Laboratory Drying Procedures and the Permeability of Tight Sandstone Core

Abstract

Summary: Core analysis of identical, side-by-side samples of Rocky Mountain tight sandstone indicates that laboratory drying technique strongly affects the measured core permeabilities to gas. Samples dried to constant weight at 140°F [60°C] in a vacuum oven or in an unhumidified conventional laboratory oven have significantly higher permeabilities than identical samples dried to constant weight at 140°F [60°C] under 45% relative humidity. Drying the samples maintains the proper hydration states of swelling (smectite) clays present in "dry" gas-bearing tight sands in the ground. Drying in an unhumidified oven removes this hydration water, causes the clays in the flow paths to collapse, and increases the permeability of the core. This clay damage is irreversible. We recommend that Rocky Mountain tight sands be dried in a controlled-humidity chamber at 140°F [60°C] under 45% relative humidity until the samples reach stable weight. This drying technique will remove all free pore water while maintaining the proper hydration state of swelling clays. Cores dried with this procedure will reflect more accurately the properties of actual tight gas sand reservoirs.