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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.