Q-technology, with streamer control and data density, provides seismic options

The seismic sector of the oil and gas business is engaged in constantly remaking itself and its products. The business adjusts, takes economic down- turns as opportunities to apply new thinking, and when necessary returns to its roots for a new solution. The latter is what Schlumberger may have done with the intro-duction of Q-technology, specifically the Q-marine acquisition system. The re-think has taken two basic forms:

• The way that signals are collected
• The way that streamers are controlled.

The company re-thought the basics of signal-to-noise and the standard process of trace summing used by the industry. Standard practice in the industry is to add more hydrophones to the sensor group and trace-sum those hydrophones to boost the signal coming from a group. The group hydrophones are hard-wired together to collect the returning airgun energy and create a stronger signal. Depending on the group spacing, certain types of noise can also be dampened and controlled.

What is produced is an averaged analog trace. The stronger signal produced by summing across the group can then be muted and filtered to control induced noise, yet will retain enough signal to produce good images. This summing creates a problem. Discrete data always gets smeared in the process of averaging. Bad sensors harm the whole group and some forms of noise cannot be removed from the signal since it is embedded in the averaged signal.

To overcome these problems the new system recreates a much older form of seismic acquisition. It captures individual hydrophone signals, models coherent noise, and removes the noise from the signal.

What is produced is a data stream that has a cleaner signal without the embedded noise that summing traces produces. Capturing individual traces also creates a new form of flexibility for the data volume. Dead or poor quality sensor readings can be eliminated; traces can be grouped and re-grouped at will with software instead of reconfiguring steamer hardware and re-shooting a survey. Costly field manipulations are now handled in the processing center. Regrouping rather than re-shooting can now focus the survey for shallower or deeper horizons to improve imaging.

Streamer control

Controlling the streamer creates more consistent bin spacing and permits the recreation of survey paths for 4D production surveys.

One of the biggest problems that marine vessels encounter is maintaining control of the "spaghetti" towed behind them. The streamers must remain in tension at all times, but are at the whim of currents and sea state. Turning is a particular problem and multi-streamer tows require a large turn radius to keep from tangling the streamers. These problems are solved by intelligent streamer steering.

The new system integrates a network of steering "birds" into each streamer which compensate for lateral current effects. Each bird has two independent vanes that can be controlled to both twist and shift the streamer laterally as needed to overcome feathering and maintain a straight survey line behind the vessel.

All birds attached to the streamer create noise. The new birds are integrated into the streamer rather than hanging below the streamer, so they have a lower noise aspect at the start. It is important to note that most types of streamer noise seem to be random, but when well enough sampled, turn out to be coherent. When properly recorded and processed, bird-related noise can be removed from the recorded signal.

Whole network acoustics throughout the streamers and GPS at the front and tail monitor the bird positions. This means that individual streamer paths can be recreated in the future when required. This opens up significant possibilities for 4D production seismic surveys.

4D surveys

Generating 4D surveys by comparing 3D surveys collected at different times and in different directions, not to mention different parameters, is very difficult. Numerous papers at recent technical meetings have demonstrated the extreme digital manipulations needed to compare data volumes and track fluid levels within reservoirs. A good, high-quality baseline survey with future repeatability is what the market has needed. This is now within reach.

The new system will shorten the cycle time on 4D projects allowing quicker identification of bypassed zones. It will eliminate major normalizing efforts required for comparing older 3D datasets. It will return answers in record time, allowing closer monitoring, and open possibilities for detailed attribute mapping of individual reservoirs.

Business implications

The seismic acquisition and processing business has been in difficult straits the last two years and is just beginning the process of consolidation. While this may be necessary to control costs and bring supply and demand back into balance, it does little to add value for the consumer - the oil companies. The process shrinks the number of market players and generates a bulk shift upward in prices for data through reduced competition.

This is necessary for the survival of seismic acquisition companies and in large measure is a result of oil company's excessive focus on squeezing survey costs. Cheap is good until you can not get what you need because the corporate expertise has been driven out of business. This new acquisition system changes the game and brings new value to the market by:

• Producing cleaner, flexible datasets
• Creating the potential for truly repeatable surveys
• Preserving regional datasets at reasonable prices
• Allowing an oil company to buy the data density it needs to solve its interpretation problems
• Differentiating the surveys collected by their true usability.

In short, there is something for everyone in the new system. Those companies, who can look beyond cost to value, will find a tool worthy of their projects.