Page 46 - Primer on Enhanced Oil Recovery

Reservoir mineralogy and rock- 5

fluid interactions

Abstract

The hydrocarbons and connate water are contained in porous rock. Rock fluid interac-
tions, fluid properties and the rock porosity need to be well analysed to enhance oil recov-
ery. As a single phase oil flow through the porous media can be predicted by Darcy’s
Equation. Unfortunately the simple equation very fast fails in the real multiphase flow.
The liquids flow is also affected by the partial saturations and partial permeabilities.
Complete liquid extraction from the porous media at the oil field is impossible due to the
capillary forces.

An oil field is a combination of hydrocarbon deposits confined to one or several
traps. The traps are connected by a common geological structure. The hydrocarbon
deposit is a rock with hydrocarbons in the rock pores. The reservoir rock consists of
cemented mineral grains. The porous rock with oil can be simply represented as
shown in Fig. 5.1.
The grains can have various sizes and shapes. Not all grains would have the
same size, mineralogy and origin. Overall grain structure and composition can
change in vertical and horizontal directions or in any part of the deposit. This fact
is referred as heterogeneity. Consolidated grains inevitably have some spaces
between them and the space in an oil formation is filled with oil and connate water.
Presence of grains also has another important manifestation the material with
grains and porosity has very high surface area. It is known that for the sandstone
2
surface area can reach 5000 m /kg, while for shales it even can be up to 20 times
2
bigger than this and can reach 100,000 m /kg. This is an enormous area, a shale
cube with a site just around 3.5 m will have bigger surface area than London.
What does not change in principle that in order to contain liquids the rock should
have porosity. The porosity is some volume in-between grains, rock voids and
cracks. In general, the porosity is defined as an amount of empty volume in the
material (rock) expressed in the percent form:

V o
φ 5 3 100% (5.1)
V r 1 V o
where V 0 is volume of all pores, V r is volume of the solid matter. In the sedimenta-
tion process porosity depends on the grain shape and size distributions. If the grains
have the same size (we assume them spherical and use so named Kepler conjecture)

Primer on Enhanced Oil Recovery. DOI: https://doi.org/10.1016/B978-0-12-817632-0.00005-0
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