7/29/2019 The operations that prepare a well bore for producing
oil or gas from the reservoir
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The operations that prepare a well bore for producing oil or gas
from the reservoir. The goal
of these operations is to optimize the flow of the reservoir
fluids into the well bore, up
through the producing string, and into the surface collection
system. See alsoOil and gasfield exploitation;Oil and gas well
drilling.
The well bore is lined (cased) with steel pipe, and
theannulusbetween well bore and casingis filled with cement.
Properly designed and cemented casing prevents collapse of the
well
bore and protects fresh-water aquifers above the oil and gas
reservoirs frombecomingcontaminatedwith oil and gas and the oil
reservoirbrine. Similarly, the oil and
gas reservoir is prevented from becoming invaded
byextraneouswater from aquifers thatwere penetrated above or below
the productive reservoir. See alsoAquifer.
The nature of the reservoir, evaluated from a core analysis,
cuttings, or logs, or from
experience with like productive formations, determines the type
of completion to be used.In abarefoot completion, the casing is set
just above the producing formation, and the
latter is drilled out and produced with no pipe set across it.
Such a completion can be used
for hard rock formations which are notfriableand will notslough,
and when there are noopportunities for producing from another,
lower reservoir. Set-through and perforatedcompletions are also
employed for relatively well-consolidated formations from which
the
potential for sand production is small. However, theperforated
completionis used when a
long producing interval must be prevented from collapse, when
multiple intervals are to be
completed in the one borehole, or when intervening water sands
within the oil-producinginterval are to be shut off and the
oil-saturated intervals selectively perforated. SeealsoWell
logging.A string of steel tubing is lowered into the casing string
and serves as the conduit for theproduced fluids. The tubing may be
hung from the well-head or supported by a packer set
above the producing zone. The packer is used when it is
desirable to isolate the casing
string from the produced fluids because of the latter's
pressure, temperature, or corrosivity,or when such isolation may
improve production characteristics.The tops of wells from which
fluids flow as a result of the indigenous reservoir energy are
equipped with amanifoldknown as the Christmas tree. However,
only some reservoirs havesufficient pressure and sufficient gas in
solution (which is released at the lower pressureexisting in the
well bore and therefore lowers the effective density of the fluid
in the tubing)
to permit natural flow to the surface. The reservoir fluids from
other reservoirs and,afterpressure depletion, even from those which
initially flowed must be brought to thesurface by one of several
methods of artificial lift.Excessive water production increases the
cost of oil production since energy must be
expended in lifting the water to the surface. Water production
may alsojeopardizetheproduction of oil and gas bysaturatingthe
oil-productive interval with water. Such damageis more likely to
occur in low-pressure formations or formations which contain
water-
sensitive clays thatswellin an excess of water. Water exclusion
may be effected by the
application of cements of various types. If it is determined
that water is entering from thelower portion of a producing sand in
a relatively shallow, low-pressure well, a cement plugmay be placed
in the bottom of the hole so that it will cover the oil-water
interface of the
reservoir. This technique is called laying in a plug and may be
accomplished by placing thecement with a dump-bottombaileron a wire
line or by pumping cement down the drill pipe
or tubing. For deeper, higher-pressure, or moretroublesomewells,
asqueezemethod isused. Squeeze cementing is the process of applying
hydraulic pressure to force a cement
into an exposed formation or through openings in the casing or
liner. It is also used forrepairing casing leaks.
Production may be impaired from a well bore as a result of
drilling-mud invasion or of
accumulation of clays and fine silts carried by the producing
fluids to the borehole, orthelithologyof the formation itself may
have a naturally lowpermeabilityto reservoir fluids.Since the
permeability to fluids of the formation within the first few feet
of the well bore has
an exponential effect on limiting the influx of fluid, the
productivity of a well can frequently
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