WELLOG ELECTRIC LOG (E-LOG)
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All Rights Reserved
WHEN DID ELECTRIC LOGGING BEGIN?
WHY IS ELECTRIC LOGGING IMPORTANT?
Whether drilling for water, oil, minerals, environmental monitoring or exploration, the E-Log (short for Electric Log) is a fundamental tool for resistivity logging to obtain important information on what is in the earth below. In many applications, the E-LOG is the only log needed! A resistivity log is the first log ran on a new well and is considered the base log on which further logging and Engineering is based. As new resistivity methods are introduced, they will never make this fundamental method obsolete! Electrical resistivity is popular because it is a simple, low cost and efficient method. It is without doubt the most practical, cost-effective logging method available today.
Electrical resistivity is a fundamental geophysical method used in both SURFACE and SUBSURFACE geophysics. The method is legendary among Geophysical methods for exploration, development and definition of existing targets.
Note: Several sources have recently announced that “electric logging is obsolete”! The Electric logging method for measurement of resistivity has been in use for nearly a century world-wide. It is used in logging thousands of wells in the oil, water, mining, and environmental industries every day.
It is without question the most important method in use today.
WHAT ARE THE APPLICATIONS OF ELECTRIC LOGGING?
In metal mining, the measurement of resistivity (inverse of conductivity) is related to metallic/sulfide content of the rock materials and host rock formations. Quantitative information is provided on relative concentration of mineralized zones including porphyry targets.
In oil and gas well drilling, resistivity is a measurement of the electro-chemical content of the pore space of the formations surrounding the well. Oil and gas to water saturation is determined by measuring resistivity. Interpretation of resistivity data is routinely used to define the ultimate value of an oil well.
In water well drilling, resistivity is a measurement of the electro-chemical content of the pore space of the formations surrounding the well. Water quantity and quality are determined by measuring resistivity. Static water level is defined thru interpretation of resistivity data. Bedrock is considered the bottom of a potential water bearing zone called an aquifer. When the thickness of the water producing zone has been identified and the porosity is determined, then the quantity of water is known. Water varies in quality from fresh water with little or no conductive impurities to salty brine that is very conductive. Water quality – a measure of total dissolved solids can be derived from resistivity. Learn more about TDS.
Another measurement that is often obtained with an E-log is call Spontaneous Potential (SP).
View a section of E-log showing SP and resistivity curves.
Total Dissolved Solids (TDS) in water is directly related to conductivity. Conductivity ( C ) is the inverse of resistivity (r).
Resistivity, r = 1/C and Conductivity, C = 1/r.
1 Mho-meter (mho-m) = 1/1 Ohm-meter (ohm-m) = 1 Siemen (S)
As total dissolved solids increase, so does conductivity. The conduction is directly related to free mobile ions in solution. For this reason, electric logs are only ran in wells containing conductive fluid that can provide an electrical connection to the surrounding formations.
In the Coal mining industry, during exploration or definition of bed boundaries and thickness, resistivity logging is used to mark bed boundaries and indicate quality of coal.
Coal Bed Methane:
In Coal Bed Methane exploration, an E-log helps in stratigraphic correlation of coal seams.
Electric logging is based on Ohms Law. Ohms law relates electrical current, voltage and resistance in a mathematic formula that looks like this:
R = V/I
Where: V = Voltage (in volts)
I = Current (in amperes)
R = Resistance (in ohms)
It is important to note that resistance is not the same as resistivity!
Resistivity (r) includes the geometry (G) of the electrodes used in the measurement of resistance.
r = V/I x G
Where r = resistivity (in ohm-meters or ohm-feet)
G = Geometric factor (meters or feet)
When resistivity is measured in a system that uses a geometric factor in meters, then resisistivity is in units of ohm-meters. When feet are used, then the geometry results in resistivity in terms of ohm-feet.
A typical e-log tool having a spacing of 16 inches is considered to be 40 centimeters (.4 meters).
The electrode geometric factor is defined as: G = 4 * p * .4 = 5.0265
Note p = (pi) = 3.14159 (approximately)
Resistivity is measured in ohm-meters. View resistivity [model].
