Porosity-permeability changes induced by varying pore pressure and confining pressure in unconventional formations: analysis of core-scale hydro-mechanical data and simulations

Abstract:

Hydrological properties of deep subsurface strata can be strongly affected by the hydrodynamic history of that strata, especially the pore fluid pressure (P_p) history. The dynamic behavior of stress-dependent hydrological properties is fundamental to characterizing and forecasting fractures and associated permeability in unconventional reservoirs. Previous studies investigated the dependence of permeability on P_p and confining pressure (P_c) with effective stress concepts, but few studies focus on how the evolution of P_p and P_c may affect porosity. We assembled a constitutive model to quantify the effect of P_p and P_c on porosity and permeability in an emerging unconventional formation, the Cane Creek unit within the Pennsylvanian Paradox Formation, with typical matrix permeability < 0.1 mD. We especially addressed stress-dependent porosity and to what extent porosity change differs from permeability change under a range of stress- and hydrodynamic-histories.

The coupled linear poroelastic model and its governing equation is solved with both the finite difference method for fluid (brine) and the finite element model for stress and strain. This core-scale constitutive model is calibrated with experimental data. Preliminary results based on data collected from the literature indicate that porosity decreases as mechanical properties (elastic moduli and strength) increase under the static condition. We hypothesize that the dynamic behavior between porosity and stress might mimic observed (lab-measured) static hydro-mechanical correlations. The primary alteration mechanism of permeability may be explained by the associated alteration of pore surface area induced by interaction among fluid, matrix, and framework grains. One of the goals of our combined modeling and experimental studies is to elucidate the relative roles of fluid properties and rock properties, especially the time-scales for the different hydro- vs. mechanical-processes. Additionally, this study is intended to expand insight about the coupling between hydrodynamics and solid mechanics in the deep subsurface.