bluuebunny/crossref_metadata_2025 huggingface dataset:
Monitoring of Shear Failure Along Rock Fractures in Limestone and Granite with Seismic Wave Transmission
Authors: Han K. , Pyrak-Nolte L. J. , Bobet A.
Published: September 2023

A key challenge in using geophysical monitoring to assess the stability of rock fractures is the ability to identify precursory signatures to slip. Laboratory direct shear tests were conducted on induced tensile fractures in Indiana limestone and Sierra White granite under an effective normal stress of 2 MPa, to determine the geophysical signatures of slip along the fractures under both dry and saturated conditions. During the experiments, the normal load, shear load and shear displacement were recorded along with compressional (P) and shear (S) waves that were propagated across the fracture. For dry limestone and granite fractures, the amplitudes of both the transmitted P- and S-waves showed a distinct peak prior to shear failure. Such peak is taken as a precursor to shear failure. However, for saturated limestone and granite fractures, the amplitude of the P-waves displayed either a small increase or a continuous decrease, while a distinct peak in wave amplitude was still observed in the S-waves. This phenomenon can be explained by the increase in the normal fracture specific stiffness due to the presence of highly incompressible fluid, water, which leads to a decreased P-wave sensitivity to changes of the normal stiffness of the asperities under saturated conditions. Also under saturated conditions, a dramatic change in the decrease rate of the P-wave amplitudes was observed that either coincided or occurred close to the peak amplitude of the S-waves, which can be then considered also as a seismic precursor to shear failure. This observation was consistent through different tests and for both limestone and granite rocks under saturated conditions.

INTRODUCTION

Monitoring of frictional rock discontinuities, such as fractures, joints, and faults, is crucial to the evaluation of the mechanical instability of rock for the design of geotechnical structures as well as to the identification of potential natural hazards. Geophysical wave measurements, such as compressional (P) and shear (S) wave propagation, have been widely used as remote and non-destructive methods to monitor changes in the contact area and the state of stress along rock discontinuities. In laboratory experiments, changes in P- and S-waves in the transmitted, reflected, and converted modes have been used to assess local changes in frictional contact surfaces and subsequently understand slip initiation and propagation (Nakagawa et al., 2000; Hedayat et al., 2014a, 2014b; El Fil et al., 2019, 2021; Gheibi et al., 2021, Han et al., 2022a). For example, Mutlu and Bobet (2006) showed that slip along non-homogenous interfaces does not occur simultaneously at all locations along the interface. Also, significant progress has been made to identify and detect seismic precursors to shear failure of artificial rock (gypsum) discontinuities where frictional interfaces have been molded or saw-cut (Hedayat et al., 2014a, 2014b; El Fil et al., 2019, 2021).
Numerical Study on Rock-Breaking Mechanisms of Triangular-Shaped PDC Cutter
Authors: Wenhao He , Xue Li , Runqing Zhang , Zhenliang Chen , Huaizhong Shi , Zhongwei Huang , Chao Xiong , Jinming Sun , Chengtao Hu
Published: September 2023

Conventional PDC cutter usually gets broken when drilling into deep formations due to high hardness and strong abrasion. To help solve such a problem, triangular-shaped PDC cutters have been developed and utilized in the design of hybrid-cutters PDC bit, which have achieved enhanced rate of penetration and high life cycle. Nevertheless, few researches have focused on the rock-breaking mechanisms of triangular-shaped PDC cutter. This research carries out a series of numerical simulations to reveal the rock-breaking mechanisms of the triangular-shaped PDC cutter. Parametrical analysis has also been conducted to reveal effects of cutting parameters, rock strength, and cutter structural parameters on the rock breaking performances of the triangular-shaped PDC cutter. Numerical analysis results show that the triangular PDC cutter is more easily to crush the rock than the conventional PDC cutter by predominantly decreasing the cutting force. Given the same cutting depth and the cutting angle, the triangular-shaped PDC cutter can exhibit higher rock-breaking performances than that of the conventional PDC cutter. Also, optimal analysis should be done for the design of the triangular-shaped PDC cutter. These research findings help the readers understand the rock-breaking mechanisms and the further application of the triangular-shaped PDC cutter in deep hard formations.

INTRODUCTION

Along with the development of PDC bit technologies, PDC bits have accounted for more than 90% drilling footage around the world(Dan & Scott, 2015). Meanwhile, given increasing demand on energy consumption, the exploitation depth gradually shifts into deep and ultradeep formations beyond 6000 m, which are usually distinguished by high hardness and abrasion. In those formations, conventional PDC cutters usually get broken in the drilling process due to high impact, large vibration, and strong abrasion, leading to low rate of penetration (ROP), long drilling duration, and unnecessary non-productive time(NPT) (Durrand et al., 2010; J. Liu et al., 2019; Shahrul Amar et al., 2020; Shi et al., 2018; Zhu et al., 2019). To improve the drilling performances, by modifying the planar cutting surface into 3D cutting structure, innovative kinds of PDC cutters have been proposed to enhance the resistance of PDC bits to high impact and strong wear, such as the conical PDC cutter, the axe-shaped PDC cutter, and the triangular-shaped PDC cutter. In detail, the cutting structures of conical, axe-shaped, and triangular-shaped PDC cutters are respectively a conical, ridged, and trustum of pyramid cutting element. Following the available literature findings, many of the non-planar PDC cutters have exhibited better rock-breaking performances than that of the conventional planar PDC cutter.
Hydraulic Fracturing with Supercritical CO2 in the Laboratory: Ultrasonic Monitoring of Fracture Propagation at in Situ Stress and Temperature
Authors: Kovalyshen Y. , Giwelli A. , Sarout J. , Kiewiet L. , Shulakova V. , Esteban L. , Banks S. , Sarmadivaleh M. , Kato Y. , Katou M. , Takeuchi T. , Kuramoto D. , Chen Y. , Ishida T.
Published: September 2023

