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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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be increasedmanyfoldby increasing the permeability of this element of the reservoir or
removing the skin just at the face of the producing interval. This is accomplished by
acidization and fracturing, and in some instances by the use of surfactants, solvents, andexplosives.
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