Modeling the interaction between hydraulic and natural fractures using three dimensional finite element analysis

Thesis (M.S.) University of Alaska Fairbanks, 2016Natural fractures are present in almost every formation and their size and density definitely affect the hydraulic fracturing job. Some of the analysis done in the past shed light on hydraulic fracture (HF) and natural fracture (NF) geometries. The interaction of the HF with existing NF in a formation results in a denser fracture network. The volume of rock covering this fracture network is called the stimulated reservoir volume (SRV). This SRV governs the hydrocarbon production and the ultimate revenue generation. Moreover, past studies show that a microseismic interpreted SRV can be different than the actual SRV. Additionally, there is always limited subsurface access, which makes it imperative to understand the HF – NF interaction to plan and execute a successful hydraulic fracturing job. A three layered, three dimensional complex geomechanical model is built using commercially available finite element analysis (FEA) software. A propagating HF approaching mainly orthogonal NF is studied and analyzed. Cohesive pore pressure elements in FEA software capable of modeling fluid continuity at HF – NF intersection are used to model the HF – NF interaction. Furthermore, a detailed sensitivity analysis considering the effect of stress contrast, job design parameters, NF properties, and properties of the formation is conducted. The sensitivity analysis of properties such as principal horizontal stress contrast, job design parameters, NF properties and properties of target formation reveals a broad variation in the impact of the sensitivity parameters on the HF, NF, and HF-NF geometry and interaction. The observations and the corresponding conclusions were based on broadly classified sensitivity parameters. The most important parameters solely for HF resultant geometry are observed to be a high stress contrast with stress reversal, highest injection rate, and farther NF distance from the injection point. The least important parameter is observed to be the scenario with almost equal horizontal stresses. However, the most important parameter solely for resulting NF geometry is only the high stress contrast with stress reversal. Conversely, for the considered sensitivity cases, the least important parameters are the injection rate, lower injection viscosity (10 cP), higher NF leak-off coefficient, target formation thickness, Young’s modulus, and lowest value of target formation Poisson’s ratio. Collective conclusions for considering HF-NF are also obtained.Chapter 1. Introduction -- 1.1. Alaska’s Unconventional Oil and Gas Potential -- 1.2. Need for HF – NF Interaction Research -- 1.3. Outline of Present Research -- 1.4. Summary of Subsequent Chapters -- Chapter 2. Literature review -- 2.1. Hydraulic Fracture Modeling -- 2.1.1. 2D Models -- 2.1.2. 3D Models -- 2.2. Modeling Hydraulic Fractures in the Presence of Natural Fractures -- 2.3. General HF-NF Modeling Approaches -- 2.4. Commercial Software Based HF-NF Modeling Approaches -- 2.4.1. Abaqus -- 2.4.2. Unconventional Fracture Modeling (UFM) -- 2.4.3. COMSOL -- 2.4.4. FLAC 3D -- 2.4.5. Combinational Approaches -- Chapter 3. Modeling the interaction between hydraulic and natural fractures using three dimensional finite element analysis -- 3.1. Model Construction -- 3.2. Theory -- 3.2.1. Modeling the Rock Matrix -- 3.2.2. Modeling Fluid Flow -- 3.2.3. Modeling Deformation and Damage -- 3.3. Model Validation -- 3.4. Base Case -- 3.5. Sensitivity Analysis -- 3.5.1. Effect of In-Plane Stress Contrast -- 3.5.2 Effect of Job Design Parameters -- 3.5.2.1. Effect of Injection Rate -- 3.5.2.2. Effect of Injection Fluid Viscosity -- 3.5.3. Effect of NF Properties -- 3.5.3.1. Effect of NF Strength -- 3.5.3.2. Effect of NF Positioning -- 3.5.3.3. Effect of NF Orientation -- 3.5.3.4. Effect of NF Leak-off Coefficient -- 3.5.4. Effect of Formation Properties -- 3.5.4.1. Effect of HF Leak-off Coefficient -- 3.5.4.2. Effect of Target Formation Thickness -- 3.5.4.3 Effect of Target Formation Young’s Modulus -- 3.5.4.4 Effect of Target Formation Poisson’s Ratio -- 3.6 Summarized Observations -- Chapter 4. Conclusions and recommendations -- 4.1 Conclusions -- 4.2 Recommendations -- References

