Corrections and Extensions to the GRI Technique: Dual Imbibition, NMR, and SEM Image Analysis

Abstract

Listen

Translate article

The GRI (Gas Research Institute) technique for characterizing samples of unconventional reservoir formations is performed on crushed material, and typically used to measure the total porosity of unconventional reservoirs and to estimate their permeability. The addition of water and oil imbibition, NMR (Nuclear Magnetic Resonance), and SEM (Scanning Electron Microscope) imaging and image analysis allows an extension of that technique for the quantification of in situ pore fluid salinity, mineral hosted (water wet) and organic-matter hosted porosity (oil wet), and corrections to total porosity for fractures generated during the sample preparation process. Comparisons between total porosity (GRI Boyle’s law) and imbibed water volumes, suggest that the crushed samples imbibe considerably more water than the measured total porosity. Detailed sample characterization (including ICP (Inductively Couple Plasma) on supernatant fluids after conductivity equilibration is observed, post-imbibition NMR, and image analysis) indicate that multiple corrections to imbibed volumes must be applied. These corrections include: the effect of dissolution pores created by anhydrite (salt) dissolution and corrections for fractures induced during sample preparation. Dissolution of anhydrite is indicated by post imbibition ICP analysis performed on the supernatant fluid. Porosity created during sample preparation (grain fractures internal to the individual particles) was observed in post-imbibition NMR results, and in SEM images. To remove this induced volume, a cutoff was defined for each sample. To be consistent this volume was selected using the inflection point on the cumulative imbibed volume curve. The proportion of induced fracture porosity was observed to be a function of carbonate content in both terrigenous argillaceous mudrocks and calcareous mudrocks. When these two corrections are applied, a comparison of pre-test Boyle’s Law measurements and imbibed water volumes results in relatively good agreement between the two measurements, although the imbibed water volume is less than the as received total porosity from Boyle’s Law. The difference between the Boyle’s Law and imbibed water porosity is interpreted to correspond to the organic matter hosted porosity. This result illustrates that water does not enter the organic matter hosted pores and suggests that at thermal maturities between 1.4-2.5% Ro, the organic pores remain oil wet. Oil imbibition performed on a separate aliquot of material, provided a measure of organic matter hosted porosity. When we add the corrected water imbibition and oil imbibition porosities together, there is good agreement between pre-test Boyle’s Law porosity and the imbibed volumes.