Role of overpressurized fluid and fluid-driven fractures in forming fracture networks

Zhang, Xi; Jeffrey, Robert G.

doi:10.1016/j.gexplo.2014.03.021

Cited by 43 publications

(19 citation statements)

References 40 publications

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“…2 The first trend focuses on improving the computational aspects of the models by employing state-of-the-art numerical techniques to develop models that overcome the limitations of their existing counterparts. Examples are studies that employed different variations of the boundary integrals method, [3][4][5][6][7][8][9][10][11] finite element method, [12][13][14][15][16] extended/generalized finite elements, [17][18][19][20][21][22][23][24][25][26][27][28][29][30] phase field methods, [31][32][33][34][35][36] and hybrid finite element/eXtended finite element-distinct element techniques (FEM-DEM or XFEM-DEM) [37][38][39][40] to develop 2D and 3D HF models. Additionally, some recent studies have coupled boundary integral or extended finite element methods with fracture tip asymptotes and presented efficient multiscale models of HFs under different propagation regimes.…”

Section: Introductionmentioning

confidence: 99%

On the undrained and drained hydraulic fracture splits

Esfahani

Gracie

2019

Numerical Meth Engineering

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Summary The simulation of hydraulic fracturing (HF) involves the solution of a hydro‐mechanically coupled system. This article presents a new iterative sequential coupling algorithm, the undrained HF split, that improves the simulation of HFs in impermeable media. A poromechanics analogy is used to derive a stable split for the hydro‐mechanically coupled system in which the mechanical subproblem is solved first. The proposed undrained HF split is applied to the simulation of cohesive HFs in an impermeable elastic medium. The cubic law is used as the constitutive model for simulating fluid flow in fractures. A minimum hydraulic aperture is assumed in the cohesive tip zone, where the mechanical aperture smoothly vanishes. While general in its nature, the undrained HF splitting scheme is employed within the context of a two‐dimensional eXtended finite element model for the fractured solid, and a regular finite element model for fluid in the fracture. The undrained HF split is successfully used to simulate self‐similar plane strain HFs as well as the propagation of HFs from a wellbore under anisotropic stress conditions. Fracture trajectories and local alteration of stress field are investigated. The solution of the undrained HF split converges to the same solution as the fully coupled model, whereas the commonly used P→W sequential algorithm, referred to in this article as the drained HF split, generates spurious oscillations and fails to converge in many problems. The undrained HF split is shown to be stable and robust in applications where the drained HF split is unstable.

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Section: Introductionmentioning

confidence: 99%

On the undrained and drained hydraulic fracture splits

Esfahani

Gracie

2019

Numerical Meth Engineering

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“…If two equal‐size new fractures are very close, one may suppress the growth of the other or both might merge to form a larger fracture. Actually simultaneous fluid‐driven growth can be possible if two fractures have different initial sizes [ Zhang and Jeffrey , ]; however, this situation is not applicable to fracture nucleation. For the results presented below, the minimum new fracture spacing D is chosen as 0.1 m. If two starter fractures are within a separation less than 0.1 m, only one fracture is considered.…”

Section: The Modelmentioning

confidence: 99%

“…[]. During hydraulic fracturing stimulation of hydrocarbon and geothermal reservoirs, the injection of fluids causes transient increases of fluid pressure to enhance fracture connectivity through fluid‐driven crack growth [ Rutledge and Phillips , ; Zhang and Jeffrey , ]. In addition, deposition of quartz and other minerals from circulating fluids in the crust can seal fracture walls and result in a buildup in fluid pressure upstream of these sealed sections [ Ramsay , ; Fyfe et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Fluid‐driven nucleation and propagation of splay fractures from a permeable fault

