The presence of expandable clay minerals within the reservoir and source rocks causes many issues over the life of the well. These include wellbore instability and increased mud losses during drilling operations, complications and decrease in production during and after hydraulic fracturing, etc. Existing standard laboratory test methods, e.g., capillary suction time test, do not adequately predict swelling potential. The present work provides the technique to determine the properties and quantify the concentration of expandable clay minerals, thus predicting the swelling potential of clay-rich rocks. A collection of ash layer and bentonite mine samples has been analyzed for the bulk and clay fraction mineral composition and swelling in completion fluids. Comparison of X-ray diffractograms recorded from air-dried and ethylene glycolated clay fractions of ash layer samples confirmed the absence of discrete smectite but revealed the presence of interstratified illite/smectite and illite. X-ray diffractograms of all the ethylene glycolated samples have characteristic complex reflections with two maximas: one between 6.8–7.8°2θ (11.3–13.1 Å), another between 9.3–9.6°2θ (9.3–9.5 Å). Both correspond to the 001 reflection of interstratified illite/smectite mineral, and not a pure endmember illite or smectite. Two properties of this mineral vary among the samples and influence their expandability: (1) illite:smectite ratio in the range 3:2–9:1, and (2) the nature of the interstratification, i.e., the degree of ordering. The shifts of the peak maximas in the regions 6.8–9.6°2θ and 42.0–48.0°2θ are used to calculate the proportion of the component layers. Proposed technique of optimized XRD analysis reveals that the samples containing less illite/smectite mineral, but the higher concentration of smectite layers in this mixed-layer mineral are more expandable. Optimized XRD analysis correctly predicts the swelling potential of a given clay-rich zone (e.g., ash layer) as confirmed by confined immersion testing. Provided technique of XRD analysis is fast, requires small quantities of sample, and could possibly be integrated to field logs in the future. Obtained knowledge can be applied by engineers and researchers working with shale formations around the world.

Hydraulic fracture modeling has progressed into a high impact decision-making tool for the unconventional petroleum industry. However, the main influence on the model results are the rock mechanical inputs, typically derived from field logs and supplemented by core measurements, and the uncertainty associated to these. The strongly layered nature of unconventional mudstones, however, requires representation of the high contrast in properties between layers, and their anisotropic elastic behavior. This investigation provides an integrated method for experimentally measuring elastic mechanical properties from new anisotropic ultrasonic measurements at sufficiently high resolution (cm scale) to honor the rock fabric along with triaxial compressional data on sample plugs. These results are analyzed, compared, and integrated using a newly developed methodology, based on St. Venant's reduced symmetry approximation, to develop a continuous representation of the elastic properties and their associated uncertainties. For cases where core is not available, we provide a method to reconstruct anisotropic properties at cm resolution using standard resolution field logs and a borehole image log. A case study on a core from the Permian basin is presented and verified against a standard workflow. Additionally, the methodology to propagate from the cored well to nearby regions (e.g., formations above the cored interval) is demonstrated for cases where logs are available, but core data are not.

Intersections in a fracture network control the connectivity of the flow paths through rock. The long near-linear geometric nature of fractures makes them difficult to identify and characterize. We present a new type of elastic wave, an intersection wave, which travels along an intersection and is sensitive to the coupling between two orthogonal fractures that define the intersection. Group theory for C-2v and C-4v point groups predicts sets of propagating elastic waves confined to the fracture intersection. Along with the use of the wave equation and displacement discontinuity boundary conditions, the dispersion relationships for intersection waves are predicted. Experimental ultrasonic measurements on a nonwelded linear intersection between two orthogonal, synthetic fractures in aluminum confirm the existence of multiple modes that travel between the speed of wedge waves (sub-Rayleigh waves) when the intersection is completely, and bulk shear waves, when the intersection is closed, as predicted by theory. Between these two limits, the intersection behaves as a nonwelded contact and yields these new intersection waves that are dispersive and sensitive to the coupling along the intersection. Intersection waves provide the foundation for new geophysical approaches for characterizing the hydraulic connectivity of fracture networks.

Simulation of elastic-wave propagation in rock requires knowledge of the elastic constants of the medium. The number of elastic constants required to describe a rock depends on the symmetry class. For example, isotropic symmetry requires only two elastic constants, whereas transversely isotropic symmetry requires five unique elastic constants. The off-diagonal elastic constant depends on a wave velocity measured along a nonsymmetry axis. The most difficult barrier when measuring these elastic constants is the ambiguity between the phase and group velocity in experimental measurements. Several methods to eliminate this difficulty have been previously proposed, but they typically require several samples, difficult machining, or complicated computational analysis. Another approach is to use the surface (Rayleigh) wave velocity to obtain the off-diagonal elastic constant. Rayleigh waves propagated along symmetry axes have phase and group velocities that are equal for materials with no frequency dispersion, thereby eliminating the ambiguity. Using a theoretical secular equation that relates the Rayleigh velocity to the elastic constants enable determination of the off-diagonal elastic constant. Laboratory measurements of the elastic constants in isotropic and anisotropic materials were made using ultrasonic transducers (central frequency of 1 MHz) for the Rayleigh-wave method and a wavefront-imaging method. The two methods indicated agreement within 1% and 3% for isotropic and transversely isotropic samples, respectively, demonstrating the ability of the Rayleigh-wave method to measure the off-diagonal elastic constant. The surface-wave approach eliminates the need for multiple samples, expensive computational calculations, and most importantly, it removes the ambiguity between the phase and group velocity in the measured data for materials with no frequency dispersion because all measurements are made along symmetry axes.

