We present a numerical model for the simultaneous initiation and subsequent propagation of multiple transverse hydraulic fractures from a horizontal wellbore. In particular, we investigate the efficiency and robustness of the multistage hydraulic fracturing technique. We restrict the created hydraulic fractures to remain radial and planar but fully account for the stress interaction between fractures, the fluid flow in the wellbore and across the different perforation clusters which are modeled via a classical relation between the friction pressure drop and the flow rate entering a given fracture. The initiation is modeled from a radial notch of given initial length using linear elastic fracture mechanics. The solver models the complete pressurization of the wellbore, the initiation of the different fractures and their propagation and interactions. The split of the fluid between the different clusters is part of the solution at each time-step. We present some validations and a case study investigating the effect of a number of heterogeneities (in-situ stress etc.) on the robustness of the limited entry technique.
The mechanics of fluid-driven fracture propagation through fracture networks is of central interest in gas and oil extraction procedures. A number of computational strategies have now been developed to simulate these processes although specific understanding of the propagation mechanics in the vicinity of pre-existing discontinuities or faults is still limited. This paper investigates the problem of formulating appropriate fluid branching logic at multiple flow path junctions and the influence of sudden contractions or expansions in the flow path channel width at discontinuity intersections. A plane strain model is assumed. A question of additional interest is the possible existence of a “fluid lag” region between the flow front and the mobilized fracture front. The paper explores some examples of flow propagation and branching through simple joint networks.
In this paper, the results of laboratory studies of hydraulic fracture in homogeneous sandstone blocks with man-made interfaces and heterogeneous shale blocks with weak natural interfaces are reported. Tests were conducted under similar stress conditions, with fluids of different viscosity and at different injection rates. The measurements and analysis allows the identification of fracture initiation and behavior. Fracturing with high viscosity fluids resulted in stable fracture propagation initiated before breakdown, while fracturing with low viscosity fluids resulted in unstable fracture propagation initiated almost simultaneously with breakdown. Analysis also allows us to measure the fluid volume entering the fracture and the fracture volume. Monitoring of Acoustic Emission (AE) hypocenter localizations, indicates the development of created fractured area including the intersection with interfaces, fluid propagation along interfaces, crossing interfaces, and approaching the boundaries of the block. We observe strong differences in hydraulic fracture behavior, fracture geometry and fracture propagation speed, when fracturing with water and high viscosity fluids. We also observed distinct differences between sandstone blocks and shale blocks, when a certain P-wave velocity ray path is intersected by the hydraulic fracture. The velocity increases in sandstones and decreases in shale.
A rock mass is neither a continuous medium nor a totally discrete medium, it is a kind of defect material which contains many cracks, joints and faults. The nonlinear deformation behavior of a rock mass is induced by the propagation and coalescence of cracks and joints under external loads. Therefore, it is of important for rock engineering to analyze the propagation and coalescence process of cracks existed in a rock mass under external loads. The study of crack initiation and propagation is important for the understanding of rock mass behaviour which, in turn, affects rock engineering applications, such as tunnels, foundations and slopes, as well as hydrocarbon and geothermal energy extraction. Cracking mechanisms can be studied experimentally in the laboratory or in the field, or numerically. In the present study, distinct element method (DEM) which is capable to model various discontinuities was employed to simulate crack initiation and propagation in a rock masse specimen containing a single open and closed flaw. Initially, a rock domain containing a closed flaw was considered to model crack propagation. The analysis was performed by sequential modelling. Firstly, a model containing a single closed or open flaw was used to verify the types of propagation shown by Park and Bobet (2009). In these analyses, both open and closed flaws were considered and analysed with different spatial distribution (i.e. flaw angle). After results verification, the effect of open flaw filling material on crack propagation was analysed numerically. This characteristic has not yet been studied in crack propagation studies. The results obtained from open and closed flaws were in good agreement with experimental ones. All cracks mentioned in experimental literatures such as wing (tensile) cracks, coplanar secondary (shear) cracks and oblique secondary cracks were modelled successfully using DEM which indicates method capability to model nonlinear behaviour of rock masses subjected to external loadings. The emphasize of the study is to investigate the effects of open flaws containing filling material on crack initiation and propagation. Weak material was modelled as filling material. The results showed that when flaw is filled with weak materials (as it encountered frequently in natural rock masses) the cracking pattern is quite different with open flaws. In these occasions, the crack propagation direction is different. This phenomenon could be described in terms of stress attenuation in weak filling material, as stress concentration in filling materials causes change in crack propagation directions as well as crack length.
Robust and reliable hydraulic fracturing models that appropriately account for random initiation of fractures, strongly nonlinear coupling among deformation, fracturing and fluid flow in fracture apertures and leakage into porous rock matrix, would be a key step toward developing a better understanding of physics associated with hydraulic fracturing process. In this paper, we present a physics-based hydraulic fracturing simulator based on coupling a quasi-static discrete element model (DEM) for deformation and fracturing with conjugate lattice network flow model for fluid flow in both fractures and porous matrix. The coupled DEM-network flow model reproduces a variety of realistic growth patterns of hydraulic fractures. The effects of in situ stress, fluid viscosity, heterogeneity of rock mechanical properties and injection rate on the fracture patterns will be presented and discussed. In particular, simulation results of multistage horizontal wellbore with multiple perforations clearly demonstrate that elastic interactions among multiple propagating fractures, strong coupling between fluid pressure fluctuations within fractures and fracturing, and lower length scale heterogeneities, collectively lead to complicating fracturing patterns.
