bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled Mechanical and Permeability Model for Grain Crushing and Pore Collapse
Authors: Papamichos E. , Berntsen A. N. , Flatebø R. E. , Kristiansen T. G. , Vejbæk O. , Omdal E.
Published: September 2023

The coupled hydromechanical behavior of porous rocks at high compressive stresses is studied. At such stresses, grain crushing, or pore collapse are the primary failure modes. The need to study and model such behaviors is necessary as exploitation of reservoirs reaches depths where stress regimes for such failures are becoming relevant. Experimental data from triaxial compression and Ko tests at different stress ratios are analyzed to model the effects of grain crushing or pore collapse on the stress-strain behavior but also on the changes in the permeability. A stress-dependent permeability model is developed to account for the large drops in permeability associated with these compaction failures. The permeability model modifies the Kozeny-Carman model for permeability as a function porosity to include a stress dependent term linked to grain crushing or pore collapse. Comparison with the experimental results show the validity of the model in capturing permeability changes during compaction failure.

INTRODUCTION

Reservoir rocks are sometimes loaded at high compressive mean stress at deep reservoirs and high depletions. Moreover, proppant or other completions that support the face of boreholes or perforations may result in both high deviatoric and high mean stresses. Such loadings can lead to grain crushing and/or pore collapse of reservoir rocks. Sandstones often experience grain crushing at high mean stresses. A combination of deviatoric and hydrostatic loading facilitates this phenomenon (Papamichos et al. 1993). High porosity chalks on the other hand experience pore collapse where the open skeleton structure of the material breaks, and destructures giving a sharp decline in porosity (e.g., Papamichos et al. 1997). Both phenomena cause fundamental changes in the structure of the rock, the pore space and thus significantly affect the rock permeability. This work proposes a permeability model that is linked to the mechanical failure of the rock due to pore collapse and grain crushing and attempts to simulate the sharp permeability decline observed during grain crushing and pore collapse and which cannot be captured with only porosity dependent permeability models løike the Kozeny-Carman. Experiments are presented for two sandstones and a chalk where these phenomena had been investigated to a certain extent. Based on the experimental results a model was advanced that appears to provide reasonable simulations of the experimental results.
Enhanced Imbibition Through Wettability Alteration During Shut-In Treatment After Hydraulic Fracturing
Authors: Fan Xibin , Wang Wenzhong , Zhu Shijie , Wang Yingwei , Bai Hao , Liang Tianbo
Published: September 2023

Tight oil reservoirs are characteristic of low porosity and permeability, complex pore-throat network, and typically oil-wet, which results in a low oil recovery rate of less than 10%. Surfactant can alter the rock wettability while maintaining an ultralow adsorption rate on rock surface, and thus can be a promising method to enhance the oil recovery. This paper uses the numerical simulation method to understand how shut-in pressure affects the imbibition of fracturing fluid when rock wettability changes. Results show that wettability has a great influence on water imbibition depth of fracturing fluid and the change of water saturation at the invasion front. There is a positive correlation between the imbibition rate and the water-oil interfacial tension in a water-wet reservoir, but a negative correlation in an oil-wet reservoir. In water-wet reservoirs, when the shut-in pressure keeps the same, the water imbibition rate is linear to sqrt (K/ϕ), and increases with the interfacial tension. When the oil-wet reservoir is altered to water-wet, although the water imbibition rate is reduced by 37%, water saturation in the invasion front significantly increases from 0%-1.75% to 8%-15%, resulting in an enhanced water imbibition efficiency of 6.25% to 15%.

INTRODUCTION

Abundant oil and gas resources are present in tight reservoir formations, which are widely distributed all over the world. Tight reservoirs have small pore radius, few effective pores, and complicated pore throats with abundant microfractures[1]. In this kind of reservoir, the traditional water flooding method is difficult to achieve the effective use of crude oil. Hydraulic fracture is needed to improve the conductivity of the matrix[2]. Research and practice indicate imbibition flooding can significantly improve recovery in fractured tight reservoirs[3]. In general, imbibition refers to the spontaneous entry of the wetting-phase fluid into the pore throat of the rock under the action of capillary forces, displacing the non-wetting phase fluid in the pores[4]. To study the imbibition characteristics and imbibition displacement mechanism of tight reservoir can provide an effective method for improving the recovery efficiency of tight reservoir.
Probabilistic Stress Prediction: Poroelastic Profiles and Paths in the Permian Basin, USA
Authors: Amalokwu K. , Crawford B. , Chhatre S. , Manchanda R. , Becker G.
Published: September 2023

Knowledge of the in-situ state of stress is fundamental to all subsurface geomechanics/rock engineering endeavors and predicting layer-to-layer variation of horizontal principal stress magnitudes is imperative for a wide range of energy-related applications. In addition, the ability to forecast subsequent changes to these horizontal stress magnitudes associated with pore fluid pressure depletion or injection (stress paths) is vital for unconventional resources development planning, including predicting maximum injection pressures for gas containment associated with cyclic "huff and puff" enhanced oil recovery (EOR). Poroelastic theory describes the fundamental mechanical behavior of fluid-bearing porous rocks including unconventional resource rocks, and as such provides a useful framework for understanding subsurface stresses and how they impact operations. However, due to the highly heterogeneous nature of unconventional resource rocks, oftentimes deterministic predictions are not sufficient for business purposes but rather a probabilistic approach is necessary to adequately explore the full range of possible outcomes. We present practical Permian Basin applications of predictive tools based on poroelastic theory and Monte Carlo simulation for rapid geomechanical assessment of susbsurface stress profiles (e.g., as input for probabilistic hydraulic fracture modeling), and to bound likely stress paths associated with EOR injection for predicting gas containment pressures and surface compressor requirements. Practical applications related to the Permian Basin, onshore USA are presented.

