137 research outputs found
Fracture characterization from attenuation of Stoneley waves across a fracture
No full textFractures contribute significantly to the permeability of a formation. It is important to understand the fracture distribution and fluid transmissivity. Though traditional well logs can image fractures intersecting the borehole, they provide little information on the lateral extent of the fractures, away from the borehole, or the fluid transmissivity. Experiments in the past demonstrated that fracture compliance can be a good proxy to fracture fluid conductivity. We describe a method to estimate fracture compliance from the attenuation of Stoneley waves across a fracture. Solving the dispersion relation in the fracture, transmission coefficient of Stoneley waves across a fracture is studied over all frequency ranges. Based on the observations from the model, we propose that measuring the transmission coefficient near a transition frequency can help constrain fracture compliance and aperture. Comparing attenuation across a finite fracture to that of an infinitely long fracture, we show that a bound on the lateral extent of the fracture can be obtained. Given the limitation on the bandwidth of acoustic logging data, we propose using the Stoneley waves generated during micro-seismic events for fracture characterization.Eni-MIT Energy Initiative Founding Member Progra
Stoneley wave modeling in heterogeneous porous media with viscous pore fluids
No full textWe implemented Biot-type porous wave equations in a pseudo-spectral
numerical modeling algorithm for the simulation of Stoneley waves
in porous media. Fourier and Chebyshev methods are used to compute
the spatial derivatives along the horizontal and vertical directions,
respectively. To prevent from overly short time steps due to the
small grid spacing at the top and bottom of the model as a consequence
of the Chebyshev operator, the mesh is stretched in the vertical
direction. As a large benefit, the Chebyshev operator allows for
an explicit treatment of interfaces. Boundary conditions can be implemented
with a characteristics approach. The characteristic variables are
evaluated at zero viscosity. We use this approach to model seismic
wave propagation at the interface between a fluid and a porous medium.
Each medium is represented by a different mesh and the two meshes
are connected through the above described characteristics domain-decomposition
method. We show an experiment for sealed pore boundary conditions,
where we first compare the numerical solution to an analytical solution.
We then show the influence of heterogeneity and viscosity of the
pore fluid on the propagation of the Stoneley wave and surface waves
in general
Free Pipe Effects On Guided Wave Propagation
No full textThe effects of free pipe on the pseudo-Rayleigh and Stoneley waves are studied by introducing a fluid annulus between the casing and cement layers of a cased borehole
model. The existence of a fluid annulus region results in a second Stoneley mode being
generated which propagates primarily in the annulus. The propagation and attenuation
of this second mode are controlled by the fluid annulus, cement, and formation
properties. The primary or central Stoneley mode becomes decoupled from the effects
of formation when the free pipe situation exists, and its propagation and attenuation
are controlled by the central fluid and casing properties only. The amplitude of the
second Stoneley mode increases as the annulus thickness increases, but in most cases
is an order of magnitude less than the primary mode amplitude. The pseudo-Rayleigh
wave becomes more dispersive as the annulus region increases in thickness.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
POLARIZZAZIONE DELLA S DEI SISMOGRAMMI
No full textISei terremoti registrati a distanze non troppo grandi si è osservato che la faseS appare prima come una SU. seguita 10 o 14 sec più lardi da una Sì'. La presentenota si propone di decidere se la doppia rifrazione è in grado di spiegarequesto fenomeno.Un modello semplice sarebbe costituito da un materiale « isotropo trasversalmente», simmetrico attorno alla direzione radiale. Si possiedono le formule per levelocità delle onde SII e SI : esse dipendono dall'angolo che il raggio forma conla normale.È improbabile che la terra possa essere anisotropa in maniera tanto rilevantequanto il berillio, che è isotropo trasversalmente; di conseguenza questo materiale,del quale si conoscono le sei costanti elastiche, è stato assunto come un esempioestremo e le velocità delle SII e delle SV per differenti angoli di incidenza sonostate « aggiustate » in modo da ottenere le velocità delle onde di distorsione nelgranito. È cos) possibile calcolare la differenza tra i tempi occorrenti <die onde perpassare da un punto della superficie terrestre a un altro punto della superficiestessa, a seconda che l'onda S nello strato superficiale è del tipo SII o SV.Si trova che, anche in questo caso estremo, uno strato di roccia anisotropospesso circa 30 km darà origine ad una differenza di tempo di solo l]/> secondi.Cioè se la terra fosse cos'i fortemente anisotropa quanto il berillio (il che è improbabile)bisognerebbe che lo strato a orientazione preferenziale si estendesse finoa una profondità di circa 300 km. Ciò è piuttosto improbabile, dimodoché nonconviene attribuire un gran credito alle spiegazioni fondate sulla doppia rifrazione
Analysis of Full Waveform Acoustic Logging Data in "Soft" Formations
No full textDirect recording of formation shear wave travel time is not possible in "soft" formations where the shear velocity is lower than the borehole fluid velocity. The borehole Stoneley wave is quite sensitive to changes in formation shear wave properties and may be used to indirectly determine shear velocity. This paper presents a method to calculate formation shear velocity through inversion of the dispersion equation for the propagation of borehole Stoneley waves. The Stoneley wave group velocity and effective attenuation are also computed in this data analysis.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumChevronTexaco (Firm)(Fellowship
POLARISATION OF THE S - PHASE OF SEISMOGRAMS
