bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of Ground-Support Interaction Using Continuum Grain-Based Models
Authors: Sinha S. , Walton G. , Shirole D.
Published: August 2024

Spalling of pillar ribs is a major hazard in the mining industry. Developing effective support guidelines requires an understanding of pillar damage mechanisms as well as the rock-support interaction. Bonded Block Models (BBMs) represent a convenient tool for this purpose, as they can reproduce the rock fracturing process reasonably well, but it is not known whether all variants of this modeling technique can replicate the impact of reinforcement (bolts) on otherwise unsupported ground. Broadly speaking, BBMs can be sub-divided into two groups: true discontinuum (implemented in DEM software) and pseudo-discontinuum (implemented in FEM software). Previous research by the authors has demonstrated the ability of DEM-based BBMs to reproduce rock-support interaction very well using real mining case studies. The current study is focused on evaluating the capability of FEM-based BBMs to replicate pillar damage mechanisms and rock-support interaction. Specifically, the stress-strain curves from BBMs developed in RS2 for three different width to height (W/H) ratio unsupported pillars (W/H = 1, 2, 3) were calibrated to match the macroscopic behavior of BBMs developed in UDEC, followed by analyzing the influence of rockbolt patterns in these models. Model outputs like stress-strain behavior, block and joint yield, and failure patterns were analyzed. With this, differences and similarities in the behavior of the two model variants have been discussed, considering their applicability in practical support design.

1. INTRODUCTION

Pillars are an integral load bearing member in underground mines and play an essential role in upholding the functional integrity of the mine openings. As mining continues to progress deeper, these pillars are subjected to ever higher stresses, which increases the incidence and severity of ground control issues, including, but not limited to spalling, buckling, pillar bursting, etc. (Kaiser et al., 1996; Diederichs, 2007). From a macroscopic failure standpoint, the design of pillars is well-studied (Hedley and Grant, 1972; Krauland and Soder, 1987; Potvin et al., 1989; Lunder and Pakalnis, 1997; Martin and Maybee, 2000). On the contrary, design of surficial supports (e.g. rockbolts, wire meshes etc.) for controlling stress-induced failures at the pillar periphery remains a topic of research, primarily because of the complex interaction between support elements and unsupported ground undergoing stress-induced damage.
LSTM Recurrent Neural Network Based Method for Identification of Drilling Operating Conditions and its Application
Authors: Chen Changchang , Ji Guodong , Zhang Hongyuan , Sun Yuqi , Wu Qiang , Lv Zehao
Published: August 2024

Polycrystalline Diamond Compact (PDC) bit underground operating conditions identification is one of the difficulties during the drilling operation. It is of great significant for the drilling improvement and accurate construction formation to accurately identify the complex operating conditions during drilling the operation such as PDC bit wearing and stick-slip vibration, etc. In this paper, data of relationship between torque, rate of penetration (ROP) signal and weight on bit (WOB), rev were obtained through lab simulation experiment, after which, a relationship database of torque, ROP signal and WOB, rev was established, a single-layer Long Short Term Memory (LSTM) recurrent neural network (LSTM) was constructed, the number of neurons, batch size and optimizer type of LSTM layer were optimized. With BPTT algorithm, LSTM model was trained and tested on the training set containing 6000 groups of data, and achieved the goal of identifying the bit operation conditions against the input of WOB, torque, rev and ROP signal parameters. Based on this model, the PDC bit underground operating conditions identification software was compiled, and was connected to the logging unit to realize real-time identification of bit underground operating conditions, and field test application was successfully carried out in Mahu 1 Well Region. The research showed that the convergence speed was fast and the time cost was at the lowest when the number of neurons of LSTM model was 50, and that the optimized Adam optimizer model could meet the requirements of convergence speed and prediction accuracy with the lowest loss value. The classification accuracy of this model on the test set reached 94% and the accuracy of the prediction method was also verified by field test application.