In this study, both experimental and numerical studies were performed to investigate the impact of a bi-material interface on crack propagation. A set of thickness values for the weak interfacial layer was used in the experiments to investigate its influence on fracture propagation. In the numerical studies, the aforementioned experimental phenomena were simulated first, with the focus on calibrating the numerical model. Second, a numerical investigation of the influence of the strength and stiffness of the interface layer on crack propagation was performed. The influence of the interface layer permeability on the propagation of hydraulically generated fractures across the interface layer was also investigated. During the numerical studies, a simplified 3D Finite Element Model was built and used. The poroelastic plastic damage model is used to simulate crack propagation. The load of the numerical model for the 1st set of calculations is the point force in 3-point bending, which simulates the experimental phenomena. Loads in the 2nd set of calculations include the gravity load which balances the initial geostress field and the fluid injection flow rate. The results obtained can be used as a reference in the design of hydraulic fracturing for laminated thin formations.
Understanding the geometry of a hydraulic fracture is key to predicting its behavior and performance. Physical measurement of field hydraulic fracture geometries beyond the borehole is difficult and typically cost prohibitive with the only published examples being mine-back studies and cores. Laboratory-scale hydraulic fracturing experiments can more accurately measure the fracture geometry due to smaller specimen size and improved monitoring capabilities. This paper presents laboratory work where hydraulic fracture treatments were performed using epoxy injection such that a propagating fracture could be stabilized and preserved at near-critical state. Constant backpressure was applied after hydraulic breakdown but before cessation of fracture extension to maintain near-critical state geometry. Preliminary results are presented giving measurement of fracture dimensions, including aperture, at the millimeter scale for a hydraulic fractured acrylic specimen. The pressure, flow rate, material strains, acoustic emissions, and video stills associated with this fracture are also presented and analyzed. A second experiment fracturing a 300×300×300 mm3 cubic foot granite block using epoxy is also discussed. Data regarding the interaction between shear and tensile dominated fractures is presented and discussed.
Rocks with natural fractures, cracks, faults and vugs have complex multi-connected pathways for fluid flow. In these systems, fluid flow, especially to production wellbores, can change as reservoir conditions change as fluids are injected to the reservoir. In typical practice, the more detailed the characterization of the fracture network, the easier it is to optimize recovery process design and well placement to maximize the recovery factor of petroleum fluids. Furthermore, for tight rocks where hydraulic fracturing is required to enable sufficient fluid mobility for economic production, it is critical to understand the placement of the induced fractures, their connectivity, extent, and interaction with natural fractures within the system. Stress anisotropy and interactions between new fractures and natural fractures in the formation can dictate the mode, orientation and size of the hydraulic fracture network. In this study, normal deformation is coupled with fluid flow to evaluate the effect of the stress anisotropy on fracture network propagation in rock. The results demonstrate that stress anisotropy and existing natural fractures networks are playing critical roles in creating fracture-network complexity and connectivity. The model developed here assumes that the flow is single-phase and isothermal, matrix permeability is zero, and that deformation arises from small normal displacement in an infinite, homogeneous, linearly elastic medium. Specifically, the model couples fluid flow and stresses induced by fracture deformation in a plane. For this purpose, a system of equations governing fracture deformation and fluid flow through a complex fracture network is solved. The results illustrate the importance of rock properties, stress magnitude, and stress orientation on fracture complexity in unconventional naturally fractured reservoirs.
We investigate the main pumping parameters that influence a fluid driven fracture in cohesive poroelastoplastic weak formation. These parameters include the fluid viscosity and the injection rate. The first parameter dominates in the mapping of the propagation regimes from toughness to viscosity while the second parameter controls the storage to leak-off dominated regime through diffusion.
The fracture is driven in weak permeable formation by injecting an incompressible viscous fluid at the fracture inlet assuming plane strain conditions. Fluid flow in the fracture is modeled by lubrication theory. Irreversible rock deformation is modeled with the Mohr-Coulomb yield criterion assuming associative flow rule. Fracture propagation criterion is based on the cohesive zone approach. Leak-off is also considered. We perform numerical calculations with the finite element method to obtain the fracture opening, length and propagation pressure versus time.
We demonstrate that pumping parameters influence the fracture geometry and fluid pressures in weak formations through the diffusion process that create back stresses and large plastic zones as the fracture propagates. We also show that the product of propagation velocity and fluid viscosity, (μv) that appears in the scaling controls the magnitude of the plastic zones and influences the net pressure and fracture geometry.
The propagation of a single fluid-filled fracture from the surface of a semi-infinite isotropic elastic solid, subjected to both a transient temperature field and a constant source fluid pressure that is less than the confining stress, is studied using a boundary element method. Fluid flow in fractures is described by the lubrication equation, while the local pressure is determined by the strong coupling between elastic deformation, heat conduction and fluid pressure. Numerical results show that the combination of cooling-induced tensile stress and the source pressure can enhance the propagation speed. Parametric studies are carried out for identifying speed regimes and show the importance of the initial fracture aperture. Three speed regimes are found to exist. If the fluid penetration into the fracture is heavily restricted, the fracture length grows exponentially at early time, and then it suddenly reaches a large speed and progressively decelerates in a finite transition time as fluid diffusion speed varies, but eventually it follows the exponential fracture growth curve at a higher index for stable fluid flow in high-permeability fractures. The time-dependent crack growth behavior does not show any signs of unstable growth, even in the high-speed transition regime. The predictions of crack growth kinetics show a good agreement with some published experimental results and highlight the stabilizing effect of fluid transport on crack growth.