One of the most important management tools is utilising ocean bottom systems ... generically know as 4D seismic ... in order to assess hydrocarbons migration within a reservoir and therefore how best to boost recovery rates, perhaps through waterflood and CO2 injection.
Since 4D and related systems came into use in the mid-1990s, there has been been a steady growth in the market and growing array of kit and techniques designed to improve data quality and therefore provide better reservoir management tools.
Latest into this stable are new systems by Stingray Geophysical (Fosar), Schlumberger (Deep Reading) and Ion Geophysical's new acquisition optimisation services arrangement with PGS.
Stingray has only just revealed that it has achieved successful results from its analysis of data recorded during the final sea trial leading to commercialization of its Fosar permanent reservoir monitoring (seismic PRM) system.
The company says that its system demonstrated "excellent geophysical response" characteristics.
The offshore evaluation was carried out in November 2008. An array of multi-component receivers, deployed in more than 260m water depth, was trenched into the seabed for optimum geophysical coupling and protection from mechanical damage. A fibre-optic riser cable joined the seabed array to an opto-electronic interrogation unit situated on surface facilities.
Interestingly, no subsea electrical power is required for the array - a feature of the Fosar system that the company says makes it inherently more robust and much more reliable than comparable electrical systems in harsh subsea environments.
A vessel, equipped with an airgun seismic source, acquired a 3D grid of shot-points over the array; with resulting data from the seabed receivers recorded using the system's compact interrogation unit.
Stingray sans that analysis of the data-set confirmed the geophysical performance characteristics of the system, notably low noise floor, high vector fidelity, and capability to record seismic signals over an 180dB dynamic range - a range 1,000 times greater than typical electrical systems.
The company says this proves that Fosar system can handle even the largest airgun arrays at close proximity to the sensor.
Fosar comprises of four-component, passive fibre-optic sensors comprising one hydrophone and three "orthogonally" mounted accelerometers per station or optical sensing unit.
It possesses proprietory, highly efficient, optical multiplexing architecture developed in-house to minimize the number of optical fibers required for a high channel-count array, giving it flexibility and making it easy to install with less impact on oilfield infrastructure than alternative seabed seismic systems.
Stingray claims that the successful sea trials mean that it has achieved all objectives of the development programme, including delivery of high-fidelity sensor performance.
The net result is that, Fosar is now available for seismic permanent reservoir monitoring applications.
Meanwhile, Schlumberger has reported the expansion of its Deep Reading portfolio with the release of the new electromagnetic DeepLook-EM enhanced "cross-well" reservoir imaging and monitoring system and the acquisition of cross-well seismic reservoir imaging technology.
The services include pre-job planning, modeling, simulation, acquisition, processing and inversion to deliver inter-well reservoir images.
"DeepLook-EM and our recently acquired Z-Seis cross-well seismic service bridge the resolution gap between well logs and surface measurements to provide customers answers at the reservoir scale," Colin Hulme, technical director at Schlumberger unit Deep Reading said at this year's Middle East Oil & Gas Conference.
"The addition of these technologies provides the opportunity to deliver advanced reservoir monitoring answers from combined measurements and by integrating them with additional data."
The second generation DeepLook-EM system provides detailed resistivity profiles between wells up to one kilmetre apart using time-proven induction logging principles. Acquisition is performed using a dynamic transmitter sonde in one well and an array of receivers in an offset well. Receivers can be deployed in open or cased wells.
In development for more than seven years, Schlumberger has now successfully completed DeepLook-EM surveys in Canada, China, Brazil, Middle East and the US.
The group says that the cross-well images are providing customers vital information on the efficiency of water- and steam-flood programmes allowing inter-well saturation to be estimated and bypassed pay zones to be identified.
Inter-well resistivity variations can be caused by changes in saturation during water-flooding, in temperature during steam-flooding or porosity reduction if subsidence occurs.
With time-lapse monitoring using DeepLook-EM, flood-front movement can be accurately depicted and tracked.
All field data are compiled within Petrel(1) seismic-to-simulation software, and are seamlessly integrated to model and interpret the reservoir volume logged.
Schlumberger says Interwell seismic imaging can provide seismic velocity profiles and seismic imaging at resolutions an order of magnitude higher than surface seismic.
This detailed information can be used to understand reservoir characterization at the reservoir scale and in a time lapse mode to monitor fluid movements such as in steam injection and CO2 sequestration.
Also fresh off the press, so to speak, ION Geophysical Corporation has, through its Concept Systems offshoot, reached a deal to provide 'acquisition optimisation services for all of Petroleum Geo-Services' (PGS) repeat, time-lapse (4D) seismic surveys this year.
The workscope covers:
- Survey design and feasibility - Evaluating the multiple acquisition scenarios and resources that influence acquisition to ensure the appropriate balance of data quality and cost efficiency.
- Coverage modelling - Utilising proprietary technology for advanced feather prediction to perform accurate coverage and repeatability modelling for any given sail track.
- Survey Optimisation - Providing experienced in-field specialists to combine predictions and acquired data with the latest technology to dynamically optimise acquisition in real-time.
The agreement further extends the recently signed 5-year, fleet-wide agreement l for PGS' use of ION's Orca command and control system, which lies at the heart of ION's Intelligent Acquisition portfolio. During these time-lapse surveys, the companies will work together to jointly develop advanced 4D features to further improve data quality and survey operations.
Successful 4D surveys acquire data with sufficient fidelity to track reservoir fluid movements over time. Due to the high repeatability requirements, 4D surveys often incur costly and time-consuming re-shoots to meet field operator specifications. ION reckons it can do this.
Concept Systems' breakthrough real-time modelling software is designed to accurately predict the impact of various acquisition options. Experienced in-field acquisition specialists select the optimal choice at every step. The comprehensive process strives to achieve maximum repeatability, productivity, and safety.
A closing snippet for this brief summary ... and link with our seismic technology review ... is StatoilHydro's contract with PGS to deploy its ground-breaking GeoStreamer technology on 4D work on the Norwegian Continental Shelf.
This will facilitate the shooting of seismic to be compared with former seismic surveys and thus provide more reliable results for 4D studies. PGS' second new S-Class Ramform vessel, will work with both steerable streamers and steerable sources in order to produce the highest possible repeatability for 4D surveys.
A measure of the value that StatoilHydro places on 4D today is that, up to mid 2008, the company had drilled 17 wells in its North Sea Gullfaks field based on this seismic technique. The company has admitted too that these wells would hardly have been drilled without the use of this technology. And the Gullfaks prize ... 60 million barrels of high grade crude.
Basically, 4D has proved a moneyspinner for StatoilHydro and is now a vital tool for accessing $billions worth of additional reserves.