Efficient geothermal reservoir exploitation often hinges on creating sufficient fluid circulation between injection and production wells. Tracers are effective for determining flow connections, but are expensive and time consuming. We explore the use of periodic (oscillatory) hydraulic testing for measuring hydraulic connectivity between wells in saturated fractured bedrock. In a periodic test, head/flow is varied periodically in one well and the head response observed in nearby wells. The amplitude attenuation and phase of the observed hydraulic signal is indicative formation permeability. We present a field example of this approach in a low permeability fractured bedrock. One well underwent periodic injection and pumping while five observation wells 30 to 47 m away were monitored. Only one well pair is reported here, however. Tests were repeated for a range of pumping oscillation periods (frequency). We measured hydraulic connection using traditional pressure transducers and very low-frequency fiber optic distributed acoustic sensing (DAS). The DAS observations were conducted in two modes. In the first, fiber optic cable was suspended in the well water column and oscillating longitudinal strain in the cable was measured in response to hydrostatic (lateral) compression. In the second, the fiber optic cable was pressed against the borehole wall using a flexible liner while oscillating strain in the cable was measured in response to dilation/contraction of rock fractures that intersect the borehole. Both modes of operation produced periodic signals using DAS in a well 36 m from the source, but the response to dilation of the rock in response to hydraulic propping produced a much clearer signal. Sensitivity in both cases was improved with larger frequency and amplitude of head oscillation. In hydrostatic mode, DAS was sensitive to heads oscillation with periods as long as 12 minutes and head amplitudes of as small as 15 cm. In dilation mode, DAS was sensitive to heads at periods as long as 18 minutes (at 15 cm head amplitude) and head amplitudes as small as 3 mm (at 2 min periods). Laboratory tests indicate that hydrostatic DAS response is linearly related to pressure when head oscillation has short periods and large amplitudes but non-linear (power law) or poorly correlated at short periods and small amplitudes. To our knowledge, this is the first application, in laboratory or field setting, where DAS has been used to measure fluid pressure. The technology has potential for real-time monitoring of pressure response in geothermal wells. If the fiber optic cable is cemented in the annulus between the casing and formation wall monitoring may be achieved outside of currently perforated intervals. Very low frequency DAS can take advantage of existing installations of fiber optic cable for DAS seismic or acoustic monitoring or even distributed temperature sensing (DTS).

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