The efficiency with which oil and gas can be stored and recovered depends not only on the characteristics of fluids in pore spaces but on the properties of pore systems in reservoir rocks. The evaluation of reservoir rocks has been determined by the conventional methods of petrography, X-ray mineralogy and core analysis up to now. In addition to these methods the more detailed and executive estimation for the reservoir quality is effectively performed by means of i) measuring the pore size distribution (i.e, capillary pressure), ii) impregnating blue-colored epoxy resins into pore spaces including in reservoir rocks, and iii) scanning electron microscopic observations of pore systems.
Oil may migrate through water-filled pore spaces in order to accumulate in reservoir rocks, and the migration may be chiefly promoted by buoyant forces but is inhibited by the capillary pressures which must be overcome for an oil globule or filament so as to pass from a rock pore through a pore throat to an adjacent pore space. For a laboratory use capillary pressure can be easily transformed to an equivalent pore-throat size with the equation, Pc=-2γcosθ/γ
where Pc is capillary pressure, γ is surface tension, θ is contact angle of mercury and γ is pore-throat radius. We can get specific surface area values from the measurement of capillary pressure simultaneously with the pore-size distribution, by the following expression empirically,
S=1/γcosθ∫PdV
where S is specific surface area, P is capillary pressure and Vis mercury volume injected in the pore spaces.
According to probability plots of pore-size distribution and specific surface area plots against mercury porosity percent, we may estimate properly the reservoir quality of rocks.
Micropores approximately smaller than a half micron (5, 000 A) have been considered to be ineffective for the oil and gas storing till now. Recently many mineralogists propose the capability of preservation of oil and gas in such micro spaces. In fact such micropores were measured to be over than a few tens of percent occupied even in porosity of excellent volcanic reservoir rocks and fine- to coarsegrained sandstone reservoir rocks in this study.
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