Earthquakes: from chemical alteration to mechanical rupture

In the standard rebound theory of earthquakes, elastic deformation energy is progressively stored in the crust until a threshold is reached at which it is suddenly released in an earthquake. We review three important paradoxes, the strain paradox, the stress paradox and the heat flow paradox, that are difficult to account for in this picture, either individually or when taken together. Resolutions of these paradoxes usually call for additional assumptions on the nature of the rupture process (such as novel modes of deformations and ruptures) prior to and/or during an earthquake, on the nature of the fault and on the effect of trapped fluids within the crust at seismogenic depths. We review the evidence for the essential importance of water and its interaction with the modes of deformations. Water is usually seen to have mainly the mechanical effect of decreasing the normal lithostatic stress in the fault core on one hand and to weaken rock materials via hydrolytic weakening and stress corrosion on the other hand. We also review the evidences that water plays a major role in the alteration of minerals subjected to finite strains into other structures in out-of-equilibrium conditions. This suggests novel exciting routes to understand what is an earthquake, that requires to develop a truly multidisciplinary approach involving mineral chemistry, geology, rupture mechanics and statistical physics.Comment: 44 pages, 1 figures, submitted to Physics Report

Modelling Hydraulic Fracturing Propagation in Heterogeneous Reservoirs Using Cohesive Zone Methods

The recent success in exploiting low permeability shale reservoirs has heavily relied on hydraulic fracturing to produce hydrocarbons economically in disadvantaged reservoir conditions. Although horizontal drilling significantly increases the contact area between the wellbore and the reservoir, the objective of hydraulic fracturing is set on creating further expanded conductive flow paths into the reservoir. This research uses cohesive zone method to numerically simulate hydraulic fracture propagation in the presence of natural fractures in two- and three-dimensional model. The Cohesive element approach limits fracture propagation to some predefined paths. However, in highly fractured formations since hydraulic fractures are growing through a network of natural fracture by placing cohesive elements through natural fractures it is possible to track the development of a network of induced hydraulic fractures. Moreover, cohesive elements remove stress singularity at the tips of fractures, which improves numerical stability of the model. Additionally, fracture models based on Griffith\u27s criterion cannot predict fracture initiation. A numerical model was developed coupling both fluid flow in fracture network and rock deformations to study the interaction between hydraulic and natural fractures at different scales. The cohesive zone method assumes the existence of a fracture process zone characterized by a traction-separation law rather than an elastic crack tip region. The cohesive finite element method provides an alternate, effective approach for quantitative analysis of fracture behavior through explicit simulation of the fracture process. Activation of natural fractures during fracturing treatment improves the effectiveness of the stimulation tremendously. Here, integrated methodology initiated with laboratory-scale fracturing properties using a semicircular bending test is presented to determine cohesive properties of rock and natural fractures. A cohesive finite element model is used to reproduce laboratory results to verify the numerical model for interaction between the hydraulic fracture and cemented natural fractures. The results suggest that the distribution of pre-existing natural fractures can play a significant role in the final geometry of the induced fracture network. Moreover, understanding of natural fracture distribution in the reservoir will have an economical impact in projects where fracture geometry is better designed according to underground conditions

An experimental and numerical investigation into hydraulic fracture propagation in naturally fractured shale gas reservoirs

Despite the large amount of research conducted to date, the performance of hydraulic fractures in naturally fractured structures and their effect on hydrocarbon production is still not well understood. To this end, this research aimed at developing a better understanding of natural fracture and hydraulic fracture interaction in shale formations by two- and three-dimensional discrete element modelling (DEM) approaches, results of which were compared against the large-scale hydraulic fracturing experiments. The samples used in the experimental research were collected from the Hope Cement Works shale quarry in Derbyshire, UK. The mineralogy as well as the mechanical, elastic, and flow properties of samples were obtained through several laboratory sample characterisation tests. The subsequent true-triaxial hydrofracturing experiments with acoustic measurements were performed on one homogeneous and one naturally fractured 0.3 × 0.3 m × 0.3 m rock samples, which reflected the temporal information on hydraulic fracture initiations. For further identification of the location and geometry of hydraulic and natural fractures, computed tomography (CT) and seismic velocity tomography analyses were conducted. The preliminary two-dimensional discrete element modelling research results were obtained using discrete fracture network (DFN) approach in two-dimensional Particle Flow Code (PFC2D). The two-dimensional model results provided a fundamental understanding of the effects of certain parameters, in particular the angle of approach, differential stress, mechanical properties as well as ubiquity and randomness of natural fractures on fracture interaction mechanisms. However, in view of the limitations of two-dimensional representation of both the laboratory and field scale applications, three-dimensional discrete element models were developed using XSite, results of which were first compared against the findings of true triaxial hydrofracturing experiments, and then extended through a parametric research. The effects of mechanical properties of natural fractures and operational parameters on fracture interaction mechanisms were then analysed using 3D XSite models. A curved shape hydraulic fracture, which propagated perpendicular to the minimum horizontal stress direction (x) in the homogeneous sample model, agreed well with the CT scan analysis and seismic wave velocity tomography results from the laboratory experiments. Similarly, the natural fracture and hydraulic fracture interaction observed in the second heterogenous/fractured sample, particularly the arrest by the main natural fracture and the subsequent crossing with offset mechanisms, were captured well by the developed 3D numerical models. Both experimental, and the parametric two- and three-dimensional particle- and lattice-based discrete element modelling research have demonstrated that the hydraulic fracture propagation in homogeneous rock with no weakness planes/natural fractures is mainly controlled by the differential stress, as it is growth is perpendicular to the minimum horizontal stress with no observed branching/diversion. The presence of natural fractures, on the other hand, introduced the additional effects of mechanical properties of natural fractures on observed interaction mechanisms in such a way that the stronger natural fractures are found to be favouring the crossing mechanism. Importantly, the ubiquity and randomness of natural fractures, which increased the complexity in hydraulic fracture growth significantly, have shown that the hydraulic fracture almost always propagates along the nearest natural fracture plane as the least resistant and shortest path, instead of being controlled by the differential stress. These findings, indeed, emphasised the dominating role of natural fractures and their dispersion within the reservoir on hydraulic fracture propagation and subsequent fracture interaction mechanisms. Regarding the operational parameters, lower flow rate and low viscosity fluids are found to be leading to arrest mechanism with increased dilation of natural fractures, while higher flow rate and high viscosity fluids resulted in direct crossing mechanisms with observable increase in total stimulated areas.Open Acces

