The influence of the cohesive process zone in hydraulic fracturing modelling

Sarris, Ernestos; Papanastasiou, Panos

doi:10.1007/s10704-010-9515-4

Cited by 109 publications

(75 citation statements)

References 33 publications

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“…In future work we would envisage to modify the Frac3D code so that it can accommodate bilinear or linear-exponential traction separation relationships. The importance of using a complete traction displacement relationship, its effect on cohesive zone size and its influence on net pressure was highlighted by Sarris and Papanastasiou (2011). Since our endeavor was to numerically investigate the effect of pore pressure keeping other variables constant, we deliberately chose a simple linear traction separation law.…”

Section: Frac3d Coupled Simulationmentioning

confidence: 99%

Pore pressure effects on fracture net pressure and hydraulic fracture containment: Insights from an empirical and simulation approach

Prabhakaran

Pater²,

Shaoul³

2017

Journal of Petroleum Science and Engineering

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A B S T R A C TPore pressure and its relationship with fracture net pressure has been reported qualitatively from both field and experimental observations. From a modeling perspective, the ubiquitously used pseudo 3D (P3D) models that are based on linear elastic fracture mechanics (LEFM) do not include the effect of reservoir depletion (or overpressure). Models that utilize effective stress as propagation criteria with a cohesive zone description, introduce the pore pressure directly into the simulation and hence can potentially capture the effect of pore pressure on fracture propagation. This work investigates the effect of pore pressure on hydraulic fracturing net pressure and geometry using empirical and numerical simulation approaches. We carried out an analysis of more than 400 datafrac injections spanning a wide range of geological ages and depositional environments in order to investigate the relationship between observed net pressure and reservoir pore pressure. The net fracture propagation pressure from the fracture treatment analysis was seen to be correlated with the effective stress in the reservoir. Fracture propagation simulations were performed using a coupled finite element -finite difference fracture simulator. The code uses a cohesive zone model (CZM) to describe fracture propagation. Four different effective stress scenarios were used to study the effect of effective stress on net pressure. The simulation results closely match the empirical relation between net pressure and effective stress as obtained from the analysis of actual frac treatment data. It is observed from the simulations that the magnitude of the effective stress also has an effect on the fracture geometry with a high effective stress leading to wider, shorter and more radial fractures. The derived empirical correlation is hence useful as a fracture design parameter. The datafrac net pressure diagnostics workflow in the pseudo 3D models can incorporate local tip pore pressure as a new pressure matching parameter. The pore pressure effect can thus explain high net pressures routinely observed in frac operations and also as a containment mechanism.

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Section: Frac3d Coupled Simulationmentioning

confidence: 99%

Pore pressure effects on fracture net pressure and hydraulic fracture containment: Insights from an empirical and simulation approach

Prabhakaran

Pater²,

Shaoul³

2017

Journal of Petroleum Science and Engineering

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“…The first hypothesis is that the high observed net pressure is related to a sharp drop of the fluid pressure [13] and to the existence of a dry region near the fracture tip of the fracture [14]. The second hypothesis together with the non linear deformation of the rock might have a strong influence in hydraulic fracturing [4,5,8,12]. In this study, the first hypothesis was dealt with the incorporation of the cohesive zone model as the fracture propagation criterion to ensure that the stresses in the vicinity of the fracture tip will remain finite and the second hypothesis was investigated by assuming the Mohr-Coulomb yield criterion to deal with plastic yielding with associated dilation.…”

Section: Resultsmentioning

confidence: 99%

“…Previous research works that detailed examined the influence of the cohesive zone in hydraulic fracturing were reported earlier in [12]. The properties of the cohesive zone are summarized in Table (1).…”

