Study of the evolution characteristics of fiber-optic strain induced by the propagation of bedding fractures in hydraulic fracturing

Shale reservoirs contain numerous bedding fractures, making the formation of complex fracture networks during fracturing a persistent technical challenge in evaluating shale fracture morphology. Distributed optical fiber sensing technology can effectively capture the process of fracture initiation and propagation, yet the evaluation method for the initiation and propagation of bedding fractures remains immature. This study integrates a distributed optical fiber sensing device based on optical frequency domain reflectometry (OFDR) with a large-scale true tri-axial fracturing physical simulation apparatus to conduct real-time monitoring experiments on shale samples from the Lianggaoshan Formation in the Sichuan Basin, where bedding is well-developed. The experimental results demonstrate that two bedding fractures in the shale sample initiated and propagated. The evolution characteristics of fiberoptic strain in a horizontal adjacent well, induced by the initiation and propagation of bedding fractures, are characterized by the appearance of a tensile strain convergence zone in the middle of the optical fiber, flanked by two compressive strain convergence zones. The initiation and propagation of the distal bedding fracture causes the fiber-optic strain in the horizontal adjacent well to superimpose, with the asymmetric propagation of the bedding fracture leading to an asymmetric tensile strain convergence zone in the optical fiber. Utilizing a finite element method coupled with a cohesive element approach, a forward model of fiber-optic strain in the horizontal adjacent well induced by the initiation and propagation of hydraulic fracturing bedding fractures was constructed. Numerical simulation analyses were conducted to evaluate the evolution of fiber-optic strain in the horizontal adjacent well, confirming the correctness of the observed evolution characteristics. The presence of a "wedge-shaped" tensile strain convergence zone in the fiber-optic strain waterfall plot, accompanied by two compressive strain convergence zones, indicates the initiation and propagation of bedding fractures during the fracturing process. These findings provide valuable insights for interpreting distributed fiber-optic data in shale fracturing field applications. (c) 2024 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).