Ralston, Matt (EIWT, LLC) | Braile, Lawrence (Purdue University) | Helprin, Olivia (Humboldt State University) | Maguire, Henry (University of Vermont) | McCallister, Bryan (Wright State University) | Orubu, Akpofure (Montana Tech) | Rijfkogel, Luke (Fort Hays State University) | Schumann, Harrison (Southern Methodist University)
Three refraction traveltime tomography methods were applied to a 6.4 km crooked-line seismic profile acquired in the dry riverbed of the Rio de Truches, located in the Espanola Basin in northern New Mexico. The methods are based on ray-tracing, Fresnel volumes, and the adjoint-state. Each method was able to provide refraction statics for reflection processing that sufficiently accounted for changes in near-surface velocity and/or thickness. The purpose of this paper is to describe, compare, and contrast these tomographic methods and detail their application to a crooked-line survey. Velocity models and static solutions from each method are quantified by comparison of stacks, to which refraction statics have been applied, with a stack using delay-time refraction statics serving as a baseline.
Presentation Date: Wednesday, October 17, 2018
Start Time: 1:50:00 PM
Location: 204A (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT: Natural and injected fluids in the subsurface often vary in composition and have different physical properties such as density and viscosity. It is well known that fracture aperture distributions control flow, transport, and mixing of fluids within a fracture. In this study, the effect of fracture orientation on fluid mixing is examined for two cases when density contrast exists (1) between miscible fluids, and (2) between miscible reactive fluids that form precipitates. The experimental observations found very different distribution of fluids, as well as precipitation, that depended on the inclination of the fracture plane. The density contrast confined the less dense fluid to a narrow runlet, in turn producing a velocity contrast between the two fluids. The contrast in density and velocity were observed to produce hydrodynamic instabilities that affected the shape of the flow paths. The gravity-control on the shape of the interfaces affects fluid mixing and the generation of precipitates during reactive flow. The spatial distribution and thickness of precipitates was affected by gravity in two ways: (a) restriction of the mixing paths because of the density contrast between the fluids and (b) frictional stability of precipitates on the fracture surfaces. This study suggests that fractures in the subsurface may seal differently depending on their orientation.
Long-term geological sequestration and storage of CO2 relies on the mechanical, hydraulic and geochemical stability of a reservoir. The stratigraphic trapping of CO2 requires a caprock with strong integrity to prevent CO2 from escaping. Subsurface engineering and natural activities, however, might initiate or reactivate fractures and other mechanical discontinuities in the cap-rock that can form potential pathways for CO2 to escape to the surface. In addition, geochemical reactions between injected and natural fluids have the potential to either enhance leakage flow paths through geochemical dissolution or seal these pathways through mineral precipitation. Whether a fracture flow path will experience dissolution or resealing depends on the chemical composition and physical properties of the fluids and the mineralogy of rock, as well as the mixing of fluid flows, flow rates and how the fluids occupy the fracture void geometry (Dijk and Berkowitz, 1998). For example, Schuszter et al. showed that the amount and spatial distribution of precipitation can be controlled by the flow rate and the chemical concentrations of the fluids (Schuszter et al., 2016).
ABSTRACT: Natural and induced processes can produce oriented mechanical discontinuities. Compressional to shear (P-to-S) wave conversions propagated at normal incidence to a fracture (Nakagawa et al., 2000) occur as a fracture undergoes shear stress because of coupling fracture compliances. In this study, experiments and computer simulations were performed to demonstrate the link among cross-coupling stiffness, micro-crack orientation and energy partitioning into P, S, and P-S/S-P wave. 3D printed samples with linear arrays of micro-cracks oriented at 0°, ±15°, ±30°, ±45°, ±60°, ±75°, and 90° were fabricated to provide understanding of crosscoupling stiffness. Wave amplitudes were compared with 2D simulations of compressional and shear stress waves based on a Discontinuous Galerkin method. For normal incidence, wave conversions were observed for micro-crack inclinations greater than 0° and less than 90°. In addition, cracks oriented at negative and positive angles have 180°-reversed phase difference in P-to-S and S-to-P wave conversions. This finding supports a wide range of potential application for monitoring and determining the presence and inclination angle of mechanical discontinuities.
Fractures in rock are potential pathways for fluids to flow through a rock mass and planes of mechanical instability that can lead to failure. Micro-seismicity and time-lapse geophysical surveys are often used to locate and delineate fractures (e.g. Bates et al., 2001; Cicerone & Toksoz, 1995; Day-Lewis et al., 2003; Hartline et al., 2016; Queen et al., 2016), but there is a need to extract physically measurable parameters that are directly linked to hydraulic and mechanical properties of fractures. Recently, a scaling relationship between fluid flow and fracture specific stiffness for a fracture has been demonstrated to exist that accounts for spatial correlations in the fracture aperture distribution (Petrovitch et al., 2013 & 2014; Pyrak-Nolte & Nolte, 2016; Pyrak-Nolte, 2018). Fracture specific stiffness, also known as unit joint stiffness, was introduced by Goodman et al. (1968) to describe the behavior of a fracture because it could be measured in the laboratory without detailed analysis of the fracture geometry. Fracture specific stiffness is also used in theoretical models for elastic waves propagation across a fracture to capture the complexity of the fracture void geometry (i.e., spatial and probability distribution of aperture and contact area) (Mindlin, 1960; Kendall and Tabor, 1972; Schoenberg, 1980 & 1983; Kitsunezaki, 1983; Pyrak-Nolte and Cook, 1987; Pyrak-Nolte et al, 1990a&b). These studies have shown that a fracture behaves as a low pass filter resulting in transmission, reflection coefficients that are frequency-dependent and depend on fracture stiffness. A key question is there information in a transmitted/reflected signal that would provide more information on the fracture void geometry.
ABSTRACT: Pore fluids and mineralogical composition of the rock can affect caprock integrity through chemo-mechanical interactions that may alter the fracture geometry. Geo-architected rocks were fabricated to examine the role of clay on fracture generation caused by volumetric changes in the clay from dehydration. Acoustic emissions emitted by geo-architected rocks with and without Montmorillonite were monitored to investigate the behavior of clay-rich rocks during drying. During the drying, acoustic emissions were generated from two sources: movement of the fluid front during drying, and induced cracking from volumetric changes in clay from desiccation. Time lapsed 3D X-ray tomographic imaging revealed that the drying front in clay-rich rocks moved from the edge of the sample to the interior, and that the fractures formed in the dried out regions and terminated on the drying front. The peak frequencies of drying events in samples with and without clay were greater than the peak frequencies of events generated by induced cracking. Overall analysis of the different geo-architected rocks shows that the presence of clay has a significant effect on the extent and location of the fracture network.
An extensive impermeable caprock (seal) is one of the most important aspects of geological systems capable of retaining captured CO2 or other fluids in the subsurface. The impermeable confining caprock is expected to provide a rigorous barrier against fluid migration, and long-term retention of the trapped fluids. Argillaceous rocks (mudstones, clays, and shales) and evaporites (salts and anhydrite) are commonly identified caprocks for CO2 storage (Song and Zhang, 2013; Griffith, 2011). Two important macro-scale characteristics of good caprock (seal) layers are continuity and ductility (Downey, 1984). Accordingly, these seal lithologies, can be arranged by ductility as follows: salt > anhydrite > kerogen-rich shales > clay shales > silty shales (Downey, 1984).
Shale in particular is considered an ideal barrier for geological storage systems, and the mineralogy and microstructure of shale is especially interesting. Shale is comprised of feldspathic minerals, carbonates, and varied amounts of clay (kaolinite, montmorillonite- smectite, illite or chlorite) that greatly influence the hydro-chemo-mechanical behaviors of the rock. The type and percentage of clay embedded in the matrix of shale and how the clay responds to changes in the in-situ environment can influence sealing properties. That is, deformation or changes in the in situ environment may excite brittle or ductile behavior in a shale caprock.