Numerical Modeling of Fluid Migration in Hydraulically Fractured Formations

Economic production from low permeability shale gas formations has been made possible by the introduction of horizontal drilling and hydraulic fracturing. To ensure that gas production from these formations is optimized and carried out in an environmentally friendly approach, knowledge about the patterns of gas flow in the shale reservoir formation is required. This work presents the development of a shale gas reservoir model for the characterization of flow behavior in hydraulically fractured shale formations. The study also seeks to develop more computationally efficient approaches towards the modeling of complex fracture geometries. The model evaluates the migration patterns of gas in the formations, and investigates the range of physical conditions that favor the direction of gas flux towards the wellbore and decreases the probability of gas escape into the overlying formation. Two conceptual models that bypass the need for explicit fracture domains are utilized for this study, the semi-explicit conceptual model and the fractured continuum model. Fracture complexity is accounted for by modeling induced secondary hydraulic fractures. A novel approach to modeling the secondary fractures, which utilizes asymmetrical fractal representations is also implemented, and the governing equations for flow in the system are solved numerically using COMSOL Multiphysics 4.4b, a finite-element analysis software package. A parametric study is conducted on the reservoir and fracture properties and an assessment of their impacts on the production and formation leak off rates examined. The study results are presented and analyzed using a combination of transient pressure surface maps, production rate data curves and transient velocity distribution maps. Optimization of gas production rates from the studied formation is shown to be achievable by the use of long lateral fractures placed orthogonal to the wellbore. There is a need for an accounting of the distinct fracture systems present in a fractured formation for the accurate prediction of production values and flow patterns arising in the formation. This work extends the understanding associated with shale gas reservoir modeling and demonstrates the applicability of the fractured continuum model approach for the simulation of complex fractured shale formations

Frictional behavior of talc-calcite mixtures

Faults involving phyllosilicates appear weak when compared to the laboratory-derived strength of most crustal rocks. Among phyllosilicates, talc, with very low friction, is one of the weakest minerals involved in various tectonic settings. As the presence of talc has been recently documented in carbonate faults, we performed laboratory friction experiments to better constrain how various amounts of talc could alter these fault’s frictional properties. We used a biaxial apparatus to systematically shear different mixtures of talc and calcite as powdered gouge at room temperature, normal stresses up to 50 MPa and under different pore fluid saturated conditions, i.e., CaCO3-equilibrated water and silicone oil. We performed slide-hold-slide tests, 1–3000 s, to measure the amount of frictional healing and velocity-stepping tests, 0.1–1000 μm/s, to evaluate frictional stability. We then analyzed microstructures developed during our experiments. Our results show that with the addition of 20% talc the calcite gouge undergoes a 70% reduction in steady state frictional strength, a complete reduction of frictional healing and a transition from velocity-weakening to velocity-strengthening behavior. Microstructural analysis shows that with increasing talc content, deformation mechanisms evolve from distributed cataclastic flow of the granular calcite to localized sliding along talc-rich shear planes, resulting in a fully interconnected network of talc lamellae from 20% talc onward. Our observations indicate that in faults where talc and calcite are present, a low concentration of talc is enough to strongly modify the gouge’s frictional properties and specifically to weaken the fault, reduce its ability to sustain future stress drops, and stabilize slip

