The Decline and Ultimate Production of Oil Wells, with Notes on the Valuation of Oil Properties Page: 16
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16 DECLINE AND ULTIMATE PRODUCTION OF OIL WELLS.
SATURATION MIIETIHIOD.
BASIS OF METHOD.
The saturation method is based on several factors the values of
which are uncertain, these factors being the porosity, the thickness,
the extent, and the saturation of the oil sand. From these the per-
centage of oil that may be recovered is estimated. The capacity of
an oil sand of uniform thickness and porosity can be determined
with fair accuracy from samples of the sand from different wells,
especially if the wells are scattered over the area and several deter-
minations are made from a number of representative samples. The
most difficult determination in this method is the proportion of the
total oil content that may be recovered from the sand. This is closely
related to the error resulting from estimating the saturation of the
oil sand.
OIL CONTENT OF SAND AND OIL RECOVERED.
Usually more oil is left underground than is brought to the sur-
face; in fact, only a small percentage of the total oil content of a
sand is ordinarily recovered. The so-called recovery factor varies
widely according to the conditions controlling the production of oil
and gas. Hence an arbitrary value nearly always has to be assigned
to the recovery factor. As the accuracy of the whole procedure is
thereby greatly reduced, the estimate becomes little more than a
guess.
The difference between oil content and the amount of oil that may
be recovered (the ultimate production) must be kept carefully in
mind. The factors that govern the amount of oil present in a sand
of course control the amount of oil that can be recovered from it.
But other factors must be studied. The recoverable oil, or the ulti-
mate production of a sand underlying an area, is the quantity that
may actually be taken from the sand rather than the amount present
in it. This recoverable oil is a percentage of the total oil content
and varies with the conditions of occurrence of the oil and the condi-
tions of production. Unquestionably a much larger percentage of
the oil content can be recovered from a coarse porous sandstone sub-
jected to a high gas pressure than from a fine-grained denser sand-
stone under no great pressure. The coarse porous sand offers little
resistance to the flow of the oil toward the well, whereas, with a
fine sand and a low gas pressure, the production is retarded, both by
the greater frictional resistance and the lack of expulsive force.
Undoubtedly the recovery factor varies with the conditions between
these two extremes.
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Beal, Carl H. The Decline and Ultimate Production of Oil Wells, with Notes on the Valuation of Oil Properties, report, 1919; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc12371/m1/28/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.