A new sensing system moves seismic technology from an exploration tool to a production tool.

The concept of time-lapse seismic, also called 4-D seismic, is not particularly new. Neither is the debate surrounding how best to carry out the task.

But in recent years consensus seems to be building that no one application is the best answer in all cases, but rather that a combination of surface, borehole and permanent sensors can be utilized to give the best information about continuing changes in the reservoir. To that end, Weatherford has introduced Clarion, a fiber-optic seismic sensor that is installed during the completion phase and remains in place in the wellbore throughout the life of the well.

Launched at the 2003 Society of Exploration Geophysicists (SEG) convention last October, the technology has been a work in progress since the late 1990s, according to Tad Bostick, vice president of business development for Weatherford Intelligent Completion Systems. "The whole progression was, first, to demonstrate that we could actually make multicomponent seismic measurements with an optical instrument," Bostick said. "That was at the end of 1999 and into 2000.

"Then we went on to shrink everything to fit into boreholes and to implement a complex, state-of-the-art optical architecture, which allowed us to build seismic arrays with optical in-well components. After that we worked on smaller, deployable, fit-for-purpose sensors, along with an optical multi-channel architecture."

Not a small task, in other words. But proof-of-concept through product launch was less than 4 years.

The benefits are several. The system enables production optimization by making on-demand high-resolution reservoir imaging possible. This facilitates the optimal placement of development and infill wells, reduces drilling and completion costs, improves remedial action planning and implementation to manage water and gas ingress, and advances the understanding of reservoir sweep and voidage replacement.

Field trials

A number of field trials have moved the process forward. One trial focused on deploying a single-channel sensor downhole, with the primary goals of coupling the sensors to the formation, examining signal-to-noise ratios and gauging the performance of the fiber-optic sensors relative to reference vertical seismic profiling (VSP) tool measurements. Results of the study indicated that multicomponent seismic data could successfully be acquired with tubing-deployed fiber-optic sensors.

Another study in the Izaute gas storage field in southwestern France involved the permanent installation of a multi-station, multicomponent array and the subsequent acquisition of a second survey for time-lapse studies. Two types of permanent seismic surveys are being conducted in the field, VSP and extended micro-seismic monitoring. Previously completed feasibility studies using conventional wireline tools have demonstrated the ability to map the gas-water contact using VSP and to potentially correlate micro-seismic events to gas injection and production activity.

The array was installed on the outside of 4-in. production tubing inside 9 5/8-in. casing. This design allows for easy access through the tubing to the reservoir for logging tools. Fiber-optic 1/4-in. cabling links the seismic array to the surface and ties the sensor stations together.

The sensor output was interfaced in analog form to a conventional seismic recording system. Comparisons were made to field data previously acquired with wireline VSP, and a good comparison was found.

While the initial acquisition went smoothly and the data showed good repeatability, results of the time-lapse study conducted later were pending at press time. But the installation demonstrated that a distributed array of fiber-optic sensors could be readily deployed on production tubing in a cased hole with minimal impact to the well.

The next stage is to move into a more sophisticated multi-station installation offshore, which was scheduled to begin in December 2003.

Product launch

"That's how we're progressing in terms of gaining experience," Bostick said. "In the meantime, we've reached a level of confidence with this installation to go into a commercial launch phase. We launched the commercial availability at SEG to get the general community aware that we're ready to go with this."

This has not been a trivial task. Bostick said the product launch entails two main steps:

* Getting the product and the manufacturing facilities ready to go along with training the field operations and marketing staff, and
* Cross-training people across the intelligent completions business unit at Weatherford to begin to integrate Clarion with other permanent sensing products such as temperature, pressure and flow control sensors.


Weatherford has benefited from offering part of the package without trying to reinvent itself as a geophysical contractor. The product offering involves the in-well sensing systems integrated with its completions and other sensor products, along with the surface instrumentation, which is designed to interface with almost all commercial seismic systems. The processing is also done by third-party software.

"You bring the energy sources in and take active seismic surveys on a repeat basis, such as localized VSP surveys, at whatever interval you like," Bostick said.

"Also, you may have continued surface seismic operations, so when those operations are taking place, you are able to simultaneously coordinate data from the well sensors with the surface system. The whole objective in this case is to provide the badly needed calibration points."

Development hurdles

One of the key challenges in developing this technology was making the system small enough to be deployed in the wellbore without forcing well design changes to accommodate the sensors.

"One of the things we had to face was the very firm constraints from the engineers," Bostick said. "It forced us to make some trade-offs between sensitivity, bandwidth, dynamic range and all the key sensor characteristics needed for seismic measurements.

"But we ended up with a sensor that fits the constraints (1-in. diameter) and, we think, hits a sweet spot in the seismic sensing world."

The sensors have a bandwidth from 3 to 800 Hz and have very high sensitivity to handle the range of borehole seismic applications from to micro-seismic monitoring to VSP measurements and surface calibrations.

An ongoing hurdle for any production time-lapse application is the gap between the geophysical community and the engineering community. "The geophysical community is concerned with sensors, data acquisition, images and attributes," Bostick said. "But we have another set of customers, which are the completion, production and reservoir engineers. The completion engineers are concerned about how we're going to get things in the wells while being sure they're safe and efficient and don't add any significant risk to the well. Then there are the reservoir and production engineers, the ones who will ultimately benefit from the seismic information being folded back into their domain."

The degree to which these factions communicate varies from company to company, and Bostick said that often it's service companies like Weatherford who are offering an integrated service that brings the different factions together. "If you're talking about seismic production optimization, you have to get all the parties involved. Geophysicists can't do it independently. None of them can."

So is the oil and gas industry ready for fiber-optic in-well sensors? "The reception at SEG was the best we've ever had," Bostick said. "We've been sitting in the back saying we have this new technology that might be available one of these days, and we finally are ready to look at the commercial market."

Moving from exploration to production should help. "It's been a difficult road for wireline VSP and borehole seismic to compete in the exploration side of the business," Bostick said. "Those technologies have made great contributions, but their impact hasn't been as ubiquitous as you'd think it could have been in the exploration world.

"But those applications in a production world really can have an impact if the technology is right."




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