The quest for better ways of imaging oil and gas reservoirs

Paul Boughton

Jeremy Cresswell examines the latest in identifying virgin oil and gas opportunities and developing discoveries.

There can be no relenting in the quest for better ways of imaging oil and gas reservoirs, whether identifying virgin opportunities, developing discoveries, getting the best out of what is being produced or indeed carrying out assessments at the point of abandonment lest the opportunity to re-open depleted reservoirs presents at some future date, perhaps for carbon capture or gas storage.

The most urgent requirement is at the exploration phase as the hunt for oil and gas extends to smaller and smaller targets in 'conventional' geologies, or potential giants located pre-salt (subsalt) or sub-basalt and frequently also located deepwater/ultra-deepwater.

Pre-salt is currently rocketing up the agenda, powered by a string of massive discoveries made in the ultra-deepwater Espirito Santo Basin offshore Brazil, though West Africa is hinting at its potential too.

Imaging through/salt has long been recognised as a difficult task, though there has been significant progress as evidenced by the string of cluster finds made deepwater Gulf of Mexico over the past decade or so.

Today, most if not all of that accumulated knowledge is being thrown at the Espirito Santo Basin opportunities, which were fan-fared in spectacular fashion late 2007 with the giant Tupi field find made by Petrobras.

Tupi, on block BM-S-11, is now ranked as one of the 50 largest oilfields in the world. It lies 306km offshore in waters more than 2,000m deep at a sub-seafloor depth of 4000-5000m. It is estimated to hold 5-8 billion barrels of oil in a presalt reservoir that is hidden by a massive salt deposit.

Since its discovery, several other major subsalt fields have been found with a 100km radius of Tupi. All are apparently giants in their own right.

The geophysical company most associated with paving the road to Tupi is CGGVeritas and, in November, it revealed something of the technological approach used to unravel and interpret the presalt secrets of Espirito Santo, that is, Reverse Time Migration.

This was applied in the re-processing of 3D data that CGGVeritas acquired over the Tupi discovery area in 2001-02 and resulted in significantly enhanced imaging of the reservoir beneath the salt.

The company's reverse time migration was performed as a test on a small part of its 20 000sq km multi-client survey done in 2001-02 but, because it achieved such remarkable results, CGGVeritas is now reprocessing the entire survey with this technology.

Reverse time migration provides the most accurate images of the subsurface in complex areas compared to other seismic migration schemes.

By using a full two-way solution to the wave equation, and given an accurate velocity model, it will accurately construct an image using all available arrivals in the seismic record.

This technique is particularly suited to areas where direct illumination of the subsurface is limited.

The improvements achieved in subsalt and salt flank imaging have been telling as demonstrated in the reverse time migration results over Tupi.

CGGVeritas is currently using this technology for imaging most of the projects it processes in the Gulf of Mexico, offshore Brazil, and offshore West Africa.

Besides subsalt imaging, this technology also helps in imaging data in a wide range of geological settings, which ultimately help in attribute and reservoir studies.

Reverse time migration is not just the preserve of CGGVeritas; others like Western Geco have applied the technique with great success during processing.

Over on the other side of the Atlantic - offshore Nigeria to be precise - GXT Technology, a unit of ION Geophysical, revealed in October that it had secured a multi-year 2D/3D data processing contract - probably one of the plum deals of 2008.

The client is ExxonMobil, via Mobil Producing Nigeria (MPN) and on behalf of its joint venture with Nigeria National Petroleum Corporation (NNPC).

For ION, this is the largest data processing award in its history and the deal specifies that GXT and its longstanding Nigerian partner, Bulwark Services, will provide advanced imaging services for a series of 2C and 4C seabed seismic surveys that will be acquired across producing fields offshore Nigeria.

Nick Bernitsas, GXT's senior VP said of the task: "Over the next several years, we will be applying our latest proprietary processing technologies, integrating insights from our experience with full-wave (4C) seismic data and from our experience in West Africa.

"This is the latest in a series of successful ION projects offshore West Africa. Our BasinSPAN programs along the West African Margin have given us insights into the region's petroleum systems, while our reprocessing projects for West African operators have given us experience in how to apply some of the latest data processing techniques in this important hydrocarbon province. We hope to leverage these experiences and insights on this upcoming series of surveys."

The data required as feedstock for the processing will be acquired by the multi-component OBC (ocean-bottom cable) specialist Reservoir Exploration Technology (RXT), engaged via a separate contract using ION's VectorSeis Ocean (VSO) acquisition platform.

GXT and Bulwark are utilizing state-of-the-art processing techniques, including converted wave imaging, to the acquired 2C and 4C data.

Meanwhile, a name that pops up in this issue several times, Offshore Hydrocarbon Mapping (OHM), is also involved in a new interpretation-related joint industry project that involves a group of oil majors, plus the UK government's Department of Energy and Climate Change, also the Aberdeen-absed Industry Technology Facilitator (ITF).

The three-year JIP was forged mid 2008 to investigate Well Integration with Seismic and Electromagnetics (WISE). OHM's task is to develop new and innovative methods of jointly integrating and interpreting seismic, CSEM and well log data with the aim of being to provide improved maps of reservoir properties, such as hydrocarbon saturation, across a field.

Investigations include establishing the optimum algorithm for remapping CSEM results into the higher resolution seismic data and developing a joint inversion methodology for seismic and CSEM data.

According to the ITF, this groundbreaking research will further assist the industry to exploit the combined strengths of each of these valuable data types, allowing application to both existing and newly acquired data.

Ultimately, the outcomes of the project aim to help further reduce exploration risk and uncertainty in reservoir production - enabling oil producers to maximise production and increase recoverable reserves.

Oil companies initially signed up to WISE, including; Total, Chevron and DONG Energy.

Related to WISE, OHM subsidiary Rock Solid Images in November 2008 signalled that it was releasing the latest enhancement to its flagship iMOSS modeling application at the Society of Exploration Geophysicists annual meeting in Las Vegas, Nevada.

The company, created by the 2007 acquisition of US group RDSP Acquisition by OHM, said the new tool would adds electromagnetic imaging capabilities to its modeling system.

Richard Cooper, president of the unit, said at the launch: "OHM Rock Solid Images is quite unique in our ability to de-risk oil and gas exploration and exploitation using integrated seismic, well, and CSEM data.

"We believe that integration of seismic, CSEM, and well information is the key to maximising the value of these data. Scenario modeling is a major component of joint-interpretation; our latest iMOSS-em tool provides simultaneous modeling of seismic and CSEM response driven by a common earth-model which may be perturbed to test various reservoir analogues."

This tool is an early outcome of the WISE JIP, which is clearly already yielding exciting new insights into how the upstream hydrocarbons industry finds and quantifies oil and gas resources using multiple data types.

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