Applications of Fluorescent Dye Staining Techniques to Reservoir Studies of Tight Gas Sands, Mesaverde Group, Southwestern Colorado

Martha E. Cather, Norman R. Morrow, Ingrid Klich

Pore structure of tight gas sands, including sheet pores between grain boundaries and microporosity, is revealed in fine detail using the 1987 technique of Ruzyla and Jezek wherein epoxy-impregnated thin sections are surface-stained with fluorescent dye and observed under reflected fluorescent light. Successive grinding of stained surfaces showed that dye penetration averages 5-10 µm. This technique comes much closer than 30-µm thin sections providing the ideal of imaging pore structure as a two-dimensional mathematical slice.

This technique can be applied in three ways. (1) Improved thin-section porosity estimates. Pore spaces, including microporosity, are assigned values (in 25% increment) from 0% for mineral grains to 100% for empty voids. Values for partly occluded pores are estimated using either the amount of discrete pore space visible or, for areas of submicroscopic porosity, by comparing their brightness to that of open pores. Thin-section porosity estimates are within ±2% of gravimetric porosity measurements, an improvement over previous estimates from tight gas sands. (2) Pore quality evaluation. Pore quality is evaluated by using the ratio of equivalent pores (number of pores multiplied by their percent occlusion) to total pores counted. Pore quality correlates closely to BET surface area m asurements and structural information derived from desorption isotherms. (3) Investigation of changes in pore structure caused by increased overburden pressure. Permeability of tight gas sands is highly pressure sensitive; fluid flow is controlled by sheet pores of submicron size. Pressure sensitivity may be due to closure of sheet pores under pressure. Comparisons of sheet pore width in cores cured at confining pressures of 0 and 5,000 psi show that some pore closure exists in highly pressure-sensitive samples.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.