Reservoir system

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Predicting reservoir system quality and performance
Author Dan J. Hartmann, Edward A. Beaumont
Link Web page
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The term “reservoir” creates confusion between different disciplines. Explorationists apply the term to mean a porous and permeable rock regardless of the fluid it contains. Reservoir engineers apply the term to mean a rock that contains hydrocarbons and associated fluids. This difference in meanings can cause problems for multidisciplinary teams unless the terminology is clear.

Reservoir system components

Figure 1 Major components of a conventional reservoir system.

In this discussion, a reservoir system is a water–hydrocarbon system contained within the pores of a rock unit. A reservoir system has three main components: a reservoir, an aquifer, and a transition zone (interface) between the two.

  • A reservoir is a porous and permeable rock saturated with oil or gas in buoyancy pressure equilibrium with a free water level (zero buoyancy pressure). It has one or more containers and is located below a seal.
  • A transition zone is the interval of rock separating the reservoir from the aquifer; it is less than 100% saturated with water.
  • An aquifer is a porous and permeable rock 100% saturated with water. It has one or more containers that may or may not be shared with a reservoir.

The diagram in Figure 1 illustrates the major components of a conventional reservoir system.

Waste and transition zones

A waste zone may be found at the top of a reservoir, just below the seal, if there is a decrease in the size of the pore throat radii of the reservoir. It generally produces hydrocarbon and water on a production test.[1]

A transition zone is located at the base of a reservoir and forms as a result of a loss of buoyancy pressure in the hydrocarbon phase. Pore throat diameter and fluid densities determine its thickness. It generally produces hydrocarbon and water on a production test.

Free water level

The free water level is located at the base of a hydrocarbon column and the transition zone. Above this level, the reservoir produces water alone, hydrocarbon and water, or hydrocarbon alone on a production test. Below this level lies the aquifer of a water-drive reservoir system. It produces water only. Zero buoyancy pressure exists at this level or below.

References

  1. Schowalter, T. T., 1979, Mechanics of secondary hydrocarbon migration and entrapment: AAPG Bulletin, v. 63, no. 5, p. 723-760.

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