Geomechanical Properties Of Rocks Research Articles

Assessment of plastic zones surrounding the power station cavern using numerical, fuzzy and statistical models

Plastic zones evaluation around the powerhouse caverns is a very crucial issue in designing and constructing these structures and accurate determination of their related optimum support systems. Due to inherent difficulties during the field measurement of plastic zones around the powerhouse caverns and shortcomings of the available methods in this field, applying new predictive models is an attractive and helpful topic. Accordingly, plastic zones around the powerhouse caverns have been investigated in this research using numerical analysis (NA), fuzzy inference system (FIS) and multivariate regression (MVR) model. Based on the numerical simulations, a new predictive equation has been developed to determine the plastic zone at middle point of sidewall and induced key point around a cavern. The basic parameters including rock geomechanical properties and geometrical characteristics of cavern structures have been considered as input variables in plastic zones modeling at middle points of roof, floor, left sidewall and right sidewall as well as at key point. For FIS and MVR models construction, sufficient datasets were introduced based on the numerical simulations. Performance of established models has been assessed applying testing dataset and utilizing powerful statistical indices. Accordingly, it is proved that the derived results from FIS and NA models are more precise than MVR model and they are more satisfactory in plastic zone estimation. Finally, parametric study results revealed that lateral stress coefficient, depth of overburden and rock mass rating are the most effectual parameters and tensile strength is the least influencing parameter on the plastic zone around a cavern.

Factors Influencing Rock Burst Hazard in Deep Copper Ore Mine, SW Poland

In underground mines of copper ores, which are situated in Lower Silesia in SW Poland, the most common and dangerous threats pose seismic tremors and rock bursts. Copper ore exploitation is carried out in three mines, which are owned by KGHM Polish Copper JSC, i.e. Lubin mine, Rudna mine and Polkowice-Sieroszowice mine. Despite many rock burst preventive operations carried out over several decades of mining activity and aimed at counteracting seismic dynamic events, these phenomena have not been eliminated. The applied rock burst prevention measures only limit or mitigate the effects of seismic events. In mining, a concept of seismicity is used, which is defined as the tendency of an intact rock mass to seismic tremors induced by mining work in an area that was previously aseismic. Tremors and their results i.e. rock bursts affect not only excavations and underground infrastructure, but also the terrain surface. In the KGHM’s mines, seismic activity is monitored by the Mining Geophysics Stations. The number, coordinates and energy of tremors and rock bursts are registered, which helps determine the rock mass reaction to the adopted system and operation technology. The main purpose of the paper is to analyse seismic dynamic phenomena damaging mining excavations, i.e. rock bursts, in the Lubin mine, which took place from the beginning of the mine’s operation in 1975 to 2016 (over 41 years) and to identify and investigate geological and mining factors (parameters) facilitating the growth of seismic and rock burst hazards. The following factors (parameters) favourable for the occurrence of dynamic phenomena with damaging effects in excavations were taken into consideration: the seismic activity of the rock mass (spontaneous and provoked tremors), the deposit depth and thickness, geomechanical properties of rocks, exploitation systems, the method of elimination of mined-out space, the occurrence of gobs, tectonic disturbances and the life time of excavations. The results of the observation of selected parameters made it possible to determine the causes of rock bursts as well as to evaluate the impact of the exploitation system including the technology and rock burst prevention on seismic and rock burst hazard. Moreover, seismic activity of the mining division with the highest number and energy of rock bursts was analysed. The effectiveness of rock burst prevention measures used in this division was assessed too. The results of the analyses indicated some interdependencies between factors that may affect the phenomena discussed.

Development of a GEP model to assess CERCHAR abrasivity index of rocks based on geomechanical properties

The CERCHAR abrasivity test is very popular for determination of rock abrasivity. An accurate estimation of the CERCHAR abrasivity index (CAI) is useful for excavation operation costs. This paper presents a model to calculate CAI based on the gene expression programming (GEP) approach. This model is trained and tested based on a database collected from the experimental results available in the literature. The proposed GEP model predicts CAI based on two basic geomechanical properties of rocks, i.e. rock abrasivity index (RAI) and Brazilian tensile strength (BTS). Root mean square error (RMSE), mean absolute error (MAE), Nash-Sutcliffe efficiency (NSE), and coefficient of determination (R2) are used to measure the model performance. Furthermore, the developed GEP model is compared with linear and non-linear multiple regression and other existing models in the literature. The results obtained show that GEP is a strong technique for the prediction of CAI.

