Abstract
To investigate the influence of parallel fracture stress on hydraulic fracture propagation, this study pre-fabricated penny-shaped fractures in 30 cm cubic artificial cement specimens, with 4 FBG optical fibers containing a total of 20 strain measurement points positioned ahead of the fracture front. Hydraulic fracturing experiments were conducted by varying the intermediate principal stress (σ_H = 12, 21, 30 MPa) to analyze optical fiber deformation, breakdown pressure, and fracture surface characteristics during fracture initiation and propagation. Simultaneously, analytical solutions for strain at the tip of a penny-shaped hydraulic fracture were employed for analysis, leading to the following conclusions: (1) Parallel fracture stress exerts a compaction effect, inducing a plastic hardening effect in the material, which enhances the effective fracture toughness and increases both the breakdown pressure and propagation pressure. (2) As σ_H increases, mixed-mode fracture becomes more likely to occur, with fracture surfaces exhibiting curvature, step-like features, and other morphologies, and the roughness of the fracture surface increases accordingly. (3) The hydraulic fracture tip can be divided into a subcritical fracture zone, a nonlinear zone, an unloading damage zone, and an elastic zone. The fracture-tip nonlinear zone exhibits elastoplastic characteristics, manifesting as "heart-shaped" or "rabbit-ear-shaped" patterns. The fundamental reason for the increase in breakdown pressure is that parallel fracture stress enlarges the area and deformation magnitude of the fracture-tip nonlinear zone. This research holds significant importance for understanding the deformation characteristics at hydraulic fracture tips, can be used to calibrate criteria for hydraulic fracture initiation and propagation, and also provides mechanistic insights for interpreting fracture propagation patterns and fracturing impacts using DAS and DSS in field applications.
Full Text
Preamble
Investigation of Hydraulic Fracture Propagation Subjected to Crack-Parallel Compressive Stress
Zhou Dawei
College of Petroleum Engineering, China University of Petroleum (Beijing), Beijing 102249, China
Abstract
To investigate the influence of crack-parallel stress on hydraulic fracture propagation, we conducted hydraulic fracturing experiments on 30-cm cubic artificial cement specimens with pre-existing penny-shaped cracks. Four fiber Bragg grating (FBG) optical fibers, providing 20 strain measurement points, were placed ahead of the crack tip. The intermediate principal stress (σH) was varied across three levels (12, 21, and 30 MPa) to analyze fiber deformation, breakdown pressure, and fracture surface characteristics during crack initiation and propagation. Analytical solutions for strain at penny-shaped hydraulic fracture tips were also employed for comparative analysis. The following conclusions were obtained: (1) Crack-parallel stress exerts a compaction effect that induces plastic strengthening in the material, thereby enhancing the effective fracture toughness and increasing both breakdown and propagation pressures. (2) As σH increases, mixed-mode fracture becomes more likely to occur, and fracture surfaces exhibit curved profiles, step-like features, and other complex geometries, with a corresponding increase in surface roughness. (3) The hydraulic fracture tip comprises four distinct zones: a subcritical crack zone, a nonlinear zone, an unloading damage zone, and an elastic zone. The nonlinear zone exhibits elastoplastic characteristics, manifesting as a "heart-shaped" or "rabbit-ear" pattern. The fundamental mechanism underlying the increase in breakdown pressure is that crack-parallel stress enlarges both the area and deformation magnitude of the nonlinear zone at the fracture tip. This research significantly advances understanding of hydraulic fracture tip deformation characteristics, enabling calibration of fracture initiation and propagation criteria. Furthermore, it provides mechanistic insights for interpreting fracture propagation modes and fracturing-induced seismicity through distributed acoustic sensing (DAS) and distributed strain sensing (DSS) in field applications.
Keywords: parallel stress; fracture tip deformation; optical fiber; nonlinear deformation; fracture process zone
Representative Results
[FIGURE:1] Specimen with embedded FBG optical fibers and post-fracturing fracture morphology: (a) schematic diagram of artificial specimen with optical fibers; (b) specimen after fracturing.
[FIGURE:2] (a) Optical fiber responses during the fracturing process; (b) measured deformation at hydraulic fracture tip versus analytical solution for penny-shaped hydraulic fracture tip deformation.