SUMMARY Water is known to induce weakening on the static mechanical properties of rocks. However,... more SUMMARY Water is known to induce weakening on the static mechanical properties of rocks. However, injection-based operations such as EGS or EOR are commonly monitored through seismic methods involving dynamic moduli. It is therefore important to understand and quantify the effect of water-weakening on dynamic properties. In this study, we performed water injection tests on microporous carbonate rocks (two chalks from the Mons Basin) with ultrasonic monitoring of P-wave velocity and attenuation in order to observe the evolution of the rock moduli with varying water saturation. Our experimental results were interpreted through (i) a classical patchy saturation or PS model and (ii) the same model coupled with water weakening effect through modulus reduction induced by surface energy decrease induced by water in the fluid–rock system, called the WW-PS model. We show that the WW-PS model can better fit the experimental data than the PS model for both selected chalks, but also the previou...
We investigated the impact of water weakening on the mechanical behavior of Obourg Chalk and Cipl... more We investigated the impact of water weakening on the mechanical behavior of Obourg Chalk and Ciply Chalk (Mons Basin, Belgium). Different mechanical tests were conducted to estimate the unconfined compressive strength (UCS), tensile strength, Young’s modulus, mechanical strength under triaxial loading, critical pressure, fracture toughness, cohesion, and internal friction coefficient on samples either dry or saturated with water or brine. This extensive dataset allowed us to calculate wet-to-dry ratios (WDR), i.e., the ratio between any property for a dry sample to that for the water-saturated sample. For both chalks, we found that water has a strong weakening effect with WDR ranging from 0.4 to 0.75. Ciply Chalk exhibits more water weakening than Obourg Chalk. The highest water weakening effect was obtained for UCS, critical pressure, and Young’s modulus. Weakening effects are still present in brine-saturated samples but their magnitude depends on the fluid composition. The mechani...
Knowledge of the paleo-stress distribution is crucial to understand the fracture set up and orien... more Knowledge of the paleo-stress distribution is crucial to understand the fracture set up and orientations during the tectonic evolution of a basin, and thus the corresponding fluid flow patterns in a reservoir. This study aims to predict the main stress orientations and evolution during the growth of a fold by using the limit analysis method. Fourteen different steps have been integrated as 2D cross sections from an early stage to an evolved stage of a schematic and balanced propagation fold. The stress evolution was followed during the time and burial of syn tectonic layers localized in front of the thrust. Numerical simulations were used to predict the occurrence and orientation of deformation bands, i.e., compaction and shear bands, by following the kinematic of a fault-propagation fold. The case study of the Sant-Corneli-Boixols anticline was selected, located in the South Central Pyrenees in the Tremp basin, to constrain the dimension of the starting models (or prototypes) used ...
A series of laboratory tests on back-saturated specimens of Opalinus Clay was conducted to invest... more A series of laboratory tests on back-saturated specimens of Opalinus Clay was conducted to investigate the pore pressure response during undrained isotropic compression and elastic/inelastic shearing. Pore pressure measurements conducted during undrained loading utilizing a standard triaxial stress path suggest that pore pressure changes are primarily controlled by the transversal isotropic elastic behavior at low compressive loads, and the tendency of the clay shale to dilate as the differential stress exceeds the dilatancy threshold. In addition, both, poroelastic properties (i.e., Skempton’s pore pressure parameter B) and the tendency of the clay shale to dilate depend strongly on the confining stress. The stress path utilized in standard triaxial tests may substantially differ from the stress path associated with drilling or tunneling, and its influence on the effective strength is to date poorly understood. An alternative stress path used in this study, however, does not reveal significant differences with respect to strength or pore pressure evolution during shearing.
Micritic limestones exhibit large variation of (1) sedimentary texture from mudstone to packstone... more Micritic limestones exhibit large variation of (1) sedimentary texture from mudstone to packstone, (2) facies composition and (3) petrophysical properties (porosity, acoustic velocity). Those heterogeneities imply a complex distribution of fluid flow properties and a complex petrophysical signature. In the Eastern Paris Basin, Late Jurassic micritic carbonate deposits constitute a main aquifer located directly above the Callovian-Oxfordian clay-rich formation studied by the French National Radioactive Waste Management Agency (Andra) as a potential host rock for a deep geological disposal of high level radioactive wastes. A precise understanding of the factors controlling the petrophysical properties within carbonate aquifers is thus essential for rock-typing studies and fluid flow modelling. The first objective of this study is to better characterize the vertical distribution of both petrographical characteristics (texture, facies and composition) and petrophysical properties (poros...
ABSTRACT The present study provides laboratory constrains on the in situ source of shear wave bir... more ABSTRACT The present study provides laboratory constrains on the in situ source of shear wave birefringence in siltstone and sandstone lithologies from the Taiwan Chelungpu Fault Drilling Project (TCDP). Bench top measurements at ambient pressure and temperature are performed across the end faces of vertically drilled samples while the polarization angle of the source and receiver is rotated from 0 to 360° in azimuth. Shear wave birefringence in siltstone is low (˜3.5%) and controlled by low angle (˜30°) bedding dip. The sandstone samples exhibit moderate (˜15%) shear wave anisotropy due to a network of vertical microcracks with normal about the bedding strike direction. As related studies infer that the microcracks responsible for shear wave birefringence in sandstone are also open at depth, our results suggest that anisotropy in borehole measurements can simply be accounted for by the observed siltstone and sandstone microstructures, i.e., without invoking larger scale objects, nor randomly oriented cracks/fractures subjected to triaxial stress state.
