DAS Microseismic Event Location Superior to Borehole Microseismic Location?

There has been an increase in the deployment of DAS (fiber optic) in wellbores which has corresponded to an increase in participants and published statements about its advantages. One colleague of mine referred to it as the "microseismic of 15 years ago."

One article discussed the advantages of the deployment within a wellbore and offered a cursory explanation why it could more accurately locate an event than a conventional borehole seismic survey. The reason? More tools with a closer spacing. The authors wrote that they expect a resolution of 1-2 meters with a DAS system and a 30-50 meters of a borehole seismic system. Malarkey!

Borehole seismic uses 3 axial components within each geophone which results in error in 3-dimensional spaces. In a DAS system, error is only located in one direction so that portion is accurate. So what is missing? Well, the other 2-dimensions. Is the event above of below or next to your wellbore? Is it left or right of your wellbore? Is it in or out of the payzone or that aquifer you were hoping to avoid? In other words, the error not accounted for is nearly equivalent to the circumference of a circle. For example, an event that is located at a distance of 10 feet off the wellbore will have 63 feet for possible solutions. Events located 1000 feet away? A whopping 6300 feet of possible solutions. This is the actual error associated with the technique.

Let's compare with one of my borehole microseismic survey result. In this example, the monitor well is located close to the middle of the lateral and we will consider the location misfit for 4 stages of perforation events (2 on either side of the monitor well) at a distance >1600 feet. A single orientation file and velocity model was used for the entire project. The average XYZ misfit is <20 feet from the known calibration point.

The authors did point out that if they can detect the signal in the vertical section of the lateral and in the horizontal, they can constrain the solution. DAS even by the authors admission is less sensitive than borehole units, detects fewer events. So in a field where borehole viewing distances are ~2500' a borehole potentially can detect events ~5000 feet along the lateral if placed in the middle and can constrain those events within all 3 dimensions. If DAS was as sensitive as a borehole system, it could still only locate events in 3-dimensional space for 2500 feet of the lateral. As lateral sections of well are currently stretching out to 10,000 feet are becoming the standard, a borehole system can play hop to another well to record data for full coverage. With a DAS system, the events located for 1/4 of the lateral could be assumed to be the same along the wellbore. I'll leave the embolden discussion point up to Benny Hill.

*Recent discussion which has excited the community has indicated that DAS has been enhanced since the time of the original article and that it indeed as sensitive as a traditional borehole seismic deployed array.