Page 28 - MSc_thesis_R A Kil

3.3 Reservoir properties

The link between a sedimentary facies and the behaviour as a hydrocarbon producing reservoir can be
described with a number of parameters, including porosity and permeability.

Porosity is defined as the volume of open space (void space) as a fraction of the total volume of a piece of
rock. The effective porosity is used to exclude any non-connected pores, or in other words, pores that do
not contribute to the total amount of fluid that can potentially be extracted. For a porosity - permeability
relation an effective porosity is preferred, as this represents the amount of pore space contributing to the
flow trough a porous medium (Dake, 1978). Porosity is measured as either a percentage or a fraction,
and is therefore dimensionless. For calculation matters, a fraction to describe porosity is favourable, so
this will be the unit used for porosity in this report.

Permeability is a parameter that describes the ability of a fluid to flow trough a porous material. In the
oil industry this refers to the flow of oil, gas or water trough a reservoir rock. This parameter can then be
used in for example Darcy’s law to describe flow through a hydrocarbon bearing reservoir (Dake, 1978).
Permeability is measured in square meters [m2], and is therefore a sort of representative cross-sectional
area. Because permeability expressed in square meters leads to very small numbers it is more common to
use darcy [D] - one darcy is 10-12 m2 - or in most cases even millidarcy.

3.3.1 Porosity

Porosity can be estimated by using 1) an ultrapycnometer which measures the matrix volume; 2) the
measurement of a saturated core submerged in water; or 3) thin section analysis. Using multiple
measurement procedures has the advantage that a quality check can be done when comparing the results
of different methods. For simplicity, only the results from the ultrapycnometer will be used — this gave
the most accurate results — with a few quality-checks from other methods.

An ultrapycnometer uses a controlled volume of Helium gas to measure the matrix volume Vma of a plug.
The volume V of the sample container is reduced by the matrix volume when the core is placed inside the
chamber. The volume of gas is exposed to isothermal expansion from pressure p1 to p2, so the following
equation can be applied:

(V − Vma)p1 = (V − Vma + ∆V )p2 (3.1)

The chamber volume V is entered when calibrating the device. During the measurement procedure values
of p1, p2 and ∆V are recorded. The output consists of a series of matrix volumes and as a quality-check
matrix densities (derived from the weight of the plug and Vma). Porosity can then finally be calculated

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