Sonic Logging Applications Formation porosity from

Sonic Logging

Applications • Formation porosity from compressional slowness. • Formation mechanical properties from shear slowness for the well's completion and to design the well fracturing program. • Correlation to surface seismic data • Development of synthetic seismograms • Cement-bond (CBL) and variable-density (VDL) logs for cased-hole cement evaluation. 2

Basic Sonic Measurement • Sonic logging tools measure the speed of sound or its inverse, the sound slowness, in the formation around the wellbore. 3

Basic Sonic Measurement • An acoustic pulse is sent into the formation • The time it takes to reach receiver is measured • One path is 2 ft longer than the other. • The extra time that sound takes to travel those 2 ft is used to determine rock’s acoustic velocity 4

Transit Time • Transit time is: The time it takes sound to travel from the transmitter to the receiver • Measured in s • It contains : TTmud 1+TTrock+TTmud 2 Transit time depends on the geometry of the tool. 5

Delta-T • Inverse of the rock’s acoustic velocity • The difference between two transit times is the time it takes for sound to travel through a known distance of rock • measured in s/m or s/ft 6

Delta-T : Single Transmitter Far Receiver • TTmud 1 & TTmud 2 cancel out of the equation • We are left with the time it takes to travel through 2 ft of rock TT 3 -TT 4 (2 ft) TTmud 2 Near Receiver TT 4, rock TT 3, rock Transmitter TTmud 1 TT 3 TT 4

Waveforms 8

Sonic Porosity Formula • Linear equation (Wyllie) • Quadratic equation (Raymer- Gardner –Hunt) 9

Tool Hardware 10

Monopole Transmitter • A monopole source (transmitter) is a transducer that generates an acoustic wave spread around the source in all directions when electrical power is applied to it, just like a loud speaker. Magnetostrictive transmitters A steered beam transmitter can steer the sound energy towards the receiver A magnetic field is applied to a nonmagnetized ferromagnetic material, which will consequently change its size. This change in size produces an acoustic wave

Sonic Receiver • A sonic receiver is a transducer that transforms the acoustic energy into an electrical signal, just like a microphone Piezoelectric Receiver The piezoelectric crystals generate a voltage when a force is applied to them as they are hit by an acoustic wave 12

Borehole Compensated Sonic • To remove certain borehole effects, we add another transmitter/Receiver set. • Now since these are identical (but upside down) we can take the average of the two • This is the BHC Delta-T or just DT

Borehole Compensated Sonic

DDBHC – Depth Derived Bore. Hole Compensated 15

Sonde-Tilt • TT’s, decreased and separated • Zone of investigation is larger than 2’ • Signal amplitude will be dramatically smaller. • Detection becomes difficult

Sonde-Eccentered • TT’s, decreased. • Signal amplitude will be dramatically smaller. • Detection becomes difficult.

Large Boreholes • TT’s, Increased. • Signal amplitude will be dramatically smaller. • Detection becomes difficult • 16 in is the max. hole size.