Characteristics and Formation Mechanism of Giant Long‐Runout Landslide: A Case Study of the Gamisi Ancient Landslide in the Upper Minjiang River, China

The upper reaches of the Minjiang River are in the eastern margin of the Tibetan Plateau, where active faults are well developed and earthquakes frequently occur. Anomalous climate change and the extremely complex geomechanical properties of rock and soil have resulted in a number of geohazards. Based on the analysis of remote sensing interpretations, geological field surveys, geophysical prospecting and geological dating results, this paper discusses the developmental characteristics of the Gamisi ancient landslide in Songpan County, Sichuan Province, and investigates its geological age and formation mechanism. This study finds that the Gamisi ancient landslide is in the periglacial region of the Minshan Mountain and formed approximately 25 ka BP. The landslide initiation zone has a collapse and slide zone of approximately 22.65×106–31.7×106 m3 and shows a maximum sliding distance of approximately 1.42 km, with an elevation difference of approximately 310 m between the back wall of the landslide and the leading edge of the accumulation area. The landslide movement was characterized by a high speed and long runout. During the sliding process, the landslide body eroded and dammed the ancient Minjiang River valley. The ancient river channel was buried 30‐60 m below the surface of the landslide accumulation area. Geophysical prospecting and drilling observations revealed that the ancient riverbed was approximately 80‐100 m thick. After the dam broke, the Minjiang River was migrated to the current channel at the leading edge of the landslide. The Gamisi ancient landslide was greatly affected by the regional crustal uplift, topography, geomorphology and paleoclimatic change. The combined action of periglacial karstification and climate change caused the limestone at the rear edge of the landslide fractured, thus providing a lithological foundation for landslide occurrence. Intense tectonic activity along the Minjiang Fault, which runs through the middle and trailing parts of the Gamisi ancient landslide, may have been the main factor inducing landsliding. Studying the Gamisi ancient landslide is of great significance for investigating the regional response to paleoclimatic change and geomorphologic evolution of the Minjiang Fault since the late Pleistocene and for disaster prevention and mitigation.

Distributed Acoustic Sensing of Strain at Earth Tide Frequencies.

The solid Earth strains in response to the gravitational pull from the Moon, Sun, and other planetary bodies. Measuring the flexure of geologic material in response to these Earth tides provides information about the geomechanical properties of rock and sediment. Such measurements are particularly useful for understanding dilation of faults and fractures in competent rock. A new approach to measuring earth tides using fiber optic distributed acoustic sensing (DAS) is presented here. DAS was originally designed to record acoustic vibration through the measurement of dynamic strain on a fiber optic cable. Here, laboratory experiments demonstrate that oscillating strain can be measured with DAS in the microHertz frequency range, corresponding to half-day (M2) lunar tidal cycles. Although the magnitude of strain measured in the laboratory is larger than what would be expected due to earth tides, a clear signal at half-day period was extracted from the data. With the increased signal-to-noise expected from quiet field applications and improvements to DAS using engineered fiber, earth tides could potentially be measured in deep boreholes with DAS. Because of the distributed nature of the sensor (0.25 m measurement interval over kilometres), fractures could be simultaneously located and evaluated. Such measurements would provide valuable information regarding the placement and stiffness of open fractures in bedrock. Characterization of bedrock fractures is an important goal for multiple subsurface operations such as petroleum extraction, geothermal energy recovery, and geologic carbon sequestration.

