Page 115 - Petrophysics

88 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

predicted from variation in grain size and shape, and from distribution of
pore channels in the rock.
The resistivity of any formation to the electrical current flow is a
function of the amount of water in that formation and the resistivity of the
water itself. The rock grains and hydrocarbons are normally insulators.
Changes in water saturation combined with changes in the resistivity of
the fluids filling the pores create resistivity profiles in well logs. These
profiles help locate hydrocarbon-bearing formations.

POROSITY

Sand grains and particles of carbonate materials that make up sandstone
and limestone reservoirs usually never fit together perfectly due to the
high degree of irregularity in shape. The void space created throughout
the beds between grains, called pore space or interstice, is occupied by
fluids (liquids and/or gases). The porosity of a reservoir rock is defined as
that fraction of the bulk volume of the reservoir that is not occupied by the
solid framework of the reservoir. This can be expressed in mathematical
form as:

where:

41 = porosity, fraction.
Vb = bulk volume of the reservoir rock.
Vgr = grain volume.
Vp = pore volume.

According to this definition, the porosity of porous materials could
have any value, but the porosity of most sedimentary rocks is generally
lower than 50%.

EXAMPLE

A clean and dry core sample weighing 425 g was 100% saturated with a
1.07 specific gravity (y) brine. The new weight is 453 g. The core sample
is 12 cm long and 4 cm in diameter. Calculate the porosity of the rock
sample.