SUMMARY Water is known to induce weakening on the static mechanical properties of rocks. However,... more SUMMARY Water is known to induce weakening on the static mechanical properties of rocks. However, injection-based operations such as EGS or EOR are commonly monitored through seismic methods involving dynamic moduli. It is therefore important to understand and quantify the effect of water-weakening on dynamic properties. In this study, we performed water injection tests on microporous carbonate rocks (two chalks from the Mons Basin) with ultrasonic monitoring of P-wave velocity and attenuation in order to observe the evolution of the rock moduli with varying water saturation. Our experimental results were interpreted through (i) a classical patchy saturation or PS model and (ii) the same model coupled with water weakening effect through modulus reduction induced by surface energy decrease induced by water in the fluid–rock system, called the WW-PS model. We show that the WW-PS model can better fit the experimental data than the PS model for both selected chalks, but also the previou...
We investigated the impact of water weakening on the mechanical behavior of Obourg Chalk and Cipl... more We investigated the impact of water weakening on the mechanical behavior of Obourg Chalk and Ciply Chalk (Mons Basin, Belgium). Different mechanical tests were conducted to estimate the unconfined compressive strength (UCS), tensile strength, Young’s modulus, mechanical strength under triaxial loading, critical pressure, fracture toughness, cohesion, and internal friction coefficient on samples either dry or saturated with water or brine. This extensive dataset allowed us to calculate wet-to-dry ratios (WDR), i.e., the ratio between any property for a dry sample to that for the water-saturated sample. For both chalks, we found that water has a strong weakening effect with WDR ranging from 0.4 to 0.75. Ciply Chalk exhibits more water weakening than Obourg Chalk. The highest water weakening effect was obtained for UCS, critical pressure, and Young’s modulus. Weakening effects are still present in brine-saturated samples but their magnitude depends on the fluid composition. The mechani...
Knowledge of the paleo-stress distribution is crucial to understand the fracture set up and orien... more Knowledge of the paleo-stress distribution is crucial to understand the fracture set up and orientations during the tectonic evolution of a basin, and thus the corresponding fluid flow patterns in a reservoir. This study aims to predict the main stress orientations and evolution during the growth of a fold by using the limit analysis method. Fourteen different steps have been integrated as 2D cross sections from an early stage to an evolved stage of a schematic and balanced propagation fold. The stress evolution was followed during the time and burial of syn tectonic layers localized in front of the thrust. Numerical simulations were used to predict the occurrence and orientation of deformation bands, i.e., compaction and shear bands, by following the kinematic of a fault-propagation fold. The case study of the Sant-Corneli-Boixols anticline was selected, located in the South Central Pyrenees in the Tremp basin, to constrain the dimension of the starting models (or prototypes) used ...
A series of laboratory tests on back-saturated specimens of Opalinus Clay was conducted to invest... more A series of laboratory tests on back-saturated specimens of Opalinus Clay was conducted to investigate the pore pressure response during undrained isotropic compression and elastic/inelastic shearing. Pore pressure measurements conducted during undrained loading utilizing a standard triaxial stress path suggest that pore pressure changes are primarily controlled by the transversal isotropic elastic behavior at low compressive loads, and the tendency of the clay shale to dilate as the differential stress exceeds the dilatancy threshold. In addition, both, poroelastic properties (i.e., Skempton’s pore pressure parameter B) and the tendency of the clay shale to dilate depend strongly on the confining stress. The stress path utilized in standard triaxial tests may substantially differ from the stress path associated with drilling or tunneling, and its influence on the effective strength is to date poorly understood. An alternative stress path used in this study, however, does not reveal significant differences with respect to strength or pore pressure evolution during shearing.
Micritic limestones exhibit large variation of (1) sedimentary texture from mudstone to packstone... more Micritic limestones exhibit large variation of (1) sedimentary texture from mudstone to packstone, (2) facies composition and (3) petrophysical properties (porosity, acoustic velocity). Those heterogeneities imply a complex distribution of fluid flow properties and a complex petrophysical signature. In the Eastern Paris Basin, Late Jurassic micritic carbonate deposits constitute a main aquifer located directly above the Callovian-Oxfordian clay-rich formation studied by the French National Radioactive Waste Management Agency (Andra) as a potential host rock for a deep geological disposal of high level radioactive wastes. A precise understanding of the factors controlling the petrophysical properties within carbonate aquifers is thus essential for rock-typing studies and fluid flow modelling. The first objective of this study is to better characterize the vertical distribution of both petrographical characteristics (texture, facies and composition) and petrophysical properties (poros...
ABSTRACT The present study provides laboratory constrains on the in situ source of shear wave bir... more ABSTRACT The present study provides laboratory constrains on the in situ source of shear wave birefringence in siltstone and sandstone lithologies from the Taiwan Chelungpu Fault Drilling Project (TCDP). Bench top measurements at ambient pressure and temperature are performed across the end faces of vertically drilled samples while the polarization angle of the source and receiver is rotated from 0 to 360° in azimuth. Shear wave birefringence in siltstone is low (˜3.5%) and controlled by low angle (˜30°) bedding dip. The sandstone samples exhibit moderate (˜15%) shear wave anisotropy due to a network of vertical microcracks with normal about the bedding strike direction. As related studies infer that the microcracks responsible for shear wave birefringence in sandstone are also open at depth, our results suggest that anisotropy in borehole measurements can simply be accounted for by the observed siltstone and sandstone microstructures, i.e., without invoking larger scale objects, nor randomly oriented cracks/fractures subjected to triaxial stress state.
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