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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


P. Boult and J. Kaldi, eds., 2005, Evaluating fault and cap rock seals: AAPG Hedberg Series, no. 2, p. 1-12.


Copyright copy2005 by The American Association of Petroleum Geologists.

Fluid Flow, Pore Pressure, Wettability, and Leakage in Mudstone Cap Rocks

Andrew C. Aplin,1 Steve R. Larter2

1School of Civil Engineering and Geosciences, University of Newcastle, Newcastle upon Tyne, United Kingdom
2School of Civil Engineering and Geosciences, University of Newcastle, Newcastle upon Tyne, United Kingdom; Present address: Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada.


The ideas in this chapter were developed over several years with support from multiple sources: a Royal Society Research Fellowship, a European Union Training and Mobility of Researchers fellowship, a Natural Environment Research Council Realising Our Potential Award, the Polaris consortium (Exxon and Statoil), the GeoPOP consortium (Agip, Amerada Hess, Amoco, ARCO, BP, Chevron, Conoco, Elf, Enterprise, Japan National Oil Corporation, Mobil, Norsk Hydro, Phillips, Statoil, and Total), the Caprocks consortium (BP, ConocoPhillips, ENI, ExxonMobil, Norsk Hydro, Statoil, Total, and Unocal), the European Union Fourth Framework Hydrocarbon Reservoir Programme, and direct support from BP and Norsk Hydro. Additional support to SRC was provided by the Natural Sciences and Engineering Research Council of Canada and the Alberta Ingenuity Fund. We gratefully acknowledge the constructive comments of the reviewers Ben Clennell, Les Leith, and Peter Boult.


This chapter considers some of the issues surrounding the modeling of one- and two-phase fluid flow in mudstones. For single-phase flow, key relationships include those between porosity and (1) effective stress, (2) permeability, and (3) capillary breakthrough pressure. All three relationships are strongly influenced by the grain-size distribution or clay fraction of mudstones, but a quantitative description is currently only available for the porosity-effective stress relationship. The importance of lithology or clay fraction as a control on the key flow properties of mudstones indicates the practical significance of estimating clay fraction directly from geophysical logs. This chapter illustrates how artificial neural networks can be used to perform this task.

Having considered some of the basic flow properties of mudstones, the second part of the chapter discusses aspects of two-phase flow through mudstone pore systems. Rates, mechanisms, and pathways of petroleum leakage through mudstone pore systems remain poorly constrained. In this chapter, field and experimental data is combined with theoretical arguments to suggest that once a water-wet cap rock is breached, the leak path will become more oil wet as a result of sorption of hydrophilic and ultimately hydrophobic organic compounds onto mineral surfaces. Oil-water partition of hydrophilic organic compounds in reservoirs, followed by diffusion into cap rock pores may even create oil-wet pathways into cap rocks and permit leakage. In these cases, cap rocks simply retard the vertical migration of petroleum, and column height is a function of the rates of petroleum supply and loss. Modeling the rate of loss of petroleum requires a better understanding of mudstone relative permeability.

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