Page 14 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

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Reservoir Engineering

Reservoir engineering covers a broad range of subjects including the occurrence
of fluids in a gas or oil-bearing reservoir, movement of those or injected fluids,
and evaluation of the factors governing the recovery of oil or gas. The objectives
of a reservoir engineer are to maximize producing rates and to ultimately recover
oil and gas from reservoirs in the most economical manner possible.
This chapter presents the basic fundamentals useful to practical petroleum
engineers. Topics are introduced at a level that can be understood by engineers
and geologists who are not expert in this field. Various correlations are provided
where useful. Newer techniques for improving recovery are discussed.
The advent of programmable calculators and personal computers has dramatically
changed the approach of solving problems used by reservoir engineers. Many
repetitious and tedious calculations can be performed more consistently and
quickly than was possible in the past. The use of charts and graphs is being
replaced by mathematical expressions of the data that can be handled with
portable calculators or personal computers. Programs relating to many aspects
of petroleum engineering are now available. In this chapter, many of the charts
and graphs that have been historically used are presented for completeness and
for illustrative purposes. In addition, separate sections will be devoted to the
use of equations in some of the more common programs suitable for program-
mable calculators and personal computers.
BASIC PRINCIPLES, DEFINITIONS, AND DATA

Reservoir Fluids
Oil and Gas

Reservoir oil may be saturated with gas, the degree of saturation being a
function, among others, of reservoir pressure and temperature. If the reservoir
oil has dissolved in it all the gas it is capable of holding under given conditions,
it is referred to as saturated oil. The excess gas is then present in the form of
a free gas cap. If there is less gas present in the reservoir than the amount that
may be dissolved in oil under conditions of reservoir pressure and temperature,
the oil is then termed undersaturated. The pressure at which the gas begins to
come out of solution is called the saturation pressure or the bubble-point
pressure. In the case of saturated oil, the saturation pressure equals the reservoir
pressure and the gas begins coming out of solution as soon as the reservoir
pressure begins to decrease. In the case of undersaturated oil, the gas does not
start coming out of solution until the reservoir pressure drops to the level of
saturation pressure.
Apart from its function as one of the propulsive forces, causing the flow of
oil through the reservoir, the dissolved gas has other important effects on
recovery of oil. As the gas comes out of solution the viscosity of oil increases
and its gravity decreases. This makes more difficult the flow of oil through the
reservoir toward the wellbore. Thus the need is quite apparent for production

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