Seeking the Universal Hydrocarbon Indicator
Low Frequency, But High Hopes
A geophysical crew camp in Libya. The small crew size and minimal environmental footprint make for easy logistics and site access.
Imagine your comfort level when drilling a well if you knew with absolute certainty there were hydrocarbons in the target reservoir.
This is not a pipe dream.
In fact, it’s the goal of some innovative technology that evolved out of research at the University of Zurich.
Early on in their research efforts, the Zurich-based scientists recognized a relationship between low frequency spectral anomalies in seismic background wavefields and the geological characteristics of a group of reservoirs, principally in the Middle East.
The research concentrated for the most part on some enigmatic amplitude peaks gathered around 3 Hz in surface seismic data measured above hydrocarbon-bearing reservoirs -- indicating a possible universal hydrocarbon indicator (DHI).
There were skeptics aplenty -- namely the geophysicists who traditionally have thumbed their collective noses at low frequency data below 10 Hz, where conventional geophones are less effective.
Today, however, there’s evidence these folks have had a major attitude adjustment.
Indeed, these long-ignored data have gained respect for their potential to yield information that, when properly analyzed, could significantly reduce the number of dry holes drilled in the industry.
In fact, a low frequency DHI technology was developed some years back in Russia where it has been applied commercially.
Passive seismic techniques using low frequency ambient waves, which are ubiquitous in the earth’s crust, were a major topic at the 2006 EAGE workshop on passive seismic methods and applications held in Dubai. A throng of 120 professional participants representing most all major E&P firms and contractors validated the intense industry interest in this technology.
Increasing Success Rates
It is noteworthy that this evolving low frequency spectral analysis technology is a passive seismic technique as opposed to another relatively new DHI technology, multi-transient electromagnetic, which is an active electro-magnetic application, the same as the now-familiar controlled source electromagnetic (CSEM) technique used offshore.
Back in 2003, when there was nary a hint of the soon-to-come dramatic upswing in commodity prices and drilling activity, a group of entrepreneurial R&D folks decided the time was right to seize the moment and band together to specialize in DHI.
The resulting Zurich-based company -- Spectraseis -- quickly began acquiring low frequency seismic data and developing industrial applications as its research progressed.
“We felt that the industry had underinvested in R&D and was overlooking technologies that could have a major positive impact on its performance,” said Spectraseis CEO Ross Newman. “Our goal was to bring a strong body of research to the market, and it’s been really exciting to see our work being taken up so quickly by producers.”
Simply put, the company’s work is based on the observation that low frequency waves in the 1- to 10- Hz range are shaped into coherent patterns above hydrocarbon-bearing reservoirs.
“The seismic background spectrum is modified differently through interactions with subsurface reservoirs having hydrocarbon-filled pores as opposed to water-saturated formations,” Newman said. “Hydrocarbon reservoirs create characteristic patterns in the frequency spectra of surface signal, which can be analyzed to obtain information about the subsurface structures.”
Armed with such a tool to identify and map specific spectral signatures directly related to hydrocarbon accumulations, oil and gas finders would have the potential to dramatically increase their exploration success rates.
One of the challenges facing the technology providers is the lack of a full-fledged physical model for the spectral anomalies, according to Andy Jupe, an independent consultant specializing in the development and application of microseismic and passive seismic technology in the oil industry.
“Spectraseis and their partners at the University of Zurich and ETH Zurich have been really getting into the physics,” Jupe said. “Some of the hypotheses they’ve been considering seem to offer a plausible, albeit not yet proven, explanation.
“Despite this uncertainty over the physics, you can’t dismiss the fact that there is a considerable body of empirical evidence supporting the technique,” Jupe said. “This includes at least 20 case studies within the public domain, including comprehensive trials and blind tests conducted by ADCO, KOC, Petrobras and the Shell affiliates RAG. The results of these trials all seem to confirm the viability of the technique as a DHI.”
DHI technology using low frequency passive seismic data is targeted for exploration, field appraisal and production. Geologic features that are difficult to discern with 2-D or 3-D, e.g., stratigraphic traps, make good targets for low frequency passive seismic evaluation, Newman noted.
