Geomechanical Template for Distributed Acoustic Sensing Strain Patterns during Hydraulic Fracturing

Abstract

Summary Recently more distributed acoustic sensing (DAS) data have been collected during hydraulic fracturing in shale. Low-frequency DAS signals show patterns that are intuitively consistent with the understanding of the strain field around hydraulic fractures. This study uses a fracture simulator combined with a finite element solver to further understand the various patterns of the strain field caused by hydraulic fracturing. The results can serve as a “type-curve” template for the further interpretation of cross-well strain field plots. Incorporating detailed pump schedule and fracturing fluid/proppant properties, we use a hydraulic fracture simulator to generate fracture geometries, which are then passed to a finite element solver as boundary conditions for elastic-static calculation of the strain field. Because the finite element calculated strain is a tensor, it needs to be projected along the monitoring well trajectory to be comparable with the DAS strain, which is uniaxial. Moreover, the calculated strain field is transformed into a time domain using constant fracture propagation velocity. Strain rate is further derived from the simulated strain field using differentiation along the fracture propagation direction. Scenarios including a single planar hydraulic fracture, a single fracture with a discrete fracture network (DFN), and multiple planar hydraulic fractures in both vertical and horizontal directions were studied. The scenarios can be differentiated in the strain patterns on the basis of the finite element simulation results. In general, there is a tensile heart-shaped zone in front of the propagating fracture tip shown along the horizontal strain direction on both strain and strain rate plots. On the sides, there are compressional zones parallel to the fracture. The strain field projects beyond the depth where the hydraulic fracture is present. Patterns from strain rate can be used to distinguish whether the fracture is intersecting the fiber. Along the vertical direction, the transition zone depicts the upper boundary of the fracture. A complex fracture network with DFN shows a much more complex pattern compared with a single planar fracture. Multiple planar fractures show polarity reversals in horizontal fiber because of interactions between fractures. Data from the Hydraulic Fracturing Test Site 2 (HFTS2) experiment were used to validate the simulated results. The application of the study is to provide a template to better interpret hydraulic fracture characteristics using low-frequency DAS strain-monitoring data. To our understanding, there are no comprehensive templates for engineers to understand the strain signals from cross-well fiber monitoring. The results of this study will guide engineers toward better optimization of well spacing and fracturing design to minimize well interference and improve efficiency.

Hydraulic fracturing: Laboratory evidence of the brittle-to-ductile transition with depth

“…4d,e, and f). Such evidence of nonlocalized (spatially extended) plastic deformation induced by fracture propagation (stress/hydraulic) are also observed and proven in the numerical/experimental studies (Brantut et al 2011;Huang and Chen 2021;Huang and Ghassemi 2016;Liu and Brantut 2022;Parisio et al 2021;Ramos Gurjao et al 2022;Richard et al 2021;Schmidt et al 2022;Tan et al 2021;Vinci et al 2014;Wrobel et al 2022;Zhang et al 2020).…”

Autonomous Inversion of In Situ Deformation Measurement Data for CO2 Storage Decision Support

“…Murdoch et al [9] recently summarized recent advances in geomechanical instrumentation and demonstrated that instruments are now available that can reliably resolve strains as small as one nano-strain. Additionally, resent results from laboratory [3] and field tests [12] show that fiber-optic distributed acoustic sensing (DAS) measurements may be capable of detecting very small magnitude strains, though extracting quantitative quasi-static strains from DAS data is an ongoing research effort. Even though some uncertainty remains about which strain sensing approach will prove to be optimal considering trade offs of accuracy, cost, and long-term reliability, it seems quite clear that data describing the deformation of subsurface storage systems will soon be available to operators.…”