No full textIn earthquakes recorded at moderate distances it lias been ohservedthat S phase appears first as SII, folloived some 10 to 14 seconds laterby SV. The object of tliis paper is to try to decide ichether doublérefraction is likely to be the explanation of tliis jìlwnomenon.A simpie model to consider ivould be a « transversely isotropie »material, symmetrical about the radiai direction. Formulae for thevelocities of SII and SV waves are available; tliese velocities dependon the angle that the ray makes ivi t li the norma!. It is unlikely thatthe Eartli could be as markedly anisotropie as the minerai beryl, whichis transversely isotropie; aceordingly, this material, of ivhich the fi veclastic constants are knoivn is taken as an extreme example, andthe velocities of SH and SV for different angles of incidence are« scaled down » so as to match the velocity of distortional ivaves ingranite. It is then possible to calcitiate the difference in the timo takenby ivaves from one point of the surface of the Earth to anotlier pointon the surface according as the S wave in the surface layer is of SHor SV type
Dispersive surface waves along partially saturated porous media
No full textNumerical results for the velocity and attenuation of surface wave modes in fully permeable liquid/partially saturated porous solid plane interfaces are reported in a broadband of frequencies (100?Hz–1?MHz). A modified Biot theory of poromechanics is implemented which takes into account the interaction between the gas bubbles and both the liquid and the solid phases of the porous material through acoustic radiation and viscous and thermal dissipation. This model was previously verified by shock wave experiments. In the present paper this formulation is extended to account for grain compressibility. The dependence of the frequency-dependent velocities and attenuation coefficients of the surface modes on the gas saturation is studied. The results show a significant dependence of the velocities and attenuation of the pseudo-Stoneley wave and the pseudo-Rayleigh wave on the liquid saturation in the pores. Maximum values in the attenuation coefficient of the pseudo-Stoneley wave are obtained in the 10–20?kHz range of frequencies. The attenuation value and the characteristic frequency of this maximum depend on the liquid saturation. In the high-frequency limit, a transition is found between the pseudo-Stoneley wave and a true Stoneley mode. This transition occurs at a typical saturation below which the slow compressional wave propagates faster than the pseudo-Stoneley wave.GeotechnologyCivil Engineering and Geoscience
Electroseismic Logging For The Detection And Characterization Of Permeable Zones: Field Measurements And Theory
No full textA Stoneley wave propagating in a borehole generates pore fluid flow within the permeable
zones intersected by the borehole. In turn, the fluid flow induces a streaming
electrical potential. This electrical potential induced by the Stoneley wave can
be -measured in the field at frequencies from 100Hz to 4000Hz. Measurements of this
Stoneley-wave-induced electrical potential can be used to detect fractures and permeable
zones. The amplitude-versus-frequency dependence of this electroseismic phenomenon
provides a new way to test theories of the acoustics and the electrokinetics of porous
media against field measurements in real :oeks.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation
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Caractérisation des gisements d’hydrocarbures fracturés en utilisant l’outil de diagraphie acoustique EVA (R)
No full textUne large expérience acquise en diagraphies acoustiques, associée aux résultats d'une recherche théorique, ont permis de développer une nouvelle méthode de caractérisation de gisements fissurés. La méthode est fondée sur l'utilisation de l'atténuation de l'onde de cisaillement (S) et de l'onde de Stoneley, deux paramètres obtenus à partir du traitement du train complet des ondes acoustiques.La combinaison des deux paramètres permet de localiser les fractures individuelles et les zones fracturées. La propagation des ondes S et des ondes de Stoneley est fondée sur deux mécanismes physiques différents. Grâce à cela, l'utilisation combinée des deux paramètres peut aider à distinguer entre les fractures individuelles de grande ouverture et les zones fracturées conductrices (atténuation de l'onde de Stoneley) d'une part, et d'autre part les fractures individuelles d'ouverture moins importante, ou colmatées (atténuation de l'onde S). De plus nous pouvons utiliser EVA aussi bien en trou tubé qu'en trou ouvert en particulier pour l'étude de la fracturation. La méthode a été essayée avec succès sur le champ de gaz de Gaviota. L'acquisition et le traitement EVA (R) sur trois puits, ouverts ou tubés, ont aidé à localiser, ou se sont avérés la seule façon, de localiser les zones fracturées, qui, après perforation et traitements à l'acide, ont donné lieu à une production de gaz très satisfaisante
Formation Stress Estimation Using Standard Acoustic Logging
No full textIn situ formation stress directions and magnitudes are estimated by inverting the borehole flexural and Stoneley dispersions obtained from standard acoustic logging data
(dipole and monopole logs). The underlying procedure consists of the following steps:
first, we locate stressed zones in the formation by searching for crossovers in flexural
dispersions. Second, the fast shear direction is estimated from the cross-dipole waveforms. It corresponds to the direction of the maximum horizontal stress (S[subscript H]). Finally, a multi-frequency inversion of both the Stoneley and flexural dispersions yields the maximum (S[subscript H]) and minimum (S[subscript h]) horizontal stress magnitudes together with the three formation nonlinear elastic constants, C[subscript 111], C[subscript 112] and C[subscript 113], defined about the selected reference (isotropic) state. The inversion method is based on equations that relate S[subscript H] and S[subscript h] with variations in phase velocities of the borehole flexural and Stoneley waves in the stressed state from those in the assumed reference state, the state that is hydrostatically loaded and isotropic. Phase velocities of the borehole flexural and Stoneley modes as a function of frequency can be obtained from processing the cross-dipole and monopole waveforms, respectively. The borehole flexural and Stoneley dispersions in the assumed reference (isotropic) state are obtained from the solution of a standard boundary-value problem. The sensitivity functions for the inversion model are obtained from the eigenfunctions of the boundary-value problem in the reference state. Results for the stress directions and magnitudes obtained from the inversion of the Stoneley and flexural dispersions over a selected bandwidth are consistent with focal mechanism and borehole breakout data present in the world map database (Zoback, 1992).Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation
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