Page 50 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 31

correspondence between rock matrix and pore characteristics, how they are geneti-

cally and temporally related, and how they influence petrophysical attributes. Poros-
ity is measured directly from core samples and indirectly with some types of borehole

logs. Permeability is measured as the coefficient of proportionality in Darcy ’ s equa-

tion for fl uid flow through porous media. It is measured directly from core samples
and it is the yardstick by which many quality rankings are assigned to reservoirs.

Special wireline testers and pressure buildup tests can measure flow rates and
provide meaningful estimates of permeability and petrophysical experts argue that
permeability can be estimated from wireline log data. Not everyone agrees,
especially those who work on carbonate reservoirs. Bulk density is a measure of
the solid/void ratio in reservoir rocks and is measured directly in core analyses or
indirectly with wireline logs. Bulk density values can be used to aid in estimating
porosity.

2.4.1 Porosity
Reservoir rocks consist of solid material and interstitial pore spaces that may or
may not be connected, such that

V p = Pore volume
V s = Solid volume
V t = Total rock sample volume = V p + V s

Porosity is usually designated by the symbol φ and is expressed as a percentage

φ = (VV t ) ×100
p /
Reservoir specialists are primarily concerned with the fraction of total porosity

that transmits fluids, that is, the interconnected or effective porosity, φ e Effective
.
porosity is the ratio of the interconnected pore volume to the total rock volume.
Direct measurements of V p in the laboratory are measurements of effective porosity.
Not all pores are interconnected, however. Unconnected porosity is called residual
. Total porosity is the quantity
porosity, φ r , so that total porosity is the sum of φ e + φ r
derived from borehole measurements made with the various “ porosity logging ”
devices (Monicard, 1980 ). Porosity varies with texture, fabric, and fracture geometry
in the reservoir rock. Grain shape, sorting, and packing are the main variables that
affect porosity in detrital rocks , growth fabric and skeletal microstructure affect
inter - and intraparticle porosity in biogenic rocks , and porosity in fractured rocks is
determined by fracture width, fracture spacing, and presence/absence of mineraliza-
tion. Diagenesis may plug pores with cement, close pores with compaction, open
pores with dissolution, or create new pores by recrystallization or replacement.
Berg (1970) illustrated the geometrical relationship between pore size and grain
size with identical spheres in a packing arrangement with about 30% porosity.
Although the example is unrealistic in terms of “ real world ” reservoir rocks, it is a
useful demonstration of the relationships between fundamental rock properties,
texture in this case, and pore characteristics. Imagine that Berg ’ s idealized example
is an oolite grainstone with well - sorted spherical grains and unaltered depositional
porosity. Depositional pore size is a function of grain size, packing, and sorting.