?“The critical path of the instrumentation is to determine if there is a fluid signal and a desired level of porosity,” says Richard Bennett, president of Halcyon Exploration Technologies LLC. |
?Doodlebug: n, a device used in attempting to locate underground gas, water, oil or ores.
Since the first commercial oil discovery, explorationists have sought to predict depth, thickness and quality of prospective reservoirs before they commit to a drillsite. In the past, operators have used everything from a divining rod to a black-box “doodlebug” to attempt to accurately predict where to drill. Today’s E&Ps have a new trick for their toolbox. It’s called electromagneto telluric (EMT) analysis.
Denver-based Halcyon Exploration Technologies LLC uses EMT technology, refined from earlier versions of the magneto telluric (MT) concept, to enable E&Ps to electronically locate onshore hydrocarbons up to 25,000 feet deep before drilling. The system also facilitates mapping hydrocarbon distribution and continuity of the reservoir, and promotes the prioritization of drillsites to avoid drilling nonproductive or uncommercial wells.
EMT is a passive, natural-source electromagnetic geophysical method of imaging structures and fluid content below the surface. No energy is introduced into the ground because natural variations in the earth’s electromagnetic field—enhanced by outside sources such as solar winds—induce electromagnetic currents (or telluric currents) of sufficient strength for measurement and analysis.
“The entire Earth is a very noisy place,” says Richard Bennett, president. “When you listen to a geophone on the earth, what you hear is constantly fluctuating white noise. This technology and the filtering software are the keys that pull a meaningful signal out of that hash of noise.”
The EMT system takes concurrent measurements of orthogonal components of the earth’s electric and magnetic fields, allowing for the calculation of the impedance tensor (a single point or collection of isolated points), which is complex and frequency-dependent. Using this tensor, it is possible to gain insight into the resistivity structure of the surrounding material. Because the recorder operates at very low frequency (about 1 hertz), it can record sound and rationalize data at very great depths.
“The critical path of the instrumentation is to determine if there is a fluid signal and a desired level of porosity,” says Bennett. As electromagnetic energy passes through tight rock, it is drawn to the path of least resistance. If the porosity contains hydrocarbons, there is more resistance or impedance to the transmission of electromagnetic energy, which appears as a unique wave-phase distortion. If the wave-phase distortion is increased, it signals oil or gas rather than water.
“If there is no signal, it indicates that the porosity interval is rather tight and contains no fluid. Otherwise, the fluid signal will suggest water or hydrocarbons,” he says.
Halcyon has continually worked to increase the sophistication of the electromagnetic energy data processing, which has, in turn, greatly increased its success rate. The company’s modis operandi has been to go to the drillsite where the seismic or geology has determined a high probability of a pay zone. It then evaluates that location and some surrounding locations.
“It’s helpful in the case where a vertical well has been drilled and a good producer has been discovered. The operator should probably not start stepping out wells in the area without benefit of the EMT data because the next four holes could be dry. That’s the possibility that our equipment helps avoid. If we see that a proposed drillsite contains hydrocarbon, we evaluate several stations around it to ensure the drillsite will be successful and at least four or five successful offsets are likely.”
In the case in which analysis indicates nothing but water in the horizon, or extremely tight rock, Halcyon will still evaluate nearby locations. Even if the drillsite is not optimal, a site nearby in the same formation may be promising because either structure or stratigraphy may improve.
“Our clients also find it helpful for us to evaluate prospects already identified by 3-D surveys. At most depths, seismic is not able to distinguish the presence of hydrocarbon, particularly oil. Seismic simply indicates where there is an attractive structural or stratigraphic feature that may contain trapped hydrocarbons.
“If the seismic indicates three or four prospective areas, we can use this technology to prioritize those targets. More often than not, they won’t all be good. So starting with the most likely drillsite is a good way to explore, using seismic followed with EMT before drilling.”
Halcyon has conducted hundreds of surveys. Since 2007, after a total upgrade of the noise filtering and software capabilities, the process has a 70% to 80% success ratio, correctly predicting the type of fluid content, or the lack of fluid, in the objective reservoirs.
Bennett has used this technology in 19 U.S. states and in such diverse regions as the swamps of Louisiana and the fields of Romania. Halcyon will next embark on a large survey in Australia. It continues to fine-tune the filtering-system technology as additional data are collected and analyzed.
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