Research and development continues to yield impressive advances in subsurface processing and imaging technology.
A host of new tools available to operating oil and natural gas companies is enabling them to gain unprecedented efficiencies in data processing and ever-sharper clarity and detail in imaging the subsurface.
Wide-azimuth
Over the past few years, the most significant advance in seismic data imaging resulted from the drive to gain the best subsurface insights possible from data acquired with the new technique of wide-azimuth acquisition, claims Peter Whiting, CGGVeritas executive vice-president, processing and imaging.
“When you look back at data processed 10 years ago and compare it with recently imaged data, it becomes clear that advances can mean the difference between a dry hole and a discovery,” he notes. “Sometimes these advances are slow and steady and other times, they are step-changes.
“The advent of wide-azimuth acquisition resulted in a step-change improvement in some of the most challenging areas to image seismically-as an example, beneath very complex salt bodies in the deepwater Gulf of Mexico.”
Processing and imaging data acquired from wide-azimuth surveys has required a complete upgrade of the standard software tools, as well as significant enhancement to imaging algorithms, Whiting points out.
“CGGVeritas began acquisition of the first wide-azimuth survey in the deepwater Gulf of Mexico in 2004, and over the past 3 years, considerable investment and focus on processing and imaging wide-azimuth data has resulted in seismic images that are nothing short of stunning.
“In the complex geologic settings of the Gulf of Mexico, where imaging limitations previously created very high-risk profiles for prospectors, well-processed wide-azimuth surveys have unlocked new subsurface potential and are dramatically increasing interest in this potentially prolific basin.”
Conventional 3D seismic surveys today, such as those collected with narrow-azimuth geometries, are sometimes insufficient to overcome challenges imposed by the subsurface geology, points out Ruben Martinez, PGS chief geophysicist, data processing: “All these challenges exist today, and wide/multi-azimuth surveys mitigate these problems,” Martinez notes. “The inclusion of different azimuths during the data acquisition and the application of the correct data processing techniques yield results not seen otherwise with the conventional narrow-azimuth geometries or conventional 3Ds.
Martinez cites advantages of co-locating pressure and velocity sensors, such as with PGS’s new dual-sensor streamer, which yields enhanced resolution and better penetration.
“We will see more and more wide/multi-azimuth surveys in the future. The extra cost is minimal compared with the percent of risk reduction obtained with this technology.”
Reverse time migration
Reverse time migration (RTM) is an imaging technique based on the accuracy of the full two-way wave equation.
Although theoretically known for many years, computational intensity has kept it from being developed as a conventional processing option, notes Whiting.
“With the continued increase in computing capacity, CGGVeritas has developed and refined RTM so that it now a realistic option for obtaining superior images in complex areas with high dips and high frequencies,” he says. “Further, since RTM is based on the full two-way wave equation, the amplitude integrity of the images is higher than that of previous imaging algorithms and hence offers the potential for improved fluid and rock property estimates in complex areas.”
Martinez thinks that RTM will become the standard method for subsurface imaging: “RTM provides opportunities for improving the imaging accuracy not seen with conventional Kirchhoff or wave equation methods (one-way wave equation). RTM uses the two-way wave equation, and this provides the opportunity to reach geologic dips up to 90° or beyond.”
Prestack depth migration
As data volumes continue to increase, the demand for improvements in imaging accuracy and reductions in turnaround time increases as well, notes Jim Sledzik, marketing director for Schlumberger Ltd. unit WesternGeco.
“Recently, we have seen an explosion of prestack depth migration (PSDM) algorithms with more optimized computer operations and fewer earth model assumptions,” he says. “Besides the now-classic Kirchhoff PSDM and one-way wave equation migration, there is now available a wide range of PSDM algorithms dealing with various levels of subsurface complexity, and various acquisition-related implementations with a broad range of computer cost and efficiency-from algorithms such as fast-beam up to elastic [RTM].
The ultimate goal with faster computers and algorithms, says Martinez, is that “real-time depth migration-with the updates coming from the well information as they are drilling-is going to be the future.”
Controlled beam migration
Controlled beam migration (CBM) has been a breakthrough, primarily in imaging in the Gulf of Mexico but also in many other basins such as onshore US, offshore Vietnam, and offshore Western Africa, according to Whiting.
“The specialized imaging algorithm produces much clearer images of subsurface reflectors in areas where standard imaging algorithms have struggled to produce anything reliable,” he says. “CBM, combined with specialized algorithms for wide-azimuth data, forms a powerful combination for understanding reservoir potential.
