Potential field methods gain gravity
  from: Offshore Engineer
  by: Andrew McBarnet
  Thursday, October 02, 2008

Talking of a comeback might be a little misleading because potential field methods have always been in the playbook of E&P geoscientists. However, Andrew McBarnet detects renewed interest in gravity and magnetic measurements particularly in trying to divine the structural make-up of hydrocarbons prospects in complex geological settings.

A little noted missive out of San Diego recently marked the latest chapter in research by the Scripps Institution of Oceanography to explore applications of gravimetry for offshore oil and gas activities. In collaboration with Information Systems Laboratory (ISL), an R&D specialist in innovative fields like sensors, signal processing and space/missile technology, Scripps has just completed some tests using an AUV equipped with a gravimeter travelling just above the seafloor in water depths of 3000ft. Richard Miller, president and CEO of ISL, which hopes to commercialise the technology one day, said he had been encouraged by the first field tests but acknowledged that there was still work to do.

One indication of the promise of the Scripps/ISL initiative is that a number of oil companies have contributed funding. Their interest is in what successful development of the technology might bring in terms of an additional tool to accurately locate hydrocarbons in deepwater environments, especially in those areas where 3D seismic acquisition is not fully up to the task; for example, when salt or subsalt structures are encountered. It is the same mindset which has inspired the development of marine controlled source electromagnetic surveys and wide-, multiand rich-azimuth seismic data acquisition. Exploration companies believe that a good proportion of undiscovered oil and gas lies in difficult geologies found in regions such as the Gulf of Mexico (subsalt), offshore Brazil and around the Faroe Islands where volcanics are the problem, and off West Africa which exhibits complex faulting.

The AUV-mounted gravimeter project seems to reflect a new respect for what potential methods (gravity and magnetic) might be able to deliver. Marine grav/mag data acquisition has been a staple of E&P;operations for decades more or less invariably used in conjunction with 2D or 3D seismic. Gravity instruments can offer insight into the density of rocks from which inferences can be made about oil and gas prospectivity. The super low density of salt is identifiable in a way not possible using seismic alone. Magnetic field measurements are most useful for delineating basement structures which may support shallower sedimentary traps of oil and gas.

The cost of grav/mag is minimal in the E&P overall budget, less than 5% of what a company will pay for a 3D seismic survey. It’s hard to get an accurate beat on how many marine seismic surveys also carry out grav/mag measurements, but 50% was about the best figure that industry sources come up with. Often by the 3D seismic stage in the exploration cycle, the grav/mag data has already been acquired, so is unnecessary.

In fact the fascination with the high resolution data possible from 3D seismic is partly to blame for pushing grav/mag into a niche market. Today marine grav/mag acquisition is mainly subcontracted by the seismic acquisition companies to a small coterie of specialist companies, such as Fugro, Austin Exploration and Edcon-PRJ. These companies supply equipment and personnel as well as data processing and interpretation for integration into substructure models.

Surging demand

What the grav/mag service and equipment suppliers do confirm is that the boom in exploration seismic has brought them additional business. For example, to meet surging demand Houston-based Austin Exploration is just investing in another ZLS fluid damped dynamic marine gravity meter which offers fluid rather than dry damping enabling it to operate effectively in rougher waters. Platform stability is an important consideration in the acquisition of gravity data and has been a focus of R&D along with processing techniques which can compensate for disturbance.

One of the benefits of the AUV-mounted gravimeter touted by ISL and Scripps is that the torpedo-shaped vehicle operating at depth means the instrument is not affected by surface waves. More significantly, the system does much to overcome the problem that detectable changes in gravity fall off exponentially with distance from the seafloor. In other words surface-ship gravity measurements through thousands of feet of water don’t provide optimal resolution. In the equipment test offshore San Diego, where both ISL and Scripps are both based, the AUV is reported to have repeatedly crossed over a deep sedimentary basin. The research team admits that even with the relatively smooth passage of the AUV, some motion-created distortion had to be adjusted for.

Dr Mark Zumberge, who leads the Scripps research, has also been working with StatoilHydro and the Norwegian University of Science & Technology in Trondheim on time-lapse seafloor gravity measurements for reservoir monitoring which could open up a whole new market. Measurements have been taken on at least five fields in the North Sea and repeat surveys have been carried out on two of the sites with a separation of three years or more. A lot of experience has been gained. For this type of project a grid of static measuring platforms distributed around the reservoir target area has been used so that precise repositioning of the instrumentation is possible for each survey. Operationally an ROV-deployed gravity and pressure package carries the instrument to each of the permanently placed seabed benchmarks (1m diameter flat topped concrete cones). In a paper to the SEG last year the researchers said 179 measurements over 71 sites were made in 14 days.

The measurements are looking for changes in the density structure below the surface over time. In a reservoir, production causes the replacement of withdrawn fluid by water and/or subsidence of the overburden. The sensitivity of modern gravimeters is such that changes of less than 1m in the gaswater contact can be detected. In the first survey over the Troll field in 1998, the researchers obtained gravity and pressure results of 27 microGal and 14mm respectively. In 2006 over the Midgard field the measurements had improved to 3 microGal and 4mm. Better equipment was a key factor in the improved performance, highlighting the manufacturers’ optimism about the future of the grav/mag business.

Fugro, the largest provider of nonseismic geophysical methods, is clearly conscious of the current enthusiasm. In a bid to catch as much of the business as possible, it has repackaged its offerings with the creation of Fugro Gravity and Magnetic Services (FGMS). This unit will combine Fugro Airborne Services, Fugro Ground Geophysics and Fugro Robertson (marine) to deliver what is described as a full spectrum of potential field tools. Launching the change, the company made special mention of its Falcon airborne gravity gradiometry, which is a technology tipped to make some mark in the marine field. After all, if sufficiently good resolution data can be achieved using an aircraft flying over the sea, the economics should be very attractive in terms of cost and productivity.