The paper reports results of hydraulic fracturing experiments. The fracturing was completed under in situ conditions in 15 cm cube of American Black granite and Montney shale samples using supercritical CO2. Different modes of ultrasonic monitoring have been used. The final fractures were also imaged with fluorescent resin. In contrast to previously reported tests, here we use very strong rocks, high confinement, and stress contrast.

INTRODUCTION

Injection of carbon dioxide (CO2) into subsurface rock formations presents multiple benefits, including reduction in greenhouse gas emissions into the atmosphere. One way of improving the economics of geo-sequestration is to use CO2 as a fracturing fluid to enhance gas productivity in unconventional reservoirs. CO2 can also be used to enhance production of coal seam reservoirs (CO2-ECSB). Understanding fracture initiation/propagation in presence of CO2 is also crucial for the long-term containment of the storage. Monitoring remotely fracture propagation can be achieved using seismic methods. To improve the interpretation of seismic monitoring data in the field we conducted hydraulic fracturing (HF) experiments with supercritical CO2, sc-CO2, in the laboratory under true triaxial stress conditions. We used ultrasonic methods to monitor fracture propagation in cubic rock samples 150 mm in size. For verification purposes the resulting fracture network was visualised after filling it with fluorescent resin. We present here the results of experiments conducted on low permeability American Black granite and shale rock samples.

EXPERIMENTAL SETUP

Equipment

The experiments were conducted in a true triaxial stress cell (TTSC) developed at Curtin University. The maximum compressive load is 1575kN in each spatial direction, while the maximum temperature is about 60°C.

The loading stress is transferred to the sample via steel plates (see Figure 1). Further details about the TTSC and the experimental setup can be found in Wang et al. (2019). Each plate accommodates 10 ultrasonic transducers (pink circles in Fig 1). The transducers are spring loaded against the sample's surface. For this study we used commercially available P-wave ultrasonic transducers (Olympus V103-SM) with a resonant frequency of 1 MHz and a bandwidth from 0.5 to 1.5 MHz. The outer diameter of the transducer is 18 mm, and the crystal size is 13 mm. The transducers were connected to a detection/recording system developed in-house at CSIRO. Due to the system limitations at the time of testing, we could only use 32 ultrasonic transducers out of 60 available (see Figure 2).
Influence of Fault Rupture Velocity on Directivity Effect in Near-Fault Ground Motion
Authors: Zhiyuan Li , Hazarika Hemanta , Guangqi Chen , Zishuang Han , Chaofan Feng
Published: September 2023

Considering the destruction caused by the directivity effect of near-fault ground motion, finite difference models with different source modes are established to explain the directivity effect mechanism, the directivity characteristics of ground motion in the 2016 Kumamoto earthquake are analyzed, and the influences of fault rupture velocity (Vr) on the directivity effect are analyzed. The following conclusions can be made: (1) The directivity effect is mainly caused by the superposition of energy in the forward direction of the fault rupture, which is most significant when the Vr approaches the shear wave velocity (Vs). (2) The ground motion caused by the 2016 Kumamoto earthquake has a distinctive directivity. (3) The Vr is a critical factor influencing the directivity effect, the directivity effect enhances with increasing Vr.

INTRODUCTION

Near-fault ground motions have gained more attention since the 1994 Northridge earthquake, the 1995 Kobe earthquake, and the 1999 Chi-Chi earthquake (Alavi & Krawinkler, 2004; Bray & Rodriguez-Marek, 2004; Hall et al., 1995; Kalkan & Kunnath, 2006; P. G. Somerville, 2003). The ground motion in the near-fault area is significantly affected by the fault rupture mechanism, rupture process, site location, and permanent ground displacement (X. Chen & Wang, 2020; Y. Zhang et al., 2022). The ground motion in near-fault areas may differ greatly from the ground motion far from the fault areas, which is the near-fault effect of ground motion (Sun et al., 2020; Wu et al., 2021; Yang et al., 2023). The near-fault effect is generally expressed in two aspects: one is the difference with the ground motion characteristics in the area far from the fault, such as the difference in ground motion waveform, spectrum, and duration. The other aspect is the characteristics of ground motion distribution in the near-fault area, such as the distribution characteristics in peak ground velocity (PGV), peak ground acceleration (PGA), and response spectrum amplitude (Farrugia et al., 2017; Jinjun Hu et al., 2011; Mo et al., 2022). Near-fault ground motions are usually characterized by long-period velocity pulses, which have significant destructive effects on large structures and also increase the risk of landslides, debris flows and other disasters (Bray & Rodriguez-Marek, 2004; Lu et al., 2021; Quaranta et al., 2022; P. G. Somerville, 2003; P. G. Somerville et al., 1997; Song et al., 2018). The directivity effect of fault rupture propagation is one of the primary causes of velocity pulse (Bray & Rodriguez-Marek, 2004; P. G. Somerville et al., 1997; Wang et al., 2002).
Manifestation of Negative Compressibility in Partially Saturated Rocks: Theory and Experiment
Authors: Mews K. S. , Lozovyi S. , Holt R. M. , Rozhko A.
Published: September 2023

