Peter M. Roberts

Extended local Rytov Fourier migration method

We develop a novel depth‐migration method termed the extended local Rytov Fourier (ELRF) migration method. It is based on the scalar wave equation and a local application of the Rytov approximation within each extrapolation interval. Wavefields are Fourier transformed back and forth between the frequency‐space and frequency‐wavenumber domains during wavefield extrapolation. The lateral slowness variations are taken into account in the frequency‐space domain. The method is efficient due to the use of a fast Fourier transform algorithm. Under the small angle approximation, the ELRF method leads to the split‐step Fourier (SSF) method that is unconditionally stable. The ELRF method and the extended local Born Fourier (ELBF) method that we previously developed can handle wider propagation angles than the SSF method and account for the phase and amplitude changes due to the lateral variations of slowness, whereas the SSF method only accounts for the phase changes. The stability of the ELRF method is controlled ...

Enhanced DNAPL Transport in a Sand Core during Dynamic Stress Stimulation

Extraction of dense, nonaqueous-phase liquid (DNAPL) contaminants trapped in groundwater aquifers is a major problem in environmental remediation because existing field techniques, such as pump and...

Elastic wave stimulation of oil reservoirs Promising EOR technology

Roughly 60% of oil resources in the world remains unproduced, partially due to limitations in existing enhanced recovery methods. This translates to approximately half a billion tons of oil left behind annually. Anecdotal production data, as well as historic field and laboratory experiments, have shown that low-amplitude seismic waves in the frequency range of roughly 1–500 Hz can enhance oil mobility and total recovery in mature reservoirs. It is estimated that, to date, various types of seismic and acoustic oilfield stimulation activities have produced an additional 11 million tons of oil in the Former Soviet Union alone. Unfortunately, field tests with different seismic sources have often yielded mixed or inconclusive results. In some cases seismic stimulation increased production rates by 50% or more, but in other cases production was unchanged or actually declined. So far, laboratory and field experiments have not been sufficiently comprehensive to define the physical conditions under which stress-wave stimulation is most effective. An exhaustive review of observations and research on this subject from the 1950s up through 1992 was published by Beresnev and Johnson (1994). The paper summarizes the important early research, including a large body of work published only in Russian. The major conclusion of the Beresnev and Johnson review was that the seismic stimulation method “has produced promising results; however, further testing and understanding of the mechanisms are necessary.” This statement has motivated numerous sponsored research projects and industry evaluation programs over the last 10 years, particularly in the United States. Efforts in Russia have also been greatly expanded. Field tests are yielding more promising results as experimental controls and measurements are improved, but questions remain about how to predict optimum stimulation parameters for a particular reservoir, and what the statistical significance is of production changes observed during stimulation treatments. Laboratory experiments continue to …

Laboratory observations of altered porous fluid flow behavior in Berea sandstone induced by low-frequency dynamic stress stimulation

It has been observed repeatedly that low-frequency (1–500 Hz) seismic stress waves can enhance oil production from depleted reservoirs and contaminant extraction from groundwater aquifers. The physics coupling stress waves to fluid flow behavior in porous media is not understood, although numerous physical mechanisms have been proposed to explain the observations. To quantify the effects of low-frequency, dynamic-stress stimulation on multiphase fluid flow and in situ particle behavior in porous media, laboratory experiments were conducted with a core flow stimulation apparatus that allows for precise control and measurement of applied stress and strain, static confinement, and fluid flow parameters. Results are reported for experiments on stimulated single-phase and two-phase fluid flow behavior in 2.54-cm-diameter Berea sandstone cores. For all experiments, stimulation was applied to the cores in the form of sinusoidal, axial, mechanical stress coupled to the solid porous matrix at frequencies of 25 to 75 Hz. Applied stress RMS amplitudes ranged from 300 to 1200 kPa and, at these levels, produced coupled, pore-pressure fluctuations of much less than 1.2 to 4.8 kPa, respectively. During single-phase brine flow, stimulation increased the absolute permeability of the rock by 10–20%. This was caused by mobilizing in situ clay particles that were partially plugging the pore throats. During two-phase, steady-state, constant-rate flow of oil-brine and decane-brine mixtures, stimulation caused significant changes in the bulk fluid pressure drop across the core. The pressure changes showed a strong dependence on the viscosity of the nonwetting fluid phase (oil or decane) relative to the wetting phase (brine). This may indicate that relative changes in the mobility of wetting versus nonwetting fluid phases were induced by the dynamic stress. Under the specific experimental conditions used, pore-scale particle perturbation and altered wettability are possible physical mechanisms that can explain the results.

