Ion-species in pore fluids with opposite effects on limestone fracturing

At ten percent of sedimentary rocks, limestones are common geo-energy reservoirs. Being highly soluble, limestones are prone to fluid-assisted deformation and their mechanical behaviour likely susceptible to fluid chemistry. In this study, we saturated limestone samples with 0.4 M MgSO4 or 0.4 M Na2SO4 CaCO3-saturated solutions (naturally present in many reservoirs) or a reference CaCO3-saturated solution for 1, 50 or 200 days prior to mechanical testing. Triaxial deformation tests were then performed at 7, 30, and 70 MPa of confining pressure room temperature, under drained conditions. Our results show that exposure to one different cation impacts the strength of this rock (up to of dry rock strength) and its failure dynamics, associated with different microstructural damage distribution. A 200 day exposure to MgSO4 promotes strengthening whilst similar exposure to Na2SO4 leads to weakening. We posit that these strength changes may be related to changes in surface charges on the mineral surfaces. More data on fluid–rock interaction will be key to fully understand fracture propagation in natural carbonate formations

Апеляційний перегляд постанов місцевого суду, винесених за розглядом скарг на постанову про порушення кримінальної справи

Досліджуються основні проблеми апеляційної перевірки правомірності порушення кримінальної справи.Исследованы основные проблемы апелляционной проверки правомерности возбуж­дения уголовного дела.The article is dedicated to the main problems of the appellate review of instituting pros­ecution

Spinal decompensation in degenerative lumbar scoliosis

Due to the aging population, degenerative scoliosis is a growing clinical problem. It is associated with back pain and radicular symptoms. The pathogenesis of degenerative scoliosis lies in degenerative changes of the spinal structures, such as the intervertebral disc, the facet joints and the vertebrae itself. Possibly muscle weakness also plays a role. However, it is not clear what exactly causes the decompensation to occur and what determines the direction of the curve. It is known that in the normal spine a pre-existing rotation exists at the thoracic level, but not at the lumbar level. In this retrospective study we have investigated if a predominant curve pattern can be found in degenerative scoliosis and whether symptoms are predominantly present at one side relative to the curve direction. The lumbar curves of 88 patients with degenerative scoliosis were analyzed and symptoms were recorded. It was found that curve direction depended significantly on the apical level of the curve. The majority of curves with an apex above L2 were convex to the right, whereas curves with an apex below L2 were more frequently convex to the left. This would indicate that also in degenerative scoliosis the innate curvature and rotational pattern of the spine plays a role in the direction of the curve. Unilateral symptoms were not coupled to the curve direction. It is believed that the symptoms are related to local and more specific degenerative changes besides the scoliotic curve itself

Ion-species in pore fluids with opposite effects on limestone fracturing

At ten percent of sedimentary rocks, limestones are common geo-energy reservoirs. Being highly soluble, limestones are prone to fluid-assisted deformation and their mechanical behaviour likely susceptible to fluid chemistry. In this study, we saturated limestone samples with 0.4 M MgSO4 or 0.4 M Na2SO4 CaCO3-saturated solutions (naturally present in many reservoirs) or a reference CaCO3-saturated solution for 1, 50 or 200 days prior to mechanical testing. Triaxial deformation tests were then performed at 7, 30, and 70 MPa of confining pressure room temperature, under drained conditions. Our results show that exposure to one different cation impacts the strength of this rock (up to [Formula presented] of dry rock strength) and its failure dynamics, associated with different microstructural damage distribution. A 200 day exposure to MgSO4 promotes strengthening whilst similar exposure to Na2SO4 leads to weakening. We posit that these strength changes may be related to changes in surface charges on the mineral surfaces. More data on fluid–rock interaction will be key to fully understand fracture propagation in natural carbonate formations.</p

Mechano-chemical interactions in sedimentary rocks in the context of CO2 storage: Weak acid, weak effects?

International audienceDue to the corrosive nature of dissolved CO2, the potential short or long term alteration of rock properties, represents a major issue in several sites where natural CO2 circulation is observed, as well as in reservoirs targeted for storage of anthropogenic CO2. To date, this has been primarily studied from a transport-chemical perspective, with laboratory evidence of microstructural modifications together with the consequences for flow properties. Compared to the transport-chemical aspects, the mechanical-chemical aspects have been less investigated, though it is to be expected that mechanical properties (e.g. elastic properties, failure parameters, and time-dependent mechanical behaviour) could potentially be affected in a similar manner to hydraulic parameters. Yet, since CO2 is a weak acid, the pH drop is expected to be moderate with a likely lower limit close to 4.0. The buffering of pH by calcite minerals present in most reservoirs targeted for storage may further limit the pH drop, as well as confining it to a localized rock volume around the injection well. This leads to the question of the magnitude and time/spatial scales of chemically-mediated mechanical processes during CO2 sequestration. The authors propose to address this issue by reviewing recent laboratory-based studies restricted to sedimentary rocks, namely: reservoir rocks (carbonate or sandstone), intact or fractured caprocks and fault rocks. Key findings include the following: 1. the short-term impact on the elastic and inelastic behaviour of intact caprocks remains limited; 2. shear strength weakening is likely to be respectively low and low-to-moderate for shale/clay-rich and anhydrite-rich faults, but without modifying slip stability in either case; 3. the largest impact is located within carbonate reservoirs, but with a broad range of reported responses depending on hydrodynamic conditions (closed or open) and on dissolution regime (uniform or channelling); and 4. creep experiments confirm that CO2-induced dissolution may enhance long-term compaction of carbonate reservoirs, but the magnitude of acceleration (varying from non-significant to 50 times) depends to a large extent on site-specific conditions (grain size, pH, temperature, effective stress state, etc.), which renders any direct extrapolation from laboratory to reservoir scale difficult. Finally, some directions for future research studies are discuss