“At CGGVeritas, we have seen CBM produce high-quality, unambiguous images below complex salt bodies. In offshore regions of Vietnam, CBM has produced reliable images of the fractures in the buried granitic basement that are critical for successful production.”
Attenuation
A key new technology for improving attentuation is 3D surface-related multiple elimination (SRME).
For Whiting, 3D SRME provides the ability to attenuate some of the very troublesome and complex multiple events that tend to contaminate seismic images and obscure and distort the target events.
“At CGGVeritas, 3D SRME has been routinely used in recent times on very large surveys in most parts of the world, resulting in improved confidence in the interpreted estimates of the subsurface,” he says. “Recently, CGGVeritas has reengineered 3D SRME for wide-azimuth geometry and has successfully applied it to several wide-azimuth surveys in the deepwater Gulf of Mexico.
Martinez thinks SRME technology will continue to show a lot of progress: “The evolution will be in SRME-type methods that are data-driven methods; this represents a tremendous advantage over other methods, like Radon, which require a preliminary knowledge of the velocity field.”
Full-wave seismic
Full-wave seismic technology continues to mark progress with ever-increasing computational capacity and evolving multicomponent recording.
“Recording full-wave seismic, with over/under streamers or multicomponent receivers, requires adequate processing and imaging algorithms,” notes Whiting. “WesternGeco has an advanced imaging workflow, which includes model building, PP-PS joint tomography, and elastic migration.”
Martinez points out that the accuracy of four-component (4C) data is constantly improving-for example, PGS recently introduced the OptoSeis fiber optic permanent seabed 4D4C monitoring system.
“In data processing, the elusive goal continues to be the use of the elastic wave equation,” he says. “The barrier is not that the theory is unknown but that the algorithms are still expensive to run with today’s computer power. But until that happen, acceptable approximations in the algorithms are in use and giving results.”
Martinez speculates full wavefields will be used to image the subsurface in the future.
“By full wavefields I mean elastic wavefields and not only acoustic,” he clarifies. “To achieve this task, imaging methods that make use of the elastic wave equation will be in place and will include most, if not all, modes of wave propagation. Hopefully, the computer capacity will be there to be able to produce these images commercially.”
4D/real-time monitoring
The advent of 4D, or time-lapse, seismic has given the industry the capability of real-time monitoring of the subsurface.
Whiting notes that CGGVeritas has seen 4D seismic technology quietly growing to become a trusted and required tool for optimizing reservoir production.
“SeisMovie, a CGGVeritas high-resolution, high-sensitivity 4D reservoir monitoring solution using permanent arrays of sources and receivers, which operate continuously, has been applied successfully to a range of scenarios, including heavy oil production and gas storage monitoring,” he says..
Sledzik notes that the biggest impact of 4D comes in the area of seismic data acquisition, but adds, “What could potentially change the game here is the integration of processing and interpretation capable of efficiently handling multiple data volumes and the utilization of 4D measurements to discover geomechanical information about the evolution of the field.”
The ultimate goal in 4D seismic is to deliver the 4D seismic anomalies map with a fast turnaround so that the seismic information can be incorporated into the reservoir simulation process, contends Martinez.
“The turnaround time improvement can have a significant impact in the placement of the production wells to be drilled during the field’s exploitation phase,” he says. “Therefore, advances in seismic data processing with the aim of improving the turnaround time is vital to make 4D a fully accepted exploitation tool in the arsenal of the reservoir engineer’s tools. Automating the processing steps is the main challenge to date.”
Visualization
Improvements in 3D visualization have been a key enabler in measurement integration, according to Whiting.
“We have now gone one step further to using 3D visualization as an enabler for work flow integration,” he says. “We have achieved this by visualizing and manipulating together the data getting in and being produced during the stages of interpretation, processing, and modeling to imaging.”
Pete Bennion, vice-president, imaging, for TGS-Nopec, thinks that the close linkage of interpretation with imaging blurs the lines between these disciplines.
“The latest systems allow viewing of multiple attributes that add a new dimension to the analysis of seismic data,” he says. “The ability to very rapidly and efficiently look at the underlying gather data contributing to a stack trace within a 3D cube gives the interpreter powerful analysis tools to validate potential prospects.”
Martinez cites a key advance in the orders-of-magnitude improvement in computer processing capabilities, specifically “the ability to utilize more than one graphics card at once to drive one screen-even using four graphics cards as a computer bank to give the user access to what considered to be supercomputer performance just years ago on his own desktop.”
