S012-0017
Use of Multiple DAS Fiber-Optic Arrays for Microseismic Monitoring and Event Location
Use of Multiple DAS Fiber-Optic Arrays for Microseismic Monitoring and Event Location
Tuesday, 8 December 2020
Poster
Abstract:
DAS fiber optic sensors are increasingly being used for microseismic monitoring of hydraulic fracturing programs and for earthquake detection. Standard telecom-grade fiber optic cables can be used, including unused “dark” telecommunications infrastructure. In oilfield applications, fiber can be permanently installed in a borehole or temporarily deployed on a wireline.
DAS microseismic has several advantages over conventional geophone arrays. Thousands of densely spaced channels can be obtained for low incremental cost. If the fiber is permanently installed, microseismic data can be acquired without interrupting oilfield operations, translating to lower health, safety and enviromental risk. DAS can also capture large events that may exceed the dynamic range of geophone arrays, useful for monitoring induced seismicity.
One limitation of DAS arrays is lower sensitivity on a per-receiver basis than geophones, with ability to detect events down to about magnitude -2 depending on the project, where geophones can go below magnitude -4. A second constraint of DAS is that only one component of motion is recorded, unlike three-component geophones. The result is event locations cannot be determined accurately in three dimensions with a single fiber. This limitation has motivated the use of multiple fiber arrays.
We have acquired DAS microseismic data for several projects using multiple fiber-optic arrays. A typical project will use three fibers, located in two horizontal treatment wells and a vertical observation well. The use of multiple fibers allows us to map events in three dimensions. Events are mapped using an iterative grid search method, calculating semblance along ray-based traveltime trajectories from all possible source locations. Along with event locations, we estimate location uncertainty from the computed semblance volumes. We can capture induced fracturing events as well as activation of regional faults.
DAS microseismic has several advantages over conventional geophone arrays. Thousands of densely spaced channels can be obtained for low incremental cost. If the fiber is permanently installed, microseismic data can be acquired without interrupting oilfield operations, translating to lower health, safety and enviromental risk. DAS can also capture large events that may exceed the dynamic range of geophone arrays, useful for monitoring induced seismicity.
One limitation of DAS arrays is lower sensitivity on a per-receiver basis than geophones, with ability to detect events down to about magnitude -2 depending on the project, where geophones can go below magnitude -4. A second constraint of DAS is that only one component of motion is recorded, unlike three-component geophones. The result is event locations cannot be determined accurately in three dimensions with a single fiber. This limitation has motivated the use of multiple fiber arrays.
We have acquired DAS microseismic data for several projects using multiple fiber-optic arrays. A typical project will use three fibers, located in two horizontal treatment wells and a vertical observation well. The use of multiple fibers allows us to map events in three dimensions. Events are mapped using an iterative grid search method, calculating semblance along ray-based traveltime trajectories from all possible source locations. Along with event locations, we estimate location uncertainty from the computed semblance volumes. We can capture induced fracturing events as well as activation of regional faults.