Page 178 - Petroleum Geology

CHAPTER 8

THE NATURE OF PETROLEUM RESERVOIRS

SUMMARY

(1) Petroleum reservoirs contain gas and water, or oil and water, with the
petroleum usually in the central parts of the pores and the water in pendular
rings around the grain contacts. This water is apparently immobile, and the
irreducible water saturation is commonly 20-40% of the pore volume.
(2) Pressures in the petroleum are higher than normal hydrostatic water
pressures because of its smaller weight density.
(3) There are two measures of permeability: intrinsic permeability (symbol
h, dimensions L2), which is a property of the rock independent of the fluid
in it, and coefficient of permeability or hydraulic conductivity (symbol K,
dimensions LT-'), which includes the properties of the fluid. Relative perme-
ability (dimensionless) is the ratio of the effective permeability to oil or gas
to the permeability to water when the rock is fully saturated with water.
(4) Petroleum flows to a well by virtue of the effective permeability of the
rock to oil or gas, and an energy gradient induced by the well. The effective
and relative permeabilities to oil and gas decrease as the water saturation in-
creases, but the relative permeability to oil and gas may be close to unity
when the irreducible water saturation is low. Most of the oil or gas is produced
without significant water volumes.
(5) The energy of subsurface fluids is most easily represented by head
(dimension L). The total head, h, is the algebraic sum of the elevation head,
z, relative to an arbitrary datum (negative downwards), and thepressure head,
p/pg (pressure divided by weight density).
(6) A reservoir is said to have water drive if water expansion is the main
driving force replacing the volume of oil produced; gas drive, if gas-cap expan-
sion replaces the oil produced. The energy of a reservoir must usually be
maintained by injecting water below the oil/water contact, or by injecting
gas into the gas cap.
(7) About two thirds of the oil remains in the reservoir and cannot be
produced mechanically by conventional methods. There have been some
succesful attempts at recovering some of this oil with solvents.

The reservoir, as we have seen, consists of porous and permeable rock -
usually sedimentary rock, but some are of volcanic and other igneous rocks -
of which the pores and fissures are filled with water and oil, water and gas,
or (rarely) water, oil and gas. The fluids and solids are at elevated temperatures,
relative to the surface, depending on the depth and the geothermal gradient.