Sample Size Effects on the Application of Mercury Injection Capillary Pressure for Determining the Storage Capacity of Tight Gas and Oil Shales

Sample Size Effects on the Application of Mercury Injection Capillary
Pressure for Determining the Storage Capacity of Tight Gas and Oil Shales

J.T. Comisky, Apache Corp., M. Santiago, B. McCollom, Poro-Labs Inc., A. Buddhala, University of Oklahoma,
K.E. Newsham, Apache Corp.

Abstract—We measured Mercury Injection Capillary Pressure (MICP) profiles on tight shale samples with a variety of sample sizes.
The goal was to optimize the rock preparation and data reduction workflow for determining the storage properties of the rock,
particularly porosity, from MICP measurements. The rock material was taken from a whole core in the Cretaceous Eagle
Ford Formation in the form of a puck or disc. A horizontal 1 inch core plug was cut from this disc and the remaining
material was subsequently crushed and sieved through various mesh sizes. MICP profiles up to 60,000 psia were then
measured on the 1 inch plug and all of the various crushed and sieved rock particle sizes. In parallel we subsampled the plug
material to measure bulk volume, grain volume, and porosity using a crushed rock helium pycnometry method. These
additional measurements provided a comparison set of standard petrophysical properties from which we could compare the
MICP results.

In general our MICP profiles show a very strong dependence on sample size due to two reasons: pore accessibility and
conformance. We verify the conformance correction approach of Bailey (2009) which specifically accounts for the pore
volume compression of the sample before mercury has been injected into the largest set of interconnected pore throats. This
new method is preferred over the traditional method of conformance correction when compared to crushed rock helium
porosity since the latter is performed at unstressed conditions. Our results using Bailey’s (2009) method reveals that the –
20+35 sample size is optimal for determining porosity in the Eagle Ford, and potentially other tight oil and gas shales.

We use mercury injection for determining the various storage properties of tight shale because helium porosimetry is not
always possible on fine cuttings samples. There are many instances when limited cuttings may be the only source of rock
information available before a whole core is taken. Cuttings profiles also provide a key insight over long formation intervals
that may not be available from whole core. Cuttings and core profiles for use in calibrating well logs have proven to be a
requirement in these ultra-low perm systems.