Graham Gaston, Weatherford International Ltd
Weatherford and the operator installed the world’s first offshore permanent in-well optical seismic system in the operator’s injector well located in the Valhall field. The Clarion permanent in-well seismic system consists of sensitive, multicomponent miniature optical accelerometers and uses advanced optical multiplexing, based on Bragg grating technology, to augment the operator’s permanent ocean-bottom seismic system. The in-well system also includes an optical pressure/temperature gauge, deployed on the same cable.
Sensors
This North Sea installation is one of a growing number designed to provide 4D seismic by comparing images derived at different times. Changes in the images can indicate fluid movement in the reservoir, allowing the operator to determine how efficiently it is draining the formation. The sensors enhance the accuracy and understanding of those images by providing a constant reference point throughout the years-long process of 4D seismic. Ocean-bottom sensors deliver an image in time only. To translate that image to one that is representative of the real subsurface, the operator must convert time to depth – something done routinely for individual surveys. But after 4D – over five, six, 10, or 20 years – then there are variables that make it difficult to compare one survey to another. The sensors used on this platform give the operator a calibration constant that enables much better comparative images from the ocean-bottom cable.
The world’s first offshore permanent in-well optical seismic system installed in the operator’s injector well located in the Valhall field. The Valhall project is expected to continue for up to 20 years.
The tubing-deployed sensors, or seismic stations, are run much closer to the reservoir than the permanent ocean-bottom cable sensors, as they are part of the completion string. A specially designed active clamping system optimally couples each three-component sensor to the casing and substantially decouples it from the production tubing. An optical pressure/temperature gauge is deployed along with the seismic sensors on a single optical cable.
Apart from enhancing data from the surface and the ocean-bottom cable system, the sensors also produce a detailed around-the-borehole image. This allows detection and mapping of microseismic events – the opening and/or closing of fractures in rock – as they occur when fluids are produced from or injected into a formation. By following these events, operators are able to determine where fluid is moving to test predictions of fluid flow through the reservoir and to take required remedial steps early.
That ability is applicable for today’s North Sea operator, whose goal in the maturing province is production optimization through reservoir life extension and increased recoverable reserves. Operators know that monitoring the reservoir from day one provides better information about what is going on in that reservoir. The conventional method of sticking a hole in the reservoir and producing as hard as possible for as long as possible is a thing of the past. Today, reservoir monitoring helps to squeeze every bit out of the field through the use of sophisticated technologies and to best determine the optimal reservoirs to tackle.
Optics extended reliability
The Valhall project is expected to continue for up to 20 years. Given this extended time scale, the reliability of the optical sensors was of paramount concern for the operator’s engineers. By definition, to serve as a constant throughout the life of the field, the sensors must never move.
The reliability of the optics is the key. Electronic systems are restricted to a three- to five-year life span, or less; and, once anything is pulled from a well, it is extremely difficult or unlikely it can be placed back in the well in precisely the same spot. Optic systems, on the other hand, are designed to be permanent for the life of the well.
Years of development have resulted in the use of proprietary materials, chemicals, and processing solutions to combat a failure mechanism that has long plagued fiber optics in oil and gas wells. At very high temperatures and for unprotected fiber, the “hydrogen darkening” phenomenon can occur through absorption by the impure glass fiber of the highly permeating hydrogen gas. In time the glass darkens to the point where the light-dependent optical sensor is undetectable.
The tubing-deployed sensors, or seismic stations, are run much closer to the reservoir than the permanent ocean-bottom cable sensors, as they are part of the completion string.
These optical sensors have several layers of protection to make sure hydrogen does not get to the glass. Even if the glass does darken over time, the sensitivity of the system is such that it should function over a 20-year life span.
The other enemy of sensor life is vibration, a constant and extreme element in downhole production and injection environments. Traditional electronic sensors are susceptible to vibration-induced failure, as they contain numerous moving parts that can shake themselves apart in high-rate flow. Optical sensors, on the other hand, contain no moving parts.
A generic field production profile shows with and without the application of permanent in-well multi-component optical seismic systems.
And while it may be counterintuitive to think of glass being stronger than steel, the tensile strength of glass is, in fact, superior. Likewise, special techniques for engineering and manufacturing glass microstructures and packaging them for durable installation have rendered a robust system.
Acoustic baseline data
In light of the prolonged term of the Valhall field 4D seismic system, participation in the project is still in very early days. The operator has indicated that everything is going fine and the data looks good – cautiously optimistic in these early stages of processing the data. The system has given the operator more than expected in terms of downhole data.
The original purpose for the optical sensors as an enhancement to the ocean-bottom seismic system may prove to be but one part of the value eventually realized by the operator.
Initially the operator just wanted to improve ocean-bottom seismic, which is how they economically justified the optical piece. But now that the sensors are installed in the well, they are saying there is a lot more they can do with it. For now the operator has the system continually “switched on” at Valhall and is gathering accelerometer data to establish what might be thought of as an acoustic baseline for standard production operations by gathering data kilometers from the injector well in which it resides.
