Quality-assured evaluation of effective porosity using fit-for-purpose estimates of clay-mineral volume fraction

Abstract

Reservoirs that contain dispersed clay minerals traditionally have been evaluated petrophysically using either the effective or the total porosity system. The major weakness of the former is its reliance on “shale” volume fraction (Vsh) as a clay-mineral indicator in the determination of effective porosity from well logs. Downhole clay-mineral indicators have usually delivered overestimates of fractional clay-mineral volume (Vcm) because they use as a reference nearby shale beds that are often assumed to comprise clay minerals exclusively, whereas those beds also include quartzitic silts and other detritus. For this reason, effective porosity is often underestimated significantly, and this shortfall transmits to computed hydrocarbons in place and thence to estimates of ultimate recovery. The problem is overcome here by using, as proxy groundtruths, core porosities that have been upscaled to match the spatial resolutions of porosity logs. Matrix and fluid properties are established over clean intervals in the usual way. Log-derived values of Vsh are tuned so that, on average, the resulting log-derived porosities match the corresponding core porosities over an evaluation interval. In this way, Vsh is rendered fit for purpose as an indicator of clay-mineral content Vcm for purposes of evaluating effective porosity. The method is conditioned to deliver a value of effective porosity that shows overall agreement with core porosity to within the limits of uncertainty of the laboratory measurements. This is achieved through function-, reservoir- and tool-specific Vsh reduction factors that can be applied to downhole estimates of clay-mineral content over uncored intervals of similar reservoir character. As expected, the reduction factors can also vary for different measurement conditions. The reduction factors lie in the range of 0.29–0.80, which means that in its raw form, log-derived Vsh can overestimate the clay-mineral content by more than a factor of three. This exposition constitutes a major product of this paper. The implementation of the reduction factors demonstrably improves the evaluation of effective porosity from density, density–neutron and sonic logs, an exercise that also becomes more consistent across different tool types, with substantial reductions in uncertainty. This outcome brings petrophysics much closer to a verifiable equivalence of the effective and total porosity systems for enhanced quality assurance and thence a greater confidence in petrophysically-sourced reserves estimates.