Formation Of New Fractures Research Articles

Consideration of geomechanical effects in reservoir simulation for hydrocarbon field development

This paper presents a review of mathematical models and functionality for consideration of geomechanical effects during hydrocarbon field development provided by modern flow simulation software (reservoir simulators). The discussed features include variation of reservoir properties with changes in fluid pressure; modular and coupled approaches to joint simulation of fluid flow and geomechanical processes; and available options for special geomechanical effects – such as erosion or formation of new fractures and impact on characteristics of existing fractures. Possible directions for further developments in this area of reservoir simulations are underlined.

Earthquake‐Derived Seismic Velocity Changes During the 2018 Caldera Collapse of Kīlauea Volcano

AbstractThe 2018 Kīlauea caldera collapse produced extraordinary sequences of seismicity and deformation, with 62 episodic collapse events which significantly altered the landscape of the summit region. Despite decades of focused scientific studies at Kīlauea, detailed information about the internal structure of the volcano is limited. Recently developed techniques in seismic interferometry can be used to monitor the internal structure of an active volcano more directly by detecting subtle spatiotemporal changes in seismic wave velocity, but their utility relies on accurate interpretations of the underlying phenomena causing those velocity changes. Here, we retrospectively apply repeating‐earthquake‐based seismic interferometry to the 2018 Kīlauea eruption sequence. We find that seismic velocities changed over two distinct time scales: a sudden increase followed by a slower decrease in velocity in the hours following each collapse event, and a gradual, long‐term decrease in velocity over several weeks that ceased approximately 1 month prior to the end of the eruption. Modeling suggests that short‐term changes can be explained by magma reservoir pressurization which specifically closed vertical ring fractures. Long‐term changes are related to subsidence of the caldera and likely include the influence of inelastic strain from the formation of new fractures. These observations provide new insights into the evolution of Kīlauea during its progressive collapse and will inform future interpretations for near‐real‐time monitoring at hazardous volcanoes around the world using similar techniques, especially where a dominant fracture orientation is present.

Mechanical Properties of Lamellar Shale Considering the Effect of Rock Structure and Hydration from Macroscopic and Microscopic Points of View

It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.

Petrographic Record and Conditions of Expansive Hydration of Anhydrite in the Recent Weathering Zone at the Abandoned Dingwall Gypsum Quarry, Nova Scotia, Canada

In the Dingwall gypsum quarry in Nova Scotia, Canada, operating in 1933–1955, the bedrock anhydrite deposits of the Carboniferous Windsor Group have been uncovered from beneath the secondary gypsum beds of the extracted raw material. The anhydrite has been subjected to weathering undergoing hydration (gypsification), transforming into secondary gypsum due to contact with water of meteoric derivation. The ongoing gypsification is associated with a volume increase and deformation of the quarry bottom. The surface layer of the rocks is locally split from the substrate and raised, forming spectacular hydration relief. It shows numerous domes, ridges and tepee structures with empty internal chambers, some of which represent unique hydration caves (swelling caves, Quellungshöhlen). The petrographic structure of the weathering zone has been revealed by macro- and microscopic observations. It was recognized that gypsification commonly starts from a developing network of tiny fractures penetrating massive anhydrite. The gypsification advances from the fractures towards the interior of the anhydrite rocks, which are subdivided into blocks or nodules similar to corestones. Characteristic zones can be recognized at the contact of the anhydrite and the secondary gypsum: (1) massive and/or microporous anhydrite, (2) anhydrite penetrated by tiny gypsum veinlets separating the disturbed crystals and their fragments (commonly along cleavage planes), (3) gypsum with scattered anhydrite relics, and (4) secondary gypsum. The secondary gypsum crystals grow both by replacement and displacement, and also as cement. Displacive growth, evidenced by abundant deformation of the fragmented anhydrite crystals, is the direct cause of the volume increase. Crystallization pressure exerted by gypsum growth is thought to be the main factor generating volume increase and, consequently, also the formation of new fractures allowing water access to “fresh” massive anhydrite and thus accelerating its further hydration. The expansive hydration is taking place within temperature range from 0 to ~30 °C in which the solubility of gypsum is lower than that of anhydrite. In such conditions, dissolving anhydrite yields a solution supersaturated with gypsum and the dissolution of anhydrite is simultaneous with in situ replacive gypsum crystallization. Accompanying displacive growth leads to volume increase in the poorly confined environment of the weathering zone that is susceptible to upward expansion.

