Don Winterstein

Seismic detection of a hydraulic fracture from shear-wave VSP data at Lost Hills Field, California

A shear-wave (S-wave) VSP experiment was performed at Lost Hills Field, California, in an attempt to detect hydraulic fractures induced in a nearby well. The hydrofrac well was located between an impulsive, S-wave source on the surface and a receiver well containing a clamped, three-component geophone. Both direct and scattered waves were detected immediately after shut-in, when the hydraulic pumps were shut off and recording started. The scattered energy disappeared within about an hour, which is consistent with other measurements that indicate some degree of fracture closure and leak-off within that period.Although S-wave splitting was evident, no change was detected in the fast wave (polarized parallel to the fracture). However, the slow wave (polarized perpendicular to the fracture) did change over a period of about an hour, after which the prehydrofrac wavelet shape was recovered. The fact that only the wave polarized perpendicular to the fracture was affected is a dramatic confirmation of both theoretical predictions and laboratory observations of S-wave behavior in a fractured medium.Subtracting the prehydrofrac wavelet from the wavelets recorded within the first hour after shut-in revealed scattered wavelets that were diminished and phase-rotated versions of the incident (prehydrofrac) wavelet. Arrival times of the direct and scattered waves were matched by ray tracing. We accounted for the scattered-wave amplitudes by using numerical solutions of S-wave diffractions off of ribbon-shaped fractures. Amplitudes derived from full-wavefield Born scattering, however, did not match recorded amplitudes. The phase of the scattered wavelets was matched very well by Born scattering when the incident wavelet was input, but only for fracture lengths no larger than half those predicted from fracture-simulator models. These results show that a carefully controlled experiment, combined with accurate modeling, can provide important information about the geometry of induced fractures.

How shear‐wave properties relate to rock fractures: Simple cases

Shear‐wave data analysis, despite its ambiguities, is probably the most direct and sensitive method available for deducing rock fracture properties from remote measurements. Deducing fracture properties from the data requires understanding the relationships of shear‐wave (S-wave) particle motions and velocities to fracture orientations and strengths. The purpose of this article is to help the nonspecialist gain such understanding.

Some influences of stratigraphy and structure on reservoir stress orientation

The azimuth of maximum horizontal stress in a reservoir can vary significantly with depth and with position on a subsurface structure. We present and discuss evidence from field data for such variation and demonstrate both analytically and with finite‐element modeling how such changes might take place. Under boundary conditions of uniform far‐field displacement, changes in stratigraphic layering can reorient the principal stress direction if the formation is intrinsically anisotropic. If the formation stiffness is lower perpendicular to bedding than parallel to bedding (as is often the case in layered geologic media), an increase in dip will reduce the component of compressive stress in the dip azimuth direction. Folds can reorient principal stresses because flexural strain varies with depth and position. Compressive stress perpendicular to a fold axis increases with depth at the crest of an anticline and decreases with depth at the limb. When the regional stress anisotropy is weak, this change in stress ...

Vector attenuation: Some implications for plane waves in anelastic layered media

Plane waves in layered anelastic media where Q varies from one layer to the next in general will have maximum attenuation in a direction different from the direction of propagation. Lockett (1962) was first to recognize this, and several others—notably Buchen (1971), Borcherdt (1971, 1973, 1977, 1982), Krebes (1983, 1984), Richards (1984), and Wennerberg (1985)—explored and expanded the theory.

9-C time‐lapse VSP monitoring of steam injection at Cymric Oil Field

The heavy oil of Cymric Field in California’s southwest San Joaquin Basin has been produced by cyclic steaming for more than a decade, but recent efforts to optimize development and increase production have made it a top priority to know the spatial distribution of the steam. Wells are now being drilled very close together on 5/8-acre spacing. Serious problems occur when steam from one well reaches a neighboring well. For example, breakthrough can result in severe mechanical damage that can lead to abandonment of wells. The closer the wells, the more likely the problems, unless you know exactly where the steam is and adjust well locations accordingly.

Recording and processing vector wave field data: A review of the 1989 SEG Summer Research Workshop

The SEG Research Committee held its 1989 summer workshop on recording and processing vector wave field data August 13–17, 1989, in Snowbird, Utah. The workshop title proved puzzling to some, so a word of explanation may be in order. Seismic wave motion, although commonly treated as a scalar quantity, is a vector quantity because particle motions are properly represented as vectors. Three‐component detectors, but not single‐component, can faithfully record the vector motion. A vector wave field is the spatial distribution of such propagating vector waves in a rock volume. Seven half‐day workshop sessions addressed six questions.

Propagator matrix and layer stripping methods: A aomparison of shear‐wave birefringence detection on two data sets from railroad gap and Lost Hills fields

This paper compares two methods. the propagator matrix and layer stripping methods, for measuring shear-wave birefringence and its variation with depth. The comparison is done on two near offset S-wave VSPs from the southwest San Joaquin basin of California. The layer stripping method, which works from the surface by removing birefringence effects as soon as they change, is compared to the propagator matrix method, which works between two given depths without any need for information from the shallower depths. The two methods require S-wave VSPs obtained with two different horizontal source polarizations. The comparison is performed on data sets from the Railroad Gap and Lost Hills fields in California. The results of the two methods are in close agreement, especially when birefringence is large (3% to 7%). Interesting differences in results illustrate advantages and disadvantages of each method.

On: “Shear-wave splitting in cross-hole surveys: Modeling; discussion and reply

Liu et al. extended consideration of shear‐wave (S‐wave) polarization patterns in anisotropic media from the usual vertical to a predominantly horizontal direction of wave travel. Modeling was for transversely isotropic (TI) media with horizontal symmetry axes. The exposition was accurate in major concepts, but the authors could have been more precise in presenting a couple of incidental properties of TI media. These properties have to do with S‐wave polarizations and certain a priori predictions one can make about them from symmetry considerations. I state and prove two predictions here in a tutorial mode, partly to demonstrate the simplicity and power of symmetry concepts as tools for understanding wave behavior in anisotropic media.