INTRODUCTION

A wide range of energy-related applications require knowledge of in-situ stress state (and its evolution) to various levels of detail/scale. For some applications, an estimate of in situ stresses in a particular formation or region is sufficient, while for many other applications, layer to layer variation of stress is required. Knowledge of layer-to-layer variation of horizontal principal stress magnitudes is vital for a wide range of energy-related applications including drilling (wellbore stability); completions (hydraulic fracturing); mechanical seal capacity/caprock integrity (underground gas storage, carbon capture, utilization and storage), in-fill drilling and field development (parent-child well interactions, depletion planning) and abandonment (Warpinski & Teufel, 1989; Thiercelin & Plumb, 1994; Peška & Zoback, 1995).
Reconstruct Laminated Shale Cores/specimens for Numerical Modeling by Physics-Informed Image Processing
Authors: Zhao Gaobo , Ruan Mindi , Tulu Ihsan Berk , Li Xin , Xue Yuting
Published: September 2023

This paper proposes end-to-end physics-informed image processing to construct the digital twin of laminated shale cores/specimens and extract the number, spacing, length, and frequency of bedding planes. Specifically, the image stitching technique was applied to eliminate any distortion that may arise from projecting 3D objects onto 2D images. Then, based on shale physics information (gradient difference between different pixels in the image due to various mineral components), an edge detection algorithm was used to extract one-pixel-wide bedding planes from the shale image. Furthermore, a depth-first search algorithm was used to collect the point coordinates of each line. The coordinates gathered from the 2D image were used to generate the model of the laminated shale specimens/cores in UDEC. A variable sensitivity analysis of the reconstructed shale was performed using the proposed method. This work will improve the reliability of laminated shale modeling to further investigate the influence of bedding planes on the macro mechanical properties of shale rockmass.

INTRODUCTION

Laminated shale is a fissile clastic sedimentary rock whose mechanical properties depend largely on the geometrical properties of bedding planes, which are also known as beds, bedding, and laminations. Bedding planes are inevitably considered when scholars investigate different aspects of shale, such as mechanical properties, brittle failure, crack propagation, multiphase coupling, shale rock physics model, and so on, particularly in numerical modeling (Calvo et al., 1998; Amann et al., 2011; Liu and Liu, 2021; Zhao et al., 2022a,b).

In the fields of rock mechanics and mining, it has been agreed that bedding planes are the weaker component of the shale rockmass matrix (Einstein et al., 1969; Josh et al., 2012; Esterhuizen et al., 2013; Lisjak et al., 2014; Moradian et al., 2016; Seyed Ghafouri et al., 2022; Yuan et al., 2022). Generally in numerical and analytical analysis, the geometry of bedding planes is simplified as continuous, straight, and equidistant to simplify the solution approach (Sone, 2012; Morgan and Einstein, 2014; Morgan, 2015; Xue and Mishra, 2017; Zhao, 2022; Seyed Ghafouri et al., 2022). The above assumptions are feasible when simulating the preidentified macro characteristics of a specific shale rockmass, but might become unreasonable/questionable when the failure mechanism of shale rockmass is investigated on a laboratory or field scale. This is because shale specimen modeling with those assumptions is oversimplified and cannot reveal the true nature of the failure process and crack propagation.
The Hith Formation: A Potential Regional Seal for CO2 Sequestration
Authors: Sierra Hernandez Maria C. , Boehm Nicolas , van Buchem Frans , Finkbeiner Thomas , Li Zihao , Espinoza D. Nicolas
Published: September 2023

The Late Jurassic Hith Formation is an anhydrite deposit with an average thickness of > 90 m. It forms an important regional seal for major hydrocarbon accumulations over large parts of the Arabian Peninsula. In Saudi Arabia, the only location where the Hith Formation outcrops is the Dahal Hit cave south of the capital of Riyadh. The Hith is being studied as an important seal for CO2 sequestration. To achieve this, it is vital to know its geomechanical properties and chemical reactivity, especially when exposed to CO2 under reservoir temperature and pressure conditions. From a literature review of anhydrites, we obtained global value ranges for strength, stiffness and acoustic properties. All of which indicate that anhydrites are generally stiff and strong rocks. This enables us to benchmark properties from the Hith anhydrite measured on hand specimens taken from the Hit cave. Resulting compressive strength of our samples averages 145 MPa, which is within the range reported in the literature. Compressional velocities average at 5300 m/s, not as high as literature values. Shear velocities on the other hand match the literature reported values of around 3250 m/s. We further reviewed and investigated the effect of brine composition and CO2 on the chemo-mechanical behavior of the Hith anhydrite through micro-scratch testing. Experiments confirm the capability of the micro-scratch test to quantify chemomechanical alteration.