Experimental Investigation of Coupled Geomechanical, Acoustic, and Permeability Characterization of Berea Sandstone Using a Novel True Triaxial Assembly

Technological advancement of laboratory testing methods for investigating comprehensive geomechanical and transport properties of reservoir rocks is of great importance to the development of comprehensive geomechanical models that can improve operational risk assessment and production optimization. The in situ stress state can largely change due to drilling, completion, fracturing, and production, depending on the initial complexity of the geological setting, intrinsic rock anisotropy, and the degrees of artificial geomechanical and hydraulic disturbances. The versatility of a geomechanical model is therefore assured by reflecting the effects of stress magnitude and anisotropy on the considered properties. The purpose of this study was to investigate the effects of intermediate principal stress on the constitutive and flow behavior of Berea sandstone under true triaxial stress state. We present results of experiments with independent variation of intermediate principal stress under an array of octahedral normal and shear stresses, performed in dry and water-saturated cylindrical Berea sandstone core samples using a novel true triaxial testing apparatus. Data analysis indicated presence of microstructural behavior suggestive of opening and closing of microfractures, and revealed the effect of intermediate principal stress on deformation, wave velocities, and permeability. Vertical permeability displayed reduction to a smaller extent under increased vertical stress than under increased radial stress while exhibiting a steady decline under higher mean stress.

Laboratory studies of geomechanical properties of deep-level rocks

The article describes the lab-scale tests of the deformation and strength characteristics of rocks extracted from great depths. Samples of high-grade ore, cupriferous ore, disseminated ore, hornfel and limestone were subjected to tension, uniaxial compression and triaxial compression, and the failure envelopes were plotted. The change in the value of cohesion determined from Mohr’s envelopes in tension and uniaxial compression as well as in tension and triaxial compression is analyzed. The values of relative changes in cohesion and in uniaxial and triaxial compression strengths obey strong correlation.

Calculating far-field anisotropic stress from 3D seismic in the Permian Basin

Minimum horizontal stress (Sh) is the controlling parameter when hydraulic fracture stimulating tight oil formations but is next to impossible to measure quantitatively, especially in the far field and away from the wellbore. In-situ stress differences between bedding planes control fracture containment, which defines the complexity of fracture propagation and fracture geometry including orientation, height growth, width, and length. Geomechanical rock properties define elastic behavior, influencing how the subsurface will deform under induced stress. These properties include dynamic and static Young's modulus, Poisson's ratio, and Biot's coefficient. When combined with pore pressure and overburden stress, the elastic rock properties describe the mechanical earth model (MEM), which characterizes the geomechanical behavior of the subsurface. The MEM also defines key inputs for calculating Sh using the Ben Eaton stress equation, which has been commonly used by geoscientists for decades. However, calculated Sh from this simple model historically produces uncertain results when compared to field-measured stress due to an assumed homogeneous and isotropic subsurface. This is particularly contrary to tight oil formations that represent shale (or mudrock) reservoirs that are highly laminated and therefore anisotropic. Optimal parameterization of fracture geometry models for well spacing and engineered treatment design requires an anisotropic far-field in-situ stress measurement that accurately captures vertical and lateral variability of geomechanical properties in 3D space. A method is proposed herein that achieves this by using a modified version of the anisotropic Ben Eaton stress model. The method calculates minimum Sh by substitution of inverted 3D seismic volumes directly into the stress equation, replacing the bound Poisson's ratio term with an equivalent anisotropic corrected closure stress scalar (CSS) term. The CSS seismic volume is corrected for anisotropy using static triaxial core and is calibrated to multidomain data types including petrophysics, rock physics, geomechanics, and completion and reservoir engineering field measurements.

Geomechanical Upscaling Methods: Comparison and Verification via 3D Printing

Understanding geomechanical properties of rocks at multiple scales is critical and relevant in various disciplines including civil, mining, petroleum and geological engineering. Several upscaling frameworks were proposed to model elastic properties of common rock types from micro to macroscale, considering the heterogeneity and anisotropy in the samples. However, direct comparison of the results from different upscaling methods remains limited, which can question their accuracy in laboratory experiments. Extreme heterogeneity of natural rocks that arises from various existing components in them adds complexity to verifying the accuracy of these upscaling methods. Therefore, experimental validation of various upscaling methods is performed by creating simple component materials, which is, in this study, examining the predicted macroscale geomechanical properties of 3D printed rocks. Nanoindentation data were first captured from 3D printed gypsum powder and binder rock fragments followed by, triaxial compression tests on similar cylindrical core plugs to acquire modulus values in micro and macroscale respectively. Mori-Tanaka (MT) scheme, Self-Consistent Scheme (SCS) method and Differential Effective Medium (DEM) theory were used to estimate Young’s modulus in macroscale based on the results of nanoindentation experiments. The comparison demonstrated that M-T and SCS methods would provide us with more comparable results than DEM method. In addition, the potential applications of 3D printed rocks were also discussed regarding rock physics and the geomechanics area in petroleum engineering and geosciences.

Wettability Measurements on 3D Printed Sandstone Analogues and Its Implications for Fluid Transport Phenomena

Additive manufacturing technology, or 3D printing, with silica sand has enabled the manufacture of porous rock analogues for the use in experimental studies of geomechanical properties of reservoir rocks. The accurate modelling of the fluid flow phenomena within a reservoir and improving the performance of hydrocarbon recovery require an understanding of physical and chemical interactions of the reservoir fluids and the rock matrix. Therefore, for the 3D printed samples to serve as rock analogues, flow properties have to be equivalent to the petrophysical properties of their natural counterparts, such as Berea sandstone. In this study, sandstones that were 3D printed with silica sand and Poly-Furfuryl alcohol (PFA) binder, were used to investigate interactions between porous media with different fluids. Wettability preference of 3D printed samples was characterized through contact angle measurements, as well as co-current and counter-current spontaneous imbibition experiments. Results indicated that 3D printed sandstones had mixed-wet characteristics due to the high preference of silica grains for polar fluids and the affinity PFA binder to the oleic phase. Printing configurations including binder saturation were found to greatly influence the wettability preference of the 3D printed analogue rocks as higher PFA concentrations resulted in more strongly oil-wet preferences. Efforts to optimize the printing process and challenges to control the wettability preferences of the 3D printed samples are also highlighted.

Factors influencing acoustic properties of carbonate rocks: Examples from middle Jurassic carbonates, Central Saudi Arabia

The study aims to identify lithofacies, diagenetic properties, porosity and permeability, compressional and shear wave velocity, rock strength and dynamic elastic moduli of carbonate rocks. All these aspects are used together to delineate the major factors that influences acoustic and geomechanical properties of carbonate rocks of Jurassic Dhruma Formation. A total of sixty carbonate rock samples were collected, petrographic thin sections and SEM images were prepared, core plugs and small cubes were prepared for porosity, permeability, acoustic wave velocities and point load tests, Powder samples were prepared to understand the mineralogical control on these properties. Nine lithofacies were found and minor lithofacies controls on acoustic and geomechanical properties were figured out. Porosity and permeability showed poor to moderate correlations. P-wave and S-wave velocities are poorly correlated with porosity and better correlated with permeability (compared with porosity). Poor correlation between Poisson's ratio and porosity and moderate correlation between dynamic Young's modulus and porosity were found. Two failure patterns of induced fractures were found when applying point load: regular discrete (straight) pattern related to samples of higher strength index and Young's modulus, irregular patterns were found in low strength index and Young's modulus, these induced fractures are compared with natural fractures that are observed in the outcrop. Four petrographic classes were found based on pore type and diagenetic classifications control on acoustic properties: Class 1 is characterized by moldic and intraparticle pore types with limited microporosity. It shows the weakest correlation; Class 2 is characterized by intensive micro-fracturing. This class shows the best wave velocity versus porosity correlation, class 3 is intensively cemented, and Class 4 is characterized by microporosity. Vertical semi-variograms were measured for P and S-wave velocity as well as porosity. A spherical model was found for P and S-wave velocities with zero nugget and range values equal to 3 m. Hole effect reflecting the control of cyclicity was found. Porosity semi-variogram shows exponential modeling with zero nugget and range value equal to 3 m. This difference in semivariogram between velocity and porosity supports weak and moderate correlations between them.

Quartz types in shale and their effect on geomechanical properties: An example from the lower Cambrian Niutitang Formation in the Cen'gong block, South China

Abstract We identified the sources of quartz in shales of the Niutitang Formation in the Cen'gong block, northern Guizhou Province, China, based on thin-section studies using a scanning electron microscope (SEM), rock cores and field observations. By analyzing the petrology, total organic carbon (TOC), major elements, and illite “crystallinity”, we investigated the formation of silica in the Niutitang shales. Our results show that the quartz in the Niutitang shales consists mainly of detrital quartz, biogenic quartz, and quartz that transformed from clay minerals. The amount of excess SiO2 in the Niutitang shales was approximately 5–35%, and the Al/(Fe + Al + Mn) and Si/(Si + Al + Fe) ratios ranged from 0.45 to 0.85 and from 0.7 to 0.91, respectively, which indicates that the silica was not solely biogenic. The values of excess SiO2, Al/(Fe + Al + Mn), Si/(Si + Al + Fe), and TOC increased with increasing quartz content, which implies that increasing quartz concentrations were associated with gradual increases in biogenic quartz and decreases in detrital quartz. Quartz from different sources can result in varying rock geomechanical properties and fracture abundance. Compared with pure biogenic quartz or detrital quartz, an appropriate proportion of mixed biogenic quartz and detrital quartz can make rocks more brittle and therefore lead to more fractures.

Physical and mechanical property relationships of a shallow intrusion and volcanic host rock, Pinnacle Ridge, Mt. Ruapehu, New Zealand

Shallow magmatic intrusions are prevalent in volcanic settings worldwide. Understanding how these intrusions interact and influence their volcanic host rocks is therefore relevant to many engineering geology, geothermal, and volcanological applications. In this study, we present the most comprehensive dataset for a shallow intrusion and its host rock in a volcanic setting to date, detailing the mechanical and physical properties of volcanic rocks from Pinnacle Ridge, Mt. Ruapehu, New Zealand. Based on the geomechanical properties of 194 measured samples, we identify seven geotechnical units: (1) unaltered dense coherent lava, (2) altered dense coherent lava, (3) unaltered brecciated lava margin, (4) altered brecciated lava margin, (5) unaltered intrusion, (6) altered intrusion, and (7) hydrothermal veining. We detail the mineralogy (andesite compositions ranging from primary to an advanced argillic alteration assemblage), porosity (0.7–31%), permeability (10−21–10−12 m2), elastic wave velocities (1994–5615 m/s), uniaxial compressive strength (1–332 MPa) of these geotechnical units. Our laboratory analyses indicate that primary lithology is the predominant control on the physical and mechanical properties of the geotechnical units. Additionally, the data suggest that there is a correlation between distance to the largest intrusion; this is particularly evident for the measurements on the brecciated lava margin samples. Towards the largest intrusion, this breccia shows decreasing porosity (30.92 to 5.49%) and permeability (10−12 to 10−17 m2) and increasing elastic wave velocities (1994 to 4157 m/s) and uniaxial compressive strength (3 to 61 MPa). Thin-section analysis suggests that these correlations are due to mineral precipitation within fractures and pores in the brecciated lava margins. These correlations with distance to the largest intrusion are not shared by the altered intrusions or dense coherent lavas. We suggest that the high primary permeability of the unaltered breccia facilitated efficient hydrothermal fluid circulation and mineral precipitation adjacent to the intrusion. The other geotechnical units are less affected because hydrothermal fluid flow, alteration, and mineral precipitation were limited due to low initial permeability (10−21–10−16 m2). Our study shows that the initial properties of the host rock (i.e. porosity and permeability) control the extent of hydrothermal alteration and the susceptibility to modifications of rock geomechanical properties. Modifications to porosity and permeability can influence edifice-scale behaviour; for example, a reduction in permeability can result in pore pressure augmentation, which exerts a primary control on volcanic slope stability, seismicity, and eruptive behaviour. This study provides the most comprehensive and complete geomechanical properties data suite on a shallow intrusion in volcanic host rock to date and will support monitoring and modelling of volcanic hazards associated with shallow igneous intrusions.

