Distributed temperature sensing improves reservoir analysis

Ian Verhappen
Industrial Automation Networks Inc.

Do you know what is happening in your reservoir in real time? Advancements in measurement technology have made it possible to do so during all phases of a reservoir's life including the initial drilling and completion.

Well stimulation is a very common technique in the oil and gas industry with one particular technology, hydraulic fracturing or "fracking," getting significant publicity as a result of the shale gas activities that rely on this technology to increase reservoir porosity. Matrix treatments, which are typically acid based, are the other common form of stimulation.

Fracturing treatments are performed above the fracture pressure of the reservoir to cause, as the name implies, fractures or cracks in the rock and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir resulting from drilling fluids, perforations, fines migration, clay swelling, paraffin, or asphaltene deposition due to reduced reservoir pressure to the near-wellbore area. With the increasing use of horizontal wells, the industry can no longer "rely" on gravity to help with isolation of zones and estimation of "at face" conditions, which means there is an increasing demand to be able to measure the pressure and temperature downhole at the reservoir.

Instead of a single pressure gauge being installed in a well above the packer, multiple pressure gauges are being placed in the well to cover each production zone with at least one pressure and temperature measurement. In addition to pressure gauges, flow meters are added to the completion string to monitor zonal production rates and to detect potential phase fractions (oil, water, gas). These sensors can give individual zonal contribution rate measurements, while fiber based the distributed temperature sensing (DTS) offers thermal profiling and qualitative data, which can be used to identify any other potential well problem.

Fiber-based sensors based on standard optical cable have been in use since the mid-1990s. The sensing itself is based on Bragg grating light wave interference measurements, which are then correlated to provide pressure and temperature gauges and fiber-based flow meters.

One advantage of DTS is the ability to measure temperature at any point along the fiber, thereby negating the need for irreversible pre-completion sensor placement. The resulting temperature measurement to a resolution of better than 0.01°C, can be made every second, at a spatial resolution of 1 m (3 ft) along fiber optic cables up to 60 km (37 mi) long. Advances in fiber have also "eliminated" the problem of earlier fiber optic probes of "going dark" with time due to changes in bundle as a result of high temperatures and hydrogen in wellbore resulting in a sensor design life of more than 30 years, and now with improved fibers support for high-temperature performance of up to 650°C (1,202°F) as well.

Because gas has a speed of sound of around 600 m/s whereas water has a speed of sound around 1,500 m/s, the speed of sound when evaluated over a large section of the well can also measure the distributed variations of the flow composition and characteristics along the well.

Temperature measurements have been used to determine flow contributions, water injection profiles, locating cement tops, and identifying cross flow between zones, which result in the following list of "typical" applications for real-time downhole sensing:

Gas lift monitoring and optimization using field survey data to view and hear the operational status of individual gas lift valves to allow real-time adjustments and optimize production.

Flow allocation and production optimization in multi-zone completions to maximize recovery from each zone with minimal damage to the reservoir.

Flow assurance of the multi-phase transport typically found in offshore applications to ensure safe, uninterrupted, and simultaneous transport of gas, oil, and water mixtures through wells/pipelines from the reservoir to an offshore or land-based processing facility.

Avoidance/deferment of well intervention to increase overall production time while enabling better scheduling of workovers and only working on a well when it is required.

Subsea pipeline, flowline and riser monitoring, and leak detection to prevent environmental incidents.

Hydrate detection which can result in the plugging of lines due to the formation of "ice" at temperatures and pressures not normally associated with this problem.

The ability to detect the presence of sand, as well as provide a quantitative measurement of the amount of sand being produced downhole which allows distributed condition monitoring of multi-stage sand control completions to facilitate remedial steps to isolate problem zones.

Gas breakthrough identification and confirmation of subsequent isolation to maximize the production of high margin liquids while also maintaining reservoir pressure.

Electrical submersible pump (ESP) monitoring electrical motor condition monitoring using noise pattern recognition techniques.

Another benefit of fiber-based sensors is that they are relatively small and lightweight.

You can only control what you measure. As illustrated above this is important during all phases of the hydrocarbon development cycle from initial completion through production and revitalization. The good news is we can now measure and hence better control the way we develop our reservoirs and as a result improve overall recoveries.

The author

Ian Verhappen, P.Eng. is an ISA Fellow, ISA Certified Automation Professional (CAP), Automation Hall of Fame member, and a recognized authority on process analyzer sample systems, Foundation Fieldbus, and industrial communications technologies. Verhappen operates a global consultancy Industrial Automation Networks Inc. specializing in field level industrial communications, process analytics, and hydrocarbon facility automation. Feedback is always welcome via e-mail at[email protected].