INTRODUCTION

In last two centuries, there has been an almost two-fold increase in carbon dioxide (CO2) atmospheric concentration, from 280 ppm in the late 1700's to 410 ppm in 2020 (Trends in CO2, 2023; Lelandais et al., 2022). Rising CO2 emissions are believed to have caused an un-natural increase in global temperatures (Gil-Alana and Monge, 2020). Many countries – Saudi Arabia included – are committed to decrease their CO2 emissions. Despite growth in renewable energy generation, fossil fuels with their associated CO2 emissions will likely remain the primary source of energy in the near future. Furthermore, using fossil fuels combined with sequestration makes transition and use of unstable zero-carbon energy sources more efficient and economical (Zoback and Smit, 2023). Hence sequestration is necessary for competitive and efficient de-carbonization of energy production and to mitigate the impacts of today's climate crisis.
Rock Physical Modeling of Sheared Fractures: Permeability-Resistivity-Seismic Velocity Relationship Explored via Digital Rock Physics Approach
Authors: Sawayama K. , Ishibashi T. , Jiang F. , Tsuji T.
Published: September 2023

Fluid flow through faults controls subsurface transport processes, considerably impacting seismic hazards and geoengineering such as enhanced geothermal systems (EGS). Recent advances in geophysical techniques have detected changes in electrical resistivity and seismic velocity due to crustal stress changes, and possibly predicted the changes in subsurface fluid flow. However, we could not quantitatively interpret these geophysical monitoring data due to the absence of rock physical models for fractured rocks. This study investigated changes in permeability, resistivity, elastic wave velocity, and their respective relationships at elevated normal stress by performing numerical simulations of various fracture models with changing fracture surface roughness and shear displacement. The results show that permeability-resistivity and permeability-velocity relationships are less dependent on any geometric characteristics, but controlled by the percolation based on microscopic flow analysis. Our finding suggests that Overall, resistivity is more sensitive to less connected flow regime, while seismic velocities are more sensitive to connected flow regime.

INTRODUCTION

The fluid-flow characteristics of fractured geological formations are of critical interest in a number of areas including fluid resources (e.g., geothermal fluids, shale oil, and groundwater), geological storage or disposal, and seismic hazard assessment (fault reactivation, slow slip, and induced seismicity). Numerous studies have revealed that fluid-flow properties are constrained by fracture surface topography, which is also altered by shear displacement and stress (e.g., Chen et al., 2017; Durham and Bonner, 1994; Durham, 1997; Ishibashi et al., 2015; Pyrak-Nolte and Morris, 2000; Watanabe et al., 2008 Witherspoon et al., 1980; Zimmerman et al., 1992). Insitu stress is never constant during geoengineering developments or on the geological time scale, and consequently the aperture distribution and associated hydraulic properties also must change in natural settings.

Although direct measurement of permeability changes is challenging, geophysical observations are potential approaches to evaluate the permeability change. Many studies have reported changes in seismic velocities or electrical resistivity that may reflect subsurface stress changes associated with hydraulic stimulation, earthquakes or geothermal fluid production (e.g., Brenguier et al., 2008; Didana et al., 2017; Mazzella and Morrison, 1974; Nimiya et al., 2017; Park, 1991; Sánchez-Pastor et al., 2019Taira et al., 2018; Tsuji et al., 2021; Johnson et al., 2021). It would be advantageous if changes in permeability caused by subsurface stress changes could be linked to these geophysical properties that could be remotely monitored.
Towards an Efficient iPad-Based LiDAR Structural Mapping Methodology for Underground Mines
Authors: Grenon M. , Caron A. , Karampinos E. , Dorion J. F.
Published: September 2023

Structurally controlled instability is a major failure mechanism in underground hard rock mines. From a design perspective, it is important to effectively define the structural regime to assess the potential rockmass failure mechanisms. In a mining context, detailed structural mapping can be challenging due to the large extent of the underground excavations and limited access to the rock face. Structural mapping can be performed using manual mapping approaches or remote sensing techniques. Fixed photogrammetry and terrestrial laser scanning have been used in the past for rockmass characterization. However, these approaches are often time consuming and consequently not widely used in practice for structural mapping in underground mines. Mobile LiDAR systems have been increasingly used for 3D mapping applications. However, it has not been demonstrated that they can be effectively used as mapping tool in the context of a mine in production. This paper presents the results from an underground mapping campaign using a mobile LiDAR scanner approach, an iPad Pro 5th generation, in an arctic mine in northern Canada. A comparison between the results from the developed methodology and manual structural mapping demonstrated significant advantages and remaining challenges in the use of the mobile LiDAR approach for rockmass characterization.