A study to correlate LCPC rock abrasivity test results with petrographic and geomechanical rock properties

Rock abrasivity is very much influenced by the petrographic and physico-mechanical properties of rock and hence its assessment from common geomechanical rock properties is helpful for rock engineers in estimating excavation costs. In this paper LCPC and CERCHAR abrasivity tests, as well as a complete suite of mechanical and physical rock properties tests, were conducted on 51 rock samples collected from different locations in Pakistan. Moreover, petrographical studies of 48 rock specimens were performed along with the computation of Schimazek's F-value (F-value) and Rock Abrasivity Index (RAI). For the analysis of test results least square regression was employed. Initially the LCPC abrasivity coefficient (ABR) values were correlated with CERCHAR abrasivity index tests and reasonable relationships were observed. Statistically meaningful relationships were found between ABR and geotechnical wear indices (F-value and RAI). Possible correlations of LCPC test results with rock properties are also discussed. Finally multiple regression analysis was applied to find statistically significant correlation of ABR with petrographical and physico-mechanical rock properties. Test results show that RAI, Brazilian tensile strength and mean quartz grain size (Ø-qtz) prove to be the statistically best predictors of ABR from the rock properties determined. The developed correlations are specifically intended for rock engineers involved in designing excavation projects.

Geomechanical Rock Properties Using Pressure and Temperature Dependence of Elastic P- and S-Wave Velocities

It is important and meaningful for understanding the geomechanical properties of rock and providing guidance on analyzing and simulating the formation processes of engineering, geophysical, geothermal, civil and underground engineering projects. Variation in geomechanical properties of metamorphic rock, such as, seismic velocities (P-and S-wave) density deformation, is observed with the increase in pressure and temperature, Poisson’s ratio, elastic modulus, bulk modulus and shear modulus, are analyzed herein. A simple laboratory method, which measure seismic velocities has increasingly been conducted to determine the geomechanical properties of rock materials. Three metamorphic rock samples were selected for laboratory testing. Laboratory testing were done on cube samples of dry rocks in a true triaxial apparatus. First, P and S wave velocities were measured at 12–100 MPa pressure and at 20–600 °C temperature. Measurement showed that P and S wave velocities increase with increasing pressure and decrease with increasing temperature. Second, the wave velocities are used to calculate the geomechanical properties at lower to higher pressure and temperature. Furthermore, it was found that pressure and temperature have a significant effect on the change of geomechanical properties of rocks. These results contribute to a reliable estimate of geomechanical properties of rocks.

Penetration Rate and Specific Energy Prediction of Rotary–Percussive Drills Using Drill Cuttings and Engineering Properties of Selected Rock Units

This study discusses the prediction of penetration rate and specific energy of button bit equipped rotary–percussion drilling machines from drill cuttings and geo-mechanical properties of rocks. The operational parameters of drilling machines measured from selected locations were utilized for the calculation of specific energy of drilling operations. For this purpose three on-going hydropower projects and four active mining quarries of Pakistan were selected. The drill cuttings were further used to determine various descriptors of the chip size distribution including the coarseness index and Rosin–Rammler’s absolute size constant. A complete set of geo-mechanical rock tests were conducted in the laboratory and includes uniaxial compressive strength, Brazilian tensile strength, point load strength, Schmidt rebound hardness, P-wave velocity, dry density, porosity and brittleness indices. Regression analyses were performed to predict the penetration rate and specific energy of drilling from geo-mechanical properties of rocks. The models so developed were also validated by adopting the t-test and the F-test statistical techniques. Moreover, statistical models were also developed to evaluate penetration rate from various descriptors of the chip size distribution. Dependence of bit size on coarseness index and mean particle size was also discussed.

