Dipole logging in cased boreholes

Journal Article · Mon Feb 01 04:00:00 EST 1993 · Journal of the Acoustical Society of America; (United States)

DOI:https://doi.org/10.1121/1.405430· OSTI ID:6231617

Schmitt, D P ^[1]

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1. Mobil Exploration and Producing Technical Center, Dallas, TX (United States)

Shear wave logging through casing can be challenging, especially when the surrounding formation is unconsolidated. This study identifies two possible causes of the problem using synthetic microseismograms, dispersion curves, signal processing scheme based on Prony's method, and frequency-dependent sensitivity coefficients with respect to the layers parameters. The first possible cause is eccentering of the tool when the cased hole is well bonded. The second possible cause is poor bonding, which is modeled by the presence of fluid annulus. An off-center dipole source located in a well-bonded cased hole generates a Stoneley wave that is poorly coupled to the surrounding formation and propagates at a velocity close to the compressional velocity of the borefluid. This Stoneley wave can obscure the formation shear wave signal associated with a slow (unconsolidated) formation. In a vertical well-bonded cased hole, shear wave logging slow (unconsolidated) formations can be reliably performed despite the leaky character of the flexural mode if the tool is well centralized, and a low-source frequency (around 2 kHz) is used. In a highly deviated or horizontal well, where the tool cannot be efficiently centralized, reliable measurements can still be obtained provided the directivities of the source and receivers are perpendicular to the direction of eccentricity. In poorly bonded case holes, a dipole tool, regardless of its position, excites supplementary low-velocity modes that are associated with vibrations of the steel casing and the presence of an external fluid annulus. The maximum excitation of these modes is located in the low-frequency region, similar to that of unconsolidated formation signals. When the cement is not bonded to the casing, depending on the thickness of the fluid annulus, an unconsolidated formation signal may be corrupted, regardless of the tool's position within the borehole. 19 refs., 22 figs., 2 tabs.

• OSTI ID: 6231617
• Journal Information: Journal of the Acoustical Society of America; (United States), Journal Name: Journal of the Acoustical Society of America; (United States) Vol. 93:2; ISSN 0001-4966; ISSN JASMAN
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
& Exploration
99 GENERAL AND MISCELLANEOUS
990200 -- Mathematics & Computers
ALLOYS
BOREHOLES
BUILDING MATERIALS
CAVITIES
CEMENTS
EVALUATION
IRON ALLOYS
IRON BASE ALLOYS
MATERIALS
MATHEMATICAL MODELS
SEISMIC WAVES
STEELS
WELL CASINGS
WELL LOGGING