Sonic-magnetic resonance method: A sourceless porosity evaluation in gas-bearing reservoirs

For environmental reasons, there are times when the use of radioactive chem ical sources for density and neutron logging is not possible. The inability to use these logging tools seriously affects porosity determination in gas -bearing reservoirs. Several tools, such as the nuclear magnetic resonance (NMR) tool, the sonic tool, or a minitron-based tool, determine porosity wi thout using a radioactive source. These tools, however, are influenced by m any effects and, when used alone, cannot deliver an accurate gas-independen t porosity. A new methodology that combines sonic and NMR logs for improved porosity ev aluation in gas-bearing reservoirs is proposed. The first variant of the me thod uses the sonic compressional transit time and the total NMR porosity ( Phi (t.NMR)) to determine the total porosity, corrected for the gas effect, and the flushed-zone gas saturation. In this approach, a linear time-avera ged equation corrected for compaction is applied to the sonic compressional log. The simplicity of the solution, much like the previously published DM R1 Density-Magnetic Resonance interpretation Method, allows fast, easy comp utation and a complete error analysis to assess the quality of the results. In the second variant of the method, we show that the rigorous Gassman equa tion has a very similar response to the Raymer-Hunt-Gardner (RHG) equation for a water/gas mixture. This allows substitution of the complex Gassman eq uation by the much simpler RHG equation in the combined sonic-NMR (SMR) tec hnique to estimate total porosity and flushed-zone gas saturation in gas-be aring formations. Both techniques are successfully applied to an offshore gas well in Austral ia. In this well, the porosity in the well-compacted sands is in the 20 to 25 p.u. range and the compaction factor is approximately 0.77. The sonic-ma gnetic resonance results compared favorably to the established density-magn etic resonance results and also to core data. In another offshore gas well from the North Sea, the porosity in the highly uncompacted sands is in the 35 to 40 p.u. range, and the compaction factor is around 1.85. Both SMR tec hniques were able to produce a very good porosity estimate comparable to th at estimated from the: density-neutron logs.