Distributed temperature (DTS) and distributed acoustic (DAS) fibre optic sensing are now commonly used as key reservoir surveillance tools. This work shows the benefit of continuous downhole monitoring during the lifetime of a well. Fibre optic cables were permanently installed in a doublet injector/monitor well system as part of a CO2 controlled released experiment at the In-Situ Laboratory in Western Australia. During the completion and injection operations various planned and unplanned events (mud circulation, cementing, drilling, wireline logging, gas and water flows) occurred. The events were monitored from surface to reservoir with DTS and DAS fibre optic cables. The DTS was recorded continuously data starting during well completion throughout the lifetime of the wells while DAS was recorded at specific points in time, mostly associated with borehole time-lapse seismic acquisitions.
For the well completion stage, the interpretation of the DTS dataset acquired during mud circulation provided information about thief zones above the reservoir. During cementing and cement hydration, DTS highlighted areas of large breakouts and confirmed cement in-fill in those intervals. During the drilling of an unexpected cement infill, it provided a unique insight into downhole progression of the drill bit.
During the CO2 injection stage, DTS enabled monitoring the phase behaviour at the injector using distributed temperature data combined with a permanent pressure downhole gauge. At the monitoring well, the injected gas breakthrough was clearly detected at reservoir level within 1.5 day after the start of injection. Moreover, the accumulation of the CO2 over time was captured accurately at reservoir level for a further 3 days. During an unexpected leak in the casing, gas from the reservoir started entering the monitoring well leading to cyclic release of water and gas at the surface. The DAS dataset enabled to pin-point the exact moment the casing leak occurred while the DTS dataset captured the cyclic nature of water and gas leakage. The sensitivity of the tools and interpretation methods are such that the downhole location of the event can be determined within 1 m and the timing within a few seconds. More importantly, the continuous and distributed recording allowed monitoring the events developing in time over the full length of the well.
This work highlights the unique benefit of permanent distributed monitoring using distributed fibre optic sensing during the lifetime of a well. The continuous and distributed recording allowed monitoring events developing in time over the full length of the well and provides direct observations for the different events while providing near-real-time information about downhole processes.