Zhang

Jeffrey

2016

JGR Solid Earth

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Progressive development of opening‐mode splay or branch fractures along a permeable fault in an elastic medium, subject to elevated fluid pressure from a constant influx fluid source, is studied numerically using a plane‐strain hydraulic fracturing model that couples fracture deformation and fluid flow. In situ stresses are imposed so that their resultant shear stress on the fault is lower than the frictional strength. New splay fractures are initiated based on satisfying a dual criterion for both tensile strength and fracture toughness and meeting a minimum fracture spacing requirement. Numerical results demonstrate that spatial variations in permeability along faults can cause arrest of local slip and the created slip gradient can result in splay fracture initiation at a significant distance inward from the fault tips. One splay fracture generally grows first, and the kinematic coherence in displacements at the junction between it and the fault can reduce the downstream flow rate to prevent the nucleation of other splay fractures. However, the number of splay fractures can be increased when the branch growth extent is limited to a certain size; when the fault is divided into many segments, each with a linear distributed initial aperture; and if the main fault is curved. The generation of a number of splay fractures can act to increase the permeability of the rock mass. When the splay fractures are constrained by two faults, a rhomb‐shaped fault zone involving multiple high‐angle branches forms. The development of the second‐generation splay fractures on a first‐generation one is promoted by fluid penetration. Multiple fluid‐driven splay fractures can be created under the condition that fluid pressure is below or slightly above the fault confining stress. This implies that generation of complex splay fracture patterns requires less energy than generating a single opening‐mode fracture. Multiple fluid‐driven splay fracture nucleation and growth into a horsetail pattern occur when the fault‐parallel in situ normal stress becomes more tensile. In addition, the effects of fluid viscosity and tensile fracture toughness are investigated to determine their role in splay fracture initiation and growth.

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“…The technique creates simulated statistical networks (Dowd et al, 2009) to solve the disparity between three-dimensional flow and two-dimensional fracture observations in the field. Modelling in three dimensions allows geologists to treat each fracture as a discrete entity irrespective of its -full or partial -connectivity, increasing the predictive power of the method (Zhang and Jeffrey, 2014). The method discretizes the area associated with each…”

Section: 4-discrete Fracture Modellingmentioning

confidence: 99%

Modelling the structural controls of primary kaolinite formation

Tierney

Glass

2016

Geomorphology

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An abundance of kaolinite was formed within the St Austell granite pluton of Cornwall, southwest England, by the hydrous dissolution of feldspar crystals. The permeability of Cornish granites is low and alteration acts pervasively from discontinuity features, with montmorillonite recognised as an intermediate assemblage in partially kaolinised material. Structural features allowed fluids to channel through the impermeable granite and pervade deep into the rock. Areas of high structural control are hypothesised to link well with areas of advanced alteration. As kaolinisation results in a loss of competence, we present a method of utilising discontinuity orientations from nearby unaltered granites alongside the local tectonic history to calculate strain rates and delineate a discrete fracture network. Simulation of the discrete fracture network is demonstrated through a case study at Higher Moor, where kaolinite is actively extracted from a pit. Reconciliation of fracture connectivity and permeability against measured subsurface data show that higher values of modelled properties match with advanced kaolinisation observed in the field. This suggests that the technique may be applicable across various industries and disciplines.Keywords: Kaolinite, Discrete Fracture Networks, Structural Influence, Kaolinisation, 3D Modelling 1-IntroductionKaolinite is found extensively across the St Austell granite pluton of southwest England, formed from the primary, in-situ hydrous alteration of Na-feldspar and K-feldspar (Brown, 1953;Fuge and Power, A C C E P T E D M A N U S C R I P T Both equations result in the partial leaching of a silica by-product (Charoy, 1981) and an increase in hydrogen ion activity (Exley, 1976). As hydrogen ions dissolve within the circulating fluids, they corrode feldspar cleavage planes and allow ions to redistribute into silicate layers (Guilbert and Sloane, 1968). However, the change from feldspar to kaolinite is not as straightforward as the equations imply. Intermediate assemblages of alternative clay types, such as illite and smectite, are commonly found within partially kaolinised granites (Psyrillos et al., 1998(Psyrillos et al., , 2003Bristow et al., 2000; Papoulis et al., 2004;Suringar, 2004). Papoulis et al. (2004) suggest that exposure length, permeability of the deposit, and the space available for the reactions are all factors that will influence the formation of clay. Partially kaolinised samples from St Austell are found to contain Na-montmorillonite, a smectite clay (Scott et al., 1996; Psyrillos et al., 1998;Ellis and Scott, 2004), due to preferential dissolution of plagioclase end-member albite (Exley, 1976; Psyrillos et al., 1998;Bristow et al., 2000).Granite is relatively impermeable due to its interlocking crystalline texture. In order to create the wide expanses of kaolinite found around St Austell, alteration fluids must have utilised major discontinuity networks within the rock mass (e.g. joints, fractures, faults) to circulate. This is evidenced through the fact t...

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Role of overpressurized fluid and fluid-driven fractures in forming fracture networks

Cited by 43 publications

References 40 publications

On the undrained and drained hydraulic fracture splits

On the undrained and drained hydraulic fracture splits

Fluid‐driven nucleation and propagation of splay fractures from a permeable fault

Modelling the structural controls of primary kaolinite formation

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