A density-based porosity derived from the attenuation of gamma rays created by inelastic scattering of fast neutrons has been developed1,2,3 for the Computalog PND-S pulsed neutron system. The utility of this cased-hole, density-based porosity is demonstrated in reservoir analysis of sand-shale sequences in western Canada, Texas gulf coast, and Montana. Applications (in combination with the cased-hole neutron porosity) include providing porosities where hole conditions make open-hole logging unviable, and in old wells where no modern porosity logs exist or the data is of questionable quality. Low porosity can be differentiated from gas-filled porosity with the use of this measurement in cased-hole prospecting.
In shaly sand analyses, the addition of this novel density measurement to the neutron porosity and SIGMA facilitates a more comprehensive description of the reservoir. LRHO, the pulsed neutron density parameter, and RATIO, the near-to-far thermal ratio, are shown as overlays to identify gas zones. The pulsed neutron density porosity IPHI, which is used in the log analysis examples, is generated by specifically characterizing LRHO for a well or field. A prototypical borehole compensated (BHC) algorithm, that generates neutron (AIPN) and density (AIPD) porosities characterized for specific casing sizes and lithologies, is also used in the log analysis examples. The measurement theory of the PN density is reviewed as it pertains to understanding the log examples.
The correlation with open-hole porosities is demonstrated in several of the log examples. Through application to some typical reservoir description and optimization problems, the log examples show the utility of these measurements in cased-hole reservoir analysis.