Role of shale thickness on vertical connectivity of fractures: application of crack-bridging theory to the Austin Chalk, Texas

Rijken, P.M.; Cooke, Michele L.

doi:10.1016/s0040-1951(01)00107-x

Cited by 110 publications

(47 citation statements)

References 28 publications

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“…For example, some calcite veins in marly shale layers are offset (with very thin connections between the segments) at bedding contacts ( Figure 13B). Similar offsets can be observed for many joints ( Figure 13C; Helgeson and Aydin, 1991;Cooke and Underwood, 2001;Rijken and Cooke, 2001;Larsen et al, 2008). These and other field observations indicate that the arrest or, more generally, offset of hydrofractures at contacts in mechanically layered rocks is very common.…”

Section: Field Observations and Numerical Model On Hydrofracture Emplsupporting

confidence: 83%

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

2013

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Fractures generated by internal fluid pressure, for example, dykes, mineral veins, many joints and man-made hydraulic fractures, are referred to as hydrofractures. Together with shear fractures, they contribute significantly to the permeability of fluid reservoirs such as those of petroleum, geothermal water, and groundwater. Analytical and numerical models show that-in homogeneous host rocks-any significant overpressure in hydrofractures theoretically generates very high crack tip tensile stresses. Consequently, overpressured hydrofractures should propagate and help to form interconnected fracture systems that would then contribute to the permeability of fluid reservoirs. Field observations, however, show that in heterogeneous and anisotropic, e.g., layered, rocks many hydrofractures become arrested or offset at layer contacts and do not form vertically interconnected networks. The most important factors that contribute to hydrofracture arrest are discontinuities (including contacts), stiffness changes between layers, and stress barriers, where the local stress field is unfavorable to hydrofracture propagation. A necessary condition for a hydrofracture to propagate to the surface is that the stress field along its potential path is everywhere favorable to extension-fracture formation so that the probability of hydrofracture arrest is minimized. Mechanical layering and the resulting heterogeneous stress field largely control whether evolving hydrofractures become confined to single layers (stratabound fractures) or not (non-stratabound fractures) and, therefore, if a vertically interconnected fracture system forms. Non-stratabound hydrofractures may propagate through many layers and generate interconnected fracture systems. Such systems commonly reach the percolation threshold and largely control the overall permeability of the fluid reservoirs within which they develop.

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Section: Field Observations and Numerical Model On Hydrofracture Emplsupporting

confidence: 83%

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

2013

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“…Ameen & Cosgrove 1990;Koestler & Ehrmann 1991;Rawnsley et al 1992;Ameen 1995;Gross & Engelder 1995;Gillespie et al 2001;Rijken & Cooke 2001;Richard et al 2002;Belayneh 2004;Peacock 2004;Belayneh et al 2007;Gross & Eyal 2007). However, the fracture geometries developed in many of these studies are remarkably similar, and there are several key geometric patterns that recur in different lithologies and settings.…”

Section: Previous Studiesmentioning

confidence: 99%

“…This strike generally reflects a regional tectonic in situ stress field, although it may reflect a more local stress field caused by flexure on the outer arc of a large fold (Pollard & Aydin 1988;Lorenz et al 1991), for example. Fracture sets are often restricted within a single stratigraphic unit (typically a brittle bed surrounded by more ductile layers: Gross & Engelder 1995;Rijken & Cooke 2001;Peacock 2004), and the spacing of the fractures is generally proportional to the thickness of that unit, so that the layer-bound fracture density in thin beds is much higher than in thick beds (Bai & Pollard 2000a, b). The fractures may be vertical or they may be inclined, forming conjugate pairs.…”

Section: Previous Studiesmentioning

confidence: 99%

Using mechanical models to investigate the controls on fracture geometry and distribution in chalk

Welch¹,

Souque

Davies³

et al. 2014

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Chalk is an important reservoir rock. However, owing to its low permeability, fractures are key to producing hydrocarbons from chalk reservoirs. Fractures in chalk usually form one of three geometric patterns: localized fractures (commonly concentric rings) developed around tips, bends and splays in larger faults; regularly spaced regional fracture sets; and fracture corridors comprising narrow zones of closely spaced parallel fractures. Localized fracture patterns are likely to give only local permeability enhancement; regional fracture sets and, especially, fracture corridors may provide long, high-permeability flow pathways through the chalk. Field mapping shows that both localized fracture patterns and fracture corridors often nucleate around larger faults; however, the fracture corridors rapidly propagate away from the faults following the regional stress orientation. It is therefore not necessary to know the detailed fault geometry to predict the geometry of the fracture corridors, although the fault density can help to predict the spacing of the fracture corridors. Mechanical modelling shows that while localized fracture patterns can form under normal fluid pressure conditions as a result of local stress anomalies around fault bends, tips and splays, fracture corridors can only form under conditions of fluid overpressure. Once they nucleate, they will continue to propagate until they either intersect another fault or the fluid pressure in them is dissipated.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.Cretaceous chalk is an important reservoir unit in the North Sea, and hosts a number of large fields in the Norwegian, Danish and UK sectors. However, the matrix permeability of chalk is generally very low (typically ,1 mD), and, hence, fractures are often important in controlling fluid flow in the subsurface and are key to producing these fields economically ( Aims and objectivesIn this study we use mechanical modelling techniques to examine the controls on fracture development and resulting geometry in chalk, and compare these results with fracture patterns observed in two chalk outcrops from southern and NE England. In particular, we will examine the relationship between fractures, fracture corridors and larger scale faults, and the impact of fluid pressure on fracture development. We will use elastic dislocation and finite-element models to investigate the in situ stress patterns developed around larger faults under different conditions of deformation and compare these with the observed fracture patterns. The elastic dislocation models simply assume the chalk to be a continuum elastic material, and take no account of fluid pressure. However, the finite-element models allow us to incorporate the effects of fluid overpressure, leading to dilation of the faults and fractures. We model two end-member cases of overpressured chalk: a case with a permeable host rock, in which the fluid overpressure results in a decrease in the effective horizontal stress; and a...

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“…Mimicking internal pressures with effective stresses or effective strains on the boundaries of models is a widely used approach (Olson et al 2009;Philip et al 2005;Rijken and Cooke 2001), and concerning numerical modelling, results in far less complicated mesh geometries. This implies that, the results presented depict static solutions of the applied boundary conditions and implemented fractures.…”

Section: Model Setupmentioning

confidence: 99%

Mechanical Factors Controlling the Development of Orthogonal and Nested Fracture Network Geometries

et al. 2018

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Orthogonal fracture networks form an arrangement of open well-connected fractures which have perpendicular abutment angles and sometimes show topological relations by which fracture sets abut against each other, thus forming a nested network. Previous modelling studies have shown that orthogonal fractures may be caused by a local stress perturbation rather than a rotation in remote stresses. In this study, we expand on the implications of these local stress perturbations using a static finite element approach. The derived stress field is examined to assess the development of implemented microfractures. The results show that the continuous infill of fractures leads to a gradual decrease in the local tensile stresses and strain energies, and, therefore, results in the development of a saturated network, at which further fracture placement is inhibit. The geometry of this fully developed network is dependent on the remote effective stresses and partly on the material properties. Saturated networks range from: (1) a set of closely spaced parallel fractures; (2) a ladder-like geometry; and (3) an interconnected nested arrangement. Finally, we show that our modelling results at which we apply effective tension, are equivalent to having a uniformly distributed internal pore fluid pressure, when assuming static steady state conditions and no dynamic fluid behaviour.

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Role of shale thickness on vertical connectivity of fractures: application of crack-bridging theory to the Austin Chalk, Texas

Cited by 110 publications

References 28 publications

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

Using mechanical models to investigate the controls on fracture geometry and distribution in chalk

Mechanical Factors Controlling the Development of Orthogonal and Nested Fracture Network Geometries

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