Page 17 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

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formation will produce increasing amounts of water relative often have different values. Resistivity is measured by
to hydrocarbons. electric logs.
Water saturation—is the percentage of pore volume in a Conrad Schlumberger in 1912 began the first experiments
rock which ts occupied by formation water. Water saturation which led, eventually. to the development of modern day
is measured in percent and has the symbol S,. petrophysical logs. The first electric log was run September
5. 1927 by H. G. Doll in Alsace-Lorraine, France. In 1941,
formation water occupying pores
water saturation (Sy) = = —-—— : G. E. Archie with Shell Oil Company presented a paper to
. total pore space in the rock
the AIME in Dallas, Texas, which set forth the concepts
Water saturation represents an important log interpretation used as a basis for modern quantitative log interpretation
concept because you can determine the hydrocarbon (Archie, 1942).
saturation of a reservoir by subtracting water saturation Archie's experiments showed that the resistivity of
a
from the value, one (where |.0 = 100% water saturation). water-filled formation (R,). filled with water having a
Irreducible water saturation or Sy jp, is the term used to resistivity of R, can be related by means of a formation
describe the water saturation at which all the water is resistivity factor (F):
adsorbed on the grains in a rock, or is held in the capillaries
R, Ry
=
FX
by capillary pressure. At irreducible water saturation, water
will not move, and the relative permeability to water equals where the formation resistivity factor (F) is equal to the
zero. resistivity of the formation 100% water saturated (R,)
—is
Resistivity the rock property on which the entire divided by the resistivity of the formation water (R,,).
science of logging first developed. Resistance is the Archie's experiments also revealed that formation factors
inherent property of all materials, regardless of their shape can be related to porosity by the following formula:
and size, to resist the flow ofan electric current. Different
materials have different abilities to resist the flow of
electricity.
Resistivity is the measurement of resistance; the where m is a cementation exponent whose value varies with
reciprocal of resistivity is conductivity. In log interpretation. grain size, grain size distribution, and the complexity of the
hydrocarbons, the rock, and freshwater all act as insulators paths between pores (tortuosity). The higher the value for
and-are, therefore, non-conductive and highly resistive to tortuosity the higher the m value.
electric flow. Saltwater, however, is aconductor and has a Water saturation (S,,) is determined from the water filled
low resistivity. The unit of measure used for the conductor resistivity (R,) and the formation resistivity (R,) by the
is acube of the formation one meter on each edge. The following relationship:
measured units are ohm-meter2/meter, and are called
ohm-meters.

where n is the saturation exponent whose value varies from
1.8 to 2.5 but is most commonly 2.
Where: By combining the formulas: R, = F x Ry
R = resistivity (ohm-meters) and S,, = (R,/R,)!/" the water saturation formula can be
r = resistance (ohms) rewritten in the following form: Un
measurements recorded at different depths into a formation This is the formula which is most commonly referred to as
A = cross sectional area of substance being measured
FX Ry
(meters?)
Ww
R,
L = length of substance being measured (meters)
Resistivity is a basic measurement of a reservoir’s fluid
the Archie equation for water saturation (S,,). And. all
saturation and is a function of porosity, type of fluid (i.e.
present methods of interpretation involving resistivity
hydrocarbons, salt or fresh water), and type of rock.
curves are derived from this equation.
Because both the rock and hydrocarbons act as insulators
Now that the reader is introduced to some of the basic
but saltwater is conductive, resistivity measurements made
concepts of well log interpretation, our discussion can be
by logging tools can be used to detect hydrocarbons and
continued in more detail about the factors which affect
estimate the porosity of a reservoir. Because during the
logging measurements.
drilling of a well fluids move into porous and permeable
formations surrounding a borehole, resistivity
Where a hole is drilled into a formation, the rock plus the
Borehole Environment