In this study, laboratory experiments were conducted on partially saturated Mancos shale to investigate the elastic response of the sample under a simultaneous increase by the same magnitude in confining stress and pore pressure in the gas phase. Surprisingly, we observed an extensional volumetric strain of the sample and a simultaneous reduction of shear wave velocities at ultrasonic and seismic frequencies. These observations may be described by apparent effective stress coefficients significantly larger than one. A possible interpretation is a negative shale stiffness leading to the volume increase during compression at constant net stress. To further explain these results, the model of Rozhko (2021) based on the hysteresis of liquid bridges inside microcracks was applied. The stiffness associated with capillary forces is negative, indicating that capillary force may dominate the elastic response inside microcracks with a nanometer-scale aperture. These laboratory experiments and theoretical modeling contribute to a better understanding of multi-phase systems at the nanometer scale comprising of liquid, gas, and solid. Specifically, this research can help in carbon dioxide storage and monitoring through a better understanding of overburden rock's static and dynamic response.

INTRODUCTION

Understanding the mechanical behavior of multi-phase systems consisting of a liquid and a gaseous phase in overburden caprocks is necessary for Carbon Capture and Storage (CCS) monitoring, assessing migration pathways, and pore pressure predictions from seismic velocities. It is important to understand pore pressure effects not only in the liquid phase but also in the gaseous phase.

Elastic properties of porous media with empty and fluid-filled pores are described in classical poroelasticity theory (Gassmann, 1951; Biot, 1962). Brown and Korringa (1975), Rice and Cleary (1976) and Cheng (2021) pointed to the fundamental differences between unjacketed bulk modulus and unjacketed pore modulus, which is neglected in the Gassmann theory. Accounting for global and local flow in the pore space and for patchiness in saturation or rock properties introduces frequency dependence and attenuation related to the viscosity of the pore fluid and the distribution of fluid phases in the pore space. Water can significantly soften due to a change in surface energy (Parks, 1984; Tutuncu and Sharma, 1992).
Multi-Lengthscale Geomechanical Modelling and Monitoring of a Compacting Highly Depleting HPHT Field – A Case Study from Norwegian Continental Shelf
Authors: Zhou C. , Andrews J. S. , Saure-Thomassen A. , Renli L. , Andrew M. J.
Published: September 2023

The HPHT field discussed in the paper is one of the major gas production fields on the Norwegian continental shelf. Current depletion is more than 600bar and a unique and comprehensive surveillance program, at multiple length scales, has been in place to monitor the resulting geomechanical effects. This includes: 1) four well re-entries over the last 15 years to log in-situ reservoir compaction measurements based on gamma ray markers, 2) 18 years of continuous offshore platform GPS recording for seabed subsidence and 3) multi-finger caliper measurements for liner deformation at selected producers. In particular, the reservoir compaction has been monitored since 2005 using Compaction Monitoring Instrument (CMI) that measures the compaction between preplaced radioactive markers in the formation. The results from the repeated CMI logging showed that the compaction in the entire reservoir interval is not uniform and is directly influenced by rock mechanical properties. A delayed creeping effect in the producing interval was clearly revealed from core tests and supported by the most recent CMI logging in year 2021. The 1D well-based and 4D field-wide geomechanical models are validated by comparison of predicted deformation with observed casing damage, and by comparison of the predicted surface subsidence with field measurements, respectively. These monitoring results have been used by multidisciplinary teams to improve the understanding of the subsurface, such as sand production, water breakthrough, holdup depth during well intervention, re-perforation and production forecast.

INTRODUCTION

The Norwegian Continental Shelf is a critical supplier of oil and gas to the European market to support energy security. One of the most significant gas production fields on the Norwegian Continental Shelf is a High Pressure High Temperature (HPHT) field, which is the primary focus of this study. Reservoir compaction, which occurs as a result of fluid production and the associated pressure reduction, is an important geomechanical process that requires attention for HPHT fields that will undergo large depletion during primary recovery. The compaction of the reservoir rock and the resulting deformation in the burdens can cause a variety of effects, including well deformation, seabed subsidence, and changes in reservoir permeability. Understanding the effects of reservoir compaction is crucial for maximizing production and ensuring the long-term stability of the reservoir.
Characterization of Gas Transport in Fractured Rocks for Underground Nuclear Explosion Detection
Authors: Li Wenfeng , Neil Chelsea W. , Stauffer Philip H. , Carey J. William , Meng Meng , Frash Luke P.
Published: September 2023