ABSTRACT: Prismatic specimens of gypsum containing two pre-existing open flaws and a frictional interface are tested under uniaxial compression. The specimens are 203.2mm high, 101.6mm wide, and 25.4mm thick. Two flaws, with 0.1mm aperture and 12.7mm length, are placed through the thickness of the specimen. The flaws are inclined 30° from the horizontal. Unbonded interfaces, inclined 10° from the horizontal, are created by casting the specimen in two parts. The first half of the specimen is cast against a PVC block with a face inclined 80° with respect to the vertical axis of the specimen. The second half is then cast against the first half. Sandpaper attached to the PVC block provides different friction at the interface and, using a debonding agent, a cohesionless contact is obtained. In the experiments, digital image correlation (DIC) is used to monitor crack initiation and propagation on the specimen surface by measuring displacements during loading. The experimental results indicate that specimens with the smooth interface have lower initiation and coalescence stresses than with the rough interface. Also, a smooth interface seems to arrest tensile cracks reaching the interface, while shear cracks can cross it. This mechanism seems to favor crack coalescence through shear cracks when the flaws are separated by a smooth interface, rather than through tensile cracks, which is what happens when there is a rough interface.
The presence of discontinuities, i.e. fractures or cracks, plays an important role in rock mass behavior. Failures of structures in rock such as tunnels and foundations commonly originate through pre-existing discontinuities or through new discontinuities created by the stress state imposed by the engineered structures.
Experimental research has been extensively conducted on pre-cracked brittle materials to investigate crack initiation, propagation and coalescence under compression (Bobet and Einstein, 1998; Wong and Chau, 2001; Wong and Einstein, 2007; Park and Bobet, 2010). Coalescence can be described as the linkage of pre-existing flaws through the propagation of tensile cracks and/or shear cracks (Fig. 1). Tensile cracks usually propagate in a stable manner following a curvilinear path that aligns with the most compressive load; their surfaces are clean and do not contain any pulverized material. In contrast to the tensile cracks, shear cracks are characterized by the presence of crushed material and powder in their surfaces, and usually propagate in a stable manner, at least initially. They, however, may become unstable near coalescence. Shear cracks may be classified as quasi-coplanar or coplanar and oblique, depending on the angle of initiation with respect to the plane of the flaw from which they originate. Coplanar cracks make an angle of 45° or less with the flaw plane, while oblique cracks make an angle higher than 45°.
We study some aspects of the inverse boundary value problem for the wave equation with smooth wave speed. For data, we use the Neumann-to-Dirichlet map, which takes a Neumann boundary source as data and returns the Dirichlet boundary trace. Our main theoretical tool is a geometric variant of the Boundary Control (BC) method. First, we show that the travel time between an interior point and boundary points can be computed by processing the boundary data via the BC method. We stress that our method does not require a source or receiver at the interior point. Next, we consider a redatuming problem in which the wave speed is known in a near boundary region. Our redatuming procedure consists of two steps, both driven by the BC method. In the first, we extend receivers into the near boundary region, and in the second step we utilize these extended receiver measurements to move sources into the near boundary region. In both steps, the wavefields that we obtain are allowed to propagate further than the near boundary region, so they could not be obtained by directly solving the wave equation in the known region. We present computational experiments demonstrating both the travel time computations and redatuming procedure.