INTRODUCTION

In underground hard rock mines, rock mass instability poses a serious hazard to the safety of personnel and equipment. The structural regime largely controls the mechanical behaviour of jointed rock masses around the underground excavations. Under low-stress conditions, gravity failure of large blocks formed by the presence of rock discontinuities can be a significant hazard. Figure 1 shows a rehabilitated section in the ramp of an operating hard rock mine after a structurally controlled fall of ground in the back. Under these conditions, it is essential to consider the complex structural regime in the design and support of underground excavations. It is, therefore, necessary to adequately characterize the structures in the rock mass using a combination of qualitative observations and quantitative measurements.
Energy Budgeting of Laboratory Hydraulic Fracturing in Granite with Different Viscosity Injection Fluids
Authors: Butt Awais , Hedayat Ahmadreza , Moradian Omid
Published: September 2023

In this study, Barre granite cubes were hydraulically fractured in laboratory, using different viscosity injection fluids. Hydraulic Fracturing (HF) experiments were monitored with real-time acoustic emission (AE) setup which consisted of 16 calibrated Nano-30 sensors. The spectral parameters (corner frequency and low-frequency spectral plateau) were determined for each AE event by fitting the Brune Omega-squared model to the detected AE signals. Seismic parameters such as seismic moment, source radius stress drop, and seismic energy were determined after incorporating the focal mechanisms information determined through moment-tensor inversion. Higher breakdown pressures and fracture propagation times were observed for experiment conducted with higher-viscosity fluid. An inverse relationship was observed between corner frequency and seismic moment, similar to those observed for natural and induced seismicity emitted during large-scale events. The stress drop was found to be fairly constant across a wide range of seismic moment. The seismic moment, stress drop, and seismic energy were significantly higher for the higher viscosity injection fluid. However, the corner frequency and source radius were slightly larger for the lower-viscosity fluid. The seismic efficiency was <<1% (10−6-10−4 %) for both experiments but it was much lower for the experiment conducted with lower-viscosity injection fluid, implying a more aseismic response.

INTRODUCTION

Hydraulic fracturing (HF) is a technique used over the past decades, mainly to increase the reservoir rock permeability for geothermal resource extraction from kms deep crystalline granitic rock. However, HF is a complex phenomenon where it is often difficult to predict the HF growth and the underlying fracture mechanisms. In addition, the HF behavior can be significantly influenced by some key factors including hydro-mechanical properties of the host rock, in-situ stress, and injection parameters (injection fluid rate and viscosity) (Haimson and Fairhurst, 1967). Different viscosity injection fluids have been utilized for the stimulation process mainly to control the HF growth (Ishida et al. 2004). Many previous studies have shown that the attributes of produced fracture network can be drastically different depending on the viscosity of the injection fluid (Tanaka et al. 2020). For example, lower-viscosity injection fluids have been shown to produce more tortuous cracks with many secondary branches (Ishida et al. 2016). Increased stimulated reservoir volume has been shown to achieve with relatively lower-viscosity injection fluids (Warpinski et al. 2005). Understanding the characteristics of the generated HF for different viscosity injection fluids can be crucial to derive appropriate estimations for an efficient HF design.
A New Laboratory Testing Platform to Study Saturation, Swelling/contraction, Desiccation Cracking, and Permeability of Gas/liquid of Engineered Barrier Materials at High Pressure-Temperature Conditions
Authors: Noh D.-H. , Kim S. , Eun J. , Kim Y.-R.
Published: September 2023

This study introduces a new testing platform, which allows an application of high pressure (up to 5 MPa), high temperature (up to 200°C), and simultaneous measurements of vertical deformation, stress, and the permeability of gas/liquid. The test cell is designed for the radial fluid flow to minimize the risk of a preferential leak along the gap between the test specimen and the internal wall of the test cell, which can be prominent during the thermally-induced contraction of bentonite. The testing platform is calibrated by examining the swelling pressure during the saturation process and water permeability of the fully saturated bentonite specimen. After that, a temperature rise, up to 80°C, is imposed on the test cell to monitor the change in the vertical stress. With the partially saturated bentonite specimen, the temperature is raised up to ∼150°C, beyond the boiling point, to induce further desiccation cracks. After that, change in the vertical stress is continuously monitored during the temperature increase and ensuing nitrogen injection to measure the gas permeability. Once the heating period is completed, the test specimen is taken out of the cell and investigated further using the micro-CT machine to capture the distribution of internal desiccation cracks. This study also examines the effect of bentonite reinforcement using inorganic microfibers on the development of desiccation cracks.

INTRODUCTION

Bentonite has been widely used for many engineering applications, such as for the barrier of pollutant transfer, for the stability of engineered construction, etc., due to its characteristics of high swelling pressure, low hydraulic permeability, and adsorption capacity (Smith et al. 1980; Yong et al., 1986; Cho et al. 2000). These characteristics improve even further as the dry density of bentonite increases. Bentonite has also been considered the best candidate as an engineered barrier material for a geological repository of high-level nuclear wastes, including spent nuclear fuels (SNF). Most of the past geological repository concepts suggested the temperature at the canister of SNF should not exceed 100°C to better control the sealing performance of bentonite (Martin et al. 2014; Abootalebi and Siemens 2018). On the other hand, however, several current research efforts are examining the impact of higher temperatures, up to 200°C, on the safety functions of buffer materials, which in turn, can help to achieve shorter cooling times on the ground, more efficient packaging, reduced need for disposal containers and transportation, and smaller facility footprints (Pusch 1980; Gens and Garcia-Fontanet 1997; Gens et al. 2022; Chang et al. 2022).
Analysis of Rotary Steerable Systems Versus Conventional Mud Motors for Vertical and Horizontal Wells in the Caney Formation, Southwest Oklahoma
Authors: Hayes J. , Mayibeki D. , Al Dushaishi M. , Hareland G.
Published: September 2023

