The Connected Pore Space Paradigm: An Evaluation of the Effect of Crushing on Pore Volume and Structure
James Greene, NUTECH
The application of crushed rock analysis for unconventional formation evaluation has become standard in core analysis following its introduction for shale gas volumetrics by Luffel and Guidry (1992). Crushing is used to expedite the extraction, drying, and volumetric measurement processes. Critical assumptions of crushed rock analysis include: all pore space is interconnected, crushing should not create entry into any pores that previously were isolated, and the crushed particles are orders of magnitude larger than the representative pore space. The analytical procedures were established to provide reservoir rock and fluid properties, for which log interpretation methods could be developed to match the core and production results.
This study expands on the effect of crushing on core samples beyond the original Devonian shale scope of the Gas Research Institute, GRI, program. Mercury injection capillary pressure (MICP) measurements are incorporated to quantify volumetric and textural changes to the rock fabric from the crushing process. Changes in sample compressibility are also investigated to account for the removal of residual, low compressibility fluids. The objective is to understand potential fundamental changes to the rock to reconcile the crushed, cleaned ambient condition with stressed, subsurface conditions.
Fourteen core samples, at an average frequency of 18’, are selected to represent a variety of lithologies across a 200’ interval of the Wolfcamp A in the Delaware Basin. Each sample was split into three subsamples: one subsample remained intact, one subsample is coarsely crushed to +50-mesh, and the last is crushed and sieved to -20+35-mesh fraction to replicate the particle size common for many crushed rock protocols (Luffel, 1992). All subsamples were cleaned using a sequence of organic solvents and dried at 60°C to remove residual free fluid and interstitial clay bound water (Burger, 2014).
Certain facies showed a higher likelihood for pore alteration with dominant micro-scale pore features flattening, shifting, or re-distributing following the crushing and cleaning process. Mudstone samples experienced increases in compressible pore volume after crushing and extraction as total porosity converged towards GRI helium porosity. The results of this study provide characterization of the connected, effective pore volume using compressibility concepts and comparison to residual fluid volumes. The decision to crush, and the degree of crushing if so, should consider the representative pore sizes of each facies.