Teleseismic P-wave image of crust and upper mantle structure beneath the Valles Caldera, New Mexico: Initial Results from the 1993 JTEX Passive Array

Teleseismic P-wave relative arrival-time data, collected from a temporary array during the 1993 Jemez Tomography Experiment (JTEX), have been inverted to image velocity anomalies beneath the Valles caldera in northern New Mexico. Instruments were deployed in two 30-km-long profiles, one of 8 and one of 9 stations. These profiles crossed the caldera trending at azimuths of N46°W and N60°E, respectively. Two-dimensional teleseismic relative arrival time inversion of the 1993 data set, supplemented with data from an overlapping 1987 profile, confirms the existence of a mid-crustal low velocity region (-30%) beneath the Valles caldera in the depth range of 8 to 13 km (below sea level), with about a 6 km horizontal extent. This feature is interpreted to be the seismic expression of the remnant magma chamber. A shallow low velocity anomaly beneath San Antonio Mountain coincides with the region of highest thermal gradient values in the caldera. A lower crust/upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile. We tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust-mantle boundary.

Scattering of elastic waves in 2-D composite media I. Theory and test

Abstract Localised regions in the earths crust exhibiting complex variations of density and seismic wave velocities can be represented by random distribution of cavities, in a manner described by Matsunami (1983) for two-dimensional (2-D) media. In order to studt the multiple scattering of seismic waves propagating in such media, we develop an indirect boundary integral scheme with discretisation based on wave source distribution around the cavities. Numerical experiments using seven generic 2-D models and incident P, SV and SH plane waves, as well as explosive line sources, are carried out. These experiments are intended to both assess the accuracy of the method, and to examine the character of attenuation of the direct wave, coda waveforms, and travel time features that emerge from pure scattering (no intrinsic attenuation), computed in all cases for wavelengths comparable to the size of the heterogeneities. The wavefield computed for one cavity shows a remarkable diffracted wave that creeps around it, for all the incident waves, regardless of the shape of its cross-section. This wave contributes significantly to the multiple scattering caused by the direct and all reflected/converted waves in the presence of many cavities. For complex regions defined by random distribution of cavities, an explosive line source located below the region produces slight amplification of the horizontal component of the wavefield, apparently due to constructive interference, at observation points above the region, while the vertical component is strongly attenuated. The durations of the seismograms are about the same for observation points located towards both ends of the region. These results appear to be reversed when the source is above the region. In this case, the horizontal component is strongly attenuated, and the duration of the seismograms is significantly larger at observation points on the side of the incidence than at the opposite side, suggesting the dominant effect of backscattering. The amplitudes of the multiple scattered phases, the attenuation of the direct wave and the duration of the seismograms, appear to be larger when the line source is very near or within the heterogeneous region, than when it is outside. For the same geometry of the scattering region, the seismograms appear to be more complex and amplitudes of multiple scattered phases larger for a plane SH wave in a half-space.

Scattering of elastic waves in 2-D composite media II. Waveforms and spectra

The boundary integral representation of the complete seismic wavefield in two-dimensional composite media characterised by the distribution of many cavities (Paper I in this issue) is used to study the waveforms and spectra of the scattered wavefield for three models of media heterogeneity, upon the incidence of P and S plane waves and line sources. First, the case of one circular cavity and S primary waves shows that the scattered wavefield is composed mainly of S waves, and that S-P scattering can be ignored in any frequency range for all forward scattering angles (scattering angle θ measured clockwise with respect to the direction of propagation of the primary wave). The spectra of forward scattering computed for θ ⋍ 0° resemble the spectrum predicted by the Born approximation for acoustic or scalar waves for θ = 0°: of small amplitudes for small values of non-dimensional frequency kd (k is the wavenumber and d is the cavity diameter), increasing with kd, up to kd ⋍ 2, and becoming constant for larger values of kd. The spectra for backward scattering (θ ⋍ 180°) behave similarly, showing amplitudes as large as those computed for the forward cases. The non-isotropic pattern of scattering predicted by analytical solutions is also confirmed. In the case of P primary waves, P-S scattering appears to be significantly stronger than P-P scattering for most scattering angles, except for θ ⋍ 0° and θ ⋍ 180°. The computation of synthetic seismograms for models with many cavities show scattered waves of low frequency corresponding to wavelengths much larger than the size of the cavities, as well as those of high frequency due to multiple reflections and conversions at the boundaries of the cavities. A cluster of 20 cavities randomly distributed within a small region produces well-defined low frequency waves that appear to be associated with the presence of one low-velocity heterogeneous body, or soft inclusion, represented by the whole cluster. The case of 50 cavities randomly distributed within a horizontally extended region (of narrow thickness) shows coda-like wave arrivals, particularly strong in the horizontal component. Also in this case, nearly horizontally incident plane waves produce low frequency scattered waves of large amplitudes. It appears that while in the long-wavelength limit this model synthesises a coherent wave corresponding to reflection upon a horizontal interface, towards the short-wavelength limit the scattered waves show a rather complex, incoherent pattern immediately after the arrival of the incident wave, as if the region were a transitional zone of effective thickness. The analysis presented in this paper suggests that if the wavelengths are much larger than the size of the cavities, our representation of random media can be used to represent regional heterogeneity in the earths crust, associated with observed seismic scattering phenomena.

Mobilization of Colloidal Particles by Low-Frequency Dynamic Stress Stimulation

Naturally occurring seismic events and artificially generated low-frequency (1 to 500 Hz) elastic waves have been observed to alter the production rates of oil and water wells, sometimes increasing and sometimes decreasing production, and to influence the turbidity of surface and well water. The decreases in production are of particular concern, especially when artificially generated elastic waves are applied as a method for enhanced oil recovery. The exact conditions that result in a decrease in production remain unknown. Although the underlying environment is certainly complex, the observed increase in water well turbidity after natural seismic events suggests the existence of a mechanism that can affect both the subsurface flow paths and the mobilization of in situ colloidal particles. This article explores the macroscopic and microscopic effects of low-frequency dynamic stress stimulations on the release of colloidal particles from an analog core representing an infinitesimal section along the propagation paths of an elastic wave. Experiments on a column packed with 1 mm borosilicate beads and loaded with polystyrene microparticles demonstrate that axial mechanical stress oscillations enhance the mobilization of captured microparticles. Increasing the amplitude of the oscillations increases the number of microparticles released and can also result in cyclical spikes in effluent microparticle concentration during stimulation. Under a prolonged period of stimulation, the cyclical effluent spikes coincided with fluctuations in the column pressure data and continued at a diminished level after stimulation. This behavior can be attributed to rearrangements of the beads in the column, resulting in possible changes in the void space and/or tortuosity of the packing. Optical microscopy observations of the beads during low-frequency oscillations reveal that individual beads rotate, thereby rubbing against each other and scraping away portions of the adsorbed microparticles. These results support the theory that mechanical interactions between porous matrix grains are important mechanisms in flow path alteration and the mobilization of naturally occurring colloidal particles during elastic wave stimulation. These results also point to both continuous and discrete en masse releases of colloidal particles, perhaps because of circulation cells within the packing material.

Prestack Depth Migration For Complex 2D Structure Using Phase-screen Propagators

We present results for the phase-screen propagator method applied to prestack depth migration of the Marmousi synthetic data set. The data were migrated as individual common-shot records and the resulting partial images were superposed to obtain the final complete Image. Tests were performed to determine the minimum number of frequency components required to achieve the best quality image and this in turn provided estimates of the minimum computing time. Running on a single processor SUN SPARC Ultra I, high quality images were obtained in as little as 8.7 CPU hours and adequate images were obtained in as little as 4.4 CPU hours. Different methods were tested for choosing the reference velocity used for the background phase-shift operation and for defining the slowness perturbation screens. Although the depths of some of the steeply dipping, high-contrast features were shifted slightly the overall image quality was fairly insensitive to the choice of the reference velocity. Our jests show the phase-screen method to be a reliable and fast algorithm for imaging complex geologic structures, at least for complex 2D synthetic data where the velocity model is known.

Next-generation Numerical Modeling: Incorporating Elasticity, Anisotropy And Attenuation

A new effort has been initiated between the Department of Energy (DOE) and the Society of Exploration Geophysicists (SEG) to investigate what features the next generation of numerical seismic models should contain that will best address current technical problems encountered during exploration in increasingly complex geologies. This collaborative work is focused on designing and building these new models, generating synthetic seismic data through simulated surveys of various geometries, and using these data to test and validate new and improved seismic imaging algorithms. The new models will be both 2- and 3-dimensional and will include complex velocity structures as well as anisotropy and attenuation. Considerable attention is being focused on multi-component acoustic and elastic effects because it is now widely recognized that converted phases could play a vital role in improving the quality of seismic images. An existing, validated 3-D elastic modeling code is being used to generate the synthetic data. Preliminary elastic modeling results using this code are presented here along with a description of the proposed new models that will be built and tested.