Laboratory Measurement of Sorption Isotherm Under Confining Stress With Pore-Volume Effects

Abstract

Summary For 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 that combines the porosity and sorption measurements. Because the measurement is performed by use of core plugs 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 the construction of the sorption isotherm in the laboratory with the volumetric (gas expansion) method, at each pressure step, the sorbed gas taken up by the sample reduces the pore volume (PV) of the sample. As a result, the initially determined PV at low pressure must be corrected at the beginning and at the end of the pressure step. This correction can be performed 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 PV could also change as a result of pore compressibility. Our approach is based on a new analytical model of total gas storability developed to interpret the measured multiple-step pressure data on a graphical domain in which the storage-parameter estimation can be performed fast and accurately with a straight line. The approach considers both the compressibility and the sorbed-phase effects on the porosity and the sorption parameters. Experimental storage data of shale and coal samples with varying total organic content (TOC) and maturity are used to demonstrate the applicability of the analytical method to the measurements. The results show that the sorption measurements can be performed 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.