DENSITY, NEUTRON & PULSED NEUTRON
MEASUREMENTS IN LAMINAE, PART II
In this second example of laminated beds, LVPM was used to model limestone laminae whose pores alternately contain oil and then montmorillonite. The oil + limestone pore size was fixed at 0.0001 cm and the montmorillonite + limestone pore size was fixed at a much larger value of 0.1 cm. Significant differences appear between the parallel and perpendicular laminar orientations.
The bed thicknesses of both types of laminae were varied from 0.01 cm to 10 cm. In the figures of this section, the far left always refers to mainly montmorillonite saturated limestone and the far right refers to mainly oil saturated limestone. Specifically, from left to center, the limestone + montmorillonite bed thickness is fixed at 10 cm. Then, from center to right, this bed thickness drops from 10 cm to 0.01 cm. For the limestone and oil, its bed thickness varies from 0.01 cm to 10 cm from left to center and then from center to right, it is fixed at 10 cm. Thus, at the center of all these figures, both bed thicknesses are equal to 10 cm.
Again in this section, homogeneous refers to the application of classic volumetric mixing rules governed by bed thickness weighting for homogeneous media with infinitesimal pore sizes. Heterogeneous again refers to the exclusive use of the transmission probability method within LVPM to compute all physical properties for both media with finite limestone pore sizes. LVPM more accurately details the propagation of neutrons and gamma rays in these heterogeneous media. Corrections need to be applied to the apparent density and neutron logging values to properly account for finite pore sizes and bed thickness effects.
In Figure 1, note that the homogeneous/classic bulk densities and the LVPM parallel orientation bulk densities are about the same whereas the LVPM perpendicular orientation bulk densities are much lower, by an average of 0.047 g/cc, than the other two. In this example, bulk density log readings in perpendicular and parallel laminated beds need to be increased to correct for the propagation of gamma rays in these laminae. For the parallel laminae these increases are equal to those expected from classic bed thickness weighting. However, for the perpendicular lamine, such corrections are much larger.
Increases in LVPM neutron porosity in laminated beds are observed in Figure 3 as the oil and limestone bed thickness increases; these increases continue as the montmorillonite and limestone bed thickness is reduced. These porosity values are derived from a proxy model based on the neutron slowing down length (Figure 6). In this example, LVPM neutron porosities for both the parallel and perpendicular cases are about the same and both are larger than those predicted from classic bed thickness weighting. These overly optimistic neutron porosity values must be decreased to improve porosity measurement accuracy.