Model building/validation
Model building-and its validation-is one of the bottlenecks in the depth imaging process, according to Martinez.
“The standard in the industry today is the utilization of traveltime tomography,” he notes. “There are still challenges to overcome in tomography, but they are mainly related to the automation of the process.
“In the near future, we expect that the wave equation-based tomography (WE tomography) methods will gain popularity as they offer advantages over the traveltime tomography methods; the WE tomography methods do not require the intensive picking of residuals and geologic dips as does the traveltime tomography. Therefore, the WE tomography will offer better automation and better turnaround time of the velocity model to be used in depth migration.”
WesternGeco’s model-building capabilities include handling complex, hybrid earth models (combination of sharp-smooth geometrical representation) in both the model building step, as well as in tomographic inversion, notes Sledzik: “For resolving shallow subsurface anomalies, we use seismic diving wave tomography. For complete earth model, we use seismic reflection tomography. We have now started to use CSEM (controlled-source electromagnetic) inversions.
EM
Electromagnetic (EM) methods have evolved beyond the shallow subsurface, notably with seabed logging.
One advocate of seabed logging is Terje Eidesmo, CEO of Norwegian contractor ElectroMagnetic GeoServices (emgs).
“Until seabed logging came along, oil companies had to drill these wells on the basis of a single measurement: acoustic impedance derived from seismic,” he notes. “Today, they can build earth models based on a second measurement: resistivity derived from seabed logging. Furthermore, resistivity is more likely to provide information directly about pore fluids. Even where resistivity is driven by the rock matrix (carbonates, volcanics, etc.), it can provide valuable information for unraveling the real geology in many areas.”
Although seabed logging has already become a routine part of the E&P workflow at most leading oil companies, the technique is still in its infancy, Eidesmo points out: “There are still huge leaps to be made in unlocking the full information content of the full vector wavefields-electric and magnetic.
“As EM sampling densities increase, new processing techniques emerge, and equipment performance continues to improve, seabed logging will provide increasingly detailed reservoir characterization information and even time-lapse data.”
Seabed logging today is being used primarily to reduce drilling risk in prospects identified on seismic and to provide improved salt body definition to enhance subsalt seismic imaging, says Eidesmo.
“However, we think that the greatest value may lie in rapid electromagnetic scanning of frontier areas, which we see is being used more and more widely by the oil companies,” he contends. “This will identify reserves earlier in the E&P cycle and enable us to focus resources more effectively, but it will also uncover new reserves in unconventional traps that can be hard to identify from seismic data alone.”
Martinez sees step-changes in the technology used for EM acquisition, citing marine EM solutions where both the source and recording device are towed.
“Such operations could be conducted with close to the same efficiency as seismic operations and possibly enable acquisition of EM measurements from the same platform as used for seismic data,” he says. “EM technology is today at a very early stage, and we expect significant developments on acquisition, processing, and interpretation to take place over the next couple of years. If successful, the technology PGS is currently developing could represent a quantum leap in EM technology, since a towed system could be more than 10 times more efficient than current technology.”
Reservoir geophysics
Reservoir geophysics is a discipline within geophysics that continues to gain momentum, notes Martinez.
“The need to translate seismic information into rock properties and then translate these into lithology and fluids estimates has produced an important evolution in reservoir geophysics,” he says. “Techniques such as seismic inversion, amplitude vs. offset, elastic inversion, and seismic attribute analysis improve continuously to provide the link between seismic data and reservoir properties.”
Another reason to expect reservoir geophysics to greatly advance in the future is that it is required for a reliable understanding of 4D and 4C measurements, Martinez adds.
The future
Whiting foresees significant improvements continuing in the quality of subsurface images and subsurface property estimation.
“The wide-azimuth surveying technique will undoubtedly result in further refinements and provide even more reliable and extensive images of the subsurface,” he says. “Advances in computing (potentially spurred by advances in nanotechnology) will allow for higher-fidelity processing than can be considered today. The quantitative accuracy of the processed wavefield will lead to highly reliable estimates of subsurface rock and fluid properties.
“Acquisition advances in seafloor techniques and permanent monitoring will lead to an inexorable link between seismic information and successful exploitation of reservoirs. Advances in multicomponent acquisition and processing will further improve the industry’s understanding of subsurface rock and fluid properties, as well as fracture and stress information.
“Nearer-term advances at CGGVeritas, along with increasing computing power, will continue to improve our ability to process full wavefields (even more than one wavefield at the same time). This will lead to highly accurate and precise velocity models of the subsurface and correspondingly improved depth images. ]