Download Shear Acoustic Porosity in the North Sea

SHEAR ACOUSTIC POROSITY AND VELOCITY RATIO
ANALYSIS USING SONIC LWD IN THE DUNBAR FIELD
(UK NORTH SEA)
Paul Boonen, Jon Hill- PathFinder Energy Services
AFES 8th September 2004
Acoustic slowness has been used to compute the formation’s porosity since the
introduction of a weighted average transform, commonly referred to as the Wyllie
time-average equation in 1956. However, Wyllie and other compressional transforms
must be used carefully as they often require compaction correction and other
modifications to fit the local environment. In addition, because of the effect of gas on
the compressional wave, porosity from compressional is not accurate in gas sands.
The introduction of full waveform recording tools made a shear slowness available for
porosity computations. A shear slowness porosity is expected to be largely
independent of the fluid in the pore space since fluids do not support shear waves. A
shear porosity is therefore preferred in gas zones. Shear porosity is calculated from
Gassman’s shear modulus using the concept of critical porosity. At the theoretical
formation critical porosity, grain-to-grain contact is lost and the shear wave ceases to
exist.
Shear porosity is particularly useful in high angle wells where there is concern about
loss of sources, and in gas sands. A further application is in poorly consolidated
formations where the lesser sensitivity of the sonic measurement than density neutron
to washouts can enable a more continuous porosity measurement. The critical
porosity algorithm has been used widely with shear sonic over the last 2-3 years in the
North Sea in a range of formations and porosities. The results have consistently
compared well with density neutron and/or core porosity.