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

Hydraulic fracturing: a review of theory and field experience

This report summarises the current state-of-the-art knowledge of the hydraulic fracturing process used by the shale gas/oil industry using open peer-reviewed literature and from government commissioned research reports. This report has been written to make statements on our knowledge of the following questions: • How do hydrofractures form? • How far do hydrofractures extend during stimulation? • What dictates where hydrofractures propagate? • How do hydrofractures interact with the existing fracture network? • Can the size and distribution of hydrofractures be controlled? Gaps in our knowledge have been highlighted, with the largest of these resulting from differences between North American and European shale rocks

Laboratory micro-seismic signature of shear faulting and fault slip in shale

This article reports the results of a triaxial deformation experiment conducted on a transversely isotropic shale specimen. This specimen was instrumented with ultrasonic transducers to monitor the evolution of the micro-seismic activity induced by shear faulting (triaxial failure) and subsequent fault slip at two different rates. The strain data demonstrate the anisotropy of the mechanical (quasi-static) compliance of the shale; the P-wave velocity data demonstrate the anisotropy of the elastic (dynamic) compliance of the shale. The spatio-temporal evolution of the micro-seismic activity suggests the development of two distinct but overlapping shear faults, a feature similar to relay ramps observed in large-scale structural geology. The shear faulting of the shale specimen appears quasi-aseismic, at least in the 0.5 MHz range of sensitivity of the ultrasonic transducers used in the experiment. Concomitantly, the rate of micro-seismic activity is strongly correlated with the imposed slip rate and the evolution of the axial stress. The moment tensor inversion of the focal mechanism of the high quality micro-seismic events recorded suggests a transition from a non-shear dominated to a shear dominated micro-seismic activity when the rock evolves from initial failure to larger and faster slip along the fault. The frictional behaviour of the shear faults highlights the possible interactions between small asperities and slow slip of a velocity-strengthening fault, which could be considered as a realistic experimental analogue of natural observations of non-volcanic tremors and (very) low-frequency earthquakes triggered by slow slip events

Thermal damage and pore pressure effects of the brittle-ductile transition in comiso limestone

Volcanic edifices are commonly unstable, with magmatic and non‐magmatic fluid circulation, and elevated temperature gradients having influence on the mechanical strength of edifice and basement rocks. We present new mechanical characterization of the Comiso limestone of the Mount Etna Volcano (Italy) basement to constrain the effects of regional ambient conditions associated with the volcanic system: the effects of pore fluid on rock strength and the effects of distal magmatic heating (~20 °C to 600 °C) at a range of simulated depths (0.2 to 2.0 km). The presence of water promotes ductile behaviour at shallow depths and causes a significant reduction in brittle rock strength compared to dry conditions. Thermal stressing, in which specimens were heated and cooled before mechanical testing at room temperature, has a variable effect for dry and saturated cases. In dry conditions, thermal stressing up to 450 °C homogenizes the strength of the specimen such that the majority of the specimens exhibit the same peak stress; at 600 °C, the brittle failure is promoted at lower differential stress. The presence of water in thermally‐stressed specimens promotes ductile behaviour and reduces peak strength. Acoustic emission monitoring suggests that accumulated damage is associated with the heating–cooling sequence, particularly in the 300–450‐600°C. Based on conduction modeling, we estimate this temperature range could affect basement rocks up to 300 m away from minor sheet intrusions and much further with larger bodies. Considering the dyke spacing beneath Etna, these conditions may apply to a significant percentage of the basement, promoting ductile behaviour at relatively shallow depths

Ongoing evolution of submarine canyon rockwalls; examples from the Whittard Canyon, Celtic Margin (NE Atlantic)

During the CODEMAP 2015 research expedition to the Whittard Canyon, Celtic Margin (NE Atlantic), a Remotely Operated Vehicle (ROV) gathered High Definition video footage of the canyon rockwalls at depths of approximately 412–4184 m below sea level. This dataset was supplemented by predominantly carbonate rock samples collected during the dives, which were subsequently tested for key physical property characteristics in a geotechnical laboratory. The high-resolution video footage revealed small-scale rockwall slope processes that would not have been visible if shipboard geophysical equipment was solely relied upon during the survey. Of particular interest was the apparent spalling failure of mudstone and chalk rockwalls, with fresh superficial “flaking” scars and an absence of sessile fauna possibly suggesting relatively recent mass-wasting activity. Extensive talus slopes, often consisting of coarse gravel, cobble and occasionally boulder-sized clasts, were observed at the foot of slopes impacted by spalling failures; this debris was rarely colonised by biological communities, which could be an indicator of frequent rockfall events. Bio-erosion was also noted on many of the walls prone to this form of rock slope failure (RSF). As in subaerial equivalents, internal fracture networks appear to control the prevalence of RSF and the geometries of blocks, often resulting in cubic and tabular blocks (0.2–1.0 m scale) of bedrock toppling or sliding out of the cliff face. Tensile strength parameters of carbonate rock samples were determined and these may affect the mass wasting processes observed within the canyon. It was found that carbonate samples which appeared to have a higher mud content, and reduced porosity, produced significantly higher tensile strength values. It is proposed that these stronger, “muddy” carbonate units form the overhanging ledges that often provide an ideal setting for sessile species, such as Acesta excavata clams, to colonise whereas the weaker “pure” carbonate units are more easily eroded and therefore form the undercutting, receding sections of the rockwall

