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