Sensing a marinized solution

An OptaSense instrument well with a vibroseis unit conducting OptaSense DAS-VSP. Photos from OptaSense.

OptaSense is collaborating with Shell to develop a fully-marinizied DAS system. OptaSense's David Hill explains.

As the oil and gas industry enters ever more challenging and previously inaccessible plays, the financial risks can be greater. Therefore, the need to understand better the issues being faced becomes more acute.

Optimal reservoir monitoring has traditionally been a key issue, and providing a clear and accurate picture of activity is regarded as a significant enabler in achieving that.

A range of techniques have been used in the past, but, with advances in technology, the deployment of distributed fiber-optic sensing has become an increasingly prevalent, more accurate, and cost effective alternative.

Distributed temperature sensing (DTS) using fiber-optic cable has been around for several years, but the development of distributed acoustic sensing (DAS)

is a relatively recent phenomenon and, through its versatility, it has the potential to radically improve our understanding of a broad range of issues, including reservoirs, fields, geology, and asset integrity.

Working on the principle “if you can measure it you can manage it,” OptaSense, a subsidiary of QinetiQ Group, has developed a DAS system able to provide decision-ready real-time data through the conversion of any standard optical fiber into a distributed acoustic (or seismic) sensor.

Acoustic or seismic signals that strike the fiber cause minute strains. These are measured using laser interrogation, turning the fiber into a distributed acoustic/ seismic sensor. An interrogator unit (IU) fires a laser beam into the cable and measures backscatter returns from naturally occurring imperfections inherent in the optical fiber. The minute strains cause subtle modulations of the backscatter, that are then measured by the IU, thus sensing the acoustic/seismic signal.

It is five years since DAS was first used downhole and it is now delivering significant benefits in a range of completion, production, and evaluation projects, including hydraulic fracture profiling, permanent wellbore flow monitoring and seismic monitoring. OptaSense is now developing the first fully marinized and qualified DAS system, in a joint program with Shell. The system, which will be deployed in up to 10,000ft water depth, will allow highly accurate acoustic data acquisition for the first time offshore and will provide data for a wide range of subsea and deepwater applications, including pipeline surveillance and leak detection, geo-positioning, in-well monitoring, subsea assembly condition monitoring, and permanent reservoir monitoring.

The device will include functional and technical parameters, configurable in software, avoiding different hardware for settings or functions.

The marinization process will require the re-engineering of the interrogator unit, to reduce its size to fit into a highly robust pressure canister. The modified opto-electronics will be tested, to ensure they meet the rigorous and stringent temperature, vibration, shock, and electrical certifications required of subsea equipment, particularly during transportation and deployment.

In the subsea in-well monitoring application, the interrogator unit will be positioned near to the wellhead, with a processing unit, and processed data will be relayed back to shore or appropriate installation, via a further fiber-optic link in a control line.

The enabling technology to get a fiber into an offshore well has been developed and deployed. This enables fiber to pass through the wet tree and through the packer, via a downhole wetmate fiber- optic connector, into the producing zone. Once permanently deployed on the production tubing, the fiber can be used with a DAS system to continuously profile the flow along the well, measuring the inflow contribution at each producing stage.

In-well equipment, such as electrical submersible pumps, inflow control valves, and gas lift valves can be constantly monitored to optimize performance and access their condition for predicative maintenance purposes. Also the downhole fiber can be used to acquire the shot records for a vertical seismic profile (VSP). Because it is permanently in-situ, repeat VSPs can be performed easily enabling time-lapse imaging of the reservoir around the well. In EOR applications 4D VSP using DAS can be a powerful tool in understanding the effect of injection and other forms of stimulation on the reservoir.

OptaSense has been able to use its parent firm’s facilities for qualification testing. It is anticipated that the marinized unit will be ready for demonstration by the end of 2014.

An OptaSense interrogator unit. 

Further potential uses of the DAS technology subsea are currently being explored but appear wide-ranging, such as monitoring the condition of subsea equipment, by wrapping the sensors around the pressure vessel they are contained in. Onshore, fiber-optic DAS technology is being used in hydraulic fracturing operations to measure the full length of the production lateral, without having sensors inside the cased well. Clamped or strapped to the production casing and cemented in place, a fiber-optic cable can provide real-time DAS and DTS from completions through abandonment, without well intervention.

The uses of DTS, although complimentary, are limited. While DTS has been used for over a decade, borehole temperature is only part of the story when it comes to monitoring a hydraulic fracturing operation. With a DAS system, it is now possible to listen to the well at each perforation location and hear how the fluid and proppants rush through the orifices, and how the rock fractures just outside the well and beyond into the formation.

Combining DAS data with real-time pump data from the pumping trucks, it is possible to estimate how much fluid and proppant is being pumped into each perforation cluster. With the full-well continuous monitoring DAS enables, all activities during the hydraulic fracturing operation can be monitored for effectiveness including:

  • wireline tool tracking 
  • bridge plug setting
  • perforating ball drop, ball seating, sleeve sliding and isolation from previous stage
  • casing leaks and restrictions that can halt or hinder operations, and accurately report their depth.

The system can also be used to visualize flow dynamics, providing an alternative to conventional permanent production logging tool (PLT) readings, with the DAS fiber installed on casing or tubing. Through a permanently installed fiber, measurements can be taken on a continuous basis or at regular intervals without the need for costly well intervention and deferred production, using the company’s drive-by acquisition service. It is also able to provide full well bore seismic imaging.

Earlier this year, OptaSense was contracted by Petroleum Development Oman (PDO) to provide the industry’s first multi-well 4D DAS vertical seismic profiling system, to monitor and map the performance of up to 12 steam-injected oil wells in a brownfield development at South Oman salt basin.

Seismic signals have been recorded from fiber-optic cables attached to each well’s production tubing, permanently installed and linked to a surface data-gathering center. The final processed data set will be integrated into PDO’s reservoir models and assist in the determination of fluid substitution through production and contribute towards the positioning of infill wells.

Weighed against using geophones in this type of application, the DAS technology provides benefits, including lower-cost on-demand acquisition through permanent cable installation; deployment in wells inaccessible to geophones, with no well intervention required; synergies with existing systems and retrofitting capabilities; coverage over the entire length of a well and simultaneous data acquisition of multiple contiguous wells.

The development of distributed acoustic sensing through deploying fiber-optic cable represents a technological development in the monitoring arena, allowing operators to see what is happening across the wellbore in real-time. The system has multiple uses and, as it prepares for its deployment offshore, a new range of opportunities.

David Hill is a QinetiQ Senior Fellow and the Chief Technology Officer at OptaSense, a UK-based QinetiQ company, which he jointly founded. He has over 29 years research and development experience in acoustic sensing, with over half that time spent developing fiber-optic based sensors for the military, as well as oil and gas and other civil applications. He has a BSc(Hons) in Imaging Sciences from the University of Westminster, London, and a PhD in Physics, specializing in fiber-optic sensing, from the University of Kent, UK.

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