The crack damage progression in crystalline rocks is approximated in laboratory by means of rigorous strain measurement and/or monitoring of Acoustic Emission (AE) activity. When both means are used, they are treated independently for quantification of damage in the rock. This paper is investigating a new method to combine the AE and strain data in a unified function to calculate the balance of stored and released energy in the rock due to loading (strain energy) and micro-cracking respectively. This method introduces a new solution for measurement and quantification of crack damage in rock and also provides a tool to investigate the brittleness of different rock types. Unconfined Compressive Strength (UCS) testing of six different rock types with strain measurement and AE monitoring was performed for this study. The application of the new method to the data collected from the UCS tests indicates the difference between the behaviour of the various rock types in terms of sudden energy release at the onset of CI threshold and the difference in the storability of strain energy before and after CI and CD thresholds.
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.
Numerical simulations have been conducted to model the deformation, damage, and fracture growth caused by the plunge of a spherical drill bit insert into a brittle rock. The deformation of the rock, which is initially homogeneous and isotropic, is modeled using the finite element method. Fracture geometry evolves as a function of fracture growth, and the rock domain is continuously re-meshed to capture this geometric change. Contact forces are applied radially over the contact area as a function of the depth of the plunge. A series of simulations is presented, having varying initial flaw distributions, and which capture the fracture pattern formation during the progressive indentation of the insert into the rock. The ensuing patterns depict the formation of horizontal and Hertzian fractures. A large fracture density is created around the contact area. The complexity of the internal fracture structure is less apparent at the surface of the deformed rock, as compared to the internal fracture pattern. Fracturing leads to the formation of surface chips in the form of tilted elliptical domains parallel to the rock surface. Early stages of chipping are not always apparent from the fracture pattern at the surface of the rock. Results are in good agreement with experimental observations.
Dynamic loading methods promise new modes for stimulating geological resources, as the fracture patterns they produce can be tailored by the shape and nature of the pressure pulse employed. However, selecting the type of load is a difficult task: too slow and the stimulatory effect is reduced; too fast and the resource may be negatively impacted by wellbore damage, fines creation or permeability reduction. Moreover, modeling these systems proves challenging due to the myriad of length and timescales involved, combined with the need to accommodate both the generation of new fractures and propagation of preexisting fracture networks.
GEODYN-L is a massively-parallel multi-material Lagrangian code that includes advanced contact models to simulate nonlinear wave propagation through heavily-jointed rock masses, along with material model libraries specifically developed to capture the dynamic response of geologic media. We present results using GEODYN-L to simulate dynamic stimulation of geologic resources with pre-existing fracture networks and discuss the implications of these results for enhancing fracture networks with dynamic loading techniques.
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.
In this work, a hybrid discrete-continuum numerical model was used to simulate hydraulic fracture (HF) crossing and interaction with natural fractures or weakness planes. The model provided unique capabilities for investigating effects which have usually been overlooked or not able to be modeled in many of the previous studies on the subject. Multiple effects, such as the influence of stress conditions, material in-homogeneity (stiffness and strength contrast), natural fracture properties (crossing angle and friction angle), and injection parameters (injection rate and fluid viscosity) were investigated in this new work. Three types of intersection between an HF and orthogonally aligned natural fractures were identified by varying the coefficient of friction of the natural fractures and the stress ratio. In addition, the intersection angle between an HF and natural fractures or weakness planes was found to significantly affect the crossing. Decreasing the intersection angle with the natural fractures impeded direct crossing and favored the arrest of an HF. Material in-homogeneity and injection parameters were found to also greatly affect the HF crossing of natural fractures. Ultimately, the simulations showed that the geometry of an HF can be greatly affected by the interactions with adjacent natural fractures and weakness planes and that complex HF propagation patterns will occur due to complicated crossing behavior during hydraulic fracturing in naturally fractured reservoir systems.
The paper presents results of a micro-mechanical Discrete Element Method (DEM) study of the hydraulic fracture initiation and propagation in Enhanced Geothermal System (EGS) performed using the Particle Flow Code (PFC). Hydraulic fracturing is the main means to stimulate and create flow paths to extract heat in hot dry rocks with insufficient permeability to inject and circulate fluids. Hydro-thermo-mechanical coupled modeling is performed to analyze stress and strain changes on fracturing from a wellbore for improving the understanding of the role of thermal stresses on fracture propagation processes and the resulting fracture geometry. Bonded particle model (BPM) that is used for modeling the mechanical response and fracturing of solids was modified for capturing mechanical effects of temperature difference between rock and fracturing fluid infiltration in the propagating fracture. Heat exchange between fluid and rock and fracture is fully coupled processes. As the fracture propagates from the pressurized borehole, both fluid and rock adjacent to a newly formed fracture change temperature. The results show that thermally induced stresses can significantly change the fracture initiation and the fracture propagation pattern. Thermal stresses cause shallow randomly oriented cracks. The study evaluated fracture geometry and orientation with respect to fracturing fluid temperature, viscosity, density, pressure, rock parameters and in-situ stress difference.
Often, a key factor in the successful hydraulic fracture stimulation of unconventional reservoirs is the opening or shearing (and later extension) of natural fractures or weakness planes around a created hydraulic fracture. The behavior of natural fractures, or weakness planes, in response to hydraulic fracture stimulation can be complicated. Furthermore, the stimulation of these fractures and weakness planes is dependent on several critical, in-situ conditions that can increase (or decrease) the contribution of natural fractures and weakness planes to well production. The optimal economic completion, then, requires considering these factors during both stimulation design and post-stimulation evaluations.
The simplistic, and traditional, assumption that hydraulic fractures are bi-wing, planar and symmetric around the wellbore has tended to bias the interpretation of different aspects of the stimulation process. However, hydraulic fracture monitoring methods, such as microseismicity, pressure evaluations, and the coring through of hydraulic fractures, have confirmed the complex nature of fracture propagation in unconventional plays, often due to the presence of natural fractures and weakness planes. Therefore, an improved consideration of natural fracture and weakness plane behavior during hydraulic fracturing will result in a better understanding of fluid treating pressures and hydraulic fracture geometry, which will help lead to more accurate estimations of production for unconventional plays.
In this paper, the results of an extensive parametric study of in-situ stress conditions, in-situ pressure, natural fracture mechanical properties (cohesion and friction angle) and characteristics (joint orientation and initial aperture), and different operating conditions (single stage, simultaneous hydraulic fracture stages, and sequential hydraulic fracture stages) on injection (net) pressure behavior is presented. The results were generated using a 2-D distinct element model and capture the important role that, for example, initial natural fracture aperture and in-situ pressure play in the development of hydraulic fracture injection pressures in unconventional 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.