The interface between two wedges can be treated as a displacement discontinuity characterized by elastic stiffnesses. By representing the boundary between the two quarter-spaces as a displacement discontinuity, coupled wedge waves were determined theoretically to be dispersive and to depend on the specific stiffness of the non-welded contact between the two wedges. Laboratory experiments on isotropic and anisotropic aluminum confirmed the theoretical prediction that the velocity of coupled wedge waves, for a non-welded interface, ranged continuously from the single wedge wave velocity at low stress to the Rayleigh velocity as the load applied normal to the interface was increased. Elastic waves propagating along the coupled wedges of two quarter-spaces in non-welded contact are found to exist theoretically even when the material properties of the two quarter-spaces are the same.

Significant work in the past few decades has lead to a well developed understanding of seismic wave propagation in fractured media. However, previous research has focused on fractures within rock as opposed to fractures at the surface of a rock. A theoretical and experimental study was performed to examine seismic wave propagation along a fracture at the surface, i.e., along the intersection of two quarter-spaces. A theoretical model that couples two wedges, using displacement discontinuity boundary conditions, was developed that gives rise to a new guided waveform that is dispersive, depends on fracture specific sti↵ness at the intersection of a fracture with a free surface and exhibits velocities that range from the single wedge-mode velocity to the Rayleigh wave velocity at a free surface. The existence and behavior of this new guided-mode was verified using a synthetic fracture created between two aluminum blocks. This new guided-mode enables characterization of fracture specific sti↵ness of fractures, joints and other discontinuities at the surface of an outcrop.

Fractures in rock masses influence strongly the mechanical and hydraulic properties of a rock mass. Thus, the detection and characterization of fractures using geophysical methods is of critical importance for maintaining the integrity of sub-surface infrastructure and subsurface waste or storage repositories. While the effects of single fractures or sets of parallel fractures on seismic wave propagation have been studied by many scientists and engineers, little research has been performed to determine the role of fracture intersections on seismic wave attenuation and velocity. A fundamental question is whether the specific stiffness or compliance of an intersection is the same or differs from the stiffness of any of the individual fractures within two intersecting sets of fractures. In this paper, we show from experimental and numerical studies that the stiffness of fracture intersections can be less than, equal to, or greater than the stiffness of the individual fractures depending on the applied bi-axial loading conditions.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2011Fracture intersections and interface wavesAuthors: Laura J. Pyrak‐NolteBradley C. AbellFan WuLaura J. Pyrak‐NolteSearch for more papers by this author, Bradley C. AbellDepartment of Physics, Purdue University, West Lafayette, IndianaSearch for more papers by this author, and Fan WuSchool of Electrical and Computer Sciences, Purdue University, West Lafayette, IndianaSearch for more papers by this authorhttps://doi.org/10.1190/1.3627650 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Laboratory experiments were performed on synthetic orthogonal fractures to determine the effect of intersections on fracture interface waves. Compressional and shear waves were propagated along fractures as well as along an intersection for a range of normal stresses (1.4 MPa to 14). At an intersection, the bulk shear was quenched and the existence of interface waves was independent of the polarization of the shear wave source. These intersection waves were observed to be highly sensitive to stress concentrations along the intersection including the orientation of the applied stress.Permalink: https://doi.org/10.1190/1.3627650FiguresReferencesRelatedDetailsCited byElastic wave propagation in fractured media using the discontinuous Galerkin methodJonás D. De Basabe, Mrinal K. Sen, and Mary F. Wheeler6 June 2016 | GEOPHYSICS, Vol. 81, No. 4Simulation of Fracture Interface Waves using the Discontinuous Galerkin MethodJonás D. De Basabe*, Mrinal K. Sen, and Mary F. Wheeler19 August 2015 SEG Technical Program Expanded Abstracts 2011ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2011 Pages: 4424 Publisher:Society of Exploration Geophysicists HistoryPublished Online: 08 Aug 2011 CITATION INFORMATION Laura J. Pyrak‐Nolte, Bradley C. Abell, and Fan Wu, (2011), "Fracture intersections and interface waves," SEG Technical Program Expanded Abstracts : 2226-2230. https://doi.org/10.1190/1.3627650 Plain-Language Summary PDF DownloadLoading ...