# Browsing by Subject "Natural fractures"

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Item Laboratory quantification and detection of pre-existing fractures and stress-induced microfracturing through combined ultrasonic and triaxial-stress testing(2016-08) Ramos, Matthew John; Espinoza, David N.; Torres-Verdin, CarlosShow more Simultaneous triaxial stress testing and ultrasonic wave propagation were utilized to quantify natural fractures and microfracturing in Berea Sandstone and Silurian Dolomite. Experimental results indicate that the presence of fractures distinctly decreases wave velocities, with calculated dynamic elastic moduli decreasing by up to 7.5% in artificially fractured sandstone. Wave analysis of intact and artificially fractured Berea Sandstone reveal the nonlinear mechanical and geophysical response or fractured rocks subject to isotropic and deviatoric stress loading paths. Specifically, fractures increase hysteretic stress-strain behavior, and tend to amplify the stress dependence of wave attenuation and the filtering of high-frequency wave components. Additional deviatoric loading tests of Berea and Silurian samples provide evidence for the onset of stress-induced microfracturing, detected at a threshold of 1% shear wave anisotropy called the “shear wave crossover” (SWX). The SWX and subsequent increases in shear wave anisotropy evidence microstructural damage development well before quasi-static indicators such as the volumetric strain positive point of dilatancy (PPD) and yield/failure in all samples. Specifically, Berea and Silurian samples exhibit up to 5% and 7% shear wave anisotropy at the PPD, respectively. Additionally, stresses at the SWX and PPD were compared to peak axial stress to understand linkages between damage at several scales and ultimate rock strength. The SWX occurs at an average of 27% lower axial stresses, and 5% less shear wave anisotropy than the PPD, indicating that samples undergo irreversible microstructural changes earlier than previously thought. The SWX and PPD both provide meaningful estimates of failure stress, however samples must be subjected to higher stresses and strains to reach the PPD, making it less favorable for sample preservation. Furthermore, correlation between the SWX and peak stress under several different radial stresses, present a viable technique for using dynamic measurements to predict static rock failure properties, while also preserving sample competence for future tests. Linking the dynamically measured SWX to static rock failure properties provides an additional avenue for developing accurate transforms for several rock types. Therefore, the SWX can add value across industries for predicting rock behavior and maximizing the value of expensive samples and rock testing.Show more Item Numerical Investigation of Interaction Between Hydraulic Fractures and Natural Fractures(2011-02-22) Xue, WenxuShow more Hydraulic fracturing of a naturally-fractured reservoir is a challenge for industry, as fractures can have complex growth patterns when propagating in systems of natural fractures in the reservoir. Fracture propagation near a natural fracture (NF) considering interaction between a hydraulic fracture (HF) and a pre-existing NF, has been investigated comprehensively using a two dimensional Displacement Discontinuity Method (DDM) Model in this thesis. The rock is first considered as an elastic impermeable medium (with no leakoff), and then the effects of pore pressure change as a result of leakoff of fracturing fluid are considered. A uniform pressure fluid model and a Newtonian fluid flow model are used to calculate the fluid flow, fluid pressure and width distribution along the fracture. Joint elements are implemented to describe different NF contact modes (stick, slip, and open mode). The structural criterion is used for predicting the direction and mode of fracture propagation. The numerical model was used to first examine the mechanical response of the NF to predict potential reactivation of the NF and the resultant probable location for fracture re-initiation. Results demonstrate that: 1) Before the HF reaches a NF, the possibility of fracture re-initiation across the NF and with an offset is enhanced when the NF has weaker interfaces; 2) During the stage of fluid infiltration along the NF, a maximum tensile stress peak can be generated at the end of the opening zone along the NF ahead of the fluid front; 3) Poroelastic effects, arising from fluid diffusion into the rock deformation can induce closure and compressive stress at the center of the NF ahead of the HF tip before HF arrival. Upon coalescence when fluid flows along the NF, the poroelastic effects tend to reduce the value of the HF aperture and this decreases the tension peak and the possibility of fracture re-initiation with time. Next, HF trajectories near a NF were examined prior to coalesce with the NF using different joint, rock and fluid properties. Our analysis shows that: 1) Hydraulic fracture trajectories near a NF may bend and deviate from the direction of the maximum horizontal stress when using a joint model that includes initial joint deformation; 2) Hydraulic fractures propagating with higher injection rate or fracturing fluid of higher viscosity propagate longer distance when turning to the direction of maximum horizontal stress; 3) Fracture trajectories are less dependent on injection rate or fluid viscosity when using a joint model that includes initial joint deformation; whereas, they are more dominated by injection rate and fluid viscosity when using a joint model that excludes initial joint deformation.Show more Item Solving three-dimensional problems in natural and hydraulic fracture development : insight from displacement discontinuity modeling(2013-08) Sheibani, Farrokh; Olson, Jon E.Show more Although many fracture models are based on two-dimensional plane strain approximations, accurately predicting fracture propagation geometry requires accounting for the three-dimensional aspects of fractures. In this study, we implemented 3-D displacement discontinuity (DD) boundary element modeling to investigate the following intrinsically 3-D natural or hydraulic fracture propagation problems: the effect of fracture height on lateral propagation of vertical natural fractures, joint development in the vicinity of normal faults, and hydraulic fracture height growth and non-planar propagation paths. Fracture propagation is controlled by stress intensity factor (SIF) and its determination plays a central role in LEFM. The DD modeling is used to evaluate SIF in Mode I, II and III at the tip of an arbitrarily-shaped embedded crack by using crack-tip element displacement discontinuity. We examine the accuracy of SIF calculation is for rectangular, penny-shaped, and elliptical planar cracks. Using the aforementioned model for lateral propagation of overlapping fractures shows that the curving path of overlapping fractures is strongly influenced by the spacing-to-height ratio of fractures, as well as the differential stress magnitude. We show that the angle of intersection between two non-coincident but parallel en-echelon fractures depends strongly on the fracture height-to-spacing ratio, with intersection angles being asymptotic for "tall" fractures (large height-to-spacing ratios) and nearly orthogonal for "short" fractures. Stress perturbation around normal faults is three-dimensionally heterogeneous. That perturbation can result in joint development at the vicinity of normal faults. We examine the geometrical relationship between genetically related normal faults and joints in various geologic environments by considering a published case study of fault-related joints in the Arches National Park region, Utah. The results show that joint orientation is dependent on vertical position with respect to the normal fault, the spacing-to-height ratio of sub-parallel normal faults, and Poisson's ratio of the media. Our calculations represent a more physically reasonable match to measured field data than previously published, and we also identify a new mechanism to explain the driving stress for opening mode fracture propagation upon burial of quasi-elastic rocks. Hydraulic fractures may not necessarily start perpendicular to the minimum horizontal remote stress. We use the developed fracture propagation model to explain abnormality in the geometry of fracturing from misaligned horizontal wellbores. Results show that the misalignment causes non-planar lateral propagation and restriction in fracture height and fracture width in wellbore part.Show more Item The Performance of Fractured Horizontal Well in Tight Gas Reservoir(2012-02-14) Lin, JiajingShow more Horizontal wells have been used to increase reservoir recovery, especially in unconventional reservoirs, and hydraulic fracturing has been applied to further extend the contact with the reservoir to increase the efficiency of development. In the past, many models, analytical or numerical, were developed to describe the flow behavior in horizontal wells with fractures. Source solution is one of the analytical/semi-analytical approaches. To solve fractured well problems, source methods were advanced from point sources to volumetric source, and pressure change inside fractures was considered in the volumetric source method. This study aims at developing a method that can predict horizontal well performance and the model can also be applied to horizontal wells with multiple fractures in complex natural fracture networks. The method solves the problem by superposing a series of slab sources under transient or pseudosteady-state flow conditions. The principle of the method comprises the calculation of semi-analytical response of a rectilinear reservoir with closed outer boundaries. A statistically assigned fracture network is used in the study to represent natural fractures based on the spacing between fractures and fracture geometry. The multiple dominating hydraulic fractures are then added to the natural fracture system to build the physical model of the problem. Each of the hydraulic fractures is connected to the horizontal wellbore, and the natural fractures are connected to the hydraulic fractures through the network description. Each fracture, natural or hydraulically induced, is treated as a series of slab sources. The analytical solution of superposed slab sources provides the base of the approach, and the overall flow from each fracture and the effect between the fractures are modeled by applying superposition principle to all of the fractures. It is assumed that hydraulic fractures are the main fractures that connect with the wellbore and the natural fractures are branching fractures which only connect with the main fractures. The fluid inside of the branch fractures flows into the main fractures, and the fluid of the main fracture from both the reservoir and the branch fractures flows to the wellbore. Predicting well performance in a complex fracture network system is extremely challenged. The statistical nature of natural fracture networks changes the flow characteristic from that of a single linear fracture. Simply using the single fracture model for individual fracture, and then adding the flow from each fracture for the network could introduce significant error. This study provides a semi-analytical approach to estimate well performance in a complex fracture network system.Show more