Rotary steerable systems (RSS) have transformed directional drilling over the years and are becoming widely accepted throughout the world. RSS technology has been proven to provide the required strength and agility in challenging and abrasive environments as well as provide a higher rate of penetration (ROP) by allowing more weight transfer to the bit. Due to the higher price of RSS, some companies have stayed loyal to the conventional bent housing (CBH) with a performance drilling motor, regardless of the RSS benefits. This paper examines the performance and cost benefits of standalone RSS (SA-RSS), motor driven RSS (MD-RSS), and CBH systems. Three wells drilled in the Caney formation in Southwestern Oklahoma were selected for the analysis. Results showed that using RSS in the 12.25" intermediate vertical section and the 8.75"/8.5" lateral section was more cost-effective than a CBH system. The cost per foot for the CBH was 1.6 to 2.3 times higher than RSS in the vertical section and 1.7 times higher in the lateral section. The RSS ROP was 2.3 to 4.8 times faster than CBH in the vertical section and 1.6 times faster in the lateral section.

INTRODUCTION

In the USA and worldwide unconventional oil and gas wells are pushing the technical limits of existing drilling tools and technology to drill longer horizontal and extended reach wells (ERW), with ERW becoming the mainstream technology. For example, horizontal wells in the United States averaged about 10,000 feet of lateral length in the early 2000s but averaged 18,000 feet in 2019 (USA Energy Administration, 2020). One of the longest ERW wells drilled in the world (offshore Orlan, Russia) was in 2017 and reached a total depth (TD) of 49,000 feet (Slav, 2019). In Oklahoma, the test area for this paper, 1,379 horizontal wells were completed between 2020 and 2022, with an average well length of 18,624 feet and an average perforated length of 8,000 feet (Oklahoma Corporation Commission, 2023).
Mechanical Unit Control of Hydraulic Fracture Geometry
Authors: Gross M. , Dershowitz W.
Published: September 2023

This paper presents a quantitative evaluation of the effect of mechanical stratigraphy on induced hydraulic fracture shape and extent. A detailed geomechanical and hydrodynamic model was built for Hydraulic Fracture Test Site 1 (HFTS-1) in the Permian Basin, considering available geomechanical, geological and geophysical characterization data. This model was then implemented using a discrete fracture network (DFN) approach, to facilitate simulation of induced (tensile) hydraulic fractures, and reactivation of pre-existing natural fractures.

INTRODUCTION

The geometry of natural and induced tension cracks is important to civil, mining, geothermal, and energy applications. According to Griffith Crack theory for failure of brittle materials (Griffith, 1921), natural and induced tension cracks in an infinite, homogeneous, isotropic medium are circular or elliptical. Over 100 years of laboratory studies have provided empirical support for this theory. However, extensive evidence from microseismic monitoring and offset well intersections indicates that the shape of in situ hydraulic fractures vary considerably from the elliptical ideal.

GEOMECHANICAL STRATIGRAPHY

Mechanical stratigraphy refers to the subdivision of a layered rock mass into distinct units (mechanical units) that reflect internally consistent rock properties and styles of deformation (Laubach et al., 2009). In response to tectonic deformation, elements of mechanical layering often control fracture dimensions, terminations and failure mode (Gross et al., 1995, 1997; Ferrill et al., 2014). This mechanical behavior occurs across many scales, and for an unconventional oil and gas field is manifested by hydraulic fracture lengths considerably greater than their heights (Fu et al., 2022), an indication that anisotropic layering limits the vertical growth of hydraulic fractures (i.e., serve as frac barriers).

Observations of natural fractures in outcrops provide insight into the growth and geometry of induced hydraulic fractures in the subsurface (Lacazette and Engelder, 1992; Engelder, 2004; Engelder et al., 2009). Surface morphology preserved on opening-mode fractures ("joints") are manifestations of fracture initiation, propagation and arrest, as demonstrated in Figure 1. In this siltstone bed from the Appalachian Plateau in upstate New York, lineations on the fracture surface converge backwards to the initiation point (yellow dot), about one-third of the way above the bottom of the bed. The lineation pattern shows the joint grew radially in all directions, until it reached the bottom and top of the bed.
Comparison of Constitutive Models for Numerical Modeling of Strainburst Assessment
Authors: Bravo L. , Delonca A. , Robbiano F. , Orellana L. F.
Published: September 2023

Rockburst is one of the most important hazards in underground world, and therefore there is a need to adequately assess their occurrence. While different works allows the mechanism and the hazard to be better understood, there is still work to do to accurately model the phenomenon. In this paper, Mohr Coulomb and IMASS constitutive models are considered into a numerical simulation using FLAC 3D to simulate the occurrence of strainburst. Three rock types are considered, with different strength and fracturing, under different stresses and stress ratio K. In each simulation environment, the strainburst are assessed considering the zone where plasticity is observed (failed zone). The depth of failure measured is compared with well-known published empirical graphs. For K of 1, the results show that the IMASS model presents closest depth of failure estimated by the empirical graphs in high stress environment. For a K different from 1, both models are close to the empirical value, but the IMASS model presents closest values. Moreover, when considering a fractured rock mass, IMASS does not assess properly the depth of failure, but present a better assessment that the Mohr-Coulomb model for higher GSI.

