Page 200 - Petroleum Geology

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comes necessary, it will be inclined slightly towards the oil reservoir from
outside. The effect of an injection well is to impose on this potentiometric
surface a cone of impression analogous to the cone of depression imposed by
a producing well. The shape of this cone of impression is described by an
equation similar to 8.19, with the term pgk/q = K, the hydraulic conductivity,
and the sign of Q is negative.
The flow of water from an injection well is thus radial from the well, but
the symmetry will be disturbed in an injection well close to the oil/water
contact due to the low relative permeability to water at low oil saturations
about the contact, in the field of flow towards the accumulation.
Heads, being scalar quantities, are additive; and the water-injection well
being within the radius of influence of a producing well, the potentiometric
surface takes a shape that is the algebraic sum of the total heads of the sevelal
components (cf. Fig. 6-5, p. 119).
Pressure maintenance is better than trying to restore pressure lost, so water-
flood operations usually begin early in the life of a field, soon after recogni-
tion that the natural water drive is inadequate.
The uncomfortable fact that about 2/3 of the oil in place remains in the
reservoir, even after secondary recovery operations, has encouraged research
into more efficient means of recovery. The problem here is both technical
and economic: the cost of the operation must be less than the value of its
effect. There are two main factors on the technical side. First, the immiscibility
of oil and water affects the recovery because of the capillary pressures involved.
Secondly, the effects of wells are dominantly radial. Injection of solvents or
agents that promote miscibility increases the proportion of oil ultimately re-
covered, but the solvents or agents must be cheap or readily recoverable. They
must also be absolutely clean (as with the water for injection) so that they
can be injected at high rates for long periods of time without clogging the in-
jection well.
The radial tendency of the processes leaves large volumes of the reservoir
inefficiently drained, like the space between coins placed together on a table,
and anisotropy in the reservoir can (and usually does) lead to unstable dis-
placement fronts with “fingering”. Once a preferential displacement path
reaches a producing well, the greater relative permeability to the solvent or
agent (or water in water flooding) ensures that significant proportions of the
reservoir will be by-passed and will remain undrainable without drilling new
wells.

REFERENCES

Allen, D.R., 1968. Physical changes of reservoir properties caused by subsidence and re-
pressuring operations. J. Petrol. Technot., 20: 23-29.