Underground nuclear explosions produce noble gases that can migrate to the surface and become detectable by atmospheric monitoring tools. However, it is challenging to predict radionuclide gas migration in the complex engineered and natural subsurface systems due to several issues. These issues include generation of complex fracture networks near an engineered cavity, reactivation of natural fractures, coupled hydro-thermo-mechanical processes for transport of high-pressure gases in a fractured porous rock system. To improve our understanding of these technical challenges, we used a triaxial direct shear test scheme to characterize coupled hydro-mechanical gas transport in fractured porous rocks. In the experiments, we tested nitrogen (an analog noble gas) flow through two types of rocks with distinct petrophysical properties: porous Bandelier tuff and tight Climax stock granite. For each type, we measured the Biot effective stress coefficient, the rock matrix permeability, and the fracture permeability under various stress states, which allowed us to examine the stress-dependency of the Biot coefficient and investigate rock matrix-fracture interactions for transport of pressurized gas. Comparing the experimental results for the two distinct rock types, we observed some important findings for gas transport in fractured rocks. First, rock fracturing does not necessarily increase samples’ gas permeability when the rock matrix is highly porous. Furthermore, gas permeability of intact rocks can show strong exponential stress dependency even when the rock matrix is very tight. Additionally, Biot effective stress coefficients are not necessarily close to unity for highly porous rocks, especially when rocks are subjected to large effective stresses. In summary, the experimental results can be used to improve the physics in high-fidelity numerical modeling to elevate our understanding of radionuclide gas transport in fractured porous rocks after an underground nuclear explosion event.

INTRODUCTION

Surface radionuclide monitoring is the primary means of determining if an underground explosion is nuclear in nature (Maceira et al., 2017). Following an underground nuclear explosion (UNE), signature noble gas radionuclides, such as Xe-131m, Xe-133, Xe-135, and some krypton radioisotopes, will be produced by nuclear fission (Carrigan et al., 1996; De Geer, 1996; Sun and Carrigan, 2014). They are hard to contain and tend to seep from the underground explosion and migrate to the surface. Surface sampling and detection of these signature gaseous radionuclides, when above some background levels, is a strong indicator of the occurrence of an underground nuclear explosion. By comparison, seismic monitoring cannot definitively discriminate between chemical (for example, TNT) explosions and nuclear events. It is thus important to understand transport of noble gas radionuclides in the subsurface rock strata.
Buzios Field: Characteristics and Geomechanical Challenges of the Giant Carbonate Offshore Field
Authors: Falcao F. O. L. , Chaves R. A. P.
Published: September 2023

Buzios Field, a pre-salt reservoir located approximately 180 km away from Rio de Janeiro coast, in Santos Basin, is characterized by its great dimensions: Its extensions involve an area of 852 km2 and presents a 400 m of net pay in average, in a water depth of 2000 meters, below an interval of another 2000 m of salt, which is situated below an interval of 1000 m of post-salt sediments. Buzios great production potential may be illustrated by the performance of its production wells: one single well has reached the spectacular mark of 70 kboed of peak production. Along with all this production greatness, come the associated geomechanical challenges: hydromechanical behavior of faults, fractures, and karst; thermal effects due to cold water injection in a warm porous media; determination of relative permeability curves within fractures considering geomechanical effects - just to mention a few. In this way, the aim of this paper is to present the main characteristics of Buzios Field, the associated geomechanical challenges and how Petrobras’ team deals with them, taking the geomechanical role beyond safety, moving towards reservoir management.

INTRODUCTION

Buzios Field, a pre-salt reservoir located approximately 180 km away from Rio de Janeiro coast, in Santos Basin, is characterized by its great dimensions: Its extensions involve an area of 852 km2 and presents a 400 m of net pay on average, in a water depth of 2000 meters, below an interval of another 2000 m of salt, which is situated below an interval of 1000 m of post-salt sediments. It was first discovered in 2010 by the 2-ANP-1-RJS well. It had its commerciality declared in 2013 after a 3D seismic acquisition, reprocessing, and a long-term test. Three other extended tests were performed in 2015, 2016, and 2017, which confirmed its excellent oil production potential, good reservoir communication, and allowed drainage plan adaptations. In the following years four production units were installed, each of them with a processing capacity of 150 kbpd. These FPSOs (Floating Production Storage and Offloading units) made it possible to reach the outstanding mark of taking less than two years to reach 600 kbpd of production, starting from its first oil. As a reference, Tupi Field, the previous pre-salt giant carbonate reservoir, also located in Santos Basin, took almost 6 years to reach the same daily production. Marlim, the previous giant field, a turbidite sandstone, took almost 11 years (Figure 1).
Relationship Between Static and Dynamic Elastic Moduli in Carbonates
Authors: Soustelle V. , ter Heege J. , Wassing B.
Published: September 2023

