Measuring absolute adsorption in porous rocks using oscillatory motions of a spring-mass system

[Display omitted] •An oscillation-based method is presented for measuring gas stored in porous media.•Absolute adsorption (total gas stored) is obtained by subtracting added mass.•Excess adsorption is obtained from the masses of sorbing and non-sorbing gases.•Excess adsorption of propane in Niobrara shale indicated capillary condensation. We present an oscillation-based method to measure absolute adsorption, or total gas, contained in porous rocks without and with excess adsorption. Experiments conducted with a macroporous Berea sandstone sample in nitrogen where excess adsorption is negligible show that absolute adsorption can be obtained after the added mass of co-accelerated gas outside the sample is subtracted. In experiments conducted in propane with a crushed Niobrara shale sample with micro- and mesopores, absolute adsorption included significant excess adsorption. After subtracting both the added mass outside the sample and the gas that would be in the sample assuming no excess adsorption existed, estimated excess adsorption of propane is in good agreement with that projected based on capillary condensation of propane in the volume of mesopores.