Evaluation of General Resistivity-Density Based Saturation in Thin Laminated Sand-Shale Sequences

Alshannaq, Shadi; Quirein, John
Halliburton, Houston, TX.

Thin laminated sand-shale sequences can occur in many kinds of depositional environments, such as fluvial point bars, deltas, deepwater submarine fans, and turbidites, and can trap significant amounts of hydrocarbons in the thin sandy beds. Evaluation with conventional low-resolution uniaxial resistivity tools of the electrically anisotropic sand-shale sequences may result in significant underestimates of the hydrocarbon pore volume in the reservoir. A triaxial multicomponent induction tool can be used to characterize the electrical anisotropy in terms of the horizontal and vertical components of the formation resistivity. An interpretation model can be applied to convert the measured horizontal and vertical resistivities into the sand resistivity and sand volume. With the proper analysis, this translates into an enhanced hydrocarbon recovery and optimized reservoir development.

This paper provides a complete rock model and a comprehensive workflow that takes into account all of the necessary stages to estimate water saturation in a thinly bedded sand-shale sequence where both laminated and dispersed shale types can exist. It is shown that the Thomas-Stieber model can be applied, in conjunction with an approximation of total porosity, to compute the sand total and effective porosity. This enables net-pay cutoffs to be applied, based on sand volume, porosity, and water saturation. Different clay-bound water sources will be assumed. If grain density information is available from sources such as downhole geochemical logging tools or wellsite energy-dispersive x-ray fluorescence, the process can be made more accurate by iteratively solving for porosity and saturation from the bulk density along with the horizontal and vertical resistivity measurements. This is accomplished by applying a robust and efficient nonlinear solver that was been built to take into account the possible dependence of the fluid density on the saturation, as well as the dependence of the saturation and cementation exponents (N and M) on the total porosity.

Several field examples are presented for both wireline and LWD measurements. Dispersed shale is interpreted using any of the Dual-Water, Simandoux, and Indonesia equations.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012