Porosity And Fluid Saturations
Porosity is the best known physical characteristic of an oil reservoir.
It
determines the volume of oil or gas present, and all recovery computations
must
be based on knowledge of its value. Porosity of a material is defined as
that
fraction of the bulk volume of this material that is not occupied by the
solid
framework of the material.In oil reservoirs,the porosity represents
the
percentage of the total space that is available for occupancy by eighter
liquids
or gases.It determines the storage capacity of the sand and is
generally
expressed on a percentage basis or as a fraction or a decimal. One
may
distinguish two types of porosity,namely,absolute and effective:
Absolute
porosity is the percentage of total void space with respect to the
bulk volume
regardless of the interconnection of the pore voids. Effective
porosity is the
percentage of interconnected void space with respect to the
bulk volume.
Effective Porosity Measurements: Grain volume methods:In
these methods the
consolidated sample is solvent extracted and dried;the bulk
volume is determined
eighter by the displacement of a liquid which does not
penetrate the sample or
by saturating the sample and volumetrically
displacing a suitable liquid with
the saturated sample.The grain volume,or
volume of the solid framework of the
sample,maybe measured by the volumetric
displacement of a gas or a liquid,while
the pore volume may be measured by
determining the amount of liguid neccesary to
saturate the sample. It is
obvious that the percentage of porosity may be
calculated from such data by
use of eighter of the two following relationships:
Per cent
porosity=100x(bulk volume - grain volume/bulk volume) OR Per
cent
porosity=100x(pore volume/bulk volume) Bulk volume determination:The
bulk volume
of the extracted and dried samples may be determined by
volumetric displacement
of mercury. Pore gas expansion method:The
measurements of porosity may also be
made by the pore-gas expansion method,or
so-called Washburn-Bunting
method.(1922).This method makes use of a modified
Toepler pump so much in use in
high-vacuum techniques in order to produce the
barometric vacuum and remove air
from a dried core.The bulk volume of the
core must be known from other tests.
Mercury-injection method:When a rock
has a very small fraction of void space,it
is diffucult to measure it by
methods previously discussed.One may then resort
to forcing a noncompressible
liquid into the sample under very high pressure.The
original idea appears to
have been that of Horner(1944). Loss of weight
method:The measurement of the
grain volume of a core sample may also be
determined by the loss in weight of
a saturated sample plunged in a liquid.
Liquid saturation
Method:Measurements of pore volume may also be achieved by
liquid saturation.
Porosity of large core samples:A technique is advocated by
Locke and
Bliss(1950) for the measurement of porosity in large cores,i.e,cores
as
recovered from drilling operations without further reduction in size
by
sampling of small plugs. Absolute Porosity Measurement:In the
determination of
absolute porosity,it is required that all nonconnecting as
well as
interconnecting pores be accounted for. FLUID SATURATIONS: Methods
for the
determination of reservoir fluid saturations in place consist in
analyzing
reservoir core samples for water and oil,the saturation in gas
being obtained by
difference since the sum of the saturations in the three
fluids is equal to
unity. As a general practice a simple procedure is to
determine the water
saturation of a sample by a suitable technique and then
to measure the total
loss of weight of the sample on extraction and
drying.The quantity of oil
present is then calculated by subtracting the
weight of water found from the
total loss in weight.The methods which may be
recommended for this determination
will be discussed briefly. Distillation
method: Water saturation may be
determined by distillation methods such as
the ASTM method and the Dean and
Stark method. Critical solution
temperature method:A fluid saturation method
whick makes use of the critical
solutoin tempereture has been described by
Taylor(1938). Titration
method: Another procedure for saturation determination
makes use of the
titration method. Retort method: the procedures discussed above
are nearly
all combinations or modifications of methoads previously described in
the
availble literature which have been found satisfoctory in handling a
great
variety of core samples with various requirements of rapididty and
accuracy.
Other methods have been tested and found to be less desirable,
such as the
retort method( Yuster and Levine, 1938). The main objection to
the retort
method, still in common use, is the cracking of the oil with the
resluting
production of gases and the possibility of driving out water of
crystallization.
Capillary pressure method: In view of the difficulties
involved in fluid
saturation determinations from cores, Thornton and Marshall
(1947) have proposed
the use of capillary pressure curves (capillary suction
vs. brine saturation) as
a substitute for the direct measurments of connate
water. The procedure involves
the determination of air-water capillary
pressure curves on selected core
samples, the determination of the distance
above water-oil contact at which the
samples were secured, and the
application of a correction in order to reduce the
air-water capillary
presuure curve to an oil-water capillary pressure.
Summarizing, fluid
saturation determinations are probably the least reliable of
the measurements
performed on reservoir rock samples.Perhaps the best approach
to a true
solution of the problem is by correlation of quantitative information
from
various independent measuring means, to of which have been reviewed
before,
namely, by direct measurments of core samples and by indirect
measurments
through the use of capillary pressure curves.