Boris Kashtan

Localization of Seismic Events By Diffraction Stacking

The localization of seismic events is of great importance for hydro frac and reservoir monitoring. For deposits with weak 4-D signatures the passive seismic method may provide an alternative option for reservoir characterization. We introduce a new localization technique which does not require any picking of events in the individual seismograms of the recording network. The localization is performed by a modified diffraction stack of the squared amplitudes of the input seismograms resulting in the image section. The method is target oriented and is best suited for large networks of surface and/or downhole receivers. The source location is obtained from the maximum of the image section for the time window under consideration. Since the focusing analysis is performed only in this section, no optimized search procedures are required. The source time is determined in a second processing step after the source location. Initial tests with 2-D homogeneous media indicate the high potential of the method. Since the maximum of the image section is distinct even very weak events can be detected.

Head-wave Monitoring With Virtual Sources

The original applications of the Virtual Source Method (VSM) concentrated on reflected waves and demonstrated that imaging and monitoring through complex and changing overburdens can be accomplished at the expense of using downhole geophones in horizontal wells. There is number of reasons to expect even better results when head waves are restored and used for reservoir imaging and monitoring purposes. Being compared with a reflection survey, the head waves have less strict requirements for surface sources placements providing data for high resolution tomographic image for substantially larger areas. Head waves show high sensitivity to changes in the reservoir and look promising for monitoring applications. The drawback of this VSM application is in requirement of receiver lines placement close to reservoir depths.

On the role of reflections, refractions and diving waves in full-waveform inversion

Full-waveform inversion suffers from local minima, due to a lack of low frequencies in data. A reflector below the zone of interest may, however, help in recovering the long-wavelength components of a velocity perturbation, as demonstrated in a paper by Mora. With the Born approximation for a perturbation in a reference model consisting in two homogeneous isotropic acoustic half-spaces and the assumption of infinitely large apertures available in the data, analytic expressions can be found that describe the spatial spectrum of the recorded seismic signal as a function of the spatial spectrum of the inhomogeneity. Diving waves can be included if the deeper part of the homogeneous model is replaced by one that has a vertical velocity gradient. We study this spectrum in more detail by separately considering scattering of direct, reflected and head waves, as well as singly and multiply reflected diving waves for a gradient model. Taking the reflection coefficient of the deeper reflector into account, we obtain sensitivity estimates for each wavetype. Although the head waves have a relatively small contribution to the reconstruction of the velocity perturbation, compared to the other waves, they contain reliable long-wavelength information that can be beneficial for full-waveform inversion. If the deeper part has a constant positive velocity gradient with depth, all the energy eventually returns to the source-receiver line, given a sufficiently large acquisition aperture. This will improve the sensitivity of the scattered reflected and refracted wavefields to perturbations in the background model. The same happens for a zero velocity gradient but with a very high impedance contrast between the two half-spaces, which results in a large reflection coefficient.

Reconstructing head waves with virtual source method

The original applications of the Virtual Source Method (VSM) concentrate on imaging and monitoring through complex and changing overburden. This can be accomplished by correlating the wavefields recorded by downhole geophones. There are a number of reasons to expect even better results when this concept is extended to using head waves for reservoir imaging and monitoring purposes. The current practice to create virtual source data is to correlate the gated direct arrival at virtual source with total wavefield at receivers. Using a simple acoustic 2D model with two half-spaces having different velocities, we demonstrate the usefulness of correlating the head waves with different types of waves, theoretically and numerically.

Downhole acoustic surveillance of deepwater wells

Deepwater production increasingly relies on a few precious wells that are complex and expensive. Success is critically dependent on our ability to understand and manage these wells, particularly at the sandface. These wells are filled with expensive “jewelry” like sand control and production allocation systems that aim at maximizing production and minimizing risk. While this smart equipment can mitigate many anticipated dangers, it can easily fail when something unexpected happens. For example, repairing a sand control system that failed due to plugging can cost US

A new stacking operator for curved subsurface structures

30–40 million, while the costs of lost production due to long-term well impairment can be even higher. Lower-than-expected production is often referred to as “well underperformance” (Wong et al., 2003) and can be caused by various impairments: a plugged sand screen, contaminated gravel sand, clogged perforations, damaged formation around the wellbore or larger-scale compartmentalization. While 4D seismic can address large-scale compartmentalization, it has insufficient resolution to address near-well issues. Scarce downhole data from pressure and temperature gauges also cannot unambiguously characterize the impairment. This limits mitigation opportunities and prevents us from finding more effective drawdown strategies for high-rate, high ultimate-recovery deepwater wells. We strongly believe that geophysical surveillance in boreholes has a big role to play in identifying sources of well impairment and optimizing production. Here we describe one possible avenue—real-time completion monitoring (RTCM)—that utilizes acoustic signals in the fluid column to monitor changes in permeability along the completion. In essence, this is a miniaturized 4D seismic survey in a well. We illustrate the capabilities of acoustic surveillance through a series of full-scale laboratory tests with realistic completion and discuss opportunities for deployment in deepwater wells.