The application of gravity gradiometry in the E&P business has spawned a number of offshoots, one of which has become a case history in the perils of launching new technology. The gravity gradiometer was developed by Lockheed Martin for use by US Navy nuclear submarines. That was until the mid-1990s when the technology was released for commercial application. For marine application the exclusive licence for what is known as 3D full tensor gradiometry (FTG) was awarded to a Houston-based venture Bell Geospace. The company was a start-up founded to exploit the potential of FTG technology for defining complex salt structure/geometry, subsalt play resolution and re-imaging of seismic data using techniques such as pre- and post-stack depth migration. The benefit of FTG over conventional gravity acquisition comes from the multiple accelerometer feature of the system that measures five components or tensors of the gravity field while traditional methods have concentrated on the vertical component. The increased signal bandwidth containing the full spectrum facilitates improved identification and mapping of subtle density contrasts in complex geological settings.

For various reasons the original Bell Geospace business plan came unstuck, and the company underwent an uncomfortable period of restructuring before emerging with its focus directed less at marine and more towards airborne FTG surveying for the minerals exploration and water resource industries. Bell Geospace’s marine aspirations were partly the victim of bad timing in that the company was launched in 1997 on the eve of the nosedive in marine seismic business by which time it was committed to buying a number of high-priced FTG units. In addition, the industry proved less receptive than hoped to the new technology, possibly because FTG acquisition required a separate vessel operation rather than the conventional deployment from a seismic vessel. This was unfortunate because early published results, for example a survey in the Shetland-Faroe Basin, evidenced FTG’s ability to image basalt and sub-basalt geology and to offer an independent constraint for other geophysical methods deployed in the area.

Taking off

The first surveys offshore using an airborne version of gravity gradiometry were flown using the proprietary Falcon technology developed by BHP Billiton in association with Lockheed Martin following an agreement in 1999. One such survey over the Gippsland Basin in the Bass Strait, Australia written up in Exploration Geophysics offered some stateof- the-art conclusions about the possibilities. The authors stated that comparison with marine and satellite gravity data showed that ‘Falcon gravity reproduced all the information available in other surveys at wavelengths up to the survey size. At shorter wavelengths the Falcon data had higher sensitivity than all other datasets including the detailed marine gravity’. There were some qualifications of course, but the message was that Falcon could be applied to oil and gas exploration with the expectation of producing good quality data with a fast turnaround, and it was well suited to onshore and near-offshore or transition zones areas up to 200km from shore. The Falcon technology was purchased from BHP Billiton earlier this year with the seller retaining exclusivity rights with regard to mineral exploration on land, leaving the door open for Fugro to pursue the oil and gas market.

A competitor for Falcon in offshore areas could emerge from the UK company ARKeX which bills itself as the only company offering gravity gradiometry and conventional gravity and magnetics. The entity surfaced three years ago from a collaboration between Oxford Instruments Superconductors and a gravity geophysics company Ark Geophysics (which was later subsumed into the new company). To date ARKeX has been deploying two (FTGeX) gravity gradiometry instruments developed with Locheed Martin. However, the company has just raised $30 million in venture capital to accelerate the company’s services. Some of this money will doubtless be devoted to hatching its long awaited EGG (Exploration Gravity Gradiometer). The EGG will be almost an order of magnitude more sensitive than current systems, according to ARKeX, allowing more geologic settings to be imaged with clarity. It seems logical that the company could soon follow the thinking of the BHP Billiton scientists and also turn its attention offshore, particularly to areas such as transition zones which are easily accessible to airborne surveys and often poorly surveyed in the past.

ARKeX markets its airborne services as the BlueQube package with the emphasis on integration with other geophysical data such as 2D/3D seismic and electromagnetics. BlueQube itself provides a combination of gravity gradiometry, magnetic gradiometry, LiDAR digital terrain mapping and digital video.

No one can doubt that grav/mag has its attraction for the offshore oil industry, especially as the resolution of data improves to offer a valuable and cost effective constraint to seismic data.

There is also room for companies such as Getech, based in Leeds, UK which specializes in brokering gravity and magnetic data from many sources. It also increasingly provides non-exclusive integrated petroleum system studies aimed at reducing exploration risk, especially in frontier or little explored regions, by identifying the influence of regional and basin scale tectonostratigraphy on hydrocarbon prospectivity. To do this it calls upon geophysics, structural interpretation, plate tectonic modelling, petroleum geochemistry, palaeogeography, draining analysis and stratigraphy, all captured within an ArcGIS framework.

The key theme here is integration. Service providers in the potential field methods business are realistic enough to appreciate that their data must form part of a multi-disciplinary approach. An Eni paper at a workshop held last year in Capri, Italy entitled EM, gravity and mag methods: a new perspective for exploration came up with this conclusion which says it all:

‘The most used approaches for inverting potential field data usually are not able to incorporate all geological and petrophysical information in a 3D model that can be profitably used by geologists and engineers. Integrated inversion techniques have to be designed in order to avoid the typical non uniqueness problem. This implies the incorporation of all available constraints (seismic, well markers, wireline logs, petrophysics) in order to correctly model the earth interior. The integrated inversion of gravimetric, magnetic MT, EM and seismic data is the great frontier area awaiting us in the very near future. The proper definition of the geological model must contain not only the elastic properties but also the electromagnetic and gravimetric (density).’

The challenge has been stated, and Eni is not alone in promoting efforts to combine all geophysical methods which can reduce risk in exploration and provide a more comprehensive picture of the reservoir.OE

Click here to register to receive your own copy of Offshore Engineer each month.