STRESS FIELD IN A SHEAR ZONE, AND FORMATION OF THE MAIN FAULT

Using the analytical approximation method, we calculated stress field parameters for cases with different relative positions of Riedel shears and loads required for shearing. Considering an internal friction angle of 30°, and the distance between adjacent shears exceeding 0.7 of the characteristic shear length, we estimated the Coulomb stress that can lead to fracturing. In the areas between the shears, it is below the shear strength value. This means that if an increase in the external load is lacking, there are no prerequisites for the formation of new fractures that may connect adjacent shears. If the shears are spaced closer to each other (i.e. at distances less than 0.7 of the shear length), the shear strength is exceeded in the areas between them, and new shears can occur there and connect the Riedel shears to each other. Therefore, in observations of a natural system of Riedel shears, it becomes possible to assess whether this system is sufficiently stable in its current status, or, in case of a critical increase in the Coulomb stress in the areas between adjacent shears, the equilibrium can be easily disturbed, and there is a possibility that the main fault forms in the strike-slip zone under study.

Influence of supercritical CO2-H2O-caprock interactions on the sealing capability of deep coal seam caprocks related to CO2 geological storage: A case study of the silty mudstone caprock of coal seam no. 3 in the Qinshui Basin, China

We conducted simulation experiments on supercritical carbon dioxide (ScCO2)-H2O-rock interactions for caprock samples obtained from the Qinshui Basin, China, to explore the influence of carbon dioxide (CO2) injection on the sealing capability of deep unminable coal seam caprocks as it relates to CO2 geological storage. This research focused on the changes in mineral composition, pore-fissure structure, permeability, and mechanical properties of the caprocks after ScCO2-H2O treatment. These results revealed that ScCO2-H2O-rock interactions lead to the formation of dissolution pores on the contact surface and a significant increase in macropores volumes and permeability, while the precipitation of secondary minerals can prevent CO2 from entering the rock in the later reaction stage. The sealing capacity of intact caprock initially decreases and then changes slightly after being affected by ScCO2-H2O-rock interactions. The caprock integrity determines the security of the CO2 geological storage. However, both natural and induced fractures provide channels for CO2 leakage. The compressive strength of the rock was reduced by 36 %, and the toughness increased after 60 days of ScCO2-H2O treatment. Thus, the formation of new fractures and the reopening of pre-existing faults in caprock may occur before damage under stress changes and ScCO2-H2O-rock interactions. Fracturing and shedding of fault gouge and particles inside the fractures by high-pressure ScCO2-H2O fluid can expand the fractures in caprock significantly, and hence increase the risk of CO2 leakage. Therefore, deep coal seams with thick and intact caprock should be selected to increase the safety of CO2 geological storage. Furthermore, the CO2 injection pressure must be considered to prevent the formation of additional fractures and occurrence of rock failure resulting from the softening effect of long-term ScCO2-H2O-interaction.

