Page 198 - Petroleum Geology

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AT, having theoretically twice the slope of the first. It is theoretically pos-
sible to estimate the distance of the fault from the well. (The slope of the
first part extrapolates to ap* that is too small, but gives the correct value of
effective permeability ; the second line extrapolates approximately to the
true value of p * .)
If the energy of the reservoir is, or becomes, insufficient to generate flow,
or insufficient to generate optimum flow rate, it can be artificially increased
by gas-lift or by pumping. Gas-lifting involves the injection of gas through
valves into the tubing so that the effective density of the oil column in the
tubing is decreased. This has the effect of increasing the potential gradient.
Natural gas coming out of solution in the oil as it rises in the tubing to lower
pressures commonly assists production in the same way.
As oil is abstracted from the reservoir by production, its place must be
taken by water expansion or gas expansion (or both), or the pressure loss will
result in some mechanical compaction of the reservoir rock. It will be evident
from the discussion of pressure build-up in a well that water drive and gas
drive mechanisms require time, because the volume created by expansion
must flow through the reservoir rock, displacing the oil/water contact upwards
and the gas/oil contact downwards.
Compaction drive is therefore also a production mechanism, but an un-
desirable one in most oil fields because it leads to surface subsidence. Its
main interest to geologists is that it demonstrates mechanical compaction
and the qualitative validity (at least) of Terzaghi’s relationship, which we
studied in Chapter 3. The seriousness of compaction in an oil field with mul-
tiple reservoirs is well illustrated by the history of the Wilmington field in
California (Mayuga, 1970). This oil field, discovered in 1932 and found to be
one of the largest in North America, lies below the coastal districts of Los
Angeles and the city of Long Beach. The area overlying the field is industrial
and residential, and includes Long Beach harbour and a naval shipyard. By
the end of 1967, more than one thousand million barrels of oil (184 X lo6
m3) and nearly one Tcf (23.8 X 10’ m3) of gas had been produced over 30
years. However, within a decade of the beginning of major production about
1937, when the importance of the discovery became apparent, surface sub-
sidence began to threaten coastal installations. Figure 8-18 shows the pattern
of events up to 1967. Maximum subsidence, over the crest of the field struc-
ture, reached 8.8 m (29 ft), and the maximum subsidence rate was about 0.7
m/yr. This vertical movement, with horizontal displacements up to 3 m,
caused extensive damage on the surface and to oil wells at depth. Measure-
ments in boreholes (see Mayuga, 1970) showed that almost all the compac-
tion had taken place in the producing zones.
The correspondence between production rate and subsidence rate left no
doubt about the cause, so water injection was planned, with a pilot-scheme
starting in 1953. Five years later, when the major scheme started, production
rates could be increased while the rate of subsidence further decreased. In