Laboratory experiments simulating poroelastic stress changes associated with depletion and injection in low‐porosity sedimentary rocks

Abstract

We characterized the poroelastic deformation of six cores from three formations associated with the Bakken play in the Williston Basin (the Lodgepole, Middle Bakken, and Three Forks formations). All are low‐porosity, low‐permeability formations, but vary considerably in clay, kerogen, and carbonate content. The experimental program simulated reservoir stress changes associated with depletion and injection via cycling both the confining pressure (Pc) and pore pressure (Pp). We measured volumetric strain, derived the corresponding bulk modulus, and calculated the Biot coefficient (α). We found α, which generally ranges between 0.3 and 0.9, to vary systematically with Pc and Pp for each of the specimens tested. The effect of pore pressure on α is much larger at low simple effective stress (σ = Pc‐Pp) during depletion than injection. The α decreases with σ for all pore pressures. For the same Pc and Pp, the Biot coefficient is consistently higher during injection than during depletion. Given the observed variations of α with Pc and Pp, the modeling of reservoir stress changes using a constant α could be problematic as poroelastic stress changes during depletion and injection are not likely to follow the same path. Scanning electron microscope examination of microstructures suggests that the variations of the bulk modulus and the Biot coefficient can be attributed to the abundance of compliant components (pores, microcracks, clays, and organic matter) and how they are distributed throughout the rock matrix.

Comparison of stress‐dependent geophysical, hydraulic and mechanical properties of synthetic and natural sandstones for reservoir characterization and monitoring studies

“…This observation agrees with the theoretical approach of Berryman (1992), and the experimental data and modelling presented by Al-Wardy and Zimmerman (2004) for permeability in up to 10% clay-rich sandstones. Axial strains remain mostly below unity, in agreement with previous observations (e.g., Ma and Zoback 2017), showing very little variations with P diff in all cases but Syn-1. Radial strains vary significantly with the increasing stress for all samples except Syn-1; the dispersion derived by radial strains is translated into porosity as we can consider it a proxy of volumetric strain (at the relative small stress levels used in our experiments).…”

“…As expected, n values vary from one property to another (Ma and Zoback ). Theoretically, we can make a simple division of the analysed properties into two groups (Glubokovskikh and Gurevich ): scale‐dependent (permeability and resistivity), and scale‐independent (ultrasonic waves and strains) properties.…”

Coupled hydraulic and mechanical model of surface uplift due to mine water rebound: implications for mine water heating and cooling schemes

“…Deformation of saturated rock is controlled by the material-dependent properties of rock stiffness, confining stress and pore pressure (Ma & Zoback 2017), i.e. the effective stress.…”

Experimental Study of Fracture Propagation: The Application in Energy Mining

“…Previous studies on large scale hydraulic fracturing were mostly conducted on artificial cement samples with artificial materials such as cement mortar, concrete and Perspex [34,35]. Even though some studies used real natural rocks, most of the experiments used small cylindrical rock samples with 50-60 mm in diameter and 100-120 mm in height for hydraulic fracturing under a pseudo triaxial confining stress state [36][37][38]. The tensile and shear strengths of cement samples were much lower than that of granites.…”