Comparison and Limitation Analysis of Approaches for Porosity Evaluation from NMR and Three Porosity Logs in Low Permeability Gas Sands with Bad Borehole

Abstract In this paper, we compare four methods of calculating reservoir porosity in low permeability gas sandstones with bad borehole condition, they are neutron-density crossplot technology, DMR technology, piecewise scale methods from acoustic time logging and SMR technology. The principle and limitation of these methods are analysised. When the borehole condition is good, these four technologies are all available, but in the wells with bad borehole, the technology of calculating reservoir porosity by integrating interval transit time logging and NMR logging, the neutron-density crossplot are recommended. For the NMR data acquired from CMR tool, the NMR porosity must be calibrated from deep lateral resistivity or deep induction resistivity logging, but these technologies are still the most useful because the computation process are simple and the odd error is less than DMR technology. The piecewise scale methods is also not recommended because it will spend too much time for log analysts.