Distributed acoustic sensing (DAS) for hydraulic fracture monitoring in laboratory scale

Hydraulic fracturing is a widely used completion technique for unconventional reservoirs, and monitoring the growth of the fracture network is crucial to ensure safe operations and enhance oil or gas production. The microseismic signals generated during stimulation of induced and natural fractures are monitored for localization purposes. At field-scale, Distributed Acoustic Sensing (DAS) is becoming popular for detecting microseismic events, as it offers a denser spatial sampling. Conducting laboratory hydraulic fracturing experiments with DAS monitoring system provides the opportunity to control variables that cannot be directly measured in the field, allowing to optimize DAS systems and improve their reliability in field monitoring settings. In addition, other governing factors such as confining stress, well geometry, and volume and rate of injected stimulants provides a useful analogy to field completion. In this study, we utilize DAS to monitor hydraulically induced microseismic events in the laboratory within a cubic rock block of 50 cm3in size. A self-reacting triaxial loading frame provides three different confining pressures to generate a true triaxial stress state. Using ISCO pumps we inject the fracturing fluid into a borehole to simulate hydraulic stimulation. DAS fibers are distributed in three directions over six surfaces of the rock block allowing for spatial localization of the microseismic signals. Open corners are designed at each edge of the rock block so that the fiber can form a corner loop to reduce bending losses. With the help of CT-imaging technology we visualize stimulated fractures inside the rock and calibrate microseismic localization results inversed from DAS monitoring. Preliminary test results indicate that the adopted design is effective and reliable for DAS in detecting fracturing signals. Compared to conventional DAS systems, the sampling frequency in this study is increased by about ten times so that high frequency acoustic signals can be better recorded.