Mercury Injection Capillary Pressure (Micp) A Useful Tool for Improved Understanding of Porosity and Matrix Permeability Distributions in Shale Reservoirs

Robert K. Olson and Murray W. Grigg
Kerogen Reosurces, Inc., Houston, TX

Those involved in the exploitation of shale gas reservoirs are acutely aware of the need for accurate porosity estimates. Most error associated with calculation of the free gas component of OGIP (original gas in place) is attributable to its over- or under-estimate.

Throughout the shale gas industry companies generally rely on porosity values derived from tight rock analysis of conventional and side wall cores. Those values are then used to calibrate porosity logs through shale reservoirs. The purpose of this paper is to demonstrate the utility of another laboratory method- Mercury Injection Capillary Pressure analysis (MICP). This tool provides data that are equally suitable for the calibration of porosity logs and has the added advantage that the analysis can be done on fresh or archived cuttings samples as well as core. This allows for gathering of porosity data where none were previously available.

MICP analysis is performed by placing a tarred sample in the instrument chamber which is then evacuated and flooded with mercury. Pressure on the mercury is incrementally increased forcing mercury through progressively smaller pore throats. By the end of the experiment (at 60,000 psia) pores accessible through throats as small as 36Å in diameter are intruded. The volume of mercury forced into the sample is equivalent to the volume of porosity accessed. Comparison of porosities derived by this method are in very good agreement with TRA porosities.

A recent study on over 2400 samples representing twenty-five shales from thirteen basins shows that shale porosity averages 3.90% and ranges from less than 1.00% to in excess of 10%. Lower Tertiary and Upper Cretaceous shales from the Gulf Coast and several Cretaceous shales from western basins in the US and Canada consistently exhibit higher porosities then Paleozoic shales.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas