Figure 1. MTEM’s R-Land acquisition system has been used successfully in several projects. (All figures courtesy of MTEM)
Non-invasive electromagnetic (EM) survey methodology has rather quickly segued from being perceived as an esoteric technology “for the big guys only” to gaining somewhat widespread acceptance as a viable method to identify fluid content through resistivity in subsurface rocks prior to drilling.

The most recent EM technique to debut is multitransient electromagnetic (MTEM) — and it’s creating considerable buzz on both the domestic and international exploration and production scene as a means to detect and quantify hydrocarbons in the subsurface. Indeed, MTEM — which originated at the University of Edinburgh — appears to be on the cusp of becoming a must-have tool as a means to dramatically reduce the risk of drilling dry holes.

The MTEM application functions as a Direct Resistor Indicator to determine the resistivity of the subsurface, including reservoirs, before the drillbit ever turns. In fact, it’s often referred as “Logging Before Drilling.”

The method works by sending a series of electrical transient signals from the surface directly into the earth and measuring the voltage response between pairs of receiver electrodes along the recording line at different offsets from the source. The acquisition geometry entails multiple source and receiver positions. The source is pulse-encoded to allow more energy to be input in short bursts.

The signal is deconvolved to remove source and receiver effects, and the impulse response of the earth is extracted at the receivers. Signal processing and inversion techniques are used to produce a resistivity cross section — zones demonstrating high resistivity may contain hydrocarbons. On-site Real Time Appraisal provides both quality control (QC) and initial processing results within hours of recording.

Along with the capability to detect reservoirs and provide an indication of hydrocarbon volume, MTEM also has the ability to differentiate stacked low-saturation gas from commercial reservoirs.

MTEM is an active EM application (as opposed to passive, or magnetotelluric), the same as the older controlled source electromagnetic (CSEM) technique, which is band-limited and tows an electric source.

“CSEM has established the EM method as an exploration tool in deep water,” said Sarah Ryan, chief executive officer at MTEM company headquarters in Edinburgh. “MTEM is doing the same thing on land and in shallow water and extending that capacity to appraisal and monitoring.

“Unlike CSEM, the MTEM method has no airwave problems and can be used both on land and in water depths to 958 ft (300 m),” Ryan noted. “Also, the pulse encoding of the MTEM source produces a controlled broadband signal that delivers a continuous spectrum of frequencies from less than 1 Hz to the required high frequency.”

Given its sensitivity to fluid types contained within the rock, MTEM has been determined to be well-suited for regional reconnaissance and initial exploration efforts in new plays and frontiers. It can be used to high-grade exploration projects to expedite planning and maximize economics.

The system excels as a reservoir management tool, which makes it an ideal application for production and field development programs. In fact, the preferred domain for the technology is brown fields — both onshore and in shallow water — where fields in established hydrocarbon-bearing basins have already been brought on line via exploration efforts. Accurate resistivity information acquired using MTEM can be used to plan infill drilling programs to extend the productive life of the reservoirs and to identify field extension opportunities.

Varied applications of the technology for the production and development environment include bypassed pay identification, accurate imaging of resistors below low-saturation gas clouds and detection of shallow gas/drilling hazards, among others.

There’s significant industry interest in learning the outcome of the recently completed acquisition phase of an Anadarko-hosted joint industry project (JIP) using MTEM technology in the Anadarko-operated Middle Baxter Basin gas field on the Rock Springs uplift in southwestern Wyoming. The project is a test of the technology targeting three resistive horizons, with the deepest one located at 2,850 ft (870 m) subsurface.

“The JIP was the initial implementation of our new R-Land recording system for onshore,” said Jason Robinson, MTEM’s vice president for North and South America. “There’s a need to record long offsets to image deep targets, yet travel times in the near surface are short. So we designed, engineered and built entirely new acquisition systems for both onshore and offshore operations.”

The R-Land system performed superbly “right out of the box” as soon as it was put to work in the field in Wyoming, according to Robinson. In fact, he said there was only a single instrument downtime incident in the first 3 weeks of the project.

Anton Ziolkowski, technical director of MTEM, noted the initial data quality acquired during the JIP is excellent, with strong signal-to-noise ratio. He added “the real-time QC apparent resistivity pseudo-sections are also very encouraging.”

Prior to the JIP, an array of proof of concept (POC) projects demonstrated the viability of MTEM application onshore.

It is particularly noteworthy that the system has proven to be applicable for unconventional reservoirs, which have become popular targets for investment. In fact, a successfully completed POC project in Alberta focused on tar sands, which are difficult to evaluate using seismic and traditional EM methods. Because these sands are thick, resistive and close to the surface, they are ideal targets for MTEM technology.

Results of the Alberta project included the ability to see resistors in the subsurface, which were determined to be hydrocarbons and glacial deposits. The project demonstrated that inversion can delineate hydrocarbons from glacial deposits at depth.

Robinson noted MTEM shows even more promise for the deeper heavy oil areas that are beyond the range of traditional electrical methods such as Electrical Resistive Tomography and Airborne Transient EM.

Over the course of an MTEM application involving a gas storage reservoir in southern France, inversion of the data identified the depth and lateral extent of a subsurface resistor, which correlates with known gas in a relatively shallow sandstone reservoir with simple structure. Both the top and bottom of the gas-bearing structure were identified, and an independent analysis based on amplitude and shape of the response gave consistent results.


Figure 2. An impulse response showing the sharp airwave spike.
“It’s notable that the MTEM data were processed blind with no input from the client,” said Company Research Director Bruce Hobbs. “The client later revealed the position of the top of the reservoir, which was a precise match with the MTEM results.”

Another location in France was the proving ground for a 4-D application of the multitransient electromagnetic technique. Hobbs noted that an earlier land survey in a similar geophysical setting identified gas movement over time, documenting that the technique is ideal to monitor movement of hydrocarbons as well as delineating them.

Figure 3. Marine EM acquisition with an MTEM method injects a pulse encoded transient to image subsurface resistors.
Current onshore projects include an effort to image gas in Gujarat in India; the application was initiated in December 2006. Programs inked early on with clients to kick off in January 2007 are a gas sand imaging project in western India and an exploration objective in Trinidad to de-risk planned drilling efforts.

Regarding activity in the offshore arena, MTEM’s marine system — R-Marine — was field-tested in January of this year. First commercial work in the North Sea is planned to follow.
For more information, please visit www.mtem.com

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