INTRODUCTION

Due to the global increase in ore demand and the exhaustion of superficial resources, the underground mines are going deeper (Wang et al. 2021). As the depth of mining increases, stress induced rock fracturing is inevitable and when stored energy is suddenly released, rocks fail violently, leading to seismic events and rockbursts (Kaiser & Cai, 2012). The large quantity of rockburst events and the severity of those incidents are undeniable evidence that rockburst is a universal and serious problem in the underground world and therefore it is necessary to adequately assess its occurrence (Wang et al., 2021).

A rockburst occurs when a volume of rock is strained beyond its elastic limit (Sepehri et al., 2020). A definition of this phenomenon is proposed by Blake & Hedley (2003), based on the 1984 Mine Safety and Health Administration definition: rockburst is a sudden and violent failure of a large volume of overstressed rock, resulting in the instantaneous release of large amounts of accumulated energy.
Influencing Factors in Rock Cutting Using High Pressure Water Jets Under Submerged Downhole Conditions
Authors: Velmurugan N. , Gerbaud L. , Chambres C. , Naderi S. , Xiang J. , Latham J.-P.
Published: September 2023

Utilizing deep geothermal energy is an important factor in successful transition to a clean, affordable and non-intermittent source of energy. However, high costs due to low drilling performance is a showstopper in scaling geothermal energy. It has been noted that there is a significant drop in the rate of penetration (ROP) in hard crystalline granite rocks averaging between 1-2m/h. These hard rocks are found at depths greater than 4 km and increasing the ROP by a factor of 4 could reduce the drilling costs by 65%. In the framework of ORCHYD (Novel Drilling Technology Combining Hydro-Jet and Percussion for ROP Improvement in Deep Geothermal Drilling) project, we investigate ways to reduce the confining stresses of the underground rock so as the energy required to break the rock will be significantly lower and thus, increasing the ROP. A high-pressure water jet (HPWJ) of up to 250 MPa is generated in the bottomhole conditions to create peripheral groove of a few millimeters to release the rock layer from the surrounding stress regimes – reducing the energy required to break the rock using percussion drilling. Nevertheless, generating the required energy for rock slotting depends on several factors such as nozzle type, output pressure and confining pressure. In this paper, we investigate experimentally the influencing factors of HPWJ for rock slotting in bottomhole drilling conditions.

INTRODUCTION

Scaling geothermal energy is a potential solution in serving our transition needs to a clean, sustainable, non-intermittent and renewable energy source. However, to effectively utilize this energy source, it is required to drill deeper wellbores than previously done in the oil and gas industry. On an average, it is required to drill to depths greater than 4 km to achieve a temperature of above 150 °C in the reservoir rocks to be useful for electricity generation. With nearly 50% contribution to the geothermal exploration, very high drilling costs is a showstopper in making geothermal truly scalable. A majority of the drilling performance, measured as rate of penetration (ROP) in hard crystalline rocks found at such depths is 1 − 2 m/h, e.g., Soultz-sous-Forêts in France (Baujard et al., 2017), and our study shows that increasing ROP by a factor of 4 can reduce the total drilling costs by up to 65%.
Brittle Creep and Associated Acoustic Emissions in Granite Under Unconfined Compression
Authors: Zafar Sana , Hedayat Ahmadreza , Li Bing Q. , Moradian Omid
Published: September 2023

Time-dependent processes in rock govern its long-term behavior hence understanding of these processes is of great importance especially for long-lasting surface and underground structures. Results from conventional creep experiments have shown that three stages of creep (trimodal creep curve) exist for brittle materials. The first stage is the primary stage where the strain rate is inversely proportional to time. In the secondary stage, partially reversible strain occurs in which the strain rate is small and constant. The final stage is the tertiary stage where accelerated strain rate occurs, causing the failure of the rock. It is hypothesized that Acoustic emission (AE) can be used as a proxy to identify the transitions from one stage to another in creep; however, the different focal mechanisms of the fractures produced in the three stages of creep in brittle rocks have not been discussed in detail. The study presented in this paper aims at investigating how AE signatures can monitor the brittle deformations during the three stages of creep and its correlation with the mechanical observations, so that these AE signatures can be used to gain important information in the field. Creep experiments were conducted on double-flawed prismatic Barre granite specimen in the laboratory under uniaxial compression to investigate the temporal and spatial evolution of the AE events during the primary, secondary and tertiary stages of creep. Results illustrated that brittle creep in granite follows the Omori's law and inverse Omori's law in the primary and tertiary creep regimes, respectively. Non-double-couple (NDC) sources were the dominant failure mechanisms in all stages of creep. These results can be of great importance as continuous and non-destructive monitoring of structures in rock can serve as precursory indications of instability of the rock engineering structures as well as in the analysis of earthquake aftershocks and recurrences.