Effects of fault transmissivity on the potential of fault reactivation and induced seismicity: Implications for understanding induced seismicity at Pohang EGS

Enhanced Geothermal Systems (EGS) involve hydraulic stimulation of the permeability of deep low-permeable rock formations. This causes the reactivation and opening of pre-existing natural fracture networks and the formation of new fractures. During hydraulic stimulation, injection pressures at the bottom of the injection well can reach overpressures of up to several tens of MPa. The associated rise in reservoir pressures may trigger felt induced seismicity, as large-scale critically stressed fault structures can be reactivated. We here employ a 3D hydro-mechanical model coupling the software codes of TOUGHREACT and FLAC3D and combine it with Dieterich’s formulation for the rate of earthquake nucleation, to create a conceptual model to simulate the effect of stimulation activities on fault Coulomb stressing and associated induced seismicity rates. We discuss the effect of the hydromechanical properties such as fault and damage zone transmissivity and elastic properties on the relative contribution of pore pressure diffusion versus poroelasticity to fault loading. Our modelling approach shows that poroelastic effects can significantly contribute to fault loading, specifically in cases of low fault transmissivity. In this context, we discuss the potential contribution of poroelasticity to the occurrence of seismicity on a previously unmapped sealing fault associated to hydraulic stimulation at the Pohang EGS site in the Southeast of Korea. Our study demonstrates that a quantitative understanding of the stress response and induced seismicity upon injection operations such as the hydraulic stimulation at Pohang requires the incorporation of both pore pressure diffusion and poroelastic effects.

Characterization of ice shelf fracture features using ICESat-2 – A case study over the Amery Ice Shelf

Fractures are important structural features that affect the stress condition and stability of ice shelves. Previous studies have mainly focused on the measurement of fractures in the horizontal dimension. However, the vertical morphology of fractures could also be potentially important in determining their evolution and role in ice shelf stability. In this regard, the dense and high-resolution surface elevation measurements collected by the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) provide an excellent opportunity for studying fractures in the vertical dimension over a regional scale. Here we developed an object-oriented algorithm to automatically detect and characterize fracture features (including rifts, surface fractures, and the surface expressions of basal fractures) from ICESat-2 data. We successfully applied the algorithm to ICESat-2 L3A Land Ice Height (ATL06) data over the Amery Ice Shelf in East Antarctica, and retrieved the vertical structural information of two major fracture fields. The detected fracture features match well with the visible fracture lines in Landsat-8 satellite imagery, and the edges of fracture features are well-captured from the ATL06 data. We analyzed the fracture patterns in terms of surface depth, width, vertical shape, length, and orientation. We found that the fracture feature depth is a key factor determining the formation of new fractures and the spatial pattern of depth is closely related to rift formation. We anticipate that further application of the object-oriented method over other ice shelves will produce important insights regarding fracture formation and ice shelf vulnerability by combining ICESat-2-derived vertical information with the horizontal structure retrieved from satellite imagery.

In situ observation of helium and argon release during fluid-pressure-triggered rock deformation

Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale.

Hydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems

The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS.

Investigation on the Influence of Water-Shale Interaction on Stress Sensitivity of Organic-Rich Shale

Shale reservoirs are characterized by low permeability and natural fractures. In the process of reservoir development, the working fluid enters the reservoir. This may result in the formation of new fractures or expansion of natural fractures. When shale reservoirs are exploited, the fluid pressure in the fracture or pore is reduced. This destroys the stress balance of the reservoir, produces stress sensitivity damage, and reduces the reservoir permeability. Organic-rich shale from the Yanchang Formation, Chang 7 Member of the Ordos Basin, was selected for core flow experiment with helium. The effects of the type of brine, salinity, and soaking time on the stress sensitivity of an organic-rich shale reservoir were investigated. The acoustic characteristics were also investigated to study the effect of interactions between water and shale on stress sensitivity. The experimental results demonstrate that the interactions of water and shale increase the permeability of shale and reduce its stress sensitivity. Furthermore, when the permeability of the shale is excessively low, the stress sensitivity is high. In the acoustic studies, a higher attenuation coefficient of the acoustic wave corresponds to a larger variation in the shale structure and thus a larger permeability of the shale and smaller stress sensitivity coefficient. Whereas there is no apparent effect of the salt water type on the stress sensitivity, higher salinity levels cause higher stress sensitivity. After reacting with 15000 mg/L brine, the stress sensitivity coefficient of shale did not decrease significantly compared with that before action, all of which were above 0.97. However, after reacting with distilled water or 5000 mg/L brine, the stress sensitivity coefficient of shale decreased significantly, and all of them decreased to less than 0.9. Longer water exposures, corresponding to an increased duration of water-shale interactions, result in higher impacts on the stress sensitivity of shale. After 6 hours of shale-brine interaction, the stress sensitivity coefficient of shale is as high as 0.93, while after 48 hours of shale-brine interaction, the stress sensitivity coefficient of shale is reduced to 0.88. This study provides a highly effective reference with regard to the influence of the working fluid on the reservoir during drilling operations and the study of reservoir characteristics after fracturing.