Learn more about Resistivity and how it can be used to measure porosity on the WELLOG Interpretation Page.
View an [E-log tool]
NORMAL ELECTRODE ARRAY:
Typical borehole logging systems provide 16 inch “short” Normal and 32 or 64 inch “Long” Normal resistivity measurements. The electrode spacing is measured from the center of the “A” electrode at the bottom of the tool to the center of the “M” electrode; in either case 16, 32, or 64 inches.
Resistivity measurements are frequently combined with Spontaneous Potential (SP) and/or Natural Gamma to provide a four or five curve log. This combination gives Lithology, resistivity and porosity. Porosity is derived from resistivity.
ADVANTAGES OF DIFFERENT SPACING:
Defining bed boundaries:
The advantage of 64 inch spacing is that resistivity is measured deeper into the formation. The advantage of 16 or 8 inch spacing is that thin beds are better defined.
When invasion is important, It is necessary to measure at least two or more horizontal depths. Using two or more electrode spacings it is possible to define zones that have invasion and show permeability and porosity. Two resistivity curves can show relative “ability of formations to produce”. When resistivies from two spacings are the same value, then it may be assumed that no invasion has occurred. Invasion curves are available for determination of invasion related to differences in deep and shallow resistivity.
The Elog Normal resistivity measurement is affected by bed thickness. Correction charts are used to correct the logged values of resistivity.
LATERAL ELECTRODE ARRAY:
The lateral array uses an electrode at the top of the tool called the “A” electrode.
Another electrode (cable armor) located 50 feet above the “A” or at the surface called the “B” electrode.
The “A” electrode is energized to produce formation current flow. The formation current returns to the “B” electrode
In an 18 foot, 8 inch lateral tool, the midpoint of two electrodes “M” and “N” is located 18 ft, 8 inches respectively below the “A” electrode.
The “M” and “N” electrodes measure the voltage produced by formation current flow and have a distance of 16 inches (.4 meters).
The lateral array geometric factor (G) is calculated as follows:
G = 4 * p * (AO)2 * MN
AO = distance from A electrode to O
Note: “O” is the midpoint between the “M” and “N” electrodes.
MN = distance between the M electrode and N electrode, usually 16 inches = .4 meters
The 18 feet, 8 inch lateral = 6.2 meters.
Geometric factor (G) = 4 * 3.14159 * 6.2 * 6.2 * .4 = 193.22
OTHER LATERAL SPACINGS:
Lateral log spacing can be 4 feet, 8 inches; 6 feet; 9 feet; 13 feet; 15 feet; and 24 feet.
Lateral resistivity logging is used in applications where measurement of true formation resistivity beyond the invaded zone is needed.
Applications include measurement of resistivity in wells to determine static water levels or water quality assessment of different aquifers without influence of the resistivity of the invaded zone.
The lateral log has its best response in beds having a thickness more than twice the AO spacing.
The E-Log uses a bipolar alternating square wave transmitting voltage. A programmable electric log gives added flexibility. High frequencies are affected by induced polarization. With programmable frequency, and programmable sample measurement it is possible to optimize for varied borehole conditions. Low frequency square waves are more immune from the effect of induced polarization. A computer controls the transmitting frequency and measurement window for precise timing and increased accuracy. Resistivity measurements are stored in a file in the computer for log plotting, statistical plotting or analysis. Log files may be used for later interpretation using advanced methods and may be used in combination with data from other logs.
OTHER RESISTIVITY LOGS:
Three applications require the use of other types of resistivity logging tools.
CONDUCTIVE DRILLING MUD:
When a conductive (salty) drilling mud is used in frilling a well, the e-log becomes electrically short circuited by the conductive fluid in the well.
Focused resistivity tools are used in these applications.
OIL BASED DRILLING MUD:
In wells drilled will an oil based drilling mud or air drilled, the e-log cannot connect electrically with the surrounding formation. Oil and air are non conductive! In this application, resistivity tools using the property of electromagnetic induction (induction resistivity logging tools) are used.
In air drilled wells having no mud or water, the e-log cannot make electrical connection with the surrounding formation, therefore, induction resistivity tools are used.