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Distributed acoustic sensing applications: from surface to borehole
Mjehovich, Joseph A.
Mjehovich, Joseph A.
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2022
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Abstract
Distributed acoustic sensing (DAS) is a relatively new technology used in many geophysical applications. Its first notable use for geophysical monitoring includes downhole deployments for vertical seismic profiling and more recently, hydraulic fracture characterization in unconventional wells. Over the last decade, DAS has extended into broader seismic applications including structural imaging, and near-surface surveys for seismic site classification. Critical to these different applications is the availability of cost-effective methods to acquire data in logistically challenging settings. We propose two novel procedures spanning two distinct industries: (1) using the low-frequency band of DAS to diagnose multi-stage hydraulic fractures in upstream oil and gas, and (2) surface-deployed DAS optical-fiber for low-impact seismic hazard geotechnical surveys.
In 2020, 13 horizontal wells were drilled and completed at the DJ-Postle wellsite, including three wells with various fiber installation methods (permanent, wireline, and disposable) to evaluate completion design efficiency. We apply a geomechanical inversion algorithm to constrain fracture widths using low-frequency DAS (LF-DAS) recorded at an offset well to evaluate the degree of stage isolation in an injection well. LF-DAS indicates incomplete stage isolation in three of the four analyzed intervals, which is validated with distributed temperature sensing (DTS) measurements recorded in the injection well. We find high-frequency in-well DAS measurements are affected by proppant induced erosion and near-wellbore fractures, preventing reliable diagnostics. Implications of our results support LF-DAS for providing critical information for in-well diagnostic interpretations to optimize completion efficiency.
We then leverage the versatility and sensitivity to surface-waves of DAS to examine the potential for using untrenched surface deployments. We acquire continuous DAS data for one hour on a rapidly deployed fiber array composed of six parallel linear subsections laid directly on the surface with different fiber-ground contact conditions. We apply ambient interferometry and adopt a simplified spectral-analysis-of-surface waves (SASW) method to determine the average shear-wave velocity of the top 30 m (VS30). Our methodology results in VS30 estimates for each surface subsection that are consistent with collocated 1 m-depth trenched cables. The implications of these findings support DAS as a viable method for non-invasive deployment surface surveys for earthquake hazard assessment.
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