Page 252 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 228

[ ] Well Programming
2.5.6

A water aquifer is a single phase reservoir and it is saturated with
water. The term absolute permeability is a measure of the capacity of a
rock to transmit a fluid under a pressure differential when 100% satu-
rated with that fluid. It might be said that the permeability in the
aquifer is the absolute permeability.
However, most hydrocarbon reservoirs are multiphase. There are at
least two fluid phases (in some cases there are three phases): oil and
water or gas and oil. In a study of a hydrocarbon reservoir, the perme-
ability of each phase must be quantified. For each phase, there is an
effective permeability that is a measure of each specific phases ability
to flow, i.e., effective permeability, oil-effective permeability, gas-effec-
tive permeability, and water.
The most significant effect on the effective permeability of each
phase is the saturation of each phase (amount of each phase present).
The saturation of each phase is described as the residual oil saturation,
residual water saturation, and residual gas saturation. To describe the
effect of saturation on effective permeability, the term relative perme-
ability is introduced. The relative permeability for each phase is the
ratio of the effective permeability for that phase to the absolute perme-
ability. (The absolute permeability is measured in the laboratory by
using common mediums such as air, but also refined oils, synthetic
brines, gas, nitrogen, etc. It is beyond the scope of this book to provide
more detail.)
How does filtrate invasion cause damage by increasing water satu-
ration? In most rocks, oil will cease to flow when the residual oil satu-
ration decreases to the range of 20-30%. The residual saturation for oil
where flow has ceased or the relative permeability to oil has decreased
to zero is described as the irreducible oil saturation. Similarly, the effect
is seen in the variance of water saturation. The irreducible water satu-
ration is the point where water will not flow.
It can be appreciated that where a substantial increase in water sat-
uration has been caused due to substantial filtrate invasion (it also will
happen with water coning), the residual oil saturation will decrease
and thus hinder the flow of oil. Eventually the irreducible oil satura-
tion will be reached and the well will not flow oil. This condition is
described as water blockage.
A further effect that the liquid fraction of the mud can have on
productivity impairment is if there is a change in wettability (the

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