Except for early exploration efforts where large-scale screening is important, DHI technology is ordinarily used as a supplement to other technologies.
For example, a large area can be screened to detect points of specific interest where 3-D surveys might be justified versus surveying the entire area -- an always-expensive undertaking.
Seeking Specific Signatures
In order to identify and map the spectral signatures associated with suspected hydrocarbon-containing reservoirs, Spectraseis utilizes its proprietary HyMAS (Hydrocarbon Micro-tremor Analysis) technology to acquire, process and interpret low frequency data.
“Hydrocarbon microtremors represent a frequency-dependent hydrocarbon signature present in the passive wavefield,” said Rob Habiger, chief technical officer at Spectraseis. “We use our technology to analyze the changes that occur in this wavefield when the low frequency waves propagate through hydrocarbon-bearing reservoirs.
“We look for specific spectral signatures characteristic of the interaction between the reservoir and its fluid,” Habiger added, “providing the critical information -- whether the structure of interest contains hydrocarbons.”
To acquire the complex low frequency data, survey teams use portable 3C broadband seismometers, moving among a series of designated points covering a grid layout with node spacing of 250 meters to 2,000 meters, depending on the survey goals. Multiple teams can be deployed simultaneously.
All signals unrelated to the target reservoir are removed or attenuated using the company’s processing software suite. The low frequency data then can be interactively overlaid with G&G information, such as contour, fault and other attribute maps.
A number of field programs have been implemented since Spectraseis’ successful inaugural commercial survey in 2004 at a Petrobras-operated field in the onshore Potiguar Basin in Brazil.
These include three experimental surveys in collaboration with Shell-affiliate RAG in Austria, which accurately predicted the company’s first successful oil well in ten years, according to Newman.
Shell, in fact, has developed its own in-house approach to low frequency spectral analysis technology.
The company has participated via SRAK (a joint venture of Shell, Saudi Aramco and Total in Saudi Arabia’s vast Rub al-Khali desert), where a variety of acquisition applications have been used in the search for gas and associated liquids.
These applications include magnetotellurics and seismic spectroscopy, which reportedly taps the same principle as low frequency seismic albeit employing a slightly different methodology.
The various scanning tools used at Rub al-Khali aimed to provide a look at the subsurface in a quick, cost-effective manner. A paper presented at the AAPG-sponsored GEO 2006 in Bahrain indicated positive results from the low frequency passive seismic spectroscopy data, which reportedly are currently being re-evaluated.
Results will soon be in for a recent extensive pilot survey Spectraseis implemented over a Pemex-operated gas field in the Burgos Basin in Mexico.
More than 500 measurements of the omni-present wavefield at the surface were acquired over an area encompassing about 200 square kilometers, Newman said.
Following processing, the team of scientists was able to extract a hydrocarbon potential map from spectral anomalies and compare this map with available reservoir information. Newman noted they observed a high correlation of the hydrocarbon microtremor signal with the known location of a producing gas reservoir.
“The operations showed environmental and access benefits, which are important for us in Mexico,” noted Efrain Mendez, coordinator for monitoring and technology development for exploration activities at Pemex E&P. “Landowners were more willing to grant us access for the survey, and it was completed in about a third of the time we would have needed for conventional seismic.
“The next step for us will be integration of the results with 3-D seismic from the region and comparison with drilling results” Mendez added.
“If the results are good, we see potential applications in swamps, lakes and offshore Gulf of Mexico.”
Not content just to identify and map spectral signatures that can be directly correlated to hydrocarbon reservoirs, experts in the field are busy developing new techniques that provide more detailed subsurface information.
For example, Spectraseis has made advances in the realm of time reverse modeling to localize the source of the low frequency microtremors through its cooperative research program at ETH Zurich.
“The technique of time reverse analysis is a promising method to reliably localize the so-called hydrocarbon microtremors,” Habiger noted. “We have shown the potential of such an approach via a numerical feasibility study.
“By using a realistic geologic section we have encouraging results that indicate it’s possible to localize the origin of tremor signals,” Habiger said. “In the near future we believe it will be possible to distinguish between two reservoirs that are stacked.”