Fiber-optic, four-component (4-D/4-C) permanent monitoring systems for four-dimensional seismic are being tested in the Gulf of Mexico and North Sea to prove that optical systems meet the criteria established for permanent monitoring by exploration and production (E&P) operators (Figure 1). The new cable designs and three-axis optical accelerometers have been evaluated in parallel with conventional electrical cables, geophones and hydrophones. Data collected proves an all-optical system meets the performance required for 4-D/4-C monitoring, and potential advantages are seen over conventional all-electrical systems.

Electrical ocean-bottom recording systems require power, heavy cables, circuit boards and

Figure 1. An artist’s view of a permanent installation. (Images courtesy of PGS)
other components. The passive nature of the newer optical system completely eliminates the need for electronics and the potential problems associated with heavier equipment and water leaks that may impact long-term reliability. With an optoelectronic acquisition system, an optical signal is sent from a surface laser to the passive network of sensors. The cables with hydrophone and three-axis accelerometer in each sensor station are much lighter, have a noise floor below 0.0000015 psi (0.1 µBar) and are very stable to pressures equivalent to 9,840 ft (3,000 m, Figure 2).

The arrays being tested now are designed for permanent installation in deepwater applications. A sensor station is typically attached to the optical cable every 164 ft (50 m), but different station intervals can be chosen to fit the acquisition design. The fiber-optic system data shows excellent correlation with that of the electrical system. Figure 3 shows the shot gather comparison for corresponding channels during a shot line.

As the current field trials conclude in 2007/2008, work continues to extend cable lengths beyond 7.5 miles (12 km). The system allows for channel counts in excess of 2,000 per cable. The reservoir imaging capabilities of the new systems are expected to be very high thanks to the extended dynamic range and the very low channel crosstalk and distortion. The optical system is an excellent fit for, and a preferred solution for, permanently installed reservoir monitoring systems.

Why choose permanent?
A global benchmarking study of exploration and production operators conducted by PGS this year revealed several insights to selecting the right 4-D approach to monitor and manage reservoirs:
• 4-D marine streamers are chosen for reservoirs where multicomponent data are not needed or where getting data at the lowest cost or shortest time outweigh the benefits of getting multicomponent data;
• Retrievable ocean-bottom cable (OBC) surveys are chosen when multicomponent data are needed but permanent monitoring is not considered affordable;
• Nodes are viewed as a good choice for 4-D/4-C data when the surface obstructions impede marine survey efficiency or where the seafloor is not suitable for OBC or permanent systems;
• Electrical permanent systems are chosen when multicomponent data are needed; the operator wants proven technology that has already been installed and is operating in working systems; is willing to pay more for installing seafloor electronics, trenching and remotely operated vehicle operations; and the performance life of the system is expected to be less than 10 years; and
• Fiber-optic permanent systems are being evaluated for shallow and deepwater conditions where multicomponent data are needed and the operator wants lower cost cable, sensors and installation and wishes to achieve life expectancy of 20 years or longer.

While few operators are willing to disclose their detailed economics, expected monetary
Figure 2. Hydrophone and three-axis accelerometer sensor station.
value, value of information and full-cycle economics studies of permanent systems over the life of the field can be very attractive because the information influences a number of key decisions impacting recovery, costs and risks. Operators understand that fluid saturation, pressure and compaction changes observed in 4-D data impact placement of infill wells, improvements in workover and production enhancement programs, reservoir characterization studies, and identification of new step-out opportunities. Although the details of business cases are not generally available, published estimates summarizing the impact of a permanent system describe hundreds of millions of dollars in value.

As compelling as the numbers may be, many geophysicists who champion 4-D monitoring report permanent systems are seen as less attractive for capital than other exploration, drilling, subsea completions and infrastructure investments. More work is needed to educate asset managers about the potential returns of investing US $25 million to $75 million in permanent arrays that can provide information every few months about how reservoirs are changing over the life of the field. Some industry stakeholders frame the value of these systems as being equivalent to drilling one to two additional development wells — the 4-D system of the future should be seen as part of the upfront infrastructure investment needed to harvest optimum value from the discovery.

For some operators, once signal-to-noise (S/N) looks favorable, return on investment is less of an issue than reliability. Operators worry that a system costing tens of millions of dollar may fail early in its life. Rigorous testing and sea trials show, however, that fiber-optic systems should survive even deepwater environments for many years.

Variable value
In a study conducted by Shell and reported at the 2006 CSPG-CSEG-CWLS convention, the value of 4-D was charted versus the maturity of the application. Six general categories were defined (in order of maturity and value of the information):
• High porosity gas or light oil waterflood (and gas flood), carbonate reefs, cemented clastics, deepwater, deltaics, and heavy oil steamfloods;
• Pressure injection signals;
• Depleting chalk/limestone formations;
• Depleting high-pressure/high-temperature sandstones;
• Depleting stacked reservoirs (laminae); and
• Depleting normally pressured sandstones.

They noted that a reservoir may sit in more than one category. In other words, a reservoir
Figure 3. Shot record comparison, optical left, conventional right; top to bottom, hydrophone, vertical, inline and crossline components.
may have an easily interpreted waterflood response but a very complex depletion signal. Economic value, in the above cases, was dependent on having a sufficiently large S/N ratio and on the ability to quickly interpret the data to determine sweep patterns, fluid pressure, overburden stress, fault transmissibility and stratigraphic reservoir distributions, to name a few possible answers. This makes a stronger case for repeatability, which has always been an issue in 4-D acquisition schemes. The better the repeatability, the better the resolution and the ability to discriminate subtle differences in the time-lapse data.

To the extent that a passive fiber-optic 4-D/4-C system can deliver significant improvements in S/N as well as acquisition consistency and quality together with long-term reliability, it has the potential to move some of these applications up on the value curve. Besides monitoring changes in saturation, time-lapse has proven valuable in pressure monitoring, compartmentalization studies and compaction detection, adding to its usefulness for reservoir and production engineers.

An emerging technology is the tracking of the propagation of hydraulic fractures by monitoring the microseismic events generated as the rock fails.

Extremely valuable during field development, this application is being used to maximize reservoir contact, which bears significant relevance to ultimate recovery. A company with an existing 4-D/4-C fiber-optic grid in place could derive instant benefits from this application with very little additional expense.

Tying it all together
The PGS worldwide benchmarking study also revealed that no operator wants to be the system integrator who manages the overall design, procurement, logistics, installation, operation and maintenance of the system. PGS addresses these needs through
a number of business models that give the operator the opportunity to choose à la carte or to completely outsource the system from design through operations. Better acquisition, faster processing and more insightful interpretation are making 4-D/4-C a more attractive choice for a wide variety of applications. As the industry explores, drills and produces deeper targets lying beneath deeper waters, reliable, consistent monitoring is needed to maximize return on investment.