Fiber Optic Distributed Acoustic Sensing (DAS) for Hydraulic Characterization in Fractured Bedrock

A new method is presented for measuring hydraulic connectivity and hydromechanical strain in fractured bedrock. Fiber optic Distributed Acoustic Sensing (DAS), designed for measuring seismic or acoustic vibrations in the Hz-kHz frequency, is used to measure pressure oscillations in the mHz frequency. Induced periodic head oscillation in a source well is observed as strain response (fracture dilation/contraction) in a fiber optic cable installed in a monitoring wells. The fiber optic cable is mechanically coupled to the borehole walls to allow strain to be measured along the entire length of the fiber optic cable. Because the depth of hydraulic response is rarely known a priori, the ability to collect distributed strain response along the length of a borehole is a tremendous advantage for high resolution hydraulic characterization in fractured bedrock.

The technology was demonstrated in a well-characterized crystalline bedrock. Fiber optic cable was coupled borehole walls using an over-pressured flexible liner outfitted with an air coupled transducer to measure fluid pressure. DAS was used to measure strain every 0.25 m along the fiber optic cable in the monitoring wells. At one well, strain and pressure were measured simultaneously at a known fracture zone that is hydraulically connected to the pumping/injection well 30 m away. Strain amplitudes less than one nm/m were measured in response to head amplitudes of less than two mm. Clean strain signals were detected at tested periods of hydraulic oscillation ranging from 2 to 18 minutes. The response was sensitive to the fiber optic cable design with tight-buffered fiber optic cable being about twice as sensitive to strain as standard gel-filled fiber in metal tube. This first field test suggests potential for measuring hydraulic connectivity and hydromechanical behavior in fractured formations through cementing fiber optic cable in wellbores outside of well casings.