In an effort to assess the seismic potential of geothermal plays in Dinantian carbonate reservoirs in the Netherlands and Belgium, geomechanical induced seismicity models were developed. However, the static elastic moduli E (Young's modulus) and v (Poisson's ratio) formation properties are required as input for these models. Due to the lack of available core plugs from wells in the area of interest and unsatisfying static-dynamic relationship in the literature, we designed a series of experiments to build a new static-dynamic relationship for Dinantian carbonates that take into account the effective confining pressure ((Equation)). The triaxial experiments were carried out on massive limestone and karstified dolostone collected in a quarry near the Belgium Dinantian geothermal play. The static and dynamic elastic moduli Es, Ed and vs, vd were measured at various (Equation) ranging from 1 to 35 MPa (mean effective stress in the geothermal reservoirs). We established the following relationships for the karstified dolostone:

(Equation)

And the massive limestone:

(Equation)

INTRODUCTION

Geothermal heat is expected to become an important sustainable energy source to help reach the targets set by the National Climate Agreement of the Netherlands, which aims to reduce greenhouse gas emissions by 50% in 2030 and attain zero net carbon emissions by 2050. New geothermal plays, such as the Dinantian carbonates, are targeted to reach these objectives. However, they present geological characteristics quite different from the more conventional sandstone reservoir usually targeted in the Netherlands. The Dinantian carbonates are also heterogeneous and present various geological characteristics that affect the flow and mechanical properties of the reservoirs. In the southeast of the Netherlands, two geothermal doublets in the Dinantian have been stopped after a limited number of seismic events were recorded with a maximum magnitude of ML 1.7 (Baisch & Vörös, 2019). Seismicity has also been recorded in a Dinantian geothermal play near Mol in Belgium. Although the seismic activities were related to geothermal production, the causal mechanisms are still poorly constrained (Ter Heege et al., 2020).
Compaction of Crushed Salt for Safe Containment - Overview of Phase 2 of the KOMPASS Project
Authors: Friedenberg L. , Czaikowski O. , Lerch C. , Müller-Hoeppe N. , Rahmig M. , Bartol J. , Düsterloh U. , Lerche S. , Saruulbayar N. , Laurich B. , Svensson K. , Zemke K. , Thiedau J. , Liu W. , Gartzke A. K. , Popp T. , Lüdeling C. , Rölke C. , Rabbel O. , Reedlunn B. , Bean J. , Mills M. , Coulibaly J. B. , Spiers C. , De Bresser J. H. P. , Hangx S. , van Oosterhout B.
Published: September 2023

Crushed salt may be used as backfill material and sealing measures of open cavities, drifts and shafts in nuclear waste repositories located in rock salt formations. The host rock salt will gradually compact the crushed over time due to salt creep. A comprehensive understanding of the crushed salt compaction process is essential, yet crushed salt compaction involves several thermal-hydraulic-mechanical (THM) coupled processes and is influenced by internal, as well as external conditions. The current understanding has some important knowledge gaps. For example, the porosity/permeability evolution, especially for the low porosity range is not known in its entireness and the calibration of numerical models is not finished yet. The KOMPASS projects were initiated with the aim to reduce these knowledge gaps and improve predictions of crushed salt behavior. This paper gives an insight in the KOMPASS projects with focus on the outcomings of the second phase. The experimental program for a detailed investigation of crushed salt compaction is presented. In the microstructural investigations, microscale indicators were found that can clearly be related to pre-compaction treatment. The numerical work showed that deficits still exists in the simulation of crushed salt compaction with the available constitutive models.

INTRODUCTION

Rock salt is considered as a possible host rock formation for the underground disposal of high-level nuclear waste in several countries. The safety concept for a salt repository in Germany is based on a multi-barrier system including the rock salt as geological barrier, geotechnical seals ensuring the safe containment and the waste canisters (Bertrams et al., 2020). For backfilling and sealing measures of open cavities crushed salt will be used due to its favorable properties as mined-off material, its easy availability and its lithological characteristics which guarantee a maximum compatibility with the host rock. The use of crushed salt for barriers and seals represent an important paradigm shift in repository design in Germany, since crushed salt previously served only as stabilizing backfill.
Numerical Analysis for the Simulation of Crushed Salt Compaction Behavior
Authors: Friedenberg L. , Olivella S. , Düsterloh U.
Published: September 2023

In Germany, rock salt is considered as a possible host rock formation for a repository for high-level nuclear waste. Crushed salt will be used for backfilling and sealing measures of open drifts and shafts, providing the tightness of the repository and the safe containment of the waste in long-term. For safety assessment, a qualified prognosis of the crushed salts long-term behavior is crucial. This includes full knowledge and understanding of the thermal-hydraulic-mechanical coupled process, as well as a reliable numerical prediction of the compaction in long-term. In the current state uncertainties still remain with respect to the numerical simulation of crushed salt compaction processes, especially in the low porosity range (≤ % 5). This paper presents an approach for the modification of a constitutive model formulation implemented in the finite-element code CODE_BRIGHT with the objective to improve the numerical reproducibility of crushed salt compaction. The shortcomings are exemplary presented by means of the simulation of the triaxial compaction test TUC-V2. Based on the findings two approaches for the modification of the geometry-based model formulation are presented.

INTRODUCTION

In Germany, rock salt is considered as a possible host rock formation for the deep geological disposal of high-level nuclear waste. The safety concept is based on a multi-barrier system including the geological barrier rock salt, geotechnical seals, backfill material and the waste canisters (Fig. 1). Immediately after the disposal, the sealing function will be given by the waste matrix, waste canisters and geotechnical barriers. The rock salt as geological barrier and the backfill material will provide the sealing function in the long-term.

