Diagnostic Tool for Unconventionals Evaluates Near-Wellbore Fractures
Authors: Carpenter Chris 
Published: October 2022

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 2021-5408, “New Fracture Diagnostic Tool for Unconventionals: High-Resolution Distributed Strain Sensing by Rayleigh Frequency Shift During Production in Hydraulic Fracture Test 2,” by Gustavo A. Ugueto, SPE, Shell; Magdalena Wojtaszek, SPE, Brunei Shell; and Somnath Mondal, Shell, et al. The paper has not been peer reviewed.

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Fiber-optic (FO) monitoring in unconventional reservoirs has proved to be an invaluable diagnostic tool. Unfortunately, gaining detailed understanding of the near-wellbore fracture geometry or cluster or stage productivity through FO has proved to be more difficult. A new FO diagnostic method, distributed strain sensing based on Rayleigh frequency shift (DSS-RFS), provides insights about the characteristics of the near-wellbore-region (NWR) during production. The authors write that this novel FO technique can significantly improve understanding of near-wellbore hydraulic fracture characteristics and the relationships between stimulation and production from unconventional oil and gas wells.

Introduction
Successful production profiling through distributed acoustics sensing (DAS) has been reported in gas-producing wells. Regrettably, many wells do not generate strong DAS signals while flowing. Recently developed FO production-profiling data-acquisition protocols and work flows rely on collecting signals during short shut-in reopening cycles lasting only a few minutes. Therefore, a need exists to develop new work flows and algorithms using high-fidelity FO data that can be related to the production characteristics of individual perforation clusters (PCs).

DSS-RFS Method
DSS-RFS uses Rayleigh backscatter in a nonengineered single-mode fiber to measure strain changes along the fiber. When an optical fiber is manufactured, random inhomogeneities of the glass density are created in the fiber core. The random density heterogeneities manifest as a variation of refractive index along the fiber. For a certain laser frequency, the interferences between the Rayleigh backscatters cause irregular but unique amplitude fluctuations in the coherent optical time-domain reflectometer along the fiber length. For each discrete fiber segment, a unique Rayleigh scattering spectrum is obtained by scanning the fiber with a coherent optical time-domain reflectometer with a range of laser frequencies using a tunable-wavelength laser system. This unique Rayleigh scattering spectrum shifts in frequency if the temperature or strain of the fiber section changes.
DSS-RFS measures relative strain changes instead of absolute strain along the fiber. However, DSS-RFS has a higher measuring sensitivity than strain measured using Brillouin optical time-domain reflectometry. DSS-RFS has higher spatial resolution than low-frequency DAS. Finally, DSS-RFS does not have the fiber sensing length restrictions of commercially available fiber-Bragg-grating-based DSS systems.

First DSS-RFS During Shut-in and Reopening Test
In February 2020, DSS-RFS data was acquired at one of the two FO-instrumented wells in Hydraulic Fracture Test Site 2 (HFTS2). The spatial resolution of the measurement was 20 cm, with a time-sampling interval of 150 seconds. The test began after establishing a baseline measurement during normal flowing conditions. The well was shut in for 4 days and opened again in a series of 2-hour shut-in and 1-hour producing cycles. Finally, the well was reopened and DSS-RFS was acquired for approximately 6 hours. This DSS-RFS data allowed for the definition of intervals where extension has occurred corresponding to active clusters with positive strain-change signals, slightly compressed intervals with negative strain-change signals between the clusters, and intervals with inactive clusters. These signals were interpreted to be caused by fracture-aperture changes that, in turn, were caused by pressure increases during shut-in within the fracture network in the NWR.