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

Endommagement et fissuration du verre en compression triaxiale

L’objet de ce travail de recherche expérimental est de documenter l’évolution de l’endommagement du verre synthétique SON68 sous contraintes compressives triaxiales (hydrostatiques et déviatoriques). Le dispositif expérimental utilisé nous a permis de contrôler et faire varier indépendamment la pression de confinement (de 0 à 50 MPa) et la contrainte axiale (jusqu’à 680 MPa) à température ambiante. Un réseau de capteurs fixés sur chaque échantillon nous a permis de mesurer au cours des expériences : (i) la déformation axiale et radiale; (ii) les vitesses de propagation des ondes élastiques P et S; et (iii) les émissions acoustiques. De plus, nous avons introduit dans certains échantillons une population de fissures (jusqu’à une densité de 0,24) par choc thermique. Nous comparons les résultats d’endommagement sur le verre sain et les verres traités thermiquement à différentes températures. Les données obtenues en contrainte-déformation illustrent les propriétés linéaires élastiques du verre sain, même à la contrainte élevée de 680 MPa (pour une pression de confinement de 15 MPa). On observe une forte diminution à la résistance pour les verres traités thermiquement (370 MPa au maximum à la contrainte axiale pour 15 MPa de pression de confinement). L’évolution des vitesses des ondes élastiques montrent que la plupart des fissures sont fermées pour une pression d’environ 30 MPa. Dans tous les verres, sain ou pré-fissuré, l’endommagement était accompagné par la propagation dynamique des fissures, produisant ainsi des émissions acoustiques de grande énergie. Grâce à un enregistrement continu, nous avons pu localiser ces émissions acoustiques afin d’imager le schéma de microfissuration et son évolution avant la rupture macroscopique

A Novel Approach to the Laboratory Testing of Replica Discontinuities: 3D Printing Representative Morphologies

The preparation of identical synthetic samples with the same morphological and mechanical characteristics allows for repeatable and reliable testing of discontinuities under varying conditions. Studies on the behavior of replica discontinuities to date have mostly been undertaken on laboratory shearbox apparatus, allowing for the characterization of discontinuity mechanics at low stresses. This study presents a new methodology for creating representative replica discontinuities suitable for testing under triaxial conditions, allowing for characterization at elevated stresses and temperatures. The advancement of computer aided design (CAD), three dimensional (3D) scanning and 3D printing has been used to design and create 3D printed molds in acrylonitrile butadiene styrene (ABS) plastic. Synthetic materials are then cast in to the molds creating cylindrical samples with a pre-existing discontinuity of quantifiable morphology suitable for triaxial testing. Statistical and visual analyses show the morphological characteristics of the synthetic discontinuities to be highly repeatable. In addition, the mechanical behavior of different synthetic compositions in unconfined compressive strength and triaxial is compared to the behavior of natural lower strength sedimentary lithologies. The behavior of the tested synthetic materials is found to not be representative of lower strength sedimentary lithologies, with stress-strain behavior showing failure in a quasi-brittle manner

Meso-to-microscale fracture porosity in tight limestones, results of an integrated field and laboratory study

The complex fluid saturation distribution and influence of microscale and mesoscale fractures on the fluid accumulation and flow properties of carbonates are still interesting challenges for petroleum geologists. For this reason, in order to know the relative role played by the aforementioned features on the storage and migration properties of tight limestones, the present work focuses on a surface analogue cropping out in southern Italy. By first applying a deterministic Discrete Fracture Network (DFN) modelling to a 1 m3 geocellular volume, an amount of 0.3% of fracture porosity and a fracture connectivity configuration above the percolation threshold are computed. In addition to mesoscale fracture porosity, in order to gather information on matrix porosity and microscale fractures, we investigate the pore type, geometry, and textural anisotropy of selected rock plugs by mean of integrated petrophysical, ultrasonic, and optical microscopy analyses experiments. Results show values of connected porosity ranging between 1% and 6%, presence of vugs localized along pre-existing structural heterogeneities, and the predominance of stiff pores within the carbonate matrix. The rare microfractures are mainly oriented orthogonal to bedding. The estimated crack density (0.19) shows that the contribution of fracture porosity at microscale (ɸ = 0.078%) is very low if compared to that of matrix porosity and also a structural configuration above the percolation. The present study therefore documents that the carbonate matrix forms an isotropic medium, which profoundly affects the storage capability of the study limestones. In fact, the amount of storage provided by carbonate matrix and microscale fractures is greater than that due to mesoscale fractures. Moreover, we document that the matrix contribution on porosity is much more significant than the contribution provided by microfractures. Finally, the present work shows the importance of integrating different methodologies on the assessment of fracture porosity at different scales of observation. In fact, a great benefit for development and production operations can be obtained by performing studies of surface analogs, which allow the detailed investigation of rock masses below seismic resolution