Invasion Profile: Resistivity
This post is an elaboration of the "Mud Invasion Profile" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Mud Invasion Profile" slide at page 4
What is Resistivity?
Before going further to
resistivity, we should know what resistance is first. Resistance can be defined
as voltage divided by the current. The concept of resistance is parallel to the
concept of ”friction” in mechanical. It
defines how hard electric current can pass. The Ohm's Law states that ideally the
ratio of voltage and current are always constant. That’s why it is believed
that the resistance depends on material and condition being used to flow the
current. Unlike density, which will not change as the material is cut,
resistance will change if the material’s geometry or volume changes. That is
why more than just the material, the geometrical properties also affect the
resistance.
After we know about resistance,
we’ll talk about resistivity. Resistivity is the resistance of a unit material
in a certain volume. Imagine that we have a cube of ice. When we apply current
which flow through the cube (for instance, from left side to right side), resistivity
can be known by measuring the voltage for the unit cube's length, (for instance
the distance from left side to right side) Volts per meter, V/m AND by knowing the
current which flow through the cube's cross sectional area (the area of right
side) Amps per meter squared, or A/m2. Dividing these two terms will result in
units of Ohm-m2/m or Ohm-m.
Why do we need to calculate resistivity?
Water is such a good solvent it
almost always has some solute dissolved in it. Even the de-ionized water will
always have some part of ion solute. Specifically
ground water, always contain ion which is most likely salt. When water has salt,
it can conduct electricity readily, since salt separate into free ions in
aqueous solution by which an electric current can flow. That is why water has
low resistance.
Unlike water, organic compounds
like oil, phenol, alcohol, and sugar do not conduct electrical current very
well and therefore have a low conductivity when the current applied to them,
which means high resistance. From this point things are getting interesting.
Because we know the resistance of
oil is high and water is low, then we may conclude that the resistivity of rock
containing oil will be high and rock containing water will be low. That is why
formation resistivity is very important to know whether the formation contains
oil or water. We don’t want to produce water, since we have enough water from
the ocean, so we need a tool to see which part of the ground contains oil, by
seeing the resistivity. When the tools detect rock which has low resistivity,
we may believe that the rock contains hydrocaarbon.
How do we know the resistivity, and how the resistivity measurement
tool works?
If only we had created a tool
which accurately locates the exact location of rock containing oil, we have
been in the world war 3, since every country may see which other countries have
more oil, and force them to give their oil. Fortunately, the best man can make
is a tool to locate the oil location after they drill the correct location. Only
less than 70% exploration drilling in America find oil zone nowadays. That’s
why after they drill, they put in a tool to the wellbore that may show which formation
layer contains petroleum. As discussed above, this tool should be able to
measure the resistivity of the rock, and this tool is called resistivity log.
The simple explanation, resistivity log works by injecting current into the
formation from a single electrode, then the current spread out radial from the
electrode. Other electrodes, which is measuring electrodes approximates the
measurement of a constant-voltage spherical shell around the injection
electrode. The measurements of voltage and current are converted to a
resistivity measurement.
Why we need to differentiate resistivity in a borehole?
A drop of poison will alter a
glass of milk. In drilling a well, we need to put mud, which has different
resistivity from the formation water or maybe the oil (if the formation
contains oil). That’s why measurement of resistivity from the borehole must not
represent the true resistivity of the formation, since mud, mudcake, flushed
zone, and zone of transition will affect the measurement. There is no direct
measurement to get resistivity of the formation. This problem should be handled
so the true resistivity can be obtained.
How do we differentiate resistivity in a borehole?
The resistivity
measuring log can be divided based on the depth of measurement to the formation
around them that they can investigate. There are logging tools which are
specifically made to measure resistivity of flushed zone (e.g. Microlog,
microlateralog), invaded zone (e.g. short normal, spherically focused log) and
uninvaded zone (e.g. deep induction log, long normal). Flushed zone measurement
is corrected for the influence of the mudcake by estimating mudcake thickness
and resistivity. invaded and uninvaded zone resistivity measurements are
corrected for environmental effects using the charts for the tool used. There
are corrections in measurement since the borehole size and bed thickness
variations may affect the measurement. Then charts are used to know parameters
of resistivity in every zone.It is like service companies will use charts to
know the resistivity of the uninvaded zone, which is the most important
measurement, with the correction of resistivity measurement in flushed zone to estimate
the real uninvaded zone resistivity.
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