Mineral Facts and Problems: 1965 Edition Page: 673
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PETROLEUM AND NATURAL GAS
the well and attached to the casinghead. An
assembly of valves and variable chokes are
placed above a master valve on the tubing head,
referred to as the "Christmas Tree." This
assembly is arranged to direct the fluid flow
from the well to lease facilities.
Acidization is frequently applied to acid-
soluble carbonate reservoir rocks, such as lime-
stone and dolomite, to open up or enlarge flow
channels to the wellbore by dissolving barriers
and restrictions in the formation. Hydro-
chloric acid is basically the main constituent
because it is the most efficient and economical
acid available for permeability improvement.
Acids are modified in advance to suit the
One of the most outstanding well stimula-
tion methods for increasing well productivity
is hydraulic fracturing. Many fields are in
existence today because of these fracturing
techniques for, without them, many producing
horizons would very probably have been by-
passed in the past 15 years as either barren or
commercially nonproductive. Hydraulic frac-
turing of formations is produced by the appli-
cation of sufficient hydraulic pressure to the face
of the exposed subsurface productive formation
to induce fractures in the formation. The
technique is a means of increasing the effective
permeability and is particularly applicable to
formations with naturally low permeability.
In thick producing sections, parts of the sec-
tions are isolated and fractured separately to
insure maximum effectiveness.
The most common fracturing fluid used is
lease oil. Although oil is not always the most
satisfactory, it can be used more universally
than other fluids. Fracturing fluids are classi-
fied into the following general groups: (1)
fluids, (2) gels, and (3) emulsions. Fluids
used are oils, fresh water, sait water or oilfield
brines, and acids. Gels used are composed of
oil, water, or acid with a proper chemical added
to the base fluid to increase its viscosity. Emul-
sions are comprised of two fluid phases: (1) oil
and (2) water or acid. They are formed by
mixing the two phases together in correct pro-
portions in the presence of an emulsifying agent
to form either oil-in-water or water-in-oil
In addition to fluids, the fracturing medium
contains a propping agent to hold the fracture
open to increase effective flow of formation
fluids to the wellbore. Sand has been used al-
most universally for this purpose. Recently,
glass pellets, aluminum pellets, nutshells, or
plastics have been introduced as propping
A permanent completion involves placing the
lower end of the well tubing above one or more
potential producing intervals and subsequently
performing all completion and recompletion
work with small tools that pass through the
tubing. After the well easing has been set
and cemented, this type completion can be used
with any common size casing and tubing, pro-
vided the well is properly equipped.
Multiple completion is a technique which per-
mits two or more reservoirs to be produced si-
multaneously, but separately through the same
wellbore. Most multiple completions are used
only for production. Many wells of this type
serve as multiple injection wells for pressure
maintenance or secondary recovery projects.
Others are used for the injection of fluid into
one or more reservoirs while maintaining pro-
duction from others.
Recovery of Petroleum
Oil and gas accumulates underground in res-
ervoirs of porous rock masses which have imper-
meable rock boundaries. In such a reservoir
generally, free gas, if present, will occupy the
space above the oil, and water, if present will
occupy the space below the oil.
In any system fluids flow from regions of
higher to lower pressure, and within a reser-
voir the flow is toward the producing wells.
This movement is the result of: (1) propulsive
action of gas either free or in solution in the
oil which expands by lowered pressure to force
the oil and gas through the pore spaces in the
reservoir rocks; (2) displacement action of
water from adjoining parts of the sand; and (3)
gravity drainage. The space vacated by the
removal of oil is filled by expansion and move-
ment of the remaining oil, gas, and water. The
general reservoir types are classed according to
the major recovery mechanism.
(1) Gas-drive fields are reservoirs where oil
accumulations are overlain by a gas cap. Move-
ment of the oil results primarily from pressure
of gas within the gas cap, supplemented by ex-
panding gas liberated from solution in the oil.
In this mechanism, the pressure reduction ac-
companying oil production allows the com-
pressed gas in the gas cap to expand, to invade
the oil zone downstructure, and to drive oil
toward regions of reduced pressures at the pro-
ducing wells. Under efficient operations a gas
cap drive is capable of yielding approximately
20 to 60 percent of the original oil in place, de-
pending on the size of the gas cap.
(2) Water-drive fields are reservoirs in which
the space voided by the oil produced is refilled
entirely or in part by water from the adjoining
parts of the sand. No "free" gas is present in
the reservoir as long as formation pressures
are maintained above the saturation point; that
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United States. Bureau of Mines. Mineral Facts and Problems: 1965 Edition, report, 1965; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc12786/m1/679/: accessed March 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.