Effects of Mobilization Time on Occurrence of New Fractures after Vertebroplasty.

Introduction Osteoporotic vertebral fracture treatment options include vertebroplasty, in which development of new fractures is among the possible complications which may develop during the postoperative period. We aim to evaluate whether or not postoperative mobilization time has effect on occurrence of new fractures. Materials and Methods A total of 126 patients, consisting of 30 (39.7%) males and 96 (60.3%) females, who underwent sedation-assisted vertebroplasty under local anesthesia between January 2014 and June 2017 were retrospectively evaluated. Preoperative and postoperative visual analogue scores (VASs) and mobilization time (hours) were assessed. Day of new fracture occurrence during follow-up was assessed. Results The mean follow-up period was 9 months (7–13 months). The most common fracture segment was the L1 vertebra (15.9%). The preoperative VAS was 8.29 ± 0.95, and the postoperative VAS was 2.33 ± 0.91. The change in VAS was statistically significant (p=0.01, p < 0.05). Of all the patients, 21 (16.66%) had developed new fractures. No statistical difference was observed between mobilization time (hours) and formation of new fractures (p=0.48, p > 0.05). Conclusion We came to the conclusion that mobilization time (hours) was not a risk factor in the development of new fractures. In addition, there is no relationship between mobilization time and localization of new fractures.

Effects of hydration on fractures and shale permeability under different confining pressures: An experimental study

Water-rich fracturing fluids are injected into gas shale formations to promote the effective development of gas. However, the relationship between gas production decreases and the variability flow-back fluid behavior (rate and amount) remains a significant challenge to the industry. This study focused on the effects of water hydration on the rock fracture properties of a major gas shale formation in China. Shale from hydration experiments was characterized for a number of key parameters including length, volume, width, surface area, thickness and fracture frequency. X-ray computed tomography (CT) scanning was conducted on shale reservoir samples exposed to water at zero overburden stress and under three different confining pressures (725 psi, 2175 psi, and 2900 psi). CT scanning images were obtained to evaluate fracture evolutions during hydration for both the connected and unconnected fracture networks. In addition, permeabilities before and after hydration were also quantified. Results from the ambient condition experiment revealed the propagation of fractures, the formation of new fractures, and the formation of better connectivity caused by a more complicated fracture network. However, the fracture evolution of the other three samples under confining pressures are markedly different, including: more unconnected fracture networks, less fracture volume, and a poorer connectivity of fractures.

Numerical Models for Hydraulic Refracturing on Vertical Oil Wells

The paper presents an approach for simulation of refracturing treatment on vertical oil wells. The model is accounting for filtration of hydraulic fracturing fluid through the proppant packed inside the crack formed during previous hydraulic fracturing treatments. The simulations provide a possibility to estimate history of stress intensity factor appearing at the tip of the existing crack once the time profile of pressure within the wellbore is given. Introducing critical value of the stress intensity factor for the fractured media, time-to-fracture initiation (after pressure increase start) can be estimated and compared to instance of fracture event registered in real conditions. Also, the possibility of fracture reorientation through formation of new fractures at the region adjacent to the wellbore is studied.

