Cased Borehole Effect on Downhole Seismic Measurements
نویسنده
چکیده
Approximate and exact formulations are presented for the interaction of an incident wave with a cased borehole. In the approximate method, the borehole coupling theory is used to compute pressure in the fluid at a low frequency. The results are simple and explicit. They are useful in the study of cased borehole coupling and as well as borehole radiation. In the exact method, elastic potentials in each annulus are represented as a superposition of fundamental solutions to the Helmholtz equations. Continuity of displacements and stresses across layer boundaries are used to determine unknown coefficients. The global matrix method is employed to simultaneously compute these coefficients in individual layers. This method is advantageous over the ThomsonHaskell propagator matrix method in handling evanescent waves. Our results show that, in a cased borehole, the borehole effects on downhole seismic measurements are more significant than those in an open borehole, especially when the formation is soft and the casing is steel. For hard formations and frequency below 1 kHz, cased borehole influence on downhole geophone measurement is minimal, while at high frequencies, large discrepancies occur, especially at grazing incidence. For soft formations, both the pressure in the fluid and the solid displacement on the borehole wall show strong dependence on frequency and incidence angle, even at very low frequencies. Strong resonance occurs in the fluid for an SV incidence at angle 8 = cos-f3!CT where CT is the tube wave velocity in a cased borehole. This resonance is prominent even at a very high frequency and large incidence angle because the tube wave velocity is raised well above the formation shear velocity by the steel pipe. This behavior is very different from that in an open borehole. At a particular angle of incidence of a plane P wave, the pressure in the fluid is near zero at low frequencies. This angle is dependent on the casing thickness and can be computed exactly. In general the casing behaves like a shield in such a way that the amplitude of both pressure in the fluid and solid motion on the borehole wall are reduced compared to those in an open borehole. 138 Peng et a
منابع مشابه
Downhole Measurements in the AND-1B Borehole, ANDRILL McMurdo Ice Shelf Project, Antarctica
A comprehensive set of downhole measurements was collected in the AND-1B drillhole as part of the on-ice scientifi c programme defi ned for the McMurdo Ice Shelf (MIS) Project. Geophysical logs were recorded over two operation phases and consisted of calliper, temperature, fl uid conductivity, induction resistivity, magnetic susceptibility, natural gamma activity, acoustic televiewer, borehole ...
متن کاملSeismic noise emission induced by seismic waves
The in situ rock undergoing different kinds of deformation accumulates elastic energy that can be released spontaneously o r under artificial effect. Triggering of energy can also be performed by means of a dynamic excitation caused by elastic waves. W e carried out special field investigations with the use of seismic vibrators (Vibroseis-like sources) to study the effect of the energy release ...
متن کاملComprehensive downhole and core physical-property measurements at the AND-1B Drillsite, ANDRILL McMurdo Ice Shelf Project
As part of the ANDRILL McMurdo Ice Shelf (MIS) Project two comprehensive sets of geophysical data were collected on ice at the AND-1B drillsite. Whole-core physical properties were determined with high vertical resolution to a depth of 1285 mbsf. A multi-sensor-core-logger was used to determine bulk density, sonic velocity, magnetic susceptibility and electrical resistivity. After drilling, a s...
متن کاملBuilding 3D subsurface models conforming to seismic structural and stratigraphic features
Subsurface modeling from seismic and borehole data is important for reservoir prediction, geophysical exploration, and production. A reasonable model should honor borehole rock properties and conform to seismic structural and stratigraphic features. Such a subsurface model can be difficult to build in cases complicated by faults and unconformities. Automatic and semiautomatic methods have been ...
متن کامل3 D seismic image processing for subsurface modeling CWP
Subsurface modeling from seismic and borehole data is important for reservoir prediction, geophysical exploration and production. A reasonable model should honor borehole rock properties and conform to seismic structural and stratigraphic features. Such a subsurface model can be di cult to create in cases complicated by faults and unconformities. Automatic and semi-automatic methods have been p...
متن کامل