INTRODUCTION

Fracturing in rocks can happen over time; hence, it is important to understand the microcracking mechanisms of time-dependent deformation in rocks. The analysis of time-dependent effects is significant in multiple areas, including the study of seismic hazards like earthquake recurrence and aftershocks (Scholz in 1968). It is also crucial in evaluating the long-term stability of rock engineering structures, such as tunnels, mining rooms and pillars, rock slopes, and nuclear waste repositories (Diederichs and Kaiser 1999, Nara et al. 2010, Paraskevopoulou et al. 2018, Zhang et al. 2020, and Zhao et al. 2022). Moreover, time-dependent effects also have a significant impact on the exploration and utilization of resources such as oil and gas reservoirs and enhanced geothermal systems (EGS) (Demarest 1976; Cornet 2007).
Enhancing Drilling Performance of Mud Hammers by Combining High Pressure Water Jets Slotting
Authors: Gerbaud L. , Jahangir E. , Velmurugan N. , Sellami H. , Cazenave F.
Published: September 2023

Geothermal energy is a leading candidate in fulfilling our transition efforts to a clean, sustainable, non-intermittent and renewable energy source. However, high drilling costs act as a showstopper in geothermal exploration. One of the main factors contributing to this high price is the drop in drilling performance when drilling through hard crystalline granite rocks that are usually found at depths greater than 4 km. Studies show that increasing the drilling performance by a factor of 4 with respect to the current drilling rates of 1-2 m/h can reduce the drilling costs up to 65%.

Percussion drilling using down the hole (DTH) hammer has proven to be efficient in drilling through hard rocks. In ORCHYD project, we combine high-pressure water jet (HPWJ) with a DTH hammer to increase the rate of penetration (ROP) by a factor of 3 to 4. The HPWJ creates a peripheral groove in the rock surface facing the drill bit. This peripherical groove of few millimeters is expected to release the rock surface from the surrounding confining stress regimes – requiring lower energy to break the rock using percussion drilling. It is also expected that the free surface created by the groove aids in reflecting the waves created during percussion. A DTH mud hammer is modified to allow the flow of HPWJ through the hammer body and redirect it to the periphery of the drill bit. The prototype developed in this work was tested on a laboratory scale drilling test rig under realistic downhole drilling conditions. At the current stage of the project, it is demonstrated that slotting peripheral grooves using the HPWJ improves the ROP of the hammer drilling system by a factor of 2 to 2.5.

INTRODUCTION

Deep geothermal energy is a promising and increasingly important source of energy production worldwide. Geothermal electricity production is particularly attractive for both rich and poor economies due to its ability to generate both heat and electricity, which are in high demand. Enhanced Geothermal Systems (EGS) are becoming more popular as they allow for combined electricity and heat production at geothermal power plants (Angelone (2014)). However, drilling operations are a major component of the total project cost, representing up to 40% of the total cost (Angelone (2014)). For a 3 km well, the cost can reach up to 15 M€ (Angelone (2014)), which makes it challenging to extract geothermal energy in an economically viable manner.
The Damage Mechanics Challenge Results: Participant Predictions Compared with Experiment
Authors: Morris J. P. , Pyrak-Nolte L. J. , Yoon H. , Bobet A. , Jiang L.
Published: September 2023

We present results from a recent exercise where participating organizations were asked to provide model-based blind predictions of damage evolution in 3D-printed geomaterial analogue test articles. Participants were provided with a range of data characterizing both the undamaged state (e.g.: ultrasonic measurements) and damage evolution (e.g.: 3-point bending, unconfined compression, and Brazilian testing) of the material. In this paper, we focus on comparisons between the participants’ predictions and the previously secret challenge problem experimental observations. We present valuable lessons learned for the application of numerical methods to deformation and failure in brittle-ductile materials. The exercise also allows us to identify which specific types of calibration data were of most utility to the participants in developing their predictions. Further, we identify additional data that would have been useful for participants to improve the confidence of their predictions. Consequently, this work improves our understanding of how to better characterize a material to enable accurate prediction of damage and failure propagation in natural and engineered brittle-ductile materials.

INTRODUCTION

Understanding the failure of materials is particularly relevant today with the current interest in aging infrastructure and in enhanced geothermal systems which require a network of fractures to optimize production. As more and more sensors are used to monitor infrastructure and the subsurface, methods are required to detect anomalous signals in data and link these signals to the underlying physics and mechanics of failure to determine if failure is imminent. This requires robust computational methods that capture the physics of failure and identify the measurable signatures of failures. While there are many computational approaches for simulating damage evolution in materials, few have been ground-truth tested with either known experimental data or with blind data sets.