INVESTIGATING AN INNOVATIVE MODEL FOR DIMENSIONAL SEDIMENTARY ROCKS CHARACTERIZATION USING ACOUSTIC FREQUENCIES ANALYSIS DURING DRILLING

Determining geomechanical characteristics of rocks plays a significant role in all consequent designing stages of geosciences. On the other hand, drilling is one of the considerable operations in primer phases of extracting rocks. Drilling process produces acoustic signals as by-product during drilling. Then, one possible way for predicting geomechanical properties of rocks, is employing acoustic signal frequencies which are produced during drilling operation. This process helps to geoengineers for determining rock characteristics in short time and by low-cost and satisfying precision. This research tries to develop a novel computational relations between geomechanical characteristics of sedimentary rocks and produced dominant acoustic frequencies by implementing Fast Fourier Transform (FFT). For this purpose, a novel rotary drilling machine is developed by researchers. In order to introducing reliable model, 10 diverse sedimentary rock samples from various sedimentary basins of Iran are gathered in wide range of geomechanical features, and all tests are carried out on them. The results of this research could be used for sedimentary basins' characterization. Results show there are reliable mathematical relations between various characteristics of sedimentary rocks (UCS, TS, porosity and hardness) and diverse dominant frequencies.

Influence of loading and heating processes on elastic and geomechanical properties of eclogites and granulites

Increased knowledge of the elastic and geomechnical properties of rocks is important for numerous engineering and geoscience applications (e.g. petroleum geoscience, underground waste repositories, geothermal energy, earthquake studies, and hydrocarbon exploration). To assess the effect of pressure and temperature on seismic velocities and their anisotropy, laboratory experiments were conducted on metamorphic rocks. P- (Vp) and S-wave (Vs) velocities were determined on cubic samples of granulites and eclogites with an edge length of 43 mm in a triaxial multianvil apparatus using the ultrasonic pulse emission technique in dependence of changes in pressure and temperature. At successive isotropic pressure states up to 600 MPa and temperatures up to 600 °C, measurements were performed related to the sample coordinates given by the three principal fabric directions (x, y, z) representing the foliation (xy-plane), the normal to the foliation (z-direction), and the lineation direction (x-direction). Progressive volumetric strain was logged by the discrete piston displacements. Cumulative errors in Vp and Vs are estimated to be <1%. Microcrack closure significantly contributes to the increase in seismic velocities and decrease in anisotropies for pressures up to 200–250 MPa. Characteristic P-wave anisotropies of about 10% are obtained for eclogite and 3–4% in a strongly retrogressed eclogite as well as granulites. The wave velocities were used to calculate the geomechanical properties (e.g. density, Poisson's ratio, volumetric strain, and elastic moduli) at different pressure and temperature conditions. These results contribute to the reliable estimate of geomechanical properties of rocks.

Carbon geosequestration in limestone: Pore-scale dissolution and geomechanical weakening

Carbon dioxide geosequestration in deep saline aquifers or oil and gas reservoirs is a key technology to mitigate anthropogenic greenhouse gas emissions. Porous carbonate rock is a potential host rock for CO2 storage; however, carbonate rock chemically reacts when exposed to the acidic brine (which is created by the addition of CO2, CO2-saturated brine). These reactive transport processes are only poorly understood, particularly at the micrometre scale, and importantly how this affects the geomechanical rock properties. We thus imaged a heterogeneous oolitic limestone (Savonnières limestone) core before and after flooding with brine and CO2-saturated brine at representative reservoir conditions (323K temperature, 10MPa pore pressure, 5MPa effective stress) in-situ at high resolutions (3.43μm and 1.25μm voxel size) in 3D with an x-ray micro-computed tomograph; and measured the changes in nano-scale mechanical properties induced by acid exposure. Indeed the carbonate rock matrix partially dissolved, and absolute and effective porosity and permeability significantly increased. This dissolution was confined to the original flow channels and inlet points. Importantly, the rock matrix weakened significantly (- 47% in indentation modulus) due to the acid exposure.

Understanding CO2-brine-wellbore Cement-rock Interactions for CO2 Storage

There is a need to improve our understanding of wellbore integrity by conducting investigations into the flow, geomechanical and geochemical properties of cement and reservoir rocks under conditions representative of subsurface temperatures and pressures at the wellbore. A series of composite cement-host rock core samples were prepared and subjected to baseline flow and mechanical properties testing to determine porosity, permeability, strength and elastic properties. The hydrothermal experiments conducted have shown that variations in the solution profiles of Ca, Mg, and Fe were due to the dissolution of CO2 for all sets of samples. The dissolution of muscovite and montmorillonite from the composite core samples resulted in increasing concentrations of Na, K and S.