Coupled thermal−gas−mechanical (TGM) model of tight sandstone gas wells

The mechanism of CO2 fracturing under coupled thermal−gas−mechanical (TGM) conditions is important for gas production in tight sandstone gas wells. In this study, a coupled TGM model was proposed on the basis of the elastic damage principal, and the coupling relationships were expressed by the governing equations. The proposed model was then tested through comparison between the numerical uniaxial compressive test and the laboratory test. COMSOL MULTIPHYSICS was adopted to calculate the coupled numerical model; the impact of injection rate and injection gas temperature was fully investigated. The research results showed that the initiation pressure stayed stable with the increase of the breakdown pressure and injection rate over a certain range (0.106 m3 s−1–0.848 m3 s−1). The seepage area under the injection rate of 0.848 m3 s−1 was 3.45 times that under the injection rate of 0.106 m3 s−1, which resulted from the gradual destruction of the samples and the formation of new fractures at higher injection rate. The initiation pressure and the breakdown pressure decreased, since tensile cracks tended to form with the increase of injection temperature. Shear cracks formed around the hole—the crush area. The tensile cracks propagated much deeper into the surrounding rock under the higher injection temperature. This research provides a study of unconventional oil and gas extraction for reference.

Fault geometry and fluid-rock reaction: Combined controls on mineralization in the Xinli gold deposit, Jiaodong Peninsula, China

The structures and fluid-rock reaction in the Xinli gold deposit, Jiaodong Peninsula, were investigated to further understand their combined controls on the development of permeability associated with ore-forming fluid migration. Orebodies in this deposit are hosted by the moderately SE-to S-dipping Sanshandao–Cangshang fault (SCF). Variations in both dip direction and dip angle along the SCF plane produced fault bends, which controlled the fluid accumulation and ore-shoot formation. Gold mineralizations occurred in early gold–quartz–pyrite and late gold–quartz–polymetallic sulphide stages following pervasive sericitization and silicification alterations. Theoretical calculation indicates that sericitization caused 8–57% volume decrease resulting in the development/enlargement of voids, further increase of grain-scale permeability, and resultant precipitation of the early gold–quartz–pyrite pods, which destroyed permeability. The rock softening produced by alterations promoted activities of SCF secondary faults and formation of new fractures, which rebuilt the permeability and controlled the late gold–quartz–polymetallic sulfide veins. Quantitative studies on permeability distributions show that the southwestern and northeastern bend areas with similar alteration and mineralization have persistent and anti-persistent permeability networks, respectively. These were likely caused by different processes of rebuilding permeability due to different stress states resulting from changes in fault geometry.

Controls of hydrocarbon generation on the development of expulsion fractures in organic-rich shale: Based on the Paleogene Shahejie Formation in the Jiyang Depression, Bohai Bay Basin, East China

The development of expulsion fractures in organic-rich shale is closely related to hydrocarbon generation and expulsion from kerogen. Organic-rich shales from the upper part of the fourth member and the lower part of the third member of the Paleogene Shahejie Formation in the Jiyang Depression, Bohai Bay Basin, East China, are used as an example. Based on thin sections, SEM and thermal simulation experiments, the characteristics of hydrocarbon generation and the conditions supporting the development of expulsion fractures were explored. The key factors influencing these fractures include the presence of kerogens, their distribution along laminae and around particle boundaries, their exposure to heat and the build-up in pressure due to confinement by low permeability. The development of excess pore fluid pressures and intrinsic low rock fracture strength are the main influencing factors. Pressurization by rapid generation of hydrocarbon provides impetus for fracture initiation and cause bitumen to migrate quickly. The shale laminae results in distinctly lower fracture strength laminae-parallel than laminae-normal and this directs the formation of new fractures in the direction of weakness. When pore fluid pressure increases, maximum and minimum principal effective stresses decrease by different proportions with a larger reduction in the maximum principal effective stress. This increases the deviatoric stress and reduces the mean stress, thus driving the rock towards failure. Moreover, the tabular shape of the kerogen aids the generation of hydrocarbon and the initiation of expulsion fractures from the tip and edge. The resulting fractures extend along the laminae when the tensile strength is lower in the vertical direction than in the horizontal direction. Particle contact boundaries are weak and allow fractures to expand around particles and to curve as the stress/strength regime changes. When pore fluid pressure fields at different fracture tips overlap, fractures will propagate and interconnect, forming a network. This paper could provide us more detailed understanding of the forming processes of expulsion fractures and better comprehension about hydrocarbon expulsion (primary migration) in source rocks.

