R. Wiley and J. C. Patchett
Amoco Production Research
ABSTRACT
Invasion can significantly affect the response and interpretation of density/thermal neutron porosity logs, especially in gas reservoirs. This problem is due in part to the difference in the depths of investigation of these two logging devices. As a result, typical standard gas corrections can have significant errors due to invasion, lithology, and gas effects to such an extent that pay can be missed. In this study, this problem was extensively examined by modeling density/thermal neutron porosity responses in numerous lithologies containing a variety of reservoirs fluids, including hydrocarbons. Modeling has shown that invasion can impact porosity and lithology determination in oil and gas reservoirs. Moreover, the predicted density/thermal neutron responses have been observed in well log data.
Modeling was accomplished by using a three-dimensional, three-group, three-region diffusion code for both the density tool and the thermal neutron porosity tool. The analytical code used to model the density log was benchmarked against published calibration data and Monte Carlo calculations, while the thermal neutron porosity code had been previously benchmarked against published data.
The depth of investigation of the density tool is considerably less than that of the thermal neutron porosity tool. The calculated depths of investigation in a gas-filled limestone reservoir range from approximately 4.3 inches at 2.5 p.u. to 4.9 inches at 40 p.u. for the density log, while the thermal neutron porosity log ranges from approximately 15.8 inches at 2.5 p.u. to 9.1 inches at 40 p.u. The disparity in the depths of investigation between these two logging devices can result in serious interpretation problems.
These interpretation problems can be particularly acute as manifested on the density/thermal neutron crossplot when invasion has progressed to the point where the flushed zone represents an infinite medium to the density tool, while the thermal neutron porosity device is still strongly influenced by the non-invaded zone. The effects of invasion on density/thermal neutron porosity interpretation were investigated by examining four interpretation techniques for gas-filled clean sandstone and shaly sandstone. A statistical comparison of these four gas-correction methods showed serious porosity errors dependent upon the invasion depth. A third order regression of the density/thermal neutron responses produced improved porosity determination in modeled clean sandstone and shaly sandstones.