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.

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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.