Monday, August 31, 2015

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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