Experimental investigation on non-planar fractures mechanisms in hydraulic fracturing

The deflection of initial fractures and the formation of new fractures as non-planar fractures were investigated, which both are possible mechanisms for production improvement in refracturing or SRV(Stimulated reservoir volume). Firstly, initial fractures were formed under initial tri-axial stresses loading. Secondly, the formation and development of new fractures from refracturing were stimulated with changing horizontal stresses. The results show that (1) During refracturing new fractures were formed, and the initial fractures were re-opened, initiated and deflected. Whlie most of the deflection was identified as tensile and shear failure. (2) New fractures do not always result from the stress field change. This implied the smaller horizontal stress difference was, the easier the initial fractures were deflected, and the larger horizontal stress difference was, the easier the new single-wing fractures were formed on the walls of the initial fractures; (3) Deflection of initial fractures was more easier than formation of new fractures. The study indicated that, the injection pressure of deflection was lower than the initial breakdown pressure, while the formation of new fractures required higher pressure than initial breakdown pressure. (4) The injection rate contributed to a significant effect on the formation of new fractures. When injection rate was low, the formation of new fractures on the walls of the initial fractures was difficult. But when injection rate was high, new single-wing fractures formation was easily occurred on the walls of the initial fractures.

Application of vacuum impregnation with anti-freezing proteins to improve the quality of truffles

Vacuum impregnation (VI) with anti-freezing proteins was studied with the aim to minimize the loss in consistency occurring on truffles after thawing. Particularly, vacuum level, vacuum time and the relaxation time were modulated by using a Box-Behnken design. The results showed a significant weight increase of truffle samples in the range between 0.02 and 0.29 g/g proving that the impregnation occurred in all experimental conditions although the truffles showed a low porosity fraction (~6.33 %). After thawing, the samples submitted to pressure values lower than 150 mbar and vacuum times greater than 6.5 min, showed a consistency similar to the fresh samples, stating that the application of VI with a solution of anti-freezing proteins can be a useful method to reduce the loss in consistency probably caused by ice crystal formation. Moreover, after VI the samples showed an increase of porosity fraction probably due to the formation of new fractures as a consequence of the high rigidity of truffles. This phenomenon probably increased the absorption of anti-freezing proteins as a consequence of the improvement of solid–liquid surface contact area.

Brittle failure and fracture reactivation in sandstone by fluid injection

We performed laboratory experiments on sandstone specimens to study brittle failure and the reactivation of an experimentally produced failure plane induced by pore-pressure perturbations using constant force control in high compressive stress states. Here, we focus on the shear failure of a dry sample and the later on induced fracture plane reactivation due to water injection. Acoustic Emission (AE) monitoring has been used during both experiments. We also used ultrasonic wave velocities to monitor pore fluid migration through the initially dry specimen. To characterise AE source mechanisms, we analysed first motion polarities and performed full moment tensor inversion at all stages of the experiments. For the case of water injection on the dry specimen that previously failed in shear, AE activity during formation of new fractures is dominated by tensile and shear sources as opposed to the fracture plane reactivation, when compressive and shear sources are most frequent. Furthermore, during the reactivation of the latter, compressive sources involve higher compressive components compared to the shear failure case. The polarity method and the moment tensor inversion reveal similar source mechanisms but the latter provides more information on the source components.