1. INTRODUCTION

During the drilling, complex underground conditions such as PDC bit wearing and stick slip may a impact the drilling cycle and drilling cost directly, in that bit wearing reduces the rock breaking efficiency. Either too early or too late pulling out of the hole(POOH) can affect the drilling effectiveness. After the bit stick-slip vibration, the cutting teeth can not effectively eat the rock, hence reducing the rock breaking efficiency. Serious bit stick-slip vibration may cause accidents such as increase of drill string torque and even breaking of drilling tool[1-3]. Therefore, real-time and accurate prediction of underground operating conditions of PDC bit and accurate guidance of tripping operation can effectively improve the mechanical ROP, reduce NPT and shorten the well construction cycle[4].
Effect of Cyclic Freezing and Thawing on Permeability and Mechanical Properties of Red Sandstone Under Triaxial Compression Condition
Authors: Liu B. , He Y. Q. , Han Y. H. , Li W. H. , Zeng J. Y.
Published: August 2024

During the construction of coal mines in western China, many boreholes are drilled through artificially frozen Cretaceous and Jurassic formations. After experiencing one freeze-thaw cycle, these water-rich and weakly cemented rocks generally demonstrate significant changes in permeability and mechanical properties, leading to frequent occurrences of water intrusion and flooding accidents. In this study, triaxial compression test and permeability test were conducted on red sandstone under conditions of both no freeze-thaw cycle and one freeze-thaw cycle. The variation of permeability and mechanical characteristics of red sandstone induced by one freeze-thaw cycle was investigated for different confining pressures. The results indicated that, after one freeze-thaw cycle, the peak strength and elastic modulus of the red sandstone decreased but the Poisson's ratio increased. Under low confining pressures (4 and 6 MPa), the permeability of red sandstone exhibited staging characteristics. Enhanced by the freezing-thawing cycle, the permeability of red sandstone significantly increased before the onset of crack initiation. Under high confining pressures (8 and 10 MPa), the permeability showed a continuous decreasing trend, as the expansion of vertical fractures was suppressed by the high confining pressure. The test results from this work are helpful to stability analysis of the mine shaft and pre-control of seepage damage after the thawing treatment of the frozen coal mines.

1. INTRODUCTION

During the construction of coal mines using artificial freezing method in western China, the frozen boreholes often pass through the Cretaceous and Jurassic formations. These water-rich and weakly cemented rocks are characterized as late diagenesis, poor cementation, low strength, and easy to become mud when encountering water (Zheng et al., 2011; Ma et al. 2018; Weng et al.2021). After experiencing artificial freezing and natural melting process, the permeability and mechanical properties of the rock further weaken, leading to frequent occurrences of water intrusion and flooding accidents (Deng et al. 2010; Huang et al. 2022; Xiong et al. 2023). Ultimately, it can cause significant losses of economy and threaten the safety of lives and property of humans.
Rock Engineering in Canadian Mining – Highlights, Challenges and Contributions
Authors: Falmagne Véronique
Published: August 2024

Recent years have been characterized by some significant world events and the continuing evolution of mining in a mature jurisdiction, such as Canada. Topics that will be discussed include managing Covid-19 and social expectations; expanding Canadian mining into harsh environments, in the Arctic and at great depth; and the need to extract more value from every drill hole in the context of rising costs and the push to reduce uncertainty.

From March 2020 to the second half of 2022, the mining industry had to manage through the Covid 19 pandemic and is now navigating a period of inflationary pressures in addition to meeting social expectations in terms of inclusion and collaboration with local populations and environmental excellence. In the Canadian Arctic, the Covid-19 pandemic represented a tremendous challenge to maintain operations without putting workers and local populations in remote communities at risk. The response from both mine management and individuals was overwhelming in its seriousness, inventiveness, and teamwork. The measures that were put in place in record time ensured workers and community safety and demonstrated true leadership in Canada's most remote regions.

