Laboratory Measurement of Sorption Isotherm under Confining Stress with Pore Volume Effects
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AbstractFor unconventional gas resources such as coal and organic-rich shale, sorbed-phase is an important component of storage and transport calculations. Routine measurements of sorption are, however, performed separately from the porosity and permeability measurements. In this work a new gas storage measurement technique is proposed combining the porosity and sorption measurements. Because the measurement is done using core plug under confining stress, it allows investigating the storage capacity for varying effective stress and incorporating the storage data into a subsequent permeability measurement under the same conditions.During construction of the sorption isotherm in the laboratory using Boyles law setup and a volumetric method, at each pressure step, volume of the sorbed gas taken up by the sample reduces the pore volume of the sample. As a result, the initially determined pore volume at low pressure must be corrected at the beginning and at the end of the pressure step. Also known as Gibbs correction, this correction can be done relatively easily during the routine sorption measurements with the crushed samples; however, it is a challenging task with core plugs under confining stress because at each pressure step the pore volume could also change due pore compressibility. Our approach is based on a new analytical model of total gas storability developed to interpret multiple-step laboratory measured pressure data on a graphical domain where the parameter estimation can be done fast and accurately using a straight line. The approach considers both the compressibility and sorbed- phase effects on the pore volume and the sorption parameters.Experimental storage data of various shale and coal samples with varying total organic content and maturity is used to demonstrate applicability of the analytical method to the measurements. Our results show that the sorption measurements can be done with increased accuracy and relatively fast. The work is important for organic-rich sample characterization in the laboratory, and for gas-in-place and transport calculations.
