42 POWER PLANT ENGINEERING
type, the reservoir rock changes in permeability so that further movement of oil through the pores of the
reservoir rock becomes possible.
To Summaries: Hydrocarbons are generated in source rocks from the remains of marine organ-
ism deposited and buried in the rocks. They are transported by surface tension, gravitational and pres-
sure forces into reservoir rocks, where, if there are suitable traps, they accumulate in the pores of the
rock and form the reservoirs of oil and gas found today. For oil to be formed within the source rocks,
they must have been buried for a million years or more at depths over 1 km, to get the pressure and
temperature high enough, but rarely more than about 4 km or the higher temperature at those depths
would usually decompose the oil, leaving methane gas and petroleum coke.
Exploration and Production: First oil wells are only a few to hundred meters deep but most of
today’s producing accumulations lie in the depth range 500 to 3000 meters, but the deepest producing
wells are at 6500 m for oil and 7500 meter for gas. Similarly, variations in pressure from atmosphere to
1000 atm. have been found although the pressure usually increases by 100 to 150 atm. per km in depth
corresponding to the depth of the overlying column of rock pore water. Temperatures also increases with
depth at a rate given by the geothermal gradient, generally in the range 15 to 40°C per km in oil produc-
ing areas, though temperatures in oil reservoirs are usually below 110°C.
Early exploration methods like geological surveys, measuring the angles of tilt of the rock strata
that emerged at the surface, correlation of nearby drilling data, are augmented by seismic surveying;
geomagnetic and gravitational surveys; geochemical tests; geothermal, radiation and electrical conduc-
tivity surveys, etc. These exploration methods allow the identification of structure that may be traps but
they can only rarely establish the presence of oil that can be ensured only by drilling.
Oil is driven from the reservoir rocks into the borehole by the difference in pressure. Hence the
rate of production from an oil well is limited. A measure of the rate of production from a reservoir is the
reserves to production ratio (R/P) measured in years. R/P is high in the early years but tends to become
constant in the range from 5 to about 15 years. An assessment of the amount of oil that may be recovered
from a reservoir requires information on the amount of oil in place and an estimate of the recovery
factor. The amount that can be extracted is related to the conditions in the reservoir, oil composition and
the method of extraction. The world average recovery factor at present is about 25–30%.
The production of conventional oil depends on the reservoir fluids flowing under pressure out of
the reservoir rock into the borehole. Oil recovery processes are usually considered as falling into three
categories:
- Primary Recovery : The oil recovered by the natural displacement processes that occur as oil
is produced from a reservoir;
- Secondary Recovery : The additional oil recovered as a result of water/gas injection into the
reservoir to complement the naturally occurring drive processes;
- Enhanced Recovery/Tertriary Recovery : Oil recovery by processes aimed at higher dis-
placement efficiencies than those obtained through the natural processes of gas and water
drive, like use of chemicals, CO 2 and heat.
Oil Reserves. The proven reserves are defined as the quantity of oil that can be commercially
produced with existing technology. At present the total world proved reserves amount to be about 1047
barrels (1047 bbl) of which 77% lies in OPEC countries (1996 estimate). The total world consumption
of crude oil in 1996 was 71.7 million barrels per day. OPEC estimates that total world oil consumption
could reach from the 70 million barrels a day in 1995 to around 100 million barrels per day by the year
- Table 2.1 gives the world’s largest proven crude oil reserves and Table 2.2 gives the production
