Before any well is completely abandoned, some jurisdictions have governmental regulations which must be met and carried out by all companies. The main objective of these regulations is to prevent the production of oil and/or gas in the well by isolating and covering all porous zones. Monitoring with fibre-optic distributed acoustic sensing (DAS) systems allows for leak detection within the well bore and mapping migration through the cement with full-wellbore coverage. DAS provides a cost-effective method to accurately determine the depth of a leak or multiple leaks and profile gas movement due to casing failure, failure in wellhead seals, etc. In vertical wells or wells with low deviation, the optical fibre can be easily deployed by attaching a weight bar to the end of a steel tube containing fibre and running it to depth as a temporary installation or left in place as a permanent monitoring capability. Alternatively, a fibre that is already permanently installed behind the casing and cemented in place can also be used. Acoustic events from the gas movement produce a very small strain in the fibre. The strain can be measured at surface and depth-matched using the speed of light in the fibre. After characterizing the flow in the well under open and shut-in conditions, a decision can be made on how to address the leak. This paper will describe DAS technology and how it is deployed as well as show the analysis and results from a casing leak detection trial.
DAS technology utilizes fibre optic cable as a distributed microphone, with high spatial resolution over long distances. In DAS, a short pulse of light is launched into the fibre, as a probe. During the manufacture of the fibre, imperfections created during the cooling of the fibre, cause variations in the refractive index of the glass. These imperfections are very numerous and randomly distributed, and are referred to as scattering sites. When the light probe advances down the fibre, a small portion of its light becomes scattered by the scattering sites. About 0.1%, of the scattered light gets captured by the fibre and is guided back towards the launch direction, and this backscattered light is detected by a detector within the DAS system.
Figure 1 shows the relationship between the launched light probe (in blue) and its backscattered response. At time t0, a light probe is launched into the fibre and the figure shows the interaction of the light probe with the fiber, causing backscatter, denoted by red arrows, at some arbitrary time, tn. At some later time tn+1, the backscattered response (in red) has propagated back towards the launch direction, and the light probe (in blue) has continued to propagate towards the end of the fibre. As the light probe propagates down the fibre, a continuous stream of backscattered is light is generated throughout the fibre. The light probe and backscatter detection process is referred to as interrogation.