Mining in the Canadian Arctic presents a host of challenges and opportunities which lend themselves to the development of technical solutions. For example, these include managing groundwater when mining below the permafrost, introducing the first paste backfill system operating in the Arctic and optimizing cemented rockfill in sub-zero temperatures.

At the other end of the spectrum, Canadian operations are reaching unprecedented depths, mining down to 3.4 km in environments where the stress around excavations exceeds the uniaxial compressive strength of the rock. Managing seismic and rockfall risks in such environments requires robust risk management systems based on continuous monitoring and analysis, design review and research. The risk management framework for seismically active mines is presented, highlighting successes and areas of continuous improvements.

Geotechnical models are the foundation of all rock engineering hazard assessment and risk management programs. Geological and structural models form the basis of any geotechnical characterization program. With rising costs and the need to reduce uncertainty there is growing pressure to extract greater value from every opportunity to look at the rock. In most cases, this means increasing the quality and the quantity of data extracted from drill cores and drill holes. Technological advances in core scanning and borehole geophysics are leveraged to enhance geotechnical databases and models.
Experimental Research on Characteristics of CO2-CH4 Transport in Water-Bearing Carbonate Gas Reservoirs
Authors: Huang Yize , Elsworth Derek , Li Xizhe , Hu Yong , Wu Zhenkai , Guo Qimin , Pei Xiangyang
Published: August 2024

We investigate the transport properties of CO2-CH4 under high pressure conditions in both dry and variably saturated porous and fractured carbonates containing native H2O and CH4. The specific focus is on determining optimal CO2 injection rates for maximizing CH4 recovery. A series of experiments involving fluid displacement at the core scale is conducted, considering various initial H2O and CH4 saturations and CO2 injection rates. Examination of the resulting residence time distribution (RTD) curves reveals several findings: (1) the larger pores support advective transfer with diffusion from the smaller pores contributing to tailing in the RTD; (2) the porous medium to free phase diffusion coefficient ratio (D’) in dry samples is related to porosity (φ) as D’ = φ3; (3) the presence of formation water increases heterogeneity, reduces effective porosity and decreases the Peclet number of saturated samples by two orders of magnitude and transforms D’; (4) displacement by CO2, alleviates water blockage in the removal of CH4 and switches transport to that of gas-phase convection. This research enhances our understanding of fundamental phenomena governing multiphase-component transport in porous and fractured media, contributing valuable insights to enhanced gas recovery and carbon storage in variably saturated carbonate reservoirs.

1. INTRODUCTION

Injecting captured CO2 into depleted reservoirs is an essential technology to decrease carbon emissions and may concomitantly enhance recovery of heavy or condensed hydrocarbons (Oldenburg et al., 2001; Liu et al., 2013; Liu et al., 2019). Despite this clear utility, such reinjection methods have been seldom used in carbonate gas reservoirs due to unknown response. Currently, the large number of water-bearing gas reservoirs with low recovery urgently require more effective techniques to improve gas production (Honari et al., 2016; Mohammed et al., 2021; Pan et al., 2023). Successful implementation of the injection of CO2 for enhanced recovery of hydrocarbons and concurrent CO2 storage holds great promise if the fundamental response can be defined and thus potential benefits established (Pooladi-Darvish et al., 2008; Pentyala et al., 2021; Cheng et al., 2022).

We explore the transport characteristics of CO2-CH4 at high pressure (32MPa) in both dry and variably-saturated porous and fractured carbonates containing native H2O and CH4 – in particular the role of water saturation on the CO2 breakthrough profile and subsequent gas recovery. Injection rates are varied to examine both diffusive and advective response and afterward define CO2 injection rates that optimize CH4 recovery. We conduct a series of core-scale fluid displacement experiments for a spectrum of initial H2O and CH4 saturations and CO2 injection rates. Subsequently, experimental data and analytical model are fitted.
Coupling Between Weathering and Fracture: Finite Element Modeling of Granite Weathering Using Cohesive Elements
Authors: Xu Tingting , Arson Chloé
Published: August 2024

This research examines the initiation of fracture propagation by weathering and explores how an escalation in fracture density accelerates weathering rates. By integrating cohesive zone elements into an ABAQUS FEM software through user-defined subroutines, we simulate arbitrary fracture paths and examine the effect of weathering on stress redistribution and fracture propagation in the bedrock. The weathering rate is scaled based on the magnitude of fracture opening displacements, which control fluid exchanges. Simulations in a topographical setting reveal that weathering significantly affects cohesive fracture propagation, particularly in the upper 30 meters where weathering accelerates both fracture propagation and the depletion of reactant mass, in particular unweathered biotite. These findings underscore the potential of the proposed methodology for predicting the interplay between weathering and fracture development, and thus explaining the formation of landscapes as a result of climatic events and ensued environmental processes.

1 INTRODUCTION

Fractures play a crucial role in enhancing the connectivity of pore spaces, thereby significantly increasing rock permeability. This facilitation of fluid flow is instrumental in accelerating mineral dissolution processes, as evidenced by Hu et al. (2011). Fracture systems increase the specific surface of the bedrock, which escalates the likelihood of chemical reactions (St. Clair et al., 2015), thereby altering rock structure through the interplay of chemical, mechanical, and hydrological processes. Such transformations are pivotal in promoting further weathering, as was demonstrated in studies that document the coupling between bedrock weathering and fracture propagation (Navarre-Sitchler et al., 2015). Reversely, weathering enhances rock permeability (Worthington et al., 2016). A simplified homogenization model was recently introduced (Lebedeva and Brantley, 2023) to explain the impact of reactive flow in fractured rock.

Despite these developments, the feedback mechanisms between weathering and bedrock porosity evolution via microcracks and metric fractures remain under-explored. In order to address this gap, Cohesive Zone (CZ) elements are inserted within the Finite Element Method (FEM) model initially developed by Xu et al. (2022). The fracture represented by the CZ elements, comparable in length to the volume elements, are significantly larger than the microcracks depicted in the damage variable of the homogenization model implemented in the Finite Elements (FEs), offering a novel approach to examining the intricate feedback effects between weathering and porosity evolution in bedrock. The homogenization approach is summarized in Section 2. The coupling between the CZ tractionseparation law and the continuum variables of the homogenization model is explained in Section 3. Numerical results are presented in Section 4, and conclusions are drawn in Section 5.
A New Evaluation Method for Drillability of Hot Dry Rock Considering In-Situ Temperature in Formation
Authors: Li Kun , Sun Zhaowei , Wang Zikang , Wu Xiaoguang , Ye Shanlin , Liu Muchen , Wang Xiangyang
Published: August 2024

Traditional assessment approach of rock drillability is limited in deep geothermal drilling, especially in Hot Dry Rock (HDR), due to the ultra-high abrasive resistance and temperature conditions. This paper proposed a new evaluation method to assess the drillability of HDR. A series of drilling tests were conducted on Gonghe granites under in-situ high temperature conditions. The effect of five key parameters, including bit weight, RPM, torque, penetration depth and rock temperature on rate of penetration (ROP) was investigated by HDR drilling experiments. Then, an unsupervised clustering method was employed to classify the ROP and its correlated parameters reasonably. Finally, the HDR drillability levels correlated with ROP were specified. Results showed that the ROP increases with a growth in rock temperature, which is conductive to promote the bit penetration. HDR is assigned a rating from 1 to 4 taking into account the effects of rock temperature, with a higher rating corresponding greater ease of drilling. The ROP may be classified into four modes: slow (0.28-2.39mm/min), slow-medium (0.56-3.09mm/min), medium (3.31-8.93mm/min) and fast (5.63-15.93mm/min). Our works are expected to provide a theoretical basis for the optimization drilling performance in HDR.

1. INTRODUCTION

Hot dry rock (HDR) possesses the attributes of being environmentally friendly, highly efficient, and stable, making it a significant alternative energy source for the future. Efficient exploitation of geothermal resources in HDR is of great significance to the global energy transition. Drilling is the dominant technology for development HDR resources, accounting for approximately 35% to 60% of the overall engineering investment. This technology necessitates drilling holes into the high-temperature reservoirs composed of exceptionally hard crystalline rock formations, such as granite. However, the mechanical properties of HDR, i.e., high temperature, high hardness and abrasive resistance, significantly decrease the rock drillability (Kelkar et al., 2016; Lu and Wang., 2015; Zeng, 2015), resulting in bit wear and low drilling efficiency in real operations (Si et al., 2019; Wang et al., 2019). Hence, it is essential to develop an evaluation method for drillability of HDR, which is respected to enhance the drilling performance and reduce the operation costs in HDR resource excavation.
On Sweet Spot in Percussive Drilling: Single Impact Experiments
Authors: Borges Dos Santos J. V. , Richard T. , Kovalyshen Y.
Published: August 2024

This work presents results of research on down-the-hole percussive drilling and investigates the origin of the observed non-monotonic dependency of the rate of penetration (ROP) on the applied weight-on-bit (WOB). Field experiments were carried out at a mining site with a drilling rig that was instrumented with a tailored monitoring system. The field data clearly shows in several instances that the ROP is maximized for specific values of WOB; this phenomenon is commonly referred to as Sweet Spot. In order to investigate whether the Sweet Spot is intrinsic to the bit-rock interface or stems from the system dynamics, laboratory experiments were conducted with a novel apparatus that drills shallow holes with discrete events (one impact at a time); prior to each impact of the piston on the drill bit, the initial conditions are guaranteed to be re-established. The effect of drilling parameters can be studied independently with the experimental setup, such as WOB, piston impact energy, and angular displacement of the bit between successive impacts. Preliminary results show no intrinsic dependency of the rate of penetration with the applied WOB.

1 INTRODUCTION

Down-the-hole (DTH) percussive drilling is a widely employed technique in mining and civil engineering. In DTH drilling, a pneumatically or hydraulically activated piston (or hammer) strikes the drill bit, which fragments the rock surface and propagates the borehole. Concurrently, the drill bit rotates, and either compressed air or water is used to flush out rock debris and maintain bit cooling (Hustrulid and Fairhurst, 1971; Amjad, 1996; Zhao et al., 2014; Depouhon, 2014).

In the context of DTH percussive drilling, several laboratory and field experimental studies have reported the existence of optimal operating conditions that result in the highest drilling efficiency and effectiveness (Cheetham and Inett, 1953; Lundquist, 1968; Hustrulid and Fairhurst, 1972; Unger and Fumanti, 1972; Amjad, 1996; Schunnesson, 1998; Kivade et al., 2015; Ghosh et al., 2017; Shen et al., 2022). The rate of penetration (ROP) increases with the increase of weight-on-bit (WOB) up to a point and then starts to decrease with a further increase in WOB (refer to Figure 1). This phenomenon is commonly referred to in the literature as the Sweet Spot (Depouhon, 2014); in this paper, however, it will be referred to as Optimum Drilling State (ODS). Despite numerous studies investigating potential causes of the Optimum Drilling State (ODS), there remains a lack of consensus or definitive explanation for its occurrence. This absence of a precise understanding regarding the correlation between drilling parameters and the manifestation of the ODS impedes the development of optimization and control algorithms aimed at maximizing the Rate of Penetration (ROP) amidst changing borehole conditions (Song et al., 2019).
Numerical Simulation Study of Proppant Migration and Settlement Behavior in Complex Fractures
Authors: Kangjia Zhao , Jie Wang , Hualei Xu , Houshun Jiang
Published: August 2024

Hydraulic fracturing is one of the main technologies for developing unconventional oil and gas resources. Effective placement of proppant in the fracture has an important impact on the formation of channels with high conductivity. In order