The backfilling of open cavities, drifts and shafts will be done with crushed salt, since it is a long-term stable material, easily available (mined-off material) and guarantees a maximum of physo-chemical compatibility with the host rock. Due to the creep ability of the rock salt, the crushed salt backfill becomes naturally compacted with time by convergence of the open cavities. It is expected that during the compaction process, porosity, and permeability of the crushed salt decrease down to barrier properties comparable to undisturbed rock salt (Φ ≤ 1 %).
Post-Fracturing Evaluation of Fractures by Interpreting the Dynamic Matching Between Proppant Injection and Fracture Propagation
Authors: Hou Lei , Zhang Fengshou , Elsworth Derek
Published: September 2023

This study presents a new data-driven approach to interpreting fracture evolutions by analyzing the dynamic matching between proppant injection and fracture propagation. Proppant settling in low-viscosity fluids accumulates at the bottom of the fracture, compresses the flowing channel, and increases the net pressure in fractures due to increasing flow friction, which is the inherent correlation between proppant injection and fracture propagation. Evaluating the flow status of injected proppant in fractures may reveal how much fractures have been filled with proppant, and then estimate the fracture propagation indirectly - the fundamental logic of this study (using proppant as an indicator to detect the underground fractures). To accomplish this new conception, a machine-learning-based workflow is established by integrating an ensemble learning algorithm and a newly defined parameter - proppant filling index (PFI). Data from shale gas fracturing wells are collected to train the algorithm for the prediction of PFIs. The variations in PFI curves are then used to reveal the dynamic matching between proppant injection and fracture propagation, based on which the development of underground fracture volumes is estimated.

INTRODUCTION

The recognition of fracture propagation by hydraulic fracturing is a critical and generic technique for securing the injecting operation, promoting the oil and gas recovery, and evaluating the cracked reservoirs for subsequent storage of CO2 or hydrogen. In shale reservoirs, the artificial fracture system provides the essential underground channels for injection (fracturing fluids and proppant) and production (oil and gas). The safety of proppant injection and post-fracturing production rely on the flow through these artificial channels (Hou, Cheng, et al., 2022; Mayerhofer et al., 2006; Wang et al., 2018). Recent research has suggested that the fracture system is an ideal repository for CO2 or H2 in shale reservoirs, as the extremely tight matrix safely confines extraneous fluids within the fractures (Heinemann et al., 2021; Singh, 2022; Wang et al., 2022).
Seismic Source Wave Propagation Analysis Using Laboratory AE Monitoring System
Authors: Madenova Y. , Suorineni F. T. , Mukhamedyarova Z.
Published: September 2023

The source location algorithm in microseismic monitoring systems assumes a constant velocity model, which does not account for the effects of voids, fractures, backfills, and changing mine geometry in underground mines. This study investigated Acoustic Emission (AE) signal propagation characteristics towards signal receivers in laboratory physical models to understand the influence of various factors on seismic wave velocity. It was observed that the AE wave bypasses the voids and reflects off the backfill boundaries and follows the path of least resistance described by Fermat's principle and Snell's law. AE measurements show 1) similar signal travel time to sensors located across the void when the void in the concrete blocks was empty and when filled with various backfill types; 2) increasing backfill age had no influence on the signal travel time to the sensors. It was also found that the signal triggering behavior of the sensors in the proximity of the block voids is governed by Snell's law. This study concludes that the combination of the sensor network ray tracing method with Snell's law would be a promising solution to accurately determine applicable input wave velocities in seismic monitoring systems.

INTRODUCTION

The accurate location of seismic event sources is of practical importance for identifying seismic and burst-prone areas in underground mining, particularly when the ground condition and mine geometry are continuously changing. The effective tool employed by rockburst suspected underground mines to mitigate rockburst hazards is the microseismic monitoring system. The velocity model (VM) assigned to the rockmass is a fundamental parameter in microseismic monitoring systems used to define the seismic source parameters and to locate the seismic event sources. Several researchers pointed out that high uncertainties in microseismic source location come from the VM (Husen and Hardebeck, 2010; Collins et al. 2014; Reyes-Montes et al, 2016). The microseismic systems in most mines rely on a single or layered VM that is constant for source location purposes. This velocity model assumes an isotropic and homogeneous medium with the same properties throughout the volume of interest.
A Model for Optimizing Proppant Distribution Between Perforations
Authors: Dontsov E. V. , Hewson C. W. , McClure M. W.
Published: September 2023

Uniform proppant distribution between perforation clusters is one of the goals in hydraulic fracturing operations. However, this can be challenging to achieve, even if limited entry is used to acheive a nearly uniform fluid distribution. Two mechanisms contribute to a non-uniform proppant distribution between perforations. The first is particle settling in the wellbore, which becomes especially important towards the toe of the stage where the flow velocity is lower. The second mechanism is related to proppant inertia, whereby some particles are unable to follow the fluid streamlines and miss the perforation. In this paper, a mathematical model is developed to simulate both of these physical phenomena and is calibrated against available experimental and computational data. The model is then combined with an optimization algorithm to investigate perforation designs leading to nearly uniform proppant distribution between perforations. It is found that it is possible to significantly improve proppant placement by varying perforation orientation along the stage. If the same orientation is required for all perforations, then perforations located in the lower part of the wellbore with azimuths between 110° and 120° lead to more uniform proppant distribution, compared to other cases.