Compaction creep of simulated anhydrite fault gouge by pressure solution: theory v. experiments and implications for fault sealing

The sealing and healing behaviour of faults filled with anhydrite gouge, by processes such as pressure solution, is of interest in relation both to the integrity of faults cutting geological storage systems sealed by anhydrite caprocks and to seismic events that may nucleate in anhydrite-bearing sequences, such as those present in the seismogenic zone beneath the Apennines. We have developed a detailed series of kinetic models for pressure solution in anhydrite fault gouge, allowing for dissolution, diffusion and precipitation control, to estimate the time scale on which such sealing and healing effects occur. We compare the models obtained with previously reported experimental data on compaction creep rates in simulated anhydrite fault gouge, tested under wet, upper crustal conditions. The results confirm earlier indications that compaction under these conditions likely occurs by diffusion-controlled pressure solution. Applying our most rigorous model for diffusion-controlled pressure solution, constrained by the fit to the experimental data, we infer that anhydrite fault sealing will occur in a few decades at most, which is rapid compared with both CO2 storage time scales and with the recurrence interval for seismicity in the Apennines

A high resolution interferometric method to measure local swelling due to CO2 exposure in coal and shale

We present an experimental method to study time-dependent, CO2-induced, local topography changes in mm-sized composite samples, plus results showing heterogeneous swelling of coal and shale on the nano- to micrometer scale. These results were obtained using high resolution interferometry measurements of sample topography, combined with a new type of experimental microfluidic device. This device is a custom-built pressure vessel, which can contain any impermeable sample type and can be placed under any microscope. The pressure vessel itself has been tested to handle pressures up to 100 bar at room temperature conditions. For the experiments reported here we used three sample types: i) epoxy and dolomite, ii) coal, epoxy and dolomite and iii) shale. These model systems (thicknesses between 2 and 10 mm) were exposed to pressurized CO2 (20–35 bars) and subsequently deformation over time was monitored with a white light interferometer. This provided a lateral spatial resolution of 979 nm and a vertical spatial resolution of 200 nm, i.e. sufficient resolution so that coal and shale constituents can be tracked individually. Within 72 h epoxy swells homogeneously up to 11 μm, coal swells 4 ± 1 μm and dolomite is unreactive with the dry CO2 injected here, and as such is used as a reference surface. The differential swelling of coal can be correlated in space with the macerals, where macerals with an initial higher topography swell more. The average or bulk swelling exhibits an approximate t½ relation, indicative of diffusion-controlled adsorption of CO2 on the organic matter. Measurements of the differential swelling of both shale samples enabled tracking of individual patches of organic matter within the shale (max. 20 × 20 μm). These patches exhibit finite swelling of on average 250 nm in 4 h (in the Pomeranian shale) and 850 μm in 20 h (in the Green River shale), where total swelling is assumed to be related to the volume of the patches of organic matter

Porosity controls the fracturing mode in rocks in unconfined compressive strength tests

Applied Geophysics and PetrophysicsPetroleum EngineeringApplied Geolog

Effect of a singular planar heterogeneity on tensile failure

Many rocks contain planar heterogeneities, in the form of open fractures, veins and/or stylolites, but scarce data exist on how strength and fracture pattern formation is affected by the presence of a singular planar heterogeneity in an otherwise uniform matrix. The mechanics of stylolite-bearing and/or fractured limestone is of interest to several engineering applications, from quarries to subsurface gas or geothermal reservoirs. We have performed Brazilian Disc tests on pre-fractured Indiana limestone samples and Treuchtlinger Marmor discs which contain cohesive stylolites, investigating Brazilian test Strength and the resulting fracture pattern. All experiments were filmed, and where possible analyzed with particle image velocimetry. When viewed in 2D, the planar discontinuity was set at different rotation angles compared to the principal loading direction, where perpendicular to the loading direction is defined as 0⁰. The results show that all samples are weaker than their intact counterparts. For the pre-fractured Indiana limestone, there is 10–75% angle-dependent weakening. However, in the samples with a stylolite, strength is weakened by 35–75%, independent of direction. Several new cracks appeared when fracturing a stylolite-sample, where the orientation is heavily influenced by the stylolite orientation. The fracture pattern and associated stress drops are more complex for high angles. In these samples always more than one fracture formed, whereas in pre-fractured samples usually only one new fracture formed. This suggests a potential for more permeability increase when hydrofracturing a stylolite-rich interval. Comparison with Finite Element Models indicates that this difference in fracture pattern is caused by the strength contrast between the anastomosing stylolite zone and the matrix material, leading to stress concentrations effects. This causes (micro-) fracture nucleation to occur locally, promotes fracture coalescence and fracture growth at lower overall sample-load conditions compared to intact samples.Applied Geophysics and PetrophysicsReservoir Engineerin