Page 428 - Hydrocarbon Exploration and Production Second Edition

Managing Decline 415

A promising technique currently under development is downhole separation
whereby a device similar to a hydrocyclone separates oil and water in the wellbore.
The water is subsequently pumped into a zone beneath the producing interval and
only the oil is produced to surface.
In stacked reservoirs, such as those found in deltaic series, it is common to find
that some zones are not drained effectively. Through-casing logs such as thermal
neutron and GR spectroscopy devices can be run to investigate whether any layers
with original oil saturations remain. Such zones can be perforated to increase oil
production at the expense of wetter wells.
In high-permeability reservoirs, wells may produce dry oil for a limited time
following a shut-in period, during which gravity forces have segregated oil and
water near the wellbore. In fields with more production potential than production
capacity, wells can be alternately produced and shut-in (intermittent production or
cycling) to reduce the field water cut. This may still be an attractive option at
reduced rates very late in field life, if redundant facilities can be decommissioned to
reduce operating costs.

17.4.2. Gas handling
As solution gas drive reservoirs lose pressure, produced GORs increase and larger
volumes of gas require processing. Oil production can become constrained by gas
handling capacity, for example by the limited compression facilities. It may be
possible to install additional equipment, but the added operating cost towards the
end of field life is often unattractive, and may ultimately contribute to increased
abandonment costs.
If gas export or disposal is a problem, gas re-injection into the reservoir may be an
alternative, although this implies additional compression facilities. Gas production
may be reduced using well intervention methods similar to those described for
reducing water cut, though in this case up-dip wells would be isolated to cut back
gas influx. Many of the options discussed under ‘water treatment’ for multilayered
reservoirs apply equally well to the gas case.
In some undersaturated reservoirs with non-commercial quantities of gas but too
much to flare, gas has be used to fuel gas turbines and generate electricity for local use.

17.5. Incremental Development

Most oil and gas provinces are developed by exploiting the largest fields first,
since these are typically the easiest to discover. Development of the area often
involves installing a considerable infrastructure of production facilities, export
systems and processing plant. As the larger fields decline, there may be considerable
working life left in the infrastructure which can be exploited to develop smaller
fields that would be uneconomical on a stand-alone basis. If a satellite development
utilises a proportion of the existing process facilities (and carries the associated
operating costs), it may allow the abandonment rate of the mature field to be
lowered and extend its economic life.