Analysis of Interaction Between Hydraulic and Natural Fractures

The behavior of natural fractures at the hydraulic fracturing (HF) treatment is one of the most important considerations in increasing the production from this kind of reservoirs. Therefore, considering the interaction between the natural fractures and hydraulic fractures can have great impact on the analysis and design of fracturing process. Due to the existence of such natural fractures, the perturbation stress regime around the tip of hydraulic fracture leads to some deviation in the propagation of path of hydraulic fracture. Increasing the ratio of transverse stress to the interaction stress results in a reduction in the deviation of hydraulic fracturing propagation trajectory in the vicinity of natural fracture. In this study, we modeled a hydraulic fracture with the extended finite element method (XFEM) using a cohesive-zone technique. The XFEM is used to discrete the equations, allowing for the simulation of induced fracture propagation; no re-meshing of domain is required to model the interaction between hydraulic and natural fractures. XFEM results reveal that the distance and angle of natural fracture with respect to the hydraulic fracture have a direct impact on the magnitude of tensile and shear debonding. The possibility of intersection of natural fracture by the hydraulic fracture will increase with increasing the deviation angle value. At the approaching stage of hydraulic fracture to the natural fracture, hydraulic fracture tip exerts remote compressional and tensile stress on the interface of the natural fracture, which leads to the activation and separation of natural fracture walls

Numerical modeling of complex hydraulic fracture development in unconventional reservoirs

textSuccessful creations of multiple hydraulic fractures in horizontal wells are critical for economic development of unconventional reservoirs. The recent advances in diagnostic techniques suggest that multi-fracturing stimulation in unconventional reservoirs has often caused complex fracture geometry. The most important factors that might be responsible for the fracture complexity are fracture interaction and the intersection of the hydraulic and natural fracture. The complexity of fracture geometry results in significant uncertainty in fracturing treatment designs and production optimization. Modeling complex fracture propagation can provide a vital link between fracture geometry and stimulation treatments and play a significant role in economically developing unconventional reservoirs. In this research, a novel fracture propagation model was developed to simulate complex hydraulic fracture propagation in unconventional reservoirs. The model coupled rock deformation with fluid flow in the fractures and the horizontal wellbore. A Simplified Three Dimensional Displacement Discontinuity Method (S3D DDM) was proposed to describe rock deformation, calculating fracture opening and shearing as well as fracture interaction. This simplified 3D method is much more accurate than faster pseudo-3D methods for describing multiple fracture propagation but requires significantly less computational effort than fully three-dimensional methods. The mechanical interaction can enhance opening or induce closing of certain crack elements or non-planar propagation. Fluid flow in the fracture and the associated pressure drop were based on the lubrication theory. Fluid flow in the horizontal wellbore was treated as an electrical circuit network to compute the partition of flow rate between multiple fractures and maintain pressure compatibility between the horizontal wellbore and multiple fractures. Iteratively and fully coupled procedures were employed to couple rock deformation and fluid flow by the Newton-Raphson method and the Picard iteration method. The numerical model was applied to understand physical mechanisms of complex fracture geometry and offer insights for operators to design fracturing treatments and optimize the production. Modeling results suggested that non-planar fracture geometry could be generated by an initial fracture with an angle deviating from the direction of the maximum horizontal stress, or by multiple fracture propagation in closed spacing. Stress shadow effects are induced by opening fractures and affect multiple fracture propagation. For closely spaced multiple fractures growing simultaneously, width of the interior fractures are usually significantly restricted, and length of the exterior fractures are much longer than that of the interior fractures. The exterior fractures receive most of fluid and dominate propagation, resulting in immature development of the interior fractures. Natural fractures could further complicate fracture geometry. When a hydraulic fracture encounters a natural fracture and propagates along the pre-existing path of the natural fracture, fracture width on the natural fracture segment will be restricted and injection pressure will increase, as a result of stress shadow effects from hydraulic fracture segments and additional closing stresses from in-situ stress field. When multiple fractures propagate in naturally fracture reservoirs, complex fracture networks could be induced, which are affected by perforation cluster spacing, differential stress and natural fracture patterns. Combination of our numerical model and diagnostic methods (e.g. Microseismicity, DTS and DAS) is an effective approach to accurately characterize the complex fracture geometry. Furthermore, the physics-based complex fracture geometry provided by our model can be imported into reservoir simulation models for production analysis.Petroleum and Geosystems Engineerin