Porosity is the best known physical characteristic of an oil reservoir. Itdetermines the volume of oil or gas present, and all recovery computations mustbe based on knowledge of its value. Porosity of a material is defined as thatfraction of the bulk volume of this material that is not occupied by the solidframework of the material. In oil reservoirs,the porosity represents thepercentage of the total space that is available for occupancy by eighter liquidsor gases. It determines the storage capacity of the sand and is generallyexpressed on a percentage basis or as a fraction or a decimal.
One maydistinguish two types of porosity,namely,absolute and effective: Absoluteporosity is the percentage of total void space with respect to the bulk volumeregardless of the interconnection of the pore voids. Effective porosity is thepercentage of interconnected void space with respect to the bulk volume. Effective Porosity Measurements: Grain volume methods:In these methods theconsolidated sample is solvent extracted and dried;the bulk volume is determinedeighter by the displacement of a liquid which does not penetrate the sample orby saturating the sample and volumetrically displacing a suitable liquid withthe saturated sample. The grain volume,or volume of the solid framework of thesample,maybe measured by the volumetric displacement of a gas or a liquid,whilethe pore volume may be measured by determining the amount of liguid neccesary tosaturate the sample. It is obvious that the percentage of porosity may becalculated from such data by use of eighter of the two following relationships:Per cent porosity=100x(bulk volume – grain volume/bulk volume) OR Per centporosity=100x(pore volume/bulk volume) Bulk volume determination:The bulk volumeof the extracted and dried samples may be determined by volumetric displacementof mercury. Pore gas expansion method:The measurements of porosity may also bemade by the pore-gas expansion method,or so-called Washburn-Buntingmethod.
(1922). This method makes use of a modified Toepler pump so much in use inhigh-vacuum techniques in order to produce the barometric vacuum and remove airfrom 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,itis diffucult to measure it by methods previously discussed. One may then resortto forcing a noncompressible liquid into the sample under very high pressure. Theoriginal idea appears to have been that of Horner(1944).
Loss of weightmethod:The measurement of the grain volume of a core sample may also bedetermined by the loss in weight of a saturated sample plunged in a liquid. Liquid saturation Method:Measurements of pore volume may also be achieved byliquid saturation. Porosity of large core samples:A technique is advocated byLocke and Bliss(1950) for the measurement of porosity in large cores,i. e,coresas recovered from drilling operations without further reduction in size bysampling of small plugs. Absolute Porosity Measurement:In the determination ofabsolute porosity,it is required that all nonconnecting as well asinterconnecting pores be accounted for.
FLUID SATURATIONS: Methods for thedetermination of reservoir fluid saturations in place consist in analyzingreservoir core samples for water and oil,the saturation in gas being obtained bydifference since the sum of the saturations in the three fluids is equal tounity. As a general practice a simple procedure is to determine the watersaturation of a sample by a suitable technique and then to measure the totalloss of weight of the sample on extraction and drying. The quantity of oilpresent is then calculated by subtracting the weight of water found from thetotal loss in weight. The methods which may be recommended for this determinationwill be discussed briefly. Distillation method: Water saturation may bedetermined by distillation methods such as the ASTM method and the Dean andStark method. Critical solution temperature method:A fluid saturation methodwhick makes use of the critical solutoin tempereture has been described byTaylor(1938).
Titration method: Another procedure for saturation determinationmakes use of the titration method. Retort method: the procedures discussed aboveare nearly all combinations or modifications of methoads previously described inthe availble literature which have been found satisfoctory in handling a greatvariety of core samples with various requirements of rapididty and accuracy. Other methods have been tested and found to be less desirable, such as theretort method( Yuster and Levine, 1938). The main objection to the retortmethod, still in common use, is the cracking of the oil with the reslutingproduction of gases and the possibility of driving out water of crystallization.
Capillary pressure method: In view of the difficulties involved in fluidsaturation determinations from cores, Thornton and Marshall (1947) have proposedthe use of capillary pressure curves (capillary suction vs. brine saturation) asa substitute for the direct measurments of connate water. The procedure involvesthe determination of air-water capillary pressure curves on selected coresamples, the determination of the distance above water-oil contact at which thesamples were secured, and the application of a correction in order to reduce theair-water capillary presuure curve to an oil-water capillary pressure. Summarizing, fluid saturation determinations are probably the least reliable ofthe measurements performed on reservoir rock samples.
Perhaps the best approachto a true solution of the problem is by correlation of quantitative informationfrom various independent measuring means, to of which have been reviewed before,namely, by direct measurments of core samples and by indirect measurmentsthrough the use of capillary pressure curves.
