Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies—differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.

Natural fractures in shale: A review and new observations

Modeling of Hydraulic-Fracture-Network Propagation in a Naturally Fractured Formation

/pdf/development-of-an-efficient-embedded-discrete-fracture-model-24f3tauvjl.pdf

Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

Hydraulic fractures represent a particular class of tensile fractures that propagate in solid media under pre-existing compressive stresses as a result of internal pressurization by an injected viscous fluid. The main application of engineered hydraulic fractures is the stimulation of oil and gas wells to increase production. Several physical processes affect the propagation of these fractures, including the flow of viscous fluid, creation of solid surfaces, and leak-off of fracturing fluid. The interplay and the competition between these processes lead to multiple length scales and timescales in the system, which reveal the shifting influence of the far-field stress, viscous dissipation, fracture energy, and leak-off as the fracture propagates.

Mechanics of Hydraulic Fractures

Experimental study of hydraulic fracturing for shale by stimulated reservoir volume

A recently developed unconventional fracture model (UFM) is able to simulate complex fracture network propagation in a formation with pre-existing natural fractures. A method for computing the stress shadow from fracture branches in a complex hydraulic fracture network (HFN) based on an enhanced 2D displacement discontinuity method with correction for finite fracture height is implemented in UFM and is presented in detail including approach validation and examples. The influence of stress shadow effect from the HFN generated at previous treatment stage on the HFN propagation and shape at new stage is also discussed.

Numerical Modeling of Hydraulic Fractures Interaction in Complex Naturally Fractured Formations

Hydraulic Fracture Crossing Natural Fracture at Nonorthogonal Angles: A Criterion and Its Validation

Microseismic mapping (MSM) has shown that the occurrence of complex fracture growth is much more common than
initially anticipated and is becoming more prevalent with the increased development of unconventional reservoirs (shale-gas).
The nature and degree of fracture complexity must be clearly understood to select the best stimulation design and completion
strategy. Although MSM has provided significant insights into hydraulic fracture complexity, in many cases the
interpretation of fracture growth has been limited due to the absence of evaluative and predictive hydraulic fracture models.

Xiaowei Weng

Papers

Modeling of Hydraulic-Fracture-Network Propagation in a Naturally Fractured Formation

Numerical Modeling of Hydraulic Fractures Interaction in Complex Naturally Fractured Formations

Hydraulic Fracture Crossing Natural Fracture at Nonorthogonal Angles: A Criterion and Its Validation

Integrating Microseismic Mapping and Complex Fracture Modeling to Characterize Fracture Complexity

Hydraulic Fracture Crossing Natural Fracture at Non-Orthogonal Angles, A Criterion, Its Validation and Applications