Page 316 - Petrophysics 2E

FORMATION EVALUATION 287

either the progress or evaluation of an individual well [46]. The ultimate
aim of the well log interpretation, however, is the evaluation of potential
productivity of porous and permeable formations encountered by the
drill.
Electrical logging was introduced to the oil industry by Marcel and
Conrad Schlumberger in 1927 in France. Since then, due to considerable
technological and scientific advances, well logs have undergone constant
and sweeping changes. The development of recording techniques
compatible with the application of computers in well log interpretation
has removed a large number of earlier assumptions and general estimates
from well log computations. The result has been a change from a cor-
relation tool for geologists to an indispensable data source for the oil
industry. A successful logging program, along with core analysis, can
supply data for subsurface structural mapping, define the lithology,
identlfy the productive zones and accurately describe their depth and
thickness, distinguish between oil and gas, and permit a valid quanti-
tative and qualitative interpretation of reservoir characteristics, such
as fluid saturation, porosity, and permeability. Unfortunately, these
petrophysical properties cannot be measured directly and, therefore,
they must be inferred from the measurement of other parameters of the
reservoir rock, such as the resistivity of the rock, the bulk density, the
interval transit time, the spontaneous potential, the natural radioactivity,
and the hydrogen content of the rock [47].

Water Saturation
Evaluation of the amount of hydrocarbons present in the reservoir is
based on the ability of the log analyst to estimate the volume of water
present in the pore space. This requires the solution of some form of
Archie equation for the water saturation parameter S,. Because of its
simplicity and worldwide use, the Rwa method for determining S, is
the only one presented here. Water saturation in the uninvaded zone
of a clean sandstone formation having intergranular or intercrystalline
porosity can be estimated from Equation 4.53:

(4.144)

In a 100% water-saturated sand, i.e., IR = 1 and Rt = &, the water
resistivity is equal to:

Rt
R, = - (4.145)
FR