Refine
Has Fulltext
- no (3) (remove)
Year of publication
- 2021 (3) (remove)
Document Type
- Article (3)
Language
- English (3)
Is part of the Bibliography
- yes (3)
Keywords
Knowledge of pressure-dependent static and dynamic moduli of porous reservoir rocks is of key importance for evaluating geological setting of a reservoir in geo-energy applications. We examined experimentally the evolution of static and dynamic bulk moduli for porous Bentheim sandstone with increasing confining pressure up to about 190 MPa under dry and water-saturated conditions. The static bulk moduli (K-s) were estimated from stress-volumetric strain curves while dynamic bulk moduli (K-d) were derived from the changes in ultrasonic P- and S- wave velocities (similar to 1 MHz) along different traces, which were monitored simultaneously during the entire deformation. In conjunction with published data of other porous sandstones (Berea, Navajo and Weber sandstones), our results reveal that the ratio between dynamic and static bulk moduli (K-d/K-s) reduces rapidly from about 1.5 - 2.0 at ambient pressure to about 1.1 at high pressure under dry conditions and from about 2.0 - 4.0 to about 1.5 under water-saturated conditions, respectively. We interpret such a pressure-dependent reduction by closure of narrow (compliant) cracks, highlighting thatK(d)/K(s)is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space,K-d/K(s)is almost constant. The enhanced difference between dynamic and static bulk moduli under water saturation compared to dry conditions is possibly caused by high pore pressure that is locally maintained if measured using high-frequency ultrasonic wave velocities. In our experiments, the pressure dependence of dynamic bulk modulus of water-saturated Bentheim sandstone at effective pressures above 5 MPa can be roughly predicted by both the effective medium theory (Mori-Tanaka scheme) and the squirt-flow model. Static bulk moduli are found to be more sensitive to narrow cracks than dynamic bulk moduli for porous sandstones under dry and water-saturated conditions.
The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (p(c)) of 50-100 MPa, temperatures (T) between 25 and 200 degrees C and strain rates (epsilon) (over dot ) of 1 x-10(-3)-5 x-10(-6) -s(-1). Using a Paterson-type deformation apparatus, samples oriented at 0 degrees, 45 degrees and 90 degrees to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed p(c)-T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45 degrees and 90 degrees to bedding at p(c) = 100 MPa, strain is localized in discrete shear zones. Compressive strength (sigma(max)) increases with increasing pc, resulting in an internal friction coefficient of approximate to 0.31 for samples deformed at 45 degrees and 90 degrees to bedding, and approximate to 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and epsilon(over dot )do not significantly affect deformation behavior of dried samples. However, sigma(max) and Young's modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength sigma(max) and Young's modulus E at similar deformation conditions. Strength and Young's modulus of samples deformed 90 degrees and 45 degrees to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.