Distributed Acoustic Sensing: Real-time monitoring throughout the full well lifecycle
In this week’s interview we talk to Fotech’s Stuart Large – Product Line Director, Oil and Gas – about the safety and efficiency benefits Distributed Acoustic Sensing (DAS) technology can bring to shale gas producers throughout the full lifecycle of wells.
Monica Thomas (Shale Gas International): Fotech is a technology company that allows for advanced monitoring within the oil and gas sector and their flagship product is a Distributed Acoustic Sensor (DAS). Can you, perhaps tell us what this is and what are the applications of DAS technology within the oil and gas sector?
Stuart Large (Fotech): The simplest way to think of Distributed Acoustic Sensing is – it uses a fibre-optic cable which is deployed along the length of the well. DAS effectively turns the cable into tens of thousands of microphones placed along the length of the well. This allows us to listen to events that are occurring along the length of that well. The fibre responds to micro strain resulting from vibrations and noises along its length and the DAS system can interpret these vibrations.
And so, combined with Distributed Temperature Sensing, which is a well-established technology, we are able to use this fibre-optic technology to look at a number of different things occurring in the wellbore. This could be hydraulic fracture monitoring, production profiling, injection profiling, potentially also seismic data acquisition, well integrity, and sand production. Well integrity covers damage to cement – so we’re looking for leaks on the outside of the casing, as well as issues with the casing itself.
MT: Why were fibre-optics chosen for this type of task, rather than electronic solutions? Is it because they are better suited to withstand the harsh conditions in the wellbore or where there also other considerations?
SL: One of the main benefits of having a fibre is that it is able to look at the entire length of the well simultaneously. And also we are taking thousands of samples every second, and so we are able to monitor for a period of time and see the changes in the wellbore at any point along its length.
To compare it with wireline logging, for example, the wireline logging tools are a set of sensors that will be at one place in the wellbore at a given point in time, and we know that the conditions in the wellbore fluctuate. Particularly when you look at production monitoring, for example, you may find that a particular perforation or fracture is producing hydrocarbons for a period of time – maybe half an hour – and then it can stop and produce nothing for maybe an hour, and then it can come back again.
In this case, when you are logging with wireline logging tools, you either are seeing that perforation producing, or you are there at the time when it is quiet – and it’s impossible to know which. Whereas with fibre we are monitoring all the time and we can see those variations.
If we’re looking for a leak in the casing, or cement, again – these things can fluctuate. They may be sporadic events. So we can watch for a number of minutes or hours, we can locate a potential leak and then if somebody wants to diagnose that in more detail with wireline logging tools, we can direct them as to which tools to use and where to place those tools in the well to diagnose the issue more closely.
MT: So, as opposed to the alternative, DAS offers ongoing monitoring capabilities?
SL: That’s right. The fact that we have the fibre on the whole length of the well means that we are able to monitor for as long as we choose.
It is also worth understanding that the fibre can be put into a well in a number of different ways. It can be permanently installed, which means that it’s strapped to the casing and cemented in during well completion, or it can be deployed into the well at some point during the life of the well – sometimes for just a few hours – by deploying a carbon rod or a slick-line, or coil tubing. It also can be attached to the completion string if we’re talking about a frack job.
MT: Am I correct in thinking that DAS can be deployed throughout the lifecycle of the well?
SL: Yes, that’s one of the key points – it’s throughout the life of the well. So when I think about the jobs that we are doing in the U.S. right now, we’re installing the fibre initially for the purposes of hydraulic fracturing monitoring. When the fracking happens, we will be monitoring that job. We will then monitor warm-back and flow-back – as the well comes online. And then we can go back and look at the well at regular intervals of three months, six months, a year, and look at how the well performance is changing.
Using the data we have collected on well performance over time, we can define when and how we might re-frack the well in the future. Then, of course, we can monitor that re-frack. And also, through the life of that well we can use the fibre to run a VSP job, and collect seismic information. And lastly, we could be monitoring for well integrity issues to see if there are any issues with cement or with the casing that develop over time.