INTRODUCTION

Many studies have addressed the problem of particle distribution between perforations because this is a critical issue for optimization of hydraulic fracturing treatments. One of the first experimental studies (Gruesbeck and Collins, 1982) investigated the problem of slurry flow in a perforated vertical well, as is typical for conventional hydraulic fracturing treatments. It was observed that particle size and viscosity play a significant role on the resultant amount of proppant leaving through the perforation. With the rise of unconventional resource development as well as computational capabilities, Computational Fluid Dynamic (CFD) models have been used to address the same problem of particle turning into a perforation (Wu and Sharma, 2016; Wu, 2018). What is interesting is that in this study authors observed practically no dependence on particle size and fluid viscosity. A model that is employed in this study for the optimization purposes (see Dontsov (2023)) is able to explain such a contradiction by showing that the parameters used in Gruesbeck and Collins (1982) and Wu and Sharma (2016); Wu (2018) lead to dominance of different particle turning mechanisms, which in turn caused different sensitivities.
Evaluating Coupled Hydraulic-Mechanical Behavior in the Slate Geothermal Reservoir Using Discrete Element Analysis
Authors: Peng C. H. , Weng M. C. , Le H. K. , Shiu W. J. , Wu P. L.
Published: September 2023

Taiwan is rich in geothermal energy, which mainly distributes in the slate area. Due to the high anisotropy and heterogeneity of slate, the properties of rock mass are quite different from isotropic rock. This study conducted a discrete element analysis to evaluate the damage area of hydraulic stimulation in the slate for the operation of a geothermal power plant. A failure criterion for slate incorporated with a transversely isotropic model was established to describe the behavior of slate. The developed model was validated using a benchmark problem of geothermal techniques under an isotropic condition. The analysis results indicated that anisotropy significantly affects stress and strain distributions. The proposed model was further applied to analyze the injection response in a slate geothermal site. The analysis results revealed that the proposed model could reasonably simulate the anisotropic coupled hydro-mechanical behavior of slate geothermal reservoirs.

INTRODUCTION

Taiwan is located in the active plate with complex volcanic activities forming different foliated metamorphic rocks such as slate and schist. The strata also have a high potential for the development of geothermal energy. The slate was formed in the Eocene to early Miocene, and showed a high degree of anisotropy and heterogeneity, so the physical properties of hereby rock mass were different from the general isotropic ones. The existing estimation method and current constitutive model may not fully reflect the mechanical behavior of foliated slate in the geothermal site of Taiwan.

Moreover, the permeability of rock formations under high temperatures was typically low, resulting in inefficient geothermal heat for energy extraction. The Enhanced Geothermal System (EGS) used high-pressure fluid to generate artificial fractures in the rock formation, which could improve the efficiency of geothermal extraction. Therefore, understanding the mechanical characteristic of rock mass comprising the damage development and fracture propagation during and after the process of high-pressure injection was a key point for the investigation. Compared with expensive and time-consuming in-situ tests, numerical simulation was more economical and potentially applied for geothermal study in the rock mass, especially under complex geological and stress conditions.

In this study, a three-dimensional hydraulic-mechanical coupled model was constructed based on the distinct element method (DEM) software 3DEC, which was first validated by simulating a benchmark problem of the US department of energy. Besides, the mechanical behavior of foliated slate was also developed by performing a series of direct shear and pull-off tests. Finally, the 3D hydraulic-mechanical coupled simulation of the Qingshui geothermal reservoir was conducted to study the stress-strain behavior and fracture propagation of the slate under short-term hydraulic injection.
Particle Size Optimization of Lost Circulation Materials: A Comprehensive Experimental Study
Authors: Li Bingxie , Feng Yongcun , Ma Chengyun , Li Sishuai , Lai Chenxi , Zhang Shuai , Su Feiyu
Published: September 2023

Severe lost circulation in fractured formations is one of the most common and challenging problems in oil and gas drilling engineering. The lack of theoretical guidance on the selection of lost circulation materials (LCMs) for millimeter-level fractures results in unstable fracture bridging, which is prone to repeat failure. In this paper, a comprehensive laboratory experimental study was conducted with modified PPA equipment for millimeter-level fractures. The effectiveness of the LCMs is evaluated by fluid loss amount, pressure-bearing capacity, and the LCMs bridge location and integrity. Experimental results show that a single LCM size distribution cannot effectively seal millimeter-level fractures, while an optimized combination of bridging and filling particles can achieve successful sealing. It is recommended that the mass ratio of bridging particles to filling particles be 2:1. The D50 and D90 of the bridging particles should be 35-40% and 70-80% of the fracture entrance width, respectively. The D90 of the supplementary flaky materials, e.g., mica, should be slightly less than the fracture entrance width. The results of this laboratory study can provide guidance for selecting LCM size for effective fracture sealing and improved wellbore strengthening performance.

