Permanent Cap-rock Strain Measurement for a Thermal Development
Authors: Edwards John , Seiam Mahmoud , Mackay Stuart , Keshishian Avo , Singh Parvesh , Sheheimi Awadh , Bettembourag Solenn 
Published: June 2014

In a thermal development of a highly fractured carbonate the surface facilities are above the reservoir, and are sensitive to surface deformation. A coupled reservoir-geomechanical model has been history matched to synthetic-aperture radar satellite measurements of the surface uplift that occurred during the pilot, and this will be history matched to the strain measured in a vertical well observing the cap rock during the development. This geomechanical model will be used to generate forecasts of movements that could affect well or facility integrity. The observation well providing data to history match this geomechanical model needs a formation strain measurement insensitive to the steam-induced increasing temperature in the cap rock. As an alternative to dual sensor gamma ray wireline logging of the distance between pairs of radioactive tagged bullets, the permanent installation of optic fibres offers both temperature and strain without intervention, with real-time data. This technique is used to monitor geo-hazards and landslides via the strain of a shallow buried optical sensor cable to monitor onshore pipelines. This is the first time optic fibres are used to acquire measurements of formation strain across an open hole section within an observation well.

Laser interrogation of the Brillouin frequency of optic fibre bonded with resin to the inside of a control line demonstrated in the laboratory that the dynamic range of measurable strain is greater than the maximum expected formation deformation. The glass fibre reflected the strain up to the elastic limit of the metallic control line. In the observation well a pre-stressed control line containing resin-bonded optic fibre is stretched across the open hole. This pre-stressing is necessary to provide sensitivity to both negative and positive strain, to measure subsidence as well as thermal expansion. The control line strain will mimic the formation strain, and via the resin glue will pass this strain to the optic fibre that measures strain and temperature. A second free hanging fibre measures the temperature. The Brillouin frequency of optic fibre is sensitive to both strain and temperature, so an independent temperature measurement is required to remove the temperature shift on the strain measurement. These two fibres are deployed inside a specially designed completion. The cap rock is completed open hole, with tubing containing an expansion joint secured to the top and bottom of the cap rock between a packer set at the casing shoe above the cap rock and an open hole swell packer at total depth. A retrievable configuration of several bundled control lines is installed inside this tubing containing the optic fibres and a pair of hydraulically operated tubing anchors. These anchors are spaced out to correspond to the distance between the pair of packers. In a field trial of an in-situ formation strain measurement of a thermal project the completion was run as planned. It is expected that it will be several years before the system measures significant strain.