Fracture Geomechanics—Obstacles and New Perspectives

Dear Colleagues,

The objective of this Special Issue of Geosciences is to disseminate and discuss recent advances in the geomechanics of subsurface fractures by collecting high-quality original research articles, reviews, and technical notes.

The geomechanics of fractures finds applications in many subsurface engineering disciplines at the forefront of sustainable fossil-energy extraction and green-energy transition. This is because fractures strongly impact fluid flow, solute transport, seismicity, and rock mass failure in the subsurface. Hence, there is an increasing interest in the sophisticated characterization, modeling, prediction, and management of subsurface fractures. Nevertheless, the current understanding of fracture geomechanics is fairly limited, hindering safe and efficient operations in underground mining, hydrocarbon recovery, enhanced geothermal systems, geological CO₂ sequestration/mineralization, subsurface energy (H₂, Gas) storage, nuclear waste disposal, etc. To push the knowledge boundary and benefit global geo-resource utilization, we would like to invite you to submit your recent work on experimental, computational modeling, and field studies of subsurface fractures with respect to the following topics:

• Hydraulic fracturing and interactions with natural geological discontinuities;
• Hydro-mechanical coupling of fracture and fluid flow;
• Hydromechanical interactions of the fracture network and rock matrix;
• Shear fracturing and seismicity;
• Fracture geomechanics on the structural failure of underground openings (tunnels, boreholes, caves, etc.);
• Impact of geochemical reactions on fracture geomechanics;
• Constitutive behavior of fracture geomechanics;
• Fracture geomechanics in the geothermal environments;
• Monitoring of fracture geomechanical behavior;
• Application of machine learning and probabilistic modeling in fracture geomechanics.

Dr. Wenfeng Li
Dr. Shahrzad Roshankhah
Guest Editors

Manuscript Submission Information

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• hydraulic fracturing
• natural fractures
• thermo-hydro-mechanical–chemical coupling
• process monitoring systems
• acoustic emission
• seismicity
• constitutive laws
• machine learning
• flow in fractured porous media
• uncertainty quantification

Planned Paper 1:

Author: Vahid Tavakoli

Abstract: Reservoir heterogeneity plays an important role in the exploration, evaluation and drilling of hydrocarbon reservoirs. In the current research, the heterogeneity of a reservoir located in Permian–Triassic Kangan and Dalan formations were studied in two aspects namely petrophysical and geomechanical. Core-derived petrophysical data have been used to evaluate the petrophysical heterogeneity through rock typing process. Well-known methods of Lucia, FZI, and Winland were employed for this purpose. Geological properties of all rock types were considered using thin section studies. Laboratory measured rock strength characteristics and Schmidt Hammer rebound (SHR) data compared with the results of wire-line log analysis. These data were integrated in two-steps clustering for evaluating the geomechanical heterogeneity. Uniaxial compressive strength (UCS) was selected as the best strength characteristics of rocks for clustering and determination of geomechanical units (GMUs). Four GMUs were distinguished as the final results of clustering with good quality of clustering based on UCS and SHR. Distribution of each petrophysical rock type was considered in four GMUs using pie diagrams to demonstrate the effect of petrophysical on geomechanical heterogeneity. Results revealed that UCS and SHR have reverse relationships with porosity. Also, it was found that Lucia's method was more suitable than others to evaluate the geomechanical heterogeneity based on petrophysical changes in the reservoir. This method uses both geological and petrophysical properties and therefore can manage the geomechanical heterogeneity more effectively. Finally, according to the results, UCS parameter is more effective than SHR to determine the GMUs in studied reservoirs.

Planned Paper 2:

Title: Safety assessment of concrete gravity dams considering fracture propagation at the dam/foundation interface
Authors: Maria Luísa Braga Farinha; Nuno Monteiro Azevedo; Sérgio Oliveira
Affiliation: Laboratório Nacional de Engenharia Civil (LNEC), Lisbon, Portugal
Abstract: Given the potential of material and human losses associated to dam failure it is mandatory to adopt a framework that has the capacity to anticipate and prevent failures. The current approaches based on simplified limit equilibrium techniques are not able to characterize the complex dam/foundation behaviour, that leads to sliding along the dam/foundation interface or along rock mass discontinuities, or along rock mass layers of lower strength. The ability to assess the safety of the dam-foundation systems in an integrated way still needs to be improved, namely by incorporating coupled models that take into account the significant interdependence between the mechanical and hydraulic behaviour and by using adequate constitutive laws.
An explicit time-stepping small displacement coupled algorithm, Parmac2D-Fflow, is used to assess the safety of different gravity dams. This algorithm is based on a discrete representation of discontinuities, simulates the hydro-mechanical interaction and considers softening based constitutive laws that are closer to the actual behaviour of concrete, rock and dam/foundation interface. The adopted model allows two different approaches for the seepage flow: i) seepage occurs in all interfaces independently of their damage (the corresponding water pressures are installed from the beginning of the simulation on all interfaces); ii) seepage only occurs after joint failure, making it possible to model a coupled propagation failure along the dam/foundation interface due to a hypothetical dam overtopping scenario.
Parametric studies are carried out that evaluate the influence of both the mechanical and the hydraulic properties on the global safety factor for three different dam geometries. The numerical results predicted with a coupled/fracture propagation model are compared with those obtained with a strength reduction method. Also presented are the results obtained with a coupled model that considers seepage to occur from the beginning of the overtopping simulation. The results presented show that with the proposed coupled/fracture propagation model it is possible to identify more realistic failure modes of the dam/foundation system and, as expected, higher safety factors are obtained.