The Fracture Challenge concept (e.g.: Boyce et al., 2014) has demonstrated that valuable lessons can be learned by comparing computational methods and workflow approaches taken by different teams in the blind prediction of controlled fracturing experiments. Previous challenges have focused upon failure in metals. Here, we present results from a recent challenge that considered damage evolution in additively manufactured samples where the mechanical response is similar to the brittle-ductile response of rock, yet highly reproducible from one test article to another by virtue of the 3D printing process.
Experimental Investigation of Fracture Orientation in Rocks Using Shear Wave Splitting
Authors: Kholy S. M. , Lee Hunjoo P.
Published: September 2023

The understanding of the orientation of pre-existing discontinuities is crucial for subsurface engineering applications, such as drilling, well completion, and hydraulic fracturing. A variety of methods have been developed to identify fracture orientation, including wellbore imaging and monitoring of microseismic events in reservoirs. Recent research has utilized shear waves to evaluate fracture orientation and density in porous media by analyzing the shear wave splitting (SWS). The aim of this study is to assess the fracture orientation in two rock types, synthetic Hydrostone and Eagle Ford shale, under triaxial loading conditions using the SWS phenomenon. Shear wave velocities were measured using two orthogonal transducers at each end of the fractured sample for transmitting and receiving shear wave travel times. The results showed that SWS, the velocity difference between the two shear waves, was the highest at fracture orientations of 0° and 90°, when one of the transducers was parallel with the fracture. Whereas SWS was the lowest for the sample with fracture at 45°, when the split shear waves attenuated during re-orientation with their preferred propagation directions. The findings of this study could help identify fracture orientation near the wellbore and contribute to minimizing various drilling and completion issues.

INTRODUCTION

Natural fractures are abundant mechanical structures that exist in most rock types, especially in carbonates and shale (Moore and Wade, 2013). These pre-existing discontinuities could cause several drilling issues including mud losses (Razavi et al., 2017) and drilling breaks (Narr et al., 2006). In production engineering, the presence of natural fractures may enhance the local permeability, and hence the production rate (Luffel et al., 1993; Sakhaee-Pour and Bryant, 2011; Ben et al., 2012; Al-Rubaye et al., 2020). However, in the meantime, it can also increase the gas-oil ratio and water cut impairing the hydrocarbon productivity (Aguilera, 1980; Wennberg et al., 2016). Furthermore, in hydraulic fracturing applications, these natural fractures are known to have significant impact on the hydraulic fracture trajectory and the Stimulated Reservoir Volume (SRV) depending on their relative orientation to the hydraulic fracture propagation direction (Gale, et al., 2007; Zhou et al., 2008; Bahorich et al., 2012; Lee et al., 2015).

The orientation of natural fractures is determined by the paleo-stress field of the most active tectonic period (Maerten and Maerten, 2006; Zhang et al., 2022). Nevertheless, this orientation may not always align with the current stress field (Abul Khair et al., 2013; Abul Khair et al., 2015; Likrama et al., 2019). Thus, characterizing the orientation of natural fractures is crucial for effective design of various subsurface engineering applications.
Numerical Study of Hydrogen Storage in Lined Rock Cavern and Implications of Hydrogen Embrittlement
Authors: Sun Zhuang , Wu Jingtian , Salazar-Tio Rafael , Fager Andrew , Crouse Bernd
Published: September 2023

Underground hydrogen storage could provide buffer capacity to store the excess energy from renewable resources. A lined rock cavern (LRC) is one of the hydrogen geologic storage site options. It offers great flexibility because it does not rely on the existence of salt caverns, aquifer formations or depleted hydrocarbon reservoir fields. However, many important aspects have to be studied before its full-scale implementation, among them (1) the effect of spatial variation of rock mass properties and (2) hydrogen embrittlement on the steel lining. We present a numerical study on these two aspects by simulating field tests at a pilot LRC project. For the effect of rock heterogeneity, we assign a spatial variation of elastic properties based on the Gaussian covariance model. The results show a good agreement with the field measurements. The spatial correlation length of rock mass elastic properties only has a minor impact on the cavern surface deformation. Regarding hydrogen embrittlement, our simulations predict a significant hydrogen diffusion in the welded low alloy steel (S355) lining after ∼2 months of operation, which may cause hydrogen leakage and facilitate the hydrogen embrittlement. We show that advanced numerical models are powerful tools to ensure the safety of hydrogen storage in a LRC.

INTRODUCTION

Energy production from renewable sources such as wind turbines and solar cells has gained lots of attraction to reduce emissions of greenhouse gases and mitigate global warming (Pörtner et al., 2022). However, electricity from such energy sources has a strongly fluctuating nature, therefore requires storage options at sufficient scale to fulfil demand requirements (Osman et al., 2022). With the help of electrolyzer technology, the surplus electricity can be used to produce hydrogen that can be stored and used as a fuel when needed. Underground geologic storage of hydrogen enables significant storage cost reductions as well as buffer capacity to store this surplus energy generated from renewable resources (Hassanpouryouzband et al., 2021; Liebscher et al., 2016; Ozarslan, 2012; Tarkowski, 2019). The underground hydrogen storage also provides opportunities for its commercial usage in industry and transport (Lord et al., 2014).