Fundamentals of Fluid Flow in Porous Media

 

Chapter 2

The Porous Medium

Porous materials are encountered literally everywhere in everyday life, in technology and in nature. With the exception of metals, some dense rocks, and some plastics, virtually all solid and semi-solid materials are “porous” to varying degrees. A material or structure must have these two properties in order to qualify as a porous medium:

1. It must contain spaces, so-called voids or pores, free of solids, imbedded in the solid or semi-solid matrix. The pores usually contain some fluid, such as air, water, oil or a mixture of different fluids.
2. It must be permeable to a variety of fluids, i.e., fluids should be able to penetrate through one face of a sample of material and emerge on the other side.

There are many examples where porous media play important roles in technology and, conversely, many different technologies that depend on porous media. Among the most important technologies that depend on the properties of porous media are:

1. Hydrology, which relates to water movement in earth and sand structures, such as water flow to wells from water-bearing formations.
2. Petroleum engineering which is mainly concerned with petroleum and natural gas exploration and production.

The petroleum engineer is concerned with the quantities of fluid content within the rocks, transmissibility of fluids through the rocks, and other related properties. These properties depend on the rock and frequently upon the distribution of character of the fluid occurring within the rock. Knowledge of the physical properties of the rock and the existing interaction between the hydrocarbon system (gas, oil and water) and the formation is essential in understanding and evaluating the performance of a given reservoir.

Rock properties are determined by performing laboratory analyses on cores from the reservoir to be evaluated. The cores are removed from the reservoir environment through the well during the drilling operations. There are primarily two main categories of core analysis tests that are performed on core samples regarding physical properties of reservoir rocks. These are:

Routine core analysis tests

• Porosity
• Permeability
• Saturation

Special core analysis tests

• Capillary pressure
• Relative permeability
• Wettability
• Surface and Interfacial Tension
• Electrical Conductivity
• Pore size Distribution

These properties constitute a set of fundamental parameters by which the rock can be quantitatively described. They are essential for reservoir engineering calculations as they directly affect both the quantity and the distribution of hydrocarbons and, when combined with fluid properties, control the flow of the existing phases (i.e., gas, oil, and water) within the reservoir.