Page 179 - Fundamentals of Reservoir Engineering

DARCY'S LAW AND APPLICATIONS 117

The flow equations during the initial and later part of a steam soak cycle will be
described in Chapter 9, sec. 6 and Chapter 6, sec. 4, respectively, since they serve as
an interesting example of the flexibility of radial flow equations.

d) Reduction of the oil formation volume factor (B o)

As already described in Chapter 2, sec. 4; B o (rb/stb) can be minimised by choosing
the correct surface separator, or combination of separators.

e) Reduction in the ratio r e / r w

Since r e / r w appears as a logarithmic term, it has little influence on the PI and alteration
of the ratio by, for instance, underreaming the wellbore to increase r w, is seldom
considered as a means of well stimulation.

f) Increasing the well penetration (h)

It was assumed in deriving equ. (4.29) that the well was completed across the total
formation thickness and thus the flow was entirely radial. If the well is not fully
penetrating, there is a distortion of the radial flow pattern close to the well giving rise to
an additional pressure drawdown. This is generally accounted for by using the full
formation thickness in equ. (4.29) and including the effect of partial penetration as an
additional skin factor. The method of calculating this additional skin is described in
Chapter 7, sec. 9. Increasing the well penetration, if possible, will obviously increase
the Pl but in many cases wells are deliberately completed over a restricted part of the
reservoir to avoid excessive gas or water production from individual sands, or to
prevent coning.
The methods for stimulating the production of a well, described in this section, do not
necessarily increase the ultimate oil recovery from the reservoir, but rather, reduce the
time in which the recovery is obtained. As such, they are generally regarded as
acceleration projects which speed up the production, thus having a favourable effect on
the discounted cash flow.

There are exceptions. For instance, if a well has stopped producing, then any
stimulation which results in oil production can be regarded as increasing the recovery.
These methods, however, should be distinguished from the enhanced recovery
techniques, described in sec. 4.9, in which the reservoir is energised to increase the
recovery. In stimulation there is frequently no net energy increase in the reservoir. In
steam soaking, for instance, heat energy is supplied to the reservoir and is
subsequently lost during the production cycle; as opposed to continuous steam drive,
in which the aim is to keep the steam in the reservoir thus increasing the total energy of
the system.

4.8 TWO-PHASE FLOW: EFFECTIVE AND RELATIVE PERMEABILITIES

In describing Darcy's law, it has so far been assumed that the permeability is a rock
property which is a constant, irrespective of the nature of the fluid flowing through the
pores. This is correct (with the noted exception of gas flow either at low pressures or
very high rates) provided that the rock is completely saturated with the fluid in question,