Section: Methodsmentioning

confidence: 99%

Modelling of Hydraulic Fracturing in a Weak Permeable Formation

Sarris

Papanastasiou

2012

All Days

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We present numerical results of a propagating hydraulic fracture in weak rock formations. The results were obtained from a Finite Element Hydraulic Fracturing model which simulates all the involved non-linear and coupled processes. The fracture is driven by pumping of an incompressible viscous fluid with Newtonian rheology. The fracturing fluid leak-off in the host rock formation is also considered. For propagation criterion a cohesive-softening model is implemented in interface finite elements and the bulk rock deformation is modelled by Mohr-Coulomb yield criterion. The main objective of this research work is to determine the influence of the stress-field, pore pressure field and pumping parameters on rock plastic deformation and its implications on fracturing net-pressure and fracture dimensions. It has been previously shown that in weak formations plastic yielding and the associated rock dilation create shielding of the fracture tip and hence larger pressures are needed to propagate the fractures and the created fractures are shorter and wider. We extend these studies to investigate the influence of the pore pressure which has been previously ignored. We found that high anisotropic stress field and high initial pore-pressure fields increase the plastic yielding in a rock formation demanding higher net-pressures for propagating a fracture and the created fractures are shorter and wider. This non-linear analysis and results may explain the differences observed in net-pressures between field measurements and conventional model predictions. These findings are important in improving the numerical simulators of modelling hydraulic fracturing in particular for short fractures in weak formations for sand control applications. The results are also important to better prediction of vertical fracture growth and containment in shale-natural gas stimulations where there are serious environmental concerns on the risk of ground-water contamination. IntroductionThe fluid driven problem arises in hydraulic fracturing, a technique widely used in the petroleum industry to enhance the recovery of hydrocarbons from underground reservoirs. Given its wide range of applications, it is not surprising that this problem has attracted numerous research contributions for the last fifty years. Most of these efforts are been oriented in towards the development of numerical models to predict the propagation of hydraulic fractures in complex geological domains. However, significant contributions have also been made to understand the mathematical physics of the fluid driven problem using rigorous and complex analysis [1].The hydraulic fracturing process involves the pumping of a viscous-fluid from a well into the rock formation under high enough fluid pressure to fracture the reservoir. In general, a fracture will re-orient itself in the complex stress field near the wellbore and will propagate further perpendicular to the minimum compressive stress. The pumping of fluid is maintained at a rate high enough for the fluid pressure to overcome t...

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“…Essentially, in hydraulic fractures, whether man made or from a geologic source, a large (relative to the crack tip) volume around the fracture tip must transition from compression to tension in order for a fracture to extend, creating a process zone that is large relative to the geometry of the fracture [ Rubin , ]. This result has been further verified by Sarris and Papanastasiou [].…”

Section: Introductionmentioning

confidence: 99%

Fracture propagation in Indiana Limestone interpreted via linear softening cohesive fracture model

2015

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We examine the use of a linear softening cohesive fracture model (LCFM) to predict single-trace fracture growth in short-rod (SR) and notched 3-point-bend (N3PB) test configurations in Indiana Limestone. The broad goal of this work is to (a) understand the underlying assumptions of LCFM and (b) use experimental similarities and deviations from the LCFM to understand the role of loading paths of tensile fracture propagation. Cohesive fracture models are being applied in prediction of structural and subsurface fracture propagation in geomaterials. They lump the inelastic processes occurring during fracture propagation into a thin zone between elastic subdomains. LCFM assumes that the cohesive zone initially deforms elastically to a maximum tensile stress (σ max ) and then softens linearly from the crack opening width at σ max to zero stress at a critical crack opening width w 1 . Using commercial finite element software, we developed LCFMs for the SR and N3PB configurations. After fixing σ max with results from cylinder splitting tests and finding an initial Young's modulus (E) with unconfined compressive strength tests, we manually calibrate E and w 1 in the SR model against an envelope of experimental data. We apply the calibrated LCFM parameters in the N3PB geometry and compare the model against an envelope of N3PB experiments. For accurate simulation of fracture propagation, simulated off-crack stresses are high enough to require inclusion of damage. Different elastic moduli are needed in tension and compression. We hypothesize that the timing and location of shear versus extensional micromechanical failures control the qualitative macroscopic force-versus-displacement response in different tests. For accurate prediction, the LCFM requires a constant style of failure, which the SR configuration maintains until very late in deformation. The N3PB configuration does not maintain this constancy. To be broadly applicable between geometries and failure styles, the LCFM would require additional physics, possibly including elastoplastic damage in the bulk material and more complicated cohesive softening models.

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The influence of the cohesive process zone in hydraulic fracturing modelling

Cited by 109 publications

References 33 publications

Pore pressure effects on fracture net pressure and hydraulic fracture containment: Insights from an empirical and simulation approach

Pore pressure effects on fracture net pressure and hydraulic fracture containment: Insights from an empirical and simulation approach

Modelling of Hydraulic Fracturing in a Weak Permeable Formation

Fracture propagation in Indiana Limestone interpreted via linear softening cohesive fracture model

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