Acoustic waves in sand-screened deepwater completions: Comparison of experiments and modeling

Multiparameter stacking has become a standard tool for seismic reflection data processing. Different traveltime operators exist, whose accuracy depends on the offset and reflector curvature. We introduce a new, implicit stacking operator derived from evaluating Snell’s law at a locally spherical interface. Comparison of the resulting traveltime surface with those obtained from the common reflection surface and multifocusing expressions confirm high accuracy and only minor dependence on the reflector curvature. The examples show that the new method performs well for the whole range of reflector curvatures from nearly planar reflectors to the diffraction limit. INTRODUCTION Over the past years, a number of multiparameter stacking operators have been introduced as an extension of the CMP stacking technique. Examples of such operators are the common reflection surface stack (CRS, Mueller, 1999), Multifocusing (MF, Gelchinksy et al., 1999), and the shifted hyperbola (de Bazelaire, 1988). These operators describe the traveltime surface for a reflected event in the short offset limit. The accuracy of the individual methods differs and depends not only on the considered offset but also on the reflector curvature. In this work, we suggest a new stacking operator. It is derived from Snell’s law for a spherical interface and leads to an implicit expression for the traveltime surface. Although the operator can be applied in an iterative fashion, we show in our examples that already a single iteration leads to higher accuracy than the CRS and MF expressions. After a brief summary of the CRS and MF methods, we introduce a new implicit stacking operator (ISO) and examine its performance in comparison to CRS and MF. COMMON REFLECTION SURFACE The CRS stacking technique was introduced by Mueller (1999) to obtain a simulated zero offset section. The CRS stack can be considered as an extension of the classic CMP method, where stacking is carried out over offsets, while in the CRS technique the stack is applied over offsets and midpoints. This leads to a much larger number of contributing traces, and, thus, to a higher level of the signal to noise ratio. Whereas the CMP operator is a hyperbola, the corresponding CRS operator is a traveltime surface of second order that includes the CMP operator as subset. Written in midpoint (xm = x0 +∆xm) and halfoffset (h) coordinates, the CRS operator for monotypic reflections in the two-dimensional zero-offset case 248 Annual WIT report 2010

Acoustic Surveillance of Production Impairment With Real-Time Completion Monitoring

Real-timecompletionmonitoringwithacousticwaveshas been proposed recently as a method to monitor permeability changes along completions. Typical deepwater completions containadditionallayersofsandscreen,gravelsand,andperforatedcasing,whichmakethemquitedifferentfromafluidfilled open borehole. Monitoring changes in flow properties across the completion is crucial because impairment of permeabilityinanyoftheselayerscouldcausereducedwellproductivity. In contrast to an open-hole model, a sand-screened completion supports two tube waves related to an inner fluid column and a gravel suspension in the annulus. To study effects of screen and sand permeability on tube-wave signatures,weconstructsimplenumericalmodelsofvariouscompletionscenariosusingporoelasticdescriptionsofscreenand sand. Models generally predict that a fast tube wave does not attenuate at either low or high permeability, but experiences resonant attenuation at intermediate frequencies. In contrast, a slow tube wave attenuates completely above a certain permeabilityvalue.Modelsprovideaqualitativeandsometimes a semiquantitative description for signatures of the fast tube wave.However,theyareunabletoexplainwhytheslowtube wave is observed in experiments with high permeabilities of sandandscreen.Wespeculatethatabettermodelofcomplex sandscreensisrequiredtomatchexperimentaldata.

Joint inversion of seismic and magnetotelluric data with structural constraint based on dot product of image gradients

Deepwater production is challenged by well underperformance problems that are hard to diagnose early on and expensive to deal with later. Problems are amplified by reliance on few complex wells with sophisticated sand control media. New downhole data is required for better understanding and prevention of completion and formation damage. We introduce RealTime Completion Monitoring (RTCM), a new non-intrusive surveillance method for identifying impairment in sand-screened completions that utilizes acoustic signals sent via the fluid column. These signals are carried by tube waves that move borehole fluid back and forth radially across the completion layers. Such tube waves are capable of “instant” testing of the presence or absence of fluid communication across the completion and are sensitive to changes occurring in sand screens, gravel sand, perforations, and possibly reservoir. The part of the completion that has different impairment from its neighbors will carry tube waves with modified signatures (velocity, attenuation) and also would produce a reflection from the boundary where impairment changes. The method relies on permanent acoustic sensors performing acoustic soundings at the start of production and then repeating these measurements during the life of the well. Thus, it could be thought of as “miniaturized” 4D seismic and “permanent log” in an individual wellbore. In the meantime repeated conventional wireline measurements can be performed to assess the completion permeability. Motivation

Virtual refraction tomography: Application to realistic 3D model

We present an algorithm of joint inversion of seismic traveltimes and magnetotelluric impedances. To link velocity and resistivity we introduce structural constraint similar to well-known cross-gradients constraint, but more rigid one. Suggested constraint incorporates into inversion a priori information about sign of correlation between velocity and resistivity. Algorithm is tested on synthetic data.