INTRODUCTION

During the drilling process, lost circulation happens when a lot of drilling fluid leaks into the formation. Lost circulation can result in the drilling fluid's depletion and prolong the drilling cycle. Improper management of lost circulation may also lead to complex situations, such as wellbore instability, blowouts, and stuck pipes, ultimately resulting in catastrophic engineering accidents. The lost circulation rate is estimated to account for approximately 20% to 25% of the total number of drilled wells. The annual cost of well plugging can reach $4 billion USD.

In North America (the United States and Canada), the percentage of drilled wells that experience severe lost circulation in carbonate oil and gas reservoirs, shale oil and gas reservoirs, and other formations amounts to about 40% of the total. In the Middle East, the wells that experience severe lost circulation during the drilling of carbonate-fractured oil reservoirs account for more than 30% of the total, with the time loss due to lost circulation exceeding 50%.
Unloading Response of Tunnels in Deep Brittle Rock Mass Considering Ubiquitous Minor Joints
Authors: Peng S. Y. , Li X. B. , Chen G. Q. , Mitani Y.
Published: September 2023

Minor joints exist ubiquitously in deep-buried brittle rock masses subjected to high in-situ stress. These joints affect the strength and deformation properties and lead to the concentration and dissipation of stress wave propagation. Through modeling and verification, the rock mass model in the study with closable minor joints can perform the plasto-elastic behavior under static compression, and the increase of wave velocity with confining stress under dynamic load. The unloading response of tunnels is analyzed in two influencing factors: unloading rate and lateral stress coefficient. According to the results, the increase of unloading rate will intensify stress oscillation and increase additional dynamic stress disturbance of the joint rock mass near the tunnel boundary. As the vibration dissipates kinetic energy, the additional dynamic stress disturbance farther away from the tunnel boundary is decreased. Additionally, the minor joints can open or close in particular directions depending on the complex in-situ stress and the stress distribution of the surrounding rock mass. At the location along the direction of open joints and close to the stress release source, the joint rock mass may have a higher additional stress concentration than in the elastic model.

INTRODUCTION

The scarcity of shallow mineral resources and construction space has substantially escalated the depth of tunnel excavation (Li et al., 2017; Xie et al., 2015). Deep rock mass subjected to gravitational and tectonic stress reserves enormous strain energy and exhibits evident fracture development. Deep tunnel excavation is typically a dynamic unloading process (Cai, 2008; Lu et al., 2012). In particular, the strain energy will be immediately released within several milliseconds and induce a pulse load when the tunnel is excavated utilizing drilling and blasting (Yang et al., 2013). This process may lead to various observed hazard phenomena, such as spalling (Jiang et al., 2013), rock burst (Ortlepp & Stacey, 1994), large deformation (He et al., 2005), and zonal disintegration (Jia & Zhu, 2015). Meanwhile, the intensity and frequency of disasters escalate as excavation depth increases, threatening construction and operation safety (Cai, 2016). Therefore, the mechanism of unloading failure of deep tunnels in rock mass has always attracted the attention of many researchers.
Uncertainty Quantification of Young's Modulus on Core Scale: A Bayesian Analysis on a Comprehensive Geomechanical Model
Authors: Wu Zhidi , Edelman Eric , Smith Phil , Smith Sean , Irons Trevor , McPherson Brian
Published: September 2023

The ability to accurately measure the static Young's modulus is crucial for understanding subsurface storage reservoirs. However, obtaining this data can be difficult and costly. Much previous research focused on the impact of one or two factors on geomechanical properties at a single scale, but a more comprehensive understanding is needed. This study examines the impact of six rock properties - porosity, clay content, permeability, sample size, and the framework grain content to cement content (FGC/CC) ratio on Young's modulus in samples from five rock facies. The goal is to systematically quantify the effects of these secondary rock properties on a primary geomechanical property in a range of sandstones at the core scale. This study suggests that the pore abundance and the relative amount of framework grains and cements play competing roles in rock's elastic behavior. In addition, the surrogate model yields the minimum uncertainty and reflects nonlinear trends between Young's modulus and secondary rock properties. Overall, we represented a data-driven approach to quantify the uncertainty of Young's modulus of rocks using cost-effective experimental measurements.

INTRODUCTION

Understanding and minimizing uncertainties in geomechanical properties can lead to successful geological investigations and complex engineering projects and prevent accidents such as reservoir leakage and wellbore collapse. Elastic modulus is a primary property (among mechanical properties), but data are often unavailable and can be costly to measure. Alternative to direct measurements, estimating elastic modulus is more effective if the roles of secondary properties, such as geological, hydrological, and petrophysical properties, can be quantified. Previous studies often evaluate the correlation between primary and secondary properties to predict primary geomechanical properties (Han et al., 1986; Chang et al., 2006). However, the correlations between primary and secondary properties are not comprehensive, and the uncertainty associated with predictive primary geomechanical properties is generally unknown. Therefore, a framework for the Bayesian assessment of a comprehensive geomechanical model with constrained uncertainty has been developed to address this challenge. The comprehensive geomechanical model is capable of predicting elastic modulus based on main secondary properties.