Geological Constraints on Calculating Water Saturations in Gas Bearing Mudstones

The petrophysical evaluation of shale gas requires estimates of both the free gas and the adsorbed gas components. The free gas component is often determined using a modified conventional reservoir approach, where the water saturation (Sw) in the mudstone, and hence the gas saturation, is calculated using one of several available shaly sand equations. These equations require various parameter inputs such as the Archie porosity and saturation exponents (m and n). We assess variations in these Archie parameters for the Haynesville and Bossier Shale Formations and apply them to the Indonesian method (Poupon & Leveaux 1971). A new method for categorizing formation data into smaller sub-groups has also been investigated based on analyzing formation response to specific suites of parameter values. The results, from systematically modifying these values, demonstrate an improvement in the correlation between core derived Sw (Dean Stark analysis) and computed Sw.

Initial results demonstrate that no single suite of parameters can adequately represent formation properties in their entirety, but that smaller identified sub-groups show a strong response to specific suites of exponent values, resulting in a near 1:1 correlation between calculated vs. core derived Sw. These smaller sub-groups show a strong correlation of increasing exponent values with decreasing gas saturation, kerogen volume, effective porosity and matrix permeability. They also indicate a link between Archie parameter value selection and formation structure, such that the exponent values tend to increase with increasing facies structure. For example, homogenous fabrics are associated with lower parameter values, whereas normally graded thin beds are associated with higher parameter values.

This hints at the possibility of developing a predictive capability in selecting exponent and Archie parameter values based on a geological facies association, establishing a link between formation depositional systems and their petrophysical properties in mudstones. It also outlines a revised model for the application of resistivity based methods to formation evaluation in mudstones.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California