Chapter 2: Background

This chapter provides the context for the technical discussion of waterflooding technology detailed and presented in the following chapters. A few basics must be presented first so those readers for whom reservoir engineering is not their principal vocation can understand the framework from which the fundamental concepts and detailed calculation methods are developed. Some of the figures used in this section are very simplified schematic reservoir cross-sectional illustrations from a historical SPE slide presentation describing the basics of the oil and gas industry (Alford 1960).

The topics addressed in this chapter are the following:

The starting point of this discussion is a definition of what an oil reservoir is. Generally speaking, an oil reservoir is an underground porous and permeable sedimentary-rock interval, often particular rock strata or layers, in which large volumes of liquid hydrocarbons, or oil, have accumulated (see Fig. 2.1).¹ The oil has accumulated here because this porous and permeable rock interval is capped on the top and sides by other impermeable rock layers. Over geologic time, oil migrated into this rock interval, displaced its original brine, and became trapped there. In most cases, underlying the oil-bearing interval is an aquifer interval in which the pore space is filled with brine. The aquifer underlies the oil interval, or oil column, because oil at reservoir pressure and temperature conditions has a lower density than aquifer brine has; hence, the lighter fluid (oil) is found above the heavier fluid (brine).