Rabinowicz & Vigneresse, 2004), planetary evolution from accretion (e.g., Hier-Majumder & Hirschmann, 2017;Scheinberg et al, 2016), and petroleum transfer in sediments (e.g., Appold & Nunn, 2002) but also for the evolution of the inner core (Sumita et al, 1996). In the context of the inner core growth, a notable difference from most compaction studies comes from the linear dependence of gravitational acceleration on radius in the core, due to the self-gravitation.…”