Presentation Date: Monday, October 17, 2016
Start Time: 4:10:00 PM
Presentation Type: ORAL
Harper, Christopher (University of Texas - Dallas) | McBride, Kyle (University of Texas - Dallas) | Ferguson, John (University of Texas - Dallas) | Baldridge, Scott (Los Alamos National Laboratory) | Braile, Larry (Purdue University) | McPhee, Darcy (US Geological Survey) | Keithline, Nathan (College of William and Mary) | Boucher, Chloe (University of California-Santa Cruz) | Mendoza, Kevin (University of California-Merced)
Seismic and gravity data collected by the Summer of Applied Geophysical Experience (SAGE), along with energy-industry seismic data, well data, and geologic maps, go into the construction of geologic cross sections transecting the Rio Grande rift in the area of the Española basin. These cross sections reveal several key structures within the basin including the eastern bounding fault of the Los Alamos graben, and the Agua Fria fault system near the eastern boundary of the basin. Additionally, we find no evidence in the data presented here supporting the linking of the Embudo and Santa Clara faults. Finally, by flattening the geologic cross sections on several unconformity surfaces, a temporal structural progression of the study area is developed. Using the resulting paleo cross sections, we show that the trend of the Laramide Sangre de Cristo uplift likely varied markedly from the modern uplift.
Presentation Date: Wednesday, October 19, 2016
Start Time: 8:25:00 AM
Location: Lobby D/C
Presentation Type: POSTER
Xu, Zhenyu (Purdue University) | Li, Qingyun (Washington University) | Sheets, Julie (Ohio State University) | Kneafsey, Timothy J. (Lawrence Berkeley National Laboratory) | Cole, David (Ohio State University) | Jun, Young-Shin (Washington University) | Pyrak-Nolte, Laura J. (Purdue University)
Geochemical interactions during the injection/withdrawal of fluids into the subsurface can modify fracture apertures through dissolution and/or precipitation of minerals. Modification of fracture apertures by reactive flows is strongly affected by non-reactive, non-wetting fluids that limit the fracture surface area and void volume that is accessible to the reactive fluids. An experimental investigation was performed to examine the controls on mineral precipitation within a fracture when mineral precipitation occurs with and without the generation of gas. Several differences were observed in the results from the experiments using the two chemical approaches for generating precipitates. The pore-filling precipitates (particles formed in solution) were more uniform in size and were thicker than the surface adhering precipitates (particles formed on fracture surfaces). The acoustic response for fractures with pore-filling precipitates reached a steady-state after 2 hours while fractures with surface adhering precipitates took longer to equilibrate because of the presence of CO2 that was generated during the chemical reaction at liquid– surface interfaces. In surface adhering precipitation case, the generation of CO2 limited accessibility of calcium carbonate prenucleation polymers to the entire void volume of the fractures. On the other hand, fractures with pore-filling precipitates exhibited a more uniform acoustic response across the fracture plane than fractures with surface adhering precipitates. Understanding the effect of mineral precipitation on acoustic wave attenuation provides a path forward for long-term monitoring of seal integrity.
Geologic sequestration of CO2 in subsurface reservoirs relies on different mechanisms to contain the CO2 such as stratigraphic trapping, capillary/solution based trapping and mineralogical trapping. Stratigraphic trapping relies on “seals” that are low permeability rock, such as shale, that prevents the migration of CO2 to shallower depths or other regions in the subsurface. However, the integrity of seals depends on the presence of pre-existing fractures, the generation or reactivation of cracks/fractures/faults and the potential of sealing flow paths along such discontinuities through mineralization/precipitation. Sealed or partially sealed mineralized faults are observed in nature. For example, both the Refugio-Carneos fault in the Santa Barbara Basin of Southern California  and the Moab fault around Courthouse Rock and Mill Canyon in Utah  contain calcite cements. In both cases, the migration of methane-rich fluids mixed with meteoric water has resulted in the precipitation of calcium carbonate along segments of the faults.
We study a polyhedral representation of regional subsurface structures partitioned into unstructured tetrahedral mesh. The geomechanics-related discontinuities are adaptively represented as triangulated interior surfaces as seperators for blocky subdomains, or “shapes”, without asssumption on smoothness. We present a two-step framework conducting mesh deformation by evolving the interior surface based on the gradient flow derived from the Hausdorff distance between an initially guessed geometry and a target polyhedral representing the true geological structure, and then elastically deforming and modifying the volume mesh under element quality constraints. We implement a series of refinement and coarsening operations, allowing topological change, and introduce a polyhedra based level set function to retrack the interior surface, as a feedback from volume modification to surface mesh optimization, which deals with the contacting and breaking up problems.
In the full-waveform inversion (FWI) in reflection seismology, we consider a parameterization of earth structure as a piecewise smooth elastic material. The Neumann-to-Dirichlet map is considered as data, and a sparsely segmented model parameter facilitates the convergence of Landweber iteration. In the sense of sparsity, we consider a polyhedral representation of shape, that is, discontinuities in model parameters that stand for real geological structures such as fault planes, salt boundaries, ocean bottoms, sediment layers, etc. A related approach to blocky representation of geological models is found in Shin (1988), and implementation of triangular unstructured meshes can be find in Hale (2002). Such representations are initiated via segmentation using an image of material parameters as input, generating interfaces that divide subdomains. Considering the generality of geological structures, we assume no smoothness for the shape of interfaces. In general inverse problems, a piecewise constant wavespeed recovery in the finite dimenstional setting with domain partitioning is provided with a Lipschitz stability estimate in Beretta et al. (2014). Instead of recovering a continuous class of wavespeed, we hereby limit the elastic attributes to finite set of values, obtained from geological knowledge of relevant mineral types. We use a fully unstructured tetrahedral mesh with adaptive deformation, quality control, and local refinements. A hiearchical structure can be applied in determing intermediate levels of subdomains and interfaces for corresponding secondary structures.
The lowrank finite difference (FD) methods have be obtained by matching the spectral response of the FD operator with the lowrank approximate recursive integral time-extrapolation (RITE) operator. We improve the accuracy and stability of lowrank FD method for first-order and second-order acoustic wave equation by using weighted least square (WLS). The weight function is designed according to the contributions of different wavenumber components to the accuracy of the FD scheme. After a stability analysis, we also introduce a taper constraint to restrain the FD operator to satisfy the stability condition. The numerical tests indicate that the proposed WLS-based lowrank FD scheme is accurate and stable for large time step seismic wave extrapolation.
Wave extrapolation in time is an essential part of seismic imaging, modeling, and full waveform inversion. Finite-difference methods (Etgen, 1986;Wu et al., 1996) is one of the most popular and straightforward ways of implementing wave extrapolation in time. The finite-difference (FD) methods are highly efficient and easy to implement. However, the traditional FD methods are only conditionally stable and suffer from numerical dispersion (Finkelstein and Kastner, 2007).
The FD coefficients are conventionally determined through a Taylor-series expansion around the zero wavenumber (Dablain, 1986; Kindelan et al., 1990). Traditional FD methods are therefore particularly accurate for long-wavelength components. Several approaches have been proposed to improve the performance of FD method in practice. Implicit FD operators (Liu and Sen, 2009; Chu and Stoffa, 2011) can be used to achieve high numerical accuracy. Another way to control numerical errors is to use optimized FD operators (Takeuchi and Geller, 2000; Chu et al., 2009; Liu, 2013). Song et al. (2013) derived optimized coefficients of the FD operator from a lowrank approximation (Fomel et al., 2013) of the space-wavenumber extrapolation matrix.To improve the accuracy and stability and deal with the wave extrapolation of variable density, Fang et al. (2014) extend the lowrank FD methods on a staggered grid. Lowrank FD methods are accurate and stable than the usually used conventional FD methods. However, when using large time steps, wave extrapolation with lowrank FD methods still has numerical dispersion and sometimes will be unstable.
In this paper, we review the LS-based FD scheme for the first and second order wave equation. Then the WLS method is introduced to optimize the FD coefficients. We apply the stability analysis to the WLS-based FD scheme and design a taper function to improve its stability. We use lowrank decomposition to reduce the cost of optimizing FD coefficients. The numerical numerical examples test the feasibility of the proposed method.