Good bottom couple required for 4D/4C seabed surveys

New methods seek strong P and S wave signals

Victor Schmidt
International Editor

4D seismic is moving to the seabed, gathering four component data for better reservoir images. [16,695 bytes]

Time-lapse 3D seismic surveys or four-dimensional (4D) surveys are proving their worth. Most 4D work is done using marine seismic, traditional airgun, and multi-streamer data collection. While the individual surveys are good, significant problems arise when comparing any two surveys. Matching and normalizing data sets collected a year or more apart requires seismic processors to overcome many problems.

Oil companies are seeking to improve the 4D technique and are willing to fund surveys to that end. Both project money and research money are testing new ways to minimize variability in the data collection process and improve the usefulness of the data gathered.

One major improvement is to change the location of sensors, moving them from a towed streamer array to fixed seabed locations. Foinaven Field in the Atlantic Frontier is an example. The field has 36 km of seismic streamer fixed to the seabed for 4D surveys. Its next 3D dataset will be collected later this year.

Sensors on the bottom open the possibility of improving and extending the signal received. In such arrays, streamer hydrophones can be linked with multi-directional geophones, adding multi-component data to the traditional hydro phone data set. These four-component (4C) sensors are now being used in some ocean bottom cable systems.

Comparison problems

The basic problem of survey configuration exists when comparing legacy 3D surveys for production tracking. Equipment improvements and in changes technique make comparing 6-fold, dual streamer 3D data with modern 48-fold (or higher), 8-streamer data a challenge. Problems occur even when surveys are collected using similar vessel/streamer configurations. These survey differences include:

Ocean current directions and speeds
Survey shooting direction
Navigation data
Vessel tracks
Source pulse locations
Streamer feathering
Hydrophone response.

Physical changes in the mudline topography and near surface sediments also affect the surveys being campared and include:

Scouring
Slides
Fault movements
Sediment deposition
Sediment compaction.

Some very soft bottoms are sensitive to the position of the Moon because they move with the tides.These location effects and physical changes alter the seismic signal reflected from the field under examination. All create signal differences in the processing bins of the individual 3D surveys from one survey to the next.

Normalizing the surveys is a major task that includes bin alignment, signal-to-noise balancing, statics adjustments, and other corrections. Once the required adjustments are made, the normalized 3D datasets can be compared against one another for differences more closely related to oil and gas production.

4C data

Hydrophones collect pressure wave data while multi-directional geophones collect both pressure (P) and shear (S) wave data. By combining both sensor types into either ocean bottom cables or single-point sensor groups, 4C surveys can be closely tied to older seismic data.

Hydrophone data permits direct comparison to older streamer seismic and the multi-directional geophone data provide a closer link to well logs. 4C sensors also extend the range of information available to the production geophysicist.

The 4C data allow the geophysicist to better understand the bulk modulus and Poisson's ratio, two key parameters that affect the seismic signal from the producing horizon.

Bottom-couple

Bottom-couple (or the generic ground-couple) is a fundamental characteristic of all 4C sensors. It is the ability of the sensor to move in concert with the ground as the acoustic wave passes through the sensor. The ideal is a sensor system with a density that matches the seabed sediments.

P-wave information is easily collected and usually has good signal-to-noise characteristics and good couple in the water and on the seabed. S-wave information is more difficult to gather because the waves move side-to-side, perpendicular to the P-wave direction. S-waves are not transmitted through liquid, only solid media.

To get good signal-to-noise ratios for S-waves, the 4C sensor must be fixed firmly to the seabed, coupled to the bottom. This difference makes it possible to compare S-waves to P-waves for examination of reservoir fluids.

Achieving a good bottom-couple is a major problem because seabeds can be either hard or soft. Most of the seismic contractors are working to solve and resolve bottom-coupling questions:

Fixing a sensor to a hard seabed?
Getting a good couple in semi-liquid mud?
Losing a sensor in a very soft seabed?
Locating and extracting a sensor if it is deeply imbedded?
Emplacing, extracting, and moving a sensor in deep water?
Setting and maintaining sensor locations in fast current conditions?

Experimentation

Because of these problems seismic contractors are trying different collection methods: ocean bottom cable; multi-component streamers fixed to the seabed; and single sensor grids set on the seabed (multiple methods).

The advantage of seabed-fixed sensors is surface accessibility and data richness. The source vessel can maneuver around production structures and congested areas easier than a vessel towing streamers. It can move over the array with multiple passes in multiple directions to illuminate the reservoir from different angles. This is especially important when there is a preferential direction for achieving strong signals.

If the sensor array can be fully activated, a form of seismic holography can be generated. Each sensor will capture time-difference information between sensors with each source shot. These methods gather a full suite of seismic data and are a way to constrain various problems of repeatability.

Ocean bottom cable (OBC) construction has a major effect on the sensor array's effectiveness. The rigidity of the cable, the positioning of the sensors, and the distribution of weight along the cable all have a noticeable effect on bottom-coupling and the strength of the recorded signal.

Cable deployment also has an effect, whether the cable is dragged along the seabed or laid, then lifted and repositioned. High seabed currents in some areas interfere with both positioning and bottom-couple. OBC's also have depth limits due to the strength of transducer positioning systems, "pingers," on each cable.

4D/4C monitoring

Achieving good 4D/4C data is critical for maximum production from offshore fields. A good initial image of the reservoir(s) provides a close estimate of reserves-in-place. This translates directly into future dollars-in-place, which are necessary as collateral for project funding and debt service. It sets the parameters for the economic size of the field development project. 4C seismic adds a four-fold richness in data density to the survey as well as yielding:

Higher signal to-noise ratios
Higher frequency content
P and S wave comparisons
Anisotropic information (preferred flow directions)
Reservoir shape
Reservoir thickness.

Future use of 4D/4C data

The market for seismic services continues to grow even in this era of lower oil prices and limited oil company cash flow. A major reason for this continued strength is business growth downstream into reservoir monitoring and production tracking.

Oil companies are pushing 3D processing and imaging technology to identify and quantify changes in reservoir fluid volumes. The addition of 4C data to the 4D survey (4D/4C) creates a data-rich, repeatable survey that allows more complete reservoir evaluation and production tracking.

The scope of offshore projects creates a need for continued experimentation and new technique development. Many of the processes used on land are finding new life and application in the marine environment even as we are beginning to understand the relationships and special problems of the 4D/4C surveys.

As researchers develop new approaches to 4C data, we will see improved surveys, better lithologic reservoir descriptions, more closely monitored reservoirs, better reservoir drainage, and fewer bypassed barrels. In short, we will see more efficient and effective reservoir management.