Shell’s second waterflood monitor survey offshore Brazil shows technical success. Jeannie Stell reports.
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SBM Offshore’s Espirito Santo FPSO can produce up to 100,000 bo/d and inject up to 75,000 b/d of water. |
By all accounts, Shell’s first full-field, ocean-bottom, life-of-field 4D seismic system (LoFS) implemented in the operator’s BC-10 Parque das Conchas development, offshore Brazil, is a success. There, Shell’s waterflood monitor survey shows the enhanced oil-production system is hitting all targets, thanks to 4D imaging.
“At the time of its installation, as part of the BC-10 phase 2 development, this was Brazil’s first permanent LoFS network deployed on a full-field scale, and also the deepest deployment of this technology in the world (circa 1800m),” says Shell spokesperson Kayla Macke.
“These challenges proved to be valuable learning opportunities for Shell in dealing with 4D LofS,” she says. “Various factors come into play as Shell evaluates and chooses to deploy certain technologies in developing a field. In this particular case, our need to frequently monitor the behavior of the fluids inside the reservoir played a decisive cost-effective role in our decision to install a permanent network of sensors on the ocean floor.” However, it’s not just the 4D technology that is setting records.
Set the scene
Shell (50% interest) and its partners Petrobras (35%) and ONGC (15%) also set a record with the deepwater, Campos Basin, Parque das Conchas (BC-10) development by employing the first subsea installation tie back to a centrally located, turret-moored, FPSO, stationed in 1789m water depth. BC-10 is also the first of its kind, based on subsea oil and gas separation and subsea pumping.
The development includes the first application of steel tube hydraulic and multi-circuit high power umbilicals that deliver power to 1500hp pumps on the seabed, and is the first application of lazy wave steel riser technology on a turret moored FPSO.
Shell’s BC-10 development produces heavy oil, with densities ranging from 16°-24° API gravity, from four small-to-medium low-pressure, dispersed reservoirs, which requires water injection to maintain pressures.
The field’s the reservoir, consisting of unconsolidated turbidite sands with an average thickness of 25m, is relatively shallow by virtue of its top lying just 1200m below the mudline.
The FPSO Espirito Santo, a converted tanker leased by Shell and its field partners from owner and builder SBM Offshore, is the workhorse here, and is able to produce up to 100,000 b/d of oil and inject up to 75,000 b/d of water.
The field was developed with 22 wells with more than 20km of reservoir completions, 10 subsea pumps providing around 15,000hp of lift capacity, and more than 300km of subsea pipelines and umbilicals.
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Overall, the 4D enabled early observations that provided critical information to Shell, which led to optimized well rates during the early life of the BC-10 field. |
Cost-effective 4D
The seismic program includes 100km of untrenched ocean-bottom cable in 14 lines spaced 400m cross line and four component sensors placed 100m in-line covering the entire reservoir and extending over the bounding faults to the west. Trenching for the cable lines and sensors was unnecessary due to the depth of the lines, but nonetheless the cables were carefully set into place to ensure longevity.
“The network of seismic sensors installed on the ocean floor is expected to remain in place for as long as Shell continues to produce from that field,” Macke says.
The seismic cables are connected via 11km backbone cables to the existing umbilical termination assembly, which supplies power to the subsea LoFS system. Spare fiber-optics fibers in the existing umbilical are utilized to transmit data to the topsides and to provide control communications from topsides to subsea. The topside system includes a cabinet of power supplies in the FPSO turret, a dual-rack cabinet for the operating system and data storage in the main equipment room. The seismic system can be operated from either the FPSO or from Houston-based remote operations via a dedicated satellite link.
Because low-cost seismic is a crucial element to manage the costs of frequently repeated surveys for the 4D data generation, Shell used a fit-for-purpose seismic vessel to tow a single source comprised of three sub arrays of 2450cu in, with source points acquired on a 50x50m grid.
Navigation data from the shooting vessel are available shortly after completing a sail line. The data are sent via satellite to the recording system where they are merged with the seismic data. The first monitor survey was acquired within 30 days and achieved good repeatability with 93% of shots within 5m of pre-plots.
The base seismic survey was completed in November 2013 prior to the first water injection. After the water injection in 1Q 2014, the first seismic monitor survey was undertaken and completed in June 2014.
Time-lapse 4D seismic data was utilized, and was required for two reasons. First, the operator wanted to optimize the waterflood project by measuring the changes in fluid saturations and pressures. Secondly, it needed to monitor the effectiveness of the water injection process.
Data processing
Shell’s Marine Imaging Group drew on prior 4D-processing experience that it developed for ocean-bottom node surveys, with additional pre-processing, including 3-component geophone orientation. The team was able to accomplish the rapid delivery of quick-look 4D seismic volumes to the operation and development teams only three weeks after completion of the monitor survey acquisitions.
The high fold and good repeatability of the LoFS data helped reduce in-field noise to acceptable levels. The in-field noise was from rig activities and some seismic interference was generated due to another survey undertaken during the waterflood monitor.
Pressure inverted echo sounder instruments were converted to connect directly to the LoFS system for continuous power and data transmission and to provide good water velocity and tidal depth data for deepwater statics solutions.
Quick-look results
Despite the short duration between the start of the water injection and the first seismic monitor survey, significant information was obtained that confirmed that there was no out-of-zone events and that water was injected along the entire length of all laterals. The seismic confirmed the location of early water production in two wells and that no water fingering from injectors to producers was observed.
Specifically, long, narrow hardening signatures along each injector indicated the successful injection of water along the entire length of the more than 1000m laterals. Also, the 4D survey showed that two of the producers with the highest offtake rates exhibited strong softening anomalies caused by gas saturations of 2-3% due to flowing bottomhole pressures below bubble point.
Conclusion
The 4D seismic endeavor enabled early observations that provided critical information to Shell, which led to optimized well-production rates during the early life of the field. The data confirmed the absence of water fingering, which increased confidence for the continued similar and enhanced operation of the field, and allowed higher offtake rates of the relatively viscous oil while still maintaining good reservoir sweep efficiency. These observations supported the decision to operate the field at higher rates, which provided early economic benefits.
Shell reports that, beyond 4D, the baseline data will be integrated with new directional deep-reading resistivity logs from the lateral sections, as well as being integrated with inversions for reservoir properties, to build new static and dynamic models. Broadband processing will be used to aid understanding of the overburden and analyses of the reservoir depth. Passive seismic recordings will be the subject of further internal research efforts.
When asked if this technology will be deployed at similar fields, Macke says: “The LoF 4D was a cost-effective solution for BC-10, and our learnings from Brazil can be used elsewhere, if deemed appropriate for development plays in other regions.”
Based on a Shell interview and Shell’s whitepaper, OTC-25803-MS.
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