Accurate Porosity in Gas Bearing Formations

Another possible explanation is that computing a

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Unformatted text preview: nother possible explanation is that computing a weighted average assumes that the gas effect is similar on neutron and density, but as these tools see different volumes of rock, this assumption is incorrect when invasion is variable. This paper illustrates the limitation of using neutron-density measurements to compute porosity and the necessity to include other measurements to reduce uncertainty. Consonant logging In order to accurately compute porosity and adequately correct for invasion, one needs to integrate measurements that respond to similar volumes of rock. Fig. 3 shows the geometrical responses of density, neutron and two NMR tools. The Combinable Magnetic Resonance (CMR) tool has a single depth of investigation (DOI) while the Magnetic Resonance Scanner (MR Scanner) has three depths of investigation. One can observe that 80% of the density information comes from the first four inches away from the borehole while this same volume represents only 15% of the neutron response. This explains why shallow invasion effects cannot be properly compensated when combining neutron and density measurements, any small variations in the invasion will have different effects on the density and neutron. NMR is another measurement which can be combined with density to compute porosity. Similar to neutron, NMR reads low in the presence of gas. This is a consequence of gas’ low hydrogen index (HI) and of its hydrogen polarization deficit due to the long longitudinal relaxation time (T1) and the limited acquisition wait time. A quantitative workflow called Density Magnetic Resonance Porosity (DMRP) combining density and NMR measurements was developed in 1998 (Freedman et al, 1998). NMR is acquired with short wait time to boost the gas effect (apparent porosity deficit) and total porosity is computed using a weighted average of density derived and apparent NMR porosity. The weights are computed from gas properties and NMR acquisition parameters. 2 CSUG/SPE 147305 Looking at the radial responses of the various measurements, a better match can be achieved with CMR and density than with neutron and density but it’s still insufficient to obtain a porosity measurement fully compensated for invasion. Very shallow invasion will affect the density and NMR responses differently and the use of a constant weight to compute the porosity will not fully compensate for invasion. The latest NMR generation tool (MR Scanner) can acquire simultaneous measurements at three depths of investigation from 1.5” - 4” and is similar to density DOI. In terms of DOI, combining this NMR tool with the density allows for consonant logging and therefore the possibility to properly compensate for invasion. This is unfortunately not true for the vertical resolution. MR Scanner vertical resolution is poorer than density resolution and will require additional analysis, as explained later in this paper. Porosity computation workflow The first approach attempted was to apply the DMRP workf...
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