Chapter 3: Basic Concepts

The purpose of this chapter is to discuss several concepts that are basic to an understanding of the physics of waterflooding. The descriptions presented here are very simplistic but provide some necessary context for the far more complex reality of the geology, wettability, and flow characteristics of actual oil reservoirs. Many aspects of these complexities are covered in the subsequent chapters of this monograph.

The topics addressed in this chapter are

Oil reservoirs are subsurface porous media containing varying amounts of oil and water within the void spaces. Almost all of these areally extensive rock intervals were originally formed from sediments deposited as 3D layers of rock fragments, termed grains, of a range of sizes. Chemically, these grains are predominantly composed of either silica-based chemical compounds (sandstone or clastics) or carbonate-based chemicals (limestone or dolomite). Fig. 3.1 is a simplistic schematic cross-section magnified view of hypothetical porous media, and Fig. 3.2 shows lengths of 4-in.-diameter core from a sandstone oil reservoir, including the locations where 1-in.-diameter core plugs were cut. As seen in Fig. 3.1, there are three aspects to describing porous media: grains, pore bodies, and pore throats. The pore bodies are the large portion of the spaces or pores among the grains. The locations where the spaces between the grains narrow are termed pore throats. When originally deposited, all of the pore spaces, either pore bodies or pore throats, are filled with water,² whether the sediments were deposited by a river system or in the ocean.³ In an oil reservoir, the pores are filled with liquids, with varying proportions of oil and water. Fig. 3.1 shows a hypothetical distribution of oil and water within the pores, suggesting a more-continuous connate water phase and a less-continuous oil phase. When considering the 3D nature of the pore system of an oil reservoir, both the oil and water are generally continuous liquid phases.