Unconventional Applications of Archie: Does Geology Matter?
Hartigan, David; Lovell, Mike; Davies, Sarah
A significant challenge to the petrophysical evaluation of shale gas systems can be attributed to the conductivity behaviour of clay minerals and entrained clay bound waters. This is further compounded by centimeter to sub-millimeter vertical and lateral heterogeneity in formation composition and structure. Yet, despite significant variations in formation geological and thus petrophysical properties, we routinely rely on conventional resistivity methods for the determination of water saturation, and hence the free gas saturation, in gas bearing mudstones.
The application of resistivity based methods is the subject of continuing debate. There is often significant uncertainty, both in how they are applied and the saturation estimates they produce. This is partly a consequence of the view that "the quantification of the behavior of shale conductivity...has only limited geological significance" (Rider, 1986). Consequently there is a separation between our geological understanding of shale gas systems and the petrophysical rational and methods employed to evaluate them. In response to this uncertainty, petrophysicists are moving away from the use of more complex ‘shaly-sand‘ based evaluation techniques and returning to traditional Archie methods for answers. The Archie equation requires various parameter inputs such as porosity and saturation exponents (m and n), as well as values for connate fluid resistivity (Rw). Many of these parameters are difficult to determine in shale gas systems because obtaining a water sample, or carrying out laboratory experiments on recovered core is technically impractical.
Here we assess the geological implications and controls on variations in Archie parameters across three geological formations using well data spanning multiple basinal settings. The results, from systematically modifying these values to minimise the error between core derived Sw (Dean Stark analysis) and computed Sw, indicate links between sample structure and composition and inferred values for clay porosity (ΦClay) and clay bound water resistivity (RClay).
Observed results hint at the possibility of developing a predictive capability in selecting exponent and Archie parameter values based on geological facies association and log composition indicators (VClay), establishing a link between formation depositional systems and their petrophysical properties in mudstones.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013