June 2000 Vol. 221 No. 6  Exploration

Perry A. Fischer,  Engineering Editor

Exploring with lasers; seismic heartthrob

During the past few years there has been an upsurge of new surface-exploration techniques, many of which rely on the idea that hydrocarbon microseeps exist over oil and gas reservoirs. Detecting hydrocarbon microseepage using remote sensing is not new. In this column (November 1999), we reported on the history of using radar for detecting hydrocarbons in air above reservoirs. While those early experimentalists reported some success, the technique suffered due to lack of an underlying fundamental theory as to how it worked. Now, a new system of directly sensing hydrocarbons has been developed that is grounded in basic physics.

Optical absorption is based on the simple principle that when light passes through various elements and molecules, discrete wavelengths of that light will be absorbed and result in a distinct pattern that identifies those elements or molecules. Astronomers, chemists and physicists at observatories and labs use it every day. What was needed to apply it to oil and gas exploration was instrumentation that would allow a scale of investigation that was neither astronomical nor benchtop, i.e., reservoir scale.

Developed by Chimera Geophysical Corp. and funded by Laser Exploration, Inc., the new system relies on laser-based LIDAR (LIght Detection And Radar) and DIAL (DIfferential Absorption Lidar) optical absorption at very narrow wavelengths. The heart of the system is a unique, specialized laser developed by Ophir Corp. It is an eye-safe, Raman-shifted, Cr:LiSAF pulse-type laser, rapidly tunable in the 2 to 5-µm spectral region. Mounted in a 20-ft trailer complete with power supply, weather station and other equipment, the laser is bounced off a reflector that may be up to a mile away. After gathering data, the reflector is moved, say, 5–10°, and another raypath is sampled.

From the data, absorption spectra are used to detect methane, ethane and propane at, or just above, background levels. Rapid tuning minimizes transient effects from wind, dust and turbulence, while GPS receivers at both laser and reflector allow precise mapping. Besides exploration, the system has many other uses, including pipeline leak detection and pollution surveys. An excellent technical paper on the subject appears in Proceedings of the SPIE, Vol. 3757.

Of course, obstacles and rugose topography can limit the area and type of terrain on which the new technique can be used, but this still leaves vast areas where it can be applied. In the future, the company hopes to take the laser technique airborne.

While this technology shares some similarities with the new exploration method of (hyperspectral) imaging spectroscopy, such as that used by Texaco’s Alto Technology Resources or Earth Search’s Petroprobe (World Oil, May 2000), it differs in two important ways: the system is active, using laser light rather than passive (and fair-weather) sunlight; and wavelength resolution is much higher, which allows direct, sub-ppm measurement of individual atmospheric hydrocarbons.

The Geosat Committee Inc., a nonprofit consortium for research and use of remotely sensed data, has members from government, academic and private sectors that include petroleum and mining, marine mapping, environmental and other interests. The committee is currently seeking industry partners for project MIDAS (Microseep Identification by Direct Atmospheric Spectroscopy). The aim of the project is to conduct field studies to critically evaluate this promising new technology.

The studies will help optimize data-gathering methods for hydrocarbon exploration and reliably quantify – for the first time – the nature and extent of hydrocarbon anomalies that exist over known reservoirs. The ultimate goal is to aid in the discovery of commercial reservoirs using this technique. For technical questions, call Dr. Paula Wamsley at: 303 933 2200. The Geosat Committee can be reached via Dr. Rebecca Dodge at: 770 830 2376.

Seismic heartache. A report in the DOE This Month newsletter cites a story about a seismic detector system that foiled a convict’s escape. A convicted murderer, serving two life sentences at the West Tennessee State Penitentiary, was attempting to escape in the back of a truck filled with basketballs.

The seismic sensors were developed by DOE’s Oak Ridge National Laboratory and Y-12 Plant. They are attached to a vehicle and detect human heartbeats with the aid of dedicated software. Originally developed to detect unauthorized entry into DOE’s secure areas, the technology was shared with the prison industry to prevent escapes. Research is underway to improve the detector’s performance in windy conditions and to allow remote screening.

Costa Rica looks good. Harken Energy completed a 100-sq-km, 3-D seismic survey in December 1999, offshore Costa Rica. The company says that evaluation of the shoot shows Tertiary and Cretaceous target horizons in the Moin prospect, which contains a roughly 80,000-acre structure of fault-segmented, four-way closure, making it perhaps the largest unexplored Cretaceous structure in the Caribbean basin. A single well can be located structurally high to test both targets.

Geologic interpretation reveals a complex geology that could have resulted in enhanced reservoir properties. Harken holds 1.4-million acres in Costa Rican concessions.

Huge 4C nonexclusive. A sign that 4C is increasing in demand, Schlumberger completed the largest, nonexclusive 4C seismic survey ever acquired in the North Sea. The survey, acquired by the purpose-built vessels Geco Bluefin and Geco Angler, comprises 126 sq km over and around Chestnut field in UK Blocks 22/1 and 22/2.

The survey lies southeast of the 1998 Alba 4C survey, which improved definition of a producing reservoir that was poorly imaged using conventional seismic. The Chestnut survey follows other nonexclusive 4C surveys offshore UK over Blocks 28/5, 28/10 and Brent and Harding fields. The company says that these 4C surveys have delivered greatly improved imaging relative to previous attempts that used conventional, towed-streamer seismic.

Comments? Write: fischerp@gulfpub.com