Pore compressibility of a rock could be an important geomechanical consideration for accurate estimation of shale fluid storage capacity. It is, however, typically dependent on the stress conditions, and its value in a subsurface formation could change due to migration and withdrawal of the fluids or changes in the stress field. The pore compressibility is ignored during the routine measurements of porosity. In many other cases, it is oversimplified and assumed to be a constant value. In addition, the pore volume is comparatively small in shale formations. Consequently, the adsorbed and absorbed fluid phases can take up a significant portion of that volume and, hence lead to its reduction. Hence, shale fluid storage capacity measurements that do not consider the sorbed-phase correction may lead to inaccurate analyses.
In this project, a laboratory analysis is performed using Iljik and Hasandong shales considering the pore compressibility and sorbed-phase corrections on the selected shale samples. The experimental method considers multiple-step helium gas uptake by a core plug sample under effective stress, Kang et al. (2011). Confining pressure is kept constant and change in pore compressibility is observed due to changes in pore pressure. Analysis of the pore compressibility data is based on Boyle's law as outlined by Santos and Akkutlu (2013). They have used a two-step pressure-volume data to obtain the compressibility as a constant. Here, for the South Korean samples, we extended the approach to multiple (typically five) pressure-step measurement to investigate the compressibility as a pressure-dependent coefficient under varying confining stress conditions. Next, mineralogy quantification and gas storage capacity measurements are performed on the samples using methane and carbon dioxide gases as the measurement fluid. Finally pressure pulse decay is analyzed for permeability as outlined in Akkutlu and Fathi (2012).
Five shale plug samples have previously been selected for the analysis. The pore compressibility data shows strong nonlinearity to the changes in effective stress. The pore volume and sorption parameters are predicted in the presence of this nonlinearity. The laboratory analysis shows that no strong relationship has been found between the compressibility and the sample mineralogy. Measured permeability is low indicating seal rock features.