Accurate Porosity in Gas Bearing Formations

On a few wells where core data was available and is

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Unformatted text preview: ls where core data was available and is now used on all wells in the field. In order to perform this processing in a timely manner, the entire workflow was coded to a software program. Vertical resolution match filtering is handled automatically to ensure alpha processing is properly employed. This technique can also be applied when the well is drilled with water base mud. In that case the fluid invading the formation is water and this volume will still be properly measured by the NMR. Additionally the software has been coded with a switch to offer an option for both mud systems. Conclusion The examples seen in this paper highlight the complexity of porosity measurement in gas bearing formations and the importance to properly address the invasion effect. To achieve this, we need to combine tools having similar geometrical response. In the workflow detailed in this paper, this has been achieved by computing an NMR radial response similar to the density one using the multi depth of investigation capability of the NMR tool, and then handling the vertical resolution challenge by proper filtering and alpha processing technique. The final porosity computed indeed matches well with the core data. Acknowledgements We are grateful to Husky for the permission to use the data shown in this paper. We would like to thank the anonymous reviewers who helped to improve the paper’s content and readability. References Cao Minh C., Caroli E., and Sundararaman P., 2007. Estimation of Variable Fluids Mixture Density with 4D NMR Logging, SPE 109051. Freedman, R., Cao Minh, C., Gubelin, G., Freeman, J. J., McGinness, T., Terry, R. and Rawlence, D., 1998. Combining NMR and Density Logs for Petrophysical Analysis in Gas-Bearing Formations. SPWLA, 39th Annual Logging Symposium, Keystone, Colorado, United States. CSUG/SPE 147305 Fig. 1 - Gas reservoirs in the South China Sea, sand shale sequences with good porosity 5 6 Fig. 1 CSUG/SPE 147305 CSUG/SPE 147305 Fig. 2: Geometrical response of various tools Fig. 3: MR scanner antenna configuration and volume of measurements (shell) 7 8 Fig. 4: DMRP processing using the three NMR shells measurements CSUG/SPE 147305 CSUG/SPE 147305 Fig. 5: Signal to noise ratio as a function of stacking for the different shells Fig. 6: Fluid model for the simulation 9 10 Fig. 7: Modeled response of shell 1 for the fluid model described in Figure 7 Fig. 8: Modeled response of shell 8 for the fluid model described in Figure 7 CSUG/SPE 147305 CSUG/SPE 147305 Fig. 9: Expected NMR response profile versus depth of investigation Fig. 10: Initial guess of PHIT, bound water from shell-1 and oil volume from each shell Fig. 11: Extrapolation across radial distance with boundary limit (PHIT) 11 12 Fig. 12: Fluid density computation versus radial distance Fig. 13: Sensitivity function of density measurement (J-Der) CSUG/SPE 147305 CSUG/SPE 147305 Fig. 14: Alpha processing steps and final result 13...
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