Claudio Paschoa provides insights from technical sessions at this year’s Rio Oil & Gas event in Rio de Janeiro, Brazil.
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Petrobras Geophysics Manager Paulo Johann. Photo from IBP. |
At Rio Oil & Gas, Paulo Johann Ph.D., Geophysics Manager at Petrobras, explained how Petrobras is applying permanent reservoir monitoring (PRM), through high-quality 4D4C seismic reservoir surveillance, at record depths of as much as 1350m (4428ft) at the Jubarte field in the BC-60 block of the Campos basin.
The Jubarte field has a total area of 245sq km, and is located in the northern portion of the Campos basin, off the southern coast of Espírito Santo state. It sits 77km offshore in water depths ranging from 1185m-1365m. Jubarte’s oil reserves are estimated at 600MMbbl of heavy (17.1°API) oil.
Although Jubarte lies in a pre-salt area, the reservoir is post-salt. Johann says that Petrobras had taken as many as 3.8 million sensor traces per square kilometer with four-component sensors at every 50m (164ft) along the cables. “There is very high potential for enhanced oil recovery, simply by increasing our knowledge about reservoirs. If we understand them, we can learn to exploit them better,” Johann told the conference. Petrobras’ subsea seismic grid is the first and only PRM project in deep waters, and Petrobras’ primary objective is to validate the deepwater fiber optic sensing technology for detecting small impedance changes.
The target is to monitor oil and water flows inside the reservoir and pressure changes due to injection and production. Possible discontinuities, which would impact the flow and that couldn’t be identified using regular 3D seismic data, will be revealed by 4D seismic data as an anomaly limit for difference volumes. This pilot project, launched at the end of 2012, is composed of 35.6km of seismic cables, arranged in two subsea arrays that cover 9sq km in the south portion of the Jubarte field. The seismic area covers 121sq km.
The cables are anchored 300m apart to ensure good coupling, reduce noise and prevent lateral movements due to currents. They have a total of 712 4C receiver stations (three-component accelerometers and a hydrophone) at 50m intervals and were deployed in parallel lines. Jubarte PRM acquisition geometry presents a super density of seismic traces. The area of interpretable seismic data for the main reservoir is approximately 35sq km. The first acquisition was concluded in February 2013 and the processing is ongoing by Petroleum Geo-Services (PGS). Passive monitoring of seismic activity of the reservoir is also being acquired for a four-month period before the second acquisition (first monitor) scheduled for December 2014.
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Paulo Johann Petrobras Geophysics Manager speaking about the PRM. Photo from IBP. |
The Jubarte PRM seismic system uses PGS’s Optoseis technology. The system comprises a fully 4C (four-component) fiber-optic sensor array installed on the seabed and an optoelectronics controlling and recording unit installed on the FPSO P-57. Online data quality control is carried out aboard the FPSO, the seismic data is then uploaded to PGS’s processing center in Rio de Janeiro for further processing by a team of PGS and Petrobras geophysicists. Following the installation of the system in deep waters, Petrobras and PGS began acquiring active seismic data at least once a year using a seismic source vessel and passive or microseismic data between them. The PGS OptoSeis system is totally optics and is based on the Michelson interferometer sensing elements. In addition to fiber optic seismic array, the system also contains led-in cables, wet-mate connectors and optoelectronics equipment.
Permanently laid sensor cables are an alternative to other forms of repeated, or 4D, seismic data acquisition. The processed seismic data and images, allow geophysicists, geologists and reservoir engineers to produce seismic interpretations that help Petrobras optimize reservoirs management of the Jubarte field.
“The great advantage of the permanent seismic monitoring project is to enable the optimal management of Jubarte’s reservoirs, with huge potential impact on increasing the oil recovery factor,” Johann says. The PRM will enable a constant update of the geomechanical model of the Jubarte field and deepen the understanding of the value of microseismic data in deepwater projects. Data processing is in progress, and images to date have been high quality, achieving reservoir group requirements.
“4D seismic is attained through PRM by repeating 3D seismic on a regular basis, so we can create a dynamic model or film of the reservoir by adding different images above each other,” Johann explained.
Currently, Petrobras is evaluating the possibility of extension of the permanent seismic array to the north and northwest Jubarte.
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