Accurate Porosity in Gas Bearing Formations

Steps 1 start with an initial guess of porosity

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Unformatted text preview: t with an initial guess of porosity (PHITguess ) 2. Use Bound water from shell-1 and consider it constant across the depth of investigation (see Fig. 11) 3. Use oil volume from each shell, then interpolate and extrapolate the oil volume across radial distance with boundary condition (Vbound + Voil < PHITguess , Vgas > 0). (See Fig. 12) 4. Compute fluid density as a function of radial depth (see Fig. 13). Note that the gas density required for this computation is taken from pressure test measurements 5. Apply the density sensitivity function (derivative of the J-function) to the fluid density versus radial distance to compute the apparent fluid density as seen by the density measurement (see Fig. 14). 6. Compute a density porosity using this apparent fluid density and the density measurement (note to be consonant), the density measurement is filtered to the same vertical resolution of the NMR. 7. Iterate until initial guess of PHIT matches with computed porosity At the end of this iterative processing, we have an apparent fluid density and porosity computed using two tools investigating the same volume of rock. This compensates for any invasion effect but it is still a low vertical resolution result as the density measurement had to be filtered down to the vertical resolution of the NMR. The last step is to improve the vertical resolution. This is done using alpha processing performed not on the porosity but on the apparent fluid density. Neutron-density (N-D) crossplot porosity is combined with the density to compute an apparent fluid density. This apparent fluid density has the same vertical resolution as the density tool; it is not accurate due to invasion but it correlates with fluid density to a first order approximation. This is the high vertical resolution measurement for input to alpha processing. The low vertical resolution but accurate fluid density input is derived from the iterative processing of NMR and density. These two inputs are combined in a classic alpha processing technique: 1. N-D apparent fluid density is filtered down to the same vertical resolution as NMR apparent fluid density (Fig. 15, track 6: N-D fluid density non filtered in pink, N-D fluid density filtered in black, NMR fluid density in blue) 2. Alpha correction factor is computed from those two apparent fluid densities 3. The Alpha correction factor is applied to the non-filtered N-D apparent fluid density (Fig. 15, track 7: alpha corrected N-D fluid density in black) The alpha processing provides a fluid density matching the accuracy of the one computed from NMR but with the vertical resolution of the formation density. A final porosity is then computed from this fluid density and formation density. Track 8 in Fig. 15 displays the final computed porosity in green. There is now a very good agreement with the core porosity in all zones and the vertical resolution is the same as the formation density measurement. 4 CSUG/SPE 147305 This technique was tested on a few wel...
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