Numerical Modeling of Fracture Network Evolution in Organic‐Rich Shale With Rapid Internal Fluid Generation

Abstract

When low‐permeability and organic‐rich rocks such as shale experience sufficient heating, chemical reactions including shale dehydration and maturation of organic matter lead to internal fluid generation. This may cause substantial pore fluid overpressure and fracturing. In the vicinity of igneous intrusions emplaced in organic‐rich shales, temperatures of several hundred degrees accelerate these processes and lead to intense fracturing. The resulting fracture network provides hydraulic pathways, which allow fluid expulsion and affect hydrothermal fluid flow patterns. However, the evolution of these complex fracture networks and controls on geometry and connectivity are poorly understood. Here, we perform a numerical modeling study based on the extended finite element method to investigate coupled hydromechanical fracture network evolution due to fast internal fluid generation. We quantify the evolution of different initial fracture networks under varying external stresses by analyzing parameters including fracture length, opening, connectivity, and propagation angles. The results indicate a three‐phase process including (1) individual growth, (2) interaction, and (3) expulsion phase. Magnitude of external stress anisotropy and degree of fracture alignment with the largest principal stress correlate with increased fracture opening. We additionally find that although the external stress field controls the overall fracture orientation distribution, local stress interactions may cause significant deviations of fracture paths and control the coalescence characteristics of fractures. Establishing high connectivity in cases with horizontally aligned initial fractures requires stress anisotropy with σV > σH, while the initial orientation distribution is critical for connectivity if stresses are nearly isotropic.

Fracability Evaluation Method and Influencing Factors of the Tight Sandstone Reservoir

“…Yan et al [20] found that as the angle between the direction of wellbore axis and the maximum horizontal stress increases, the pressures of fracture initiation and propagation grow. Rabbel et al [21] found that the fracture opening and propagation mode are related to the magnitude of external stress anisotropy, and strongly anisotropic far-field stresses lead to highly directional connectivity, which may translate to anisotropic fracture permeability. Liu et al [22] developed a novel fracability evaluation model of hydratebearing sediments integrating hydrate saturation, brittleness, stress anisotropy, and mineral composition.…”

Development of 3‐D Curved Fracture Swarms in Shale Rock Driven by Rapid Fluid Pressure Buildup: Insights From Numerical Modeling

“…To the best of the authors' knowledge, few if any numerical studies have yet been performed to simulate the development of nonplanar 3‐D fracture swarms, except for several attempts in investigating the evolution of 2‐D planar/nonplanar fracture networks (Z. Fan et al., 2014; Jin et al., 2010; Rabbel et al., 2020; Vega et al., 2020). Previous numerical and experimental studies have recognized that development of the fracture networks will experience the following three stages: (1) individual growth; (2) interaction and coalescence; and (3) loss of fluid overpressure and fracture aperture (Kobchenko et al., 2011;2014; Rabbel et al., 2020).…”

“…Fan et al., 2014; Jin et al., 2010; Rabbel et al., 2020; Vega et al., 2020). Previous numerical and experimental studies have recognized that development of the fracture networks will experience the following three stages: (1) individual growth; (2) interaction and coalescence; and (3) loss of fluid overpressure and fracture aperture (Kobchenko et al., 2011;2014; Rabbel et al., 2020). Although 2‐D models are useful for quantifying the onset, propagation, coalescence, and evolution of fluid‐driven fracture networks (Vega et al., 2020) and the 2‐D results are easy to comprehend and visualize (Rabbel et al., 2020), absence of the third dimension would lead to a potential knowledge gap that may impede a better understanding of realistic patterns of interacting fracture swarms.…”

“…However, the scale gap may prevent practical applications (i.e., meter or kilometer scale). Precise quantification of the stress field of propagating fractures remains challenging (Rabbel et al., 2020). Consequently, numerical simulation may constitute a powerful tool to reveal mechanisms of evolving fracture swarms across scales in organic‐rich shale.…”

“…The limitation of the DEM lies in its invalidity in offering 3-D geometry as a whole and size-/ time-dependent aperture for the evolving fracture. Recently, Rabbel et al (2020) adopted the extended finite element method (XFEM) to examine fracture network evolution in organic-rich shale. Their results suggested the three-phase development of the fracture clusters during rapid internal fluid generation, that is, individual fracture propagation, interaction and coalescence, and overpressure relaxation, which achieved close accordance with earlier experimental work (Kobchenko et al, 2014).…”