Page 234 - Hydrocarbon

Reservoir Dynamic Behaviour 221

water production
water production

Stable Displacement Unstable Displacement
(M < 1) (M > 1)

water oil
Figure 9.16 Stable and unstable displacement in the horizontal plane.

water production production

oil

water water

Stable Displacement Gravity Tonguing
at low rate at high rate
Figure 9.17 Gravity tonguing.

be left unrecovered at the abandonment condition which may be dictated by a
maximum water cut.
So far we have looked only at the viscous forces (which are a measure of the
resistance to flow) acting on reservoir fluids. Another important force which
determines flow behaviour is the gravity force. The effect of the gravity force is to
separate fluids according to their density. During displacement in the reservoir, both
gravity forces and viscous forces play a major role in determining the shape of the
displacement front. Consider the following example of water displacing oil in a
dipping reservoir. Assuming a mobility ratio less than 1.0, the viscous forces will
encourage water to flow through the reservoir faster than oil, whilst the gravity forces
will encourage water to remain at the lowest point in the reservoir (Figure 9.17).
At low injection rates the displacement is stable; the gravity forces are
dominating the viscous forces. At higher rates of injection, the viscous forces are
dominating, and the water underruns the oil, forming a so-called ‘gravity tongue’.
This is a less favourable situation, since the produced fluid will be a mixture of oil
and water long before all of the oil is produced. If high water cut is an abandonment
constraint this could lead to a reduction in recovery. The steeper the dip of the