Pressure measurements have long been vital to evaluating fracture interactions and well performance in both conventional and unconventional formations. Pressure responses during stimulation can be used to evaluate characteristics such as fracture geometry and net pressure. Dynamic pressure changes which occur during drawdown, both near wellbore and far-field, provide insight into the effective geometry being drained and how it evolves over time.

The combination of horizontal wells and hydraulic stimulation have been key in unlocking vast unconventional resources across North America and beyond, but there is still much left to understand regarding how the reservoir is drained across the miles of laterals accessing the resource. The density of pressure gauges required to accurately measure the drainage pattern from a horizontal multi-stage stimulation is currently not realistic economically or technologically. In this case study, the authors will describe a method of monitoring the drainage profile of a horizontal multi-stage stimulation using optical fiber.

Optical fiber was installed on an observation lateral in combination with six reservoir-sensing pressure gauges as part of the Hydraulic Fracture Test Site I, Phase 3 in the Eagle Ford, funded by the Department of Energy (DOE). The lateral observation well was positioned approximately 225 feet away from the nearest stimulated and producing horizontal well. Using Raleigh Frequency Shift Distributed Strain Sensing (RFS-DSS), the strain change was measured in the far-field as the offset well was stimulated and first produced. RFS-DSS provides a spatial resolution of 20 cm, allowing for the monitoring of strain changes much smaller than we can accurately sense with Low Frequency Distributed Acoustic Sensing (LF-DAS) (Ugento et al., 2019). The strain change measured during the offset fracturing strongly correlates to the strain change measured during the production period.

The combination of RFS-DSS and six externally sensing pressure gauges provided a strong correlation between the total strain change and total pressure drawdown, where gauges positioned in regions of large negative strain change interpreted as drawdown showed large pressure declines and gauges in areas of small strain change saw small pressure declines. The correlation was applied to the total length of strain change, including areas without pressure gauges, to generate an estimated pressure profile for the entire length of the optical fiber.

This is the first attempt known to the authors where RFS-DSS monitored during production in a far-field observation well was translated to an estimated pressure profile. The technique will continue to be evaluated, trialed, and improved upon as additional data is collected from the pilot.

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