For decades, technicians and researchers have used variations of the above simple circuit to simulate logging cables in the shop or lab. A "dummy line" which simulates a long logging line (wireline)* is indispensable as a diagnostic tool when working on well logging equipment, but a truly accurate simulation of wireline is not trivial.Theoretical Considerations
A logging cable can be visualized as a ladder network with the side rails consisting of resistors and inductors, and the rungs consisting of an equal number of capacitors and resistors. If we knew the values of these various imaginary components distributed along the wireline, we could begin to calculate the net reactance and the net impedance. After infinite calculation, we could determine a characteristic impedance for our hypothetical cable.An Ideal Simulator
In the real world, length and temperature are two variables constantly changing during well logging operations. A wireline changes electrical length as it is reeled out and then back in because the armor contacts underlying wraps on the drum or reel. Its electrical length is thereby shortened as the wireline is reeled in even though the gross cable length is constant. This effectively shorts out one side or rail of the ladder network discussed above for all cable on the drum or reel at any given moment. Reeled out the line is more balanced, reeled in, the already imbalanced logging line becomes more imbalanced. Temperature increases with depth in wells, and resistance changes with temperature. It should be obvious that the R-L-C values and relationships are on the move! It should also be noted that reactance and impedance also changes with the aging process. Old wireline exposed to the elements has less resistance between the conductors due to insulation breakdown.
Interestingly, there is not a great deal of material available on wireline electrical characteristics and simulation. Even the logging cable manufacturers seem to avoid addressing the matter. See our Single Conductor Logging Cable (Electric Wireline) Data page for some basic information on various wireline parameters.
An ideal simulator circuit would have four components between the input node, the output node, and the shield (armor). From the input node to the output node is a variable resistance in parallel with a variable inductance. From the input node to the shield is a variable capacitance in parallel with a variable resistance. The input shield to the output shield characteristics vary as line is played out or reeled in, so an additional variable resistance in parallel with a variable inductance is required. This described circuit is referred to as a variably unbalanced ladder network. Our ideal dummy line gizmo needs six knobs, and would be a headache of epic proportions.
A Practical Dummy Line
There are a host of other factors to consider in an ideal logging line simulation. There is line noise, power supply noise, acquisition system noise, crosstalk noise, microphonic tool noise, rig noise, environmental noise, thermal noise, and of course company man / tool pusher noise that all need to be factored in. Further, line termination must also be addressed. The fact this foregoing list is probably woefully incomplete illuminates the complexity of the problem.
There was once a company with a big spool of logging cable built into a closet with a door too small to ever get it back out again without demolition work, not a really practical dummy line. But a simple resistor-capacitor "T" network has been used for decades as a practical dummy line solution. There are many subtle variations of this circuit floating around, but this
dummy line schematic shows two common versions. Each stage of this dummy line design simulates around 2,000 feet of common 5/16 inch logging cable. It has been historically recommended that one or two watt non-inductive resistors be used (traditionally carbon composition resistors), and the capacitors are usually 400 or 600 volt mylar film units.Application Notes
Final assemblies are often configured to simulate 15,000 or maybe 20,000 feet of line. Having multiple connector types for the input and output on a dummy line box is convenient, and a switch to enable and disable the dummy line can be handy. The ultimate implementation has a rotary switch to select different simulated line lengths. It is necessary to have a multiple pole rotary switch to accomplish this since the unused capacitors cannot be left in the circuit. One simple way to accomplish this design is to lift the ground on the deselected batche(s) of capacitors. Here is a
Ed "Mr. Haney" Norman's implementation of a multiple stage dummy line in his old SIE cement bond log simulator box from his lab at KFE in Kilgore, Texas.
The simple dummy line discussed above works very well for testing pulse tools, and even old style cement bond tools. It begins to fail when exploring the limits of wireline technology. It has been suggested that a few of the more modern telemetries do not work well with the simple dummy line simulators. A short piece of logging cable in series with the dummy line sometimes improves things, and other times the addition of a resistor will help.Acknowledgements
There are a few situations where a dummy line can help with actual logging operations. For instance, here in the Illinois Basin, it is not uncommon for logging companies to have short lines on their trucks due to the shallowness of existing wells, sometimes only a few thousand feet of logging cable. (There are producing wells in Kentucky less than 100 feet deep, and thousands in the range of 200 to 600 feet deep.) Some logging tools just do not like extremely short logging lines. For instance, if you are having difficulties with cement bond logging on a relatively short line, a dummy line can work miracles (the problem often manifests itself as a difficulty in adjusting the T2 floating gate travel time curve). AnaLog Services, Inc. has constructed many dummy lines for customers.
Prior to creating this page, comments were solicited on the
Wireline Reflector Mailing List. Special thanks go to Charles "Chuck" Merritt with WELLOG. This page would not have been possible without his contribution. Thanks also to Ray Ethridge with Chappel Hill Logging Systems, arguably the world's leading expert on neutron generators and related logging technology, for his observations about dummy lines and modern telemetry.|
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*Logging cable, wireline, e-line, logging line, and line are used interchangeably in this discussion.
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