Distributed fiber-optic sensing transforms an abandoned well into a permanent geophysical monitoring array: A case study from Australian South West

Abstract

Distributed temperature sensing (DTS) and distributed acoustic sensing (DAS) data recorded by a fiber-optic array installed during the decommissioning operations of the 1550 m Harvey 3 well in Western Australia reveal an abundance of valuable information about the course of the decommissioning process and the quality of the cement job. The DAS monitoring has detected vibrational disturbances during the cement's setting up, while DTS was used to assess setting up of the cement and curing times as well as uniformity of cementation from the distribution of temperature along the borehole. A weeklong trial acquisition of passive seismic data with the same array a year later shows an abundance of seismic events in a wide frequency range from below 1 mHz to above 200 Hz. The downhole DAS array provides traveltimes and amplitudes of these events, which include earthquakes, mine blasts, ocean microseisms, and local human activity. The amplitudes of waves from distant seismic events can be used to estimate and monitor physical properties of the media along the extent of the well. When used in combination with information from active vertical seismic profiling, these events can help obtain independent estimates of velocities and densities. Spectral analysis of low-frequency microseisms shows a strong correlation between passively recorded DAS and local weather observations. This shows that the ability to continuously record oceanic microseisms at low frequencies opens opportunities to employ such arrays for wave climate studies. In addition, the data contain peculiar in-hole reverberations likely caused by crossflow of groundwater behind the intermediate casing, which may indicate imperfections of the cement job. The results demonstrate that a downhole fiber-optic array installed in an abandoned well represents an opportunity to establish a permanent facility for continuous recording of passive and active geophysical data and for exploring various applications.

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“…One of these is coupling of the fibre-optic receivers to the formation. Sidenko, et al [ 9 ] reported the effect of poor cement quality in a section of a well on the DAS amplitude. This suggest that the same technique can also be employed for well integrity monitoring.…”

“…Composition of the ambient seismic energy recorded by DAS in the downhole environment was studied previously in a number of Australian wells instrumented with fibre-optic cables, including CRC-2 and CRC-3 [ 10 , 27 ], a training well at the Curtin University campus [ 8 ], and South West Hub [ 9 ] in Western Australia. These studies show strong surface waves caused by the interaction between ocean and land (oceanic microseisms), and surface and body waves generated by earthquakes, mine site blasts, and other events related to human activity.…”

“…The 174 earthquakes can either be natural seismic events or originate from human activity, such as mine site blasting. Unlike the situation close to Perth (Western Australia), where most events are attributed to mine blasting [ 8 , 9 ], in Victoria, a significant proportion of the events appear to be natural. This conclusion stems from the analysis of the distribution of these events through the time of the day as shown in Figure 2 .…”

“…In addition to signals from controlled sources, permanent receivers such as DAS record an abundance of seismic energy from various natural sources and human activities (earthquakes, mine blasts, ocean waves, machinery, and vehicular traffic) [ 8 , 9 , 10 ]. Harvesting some of this energy can potentially complement or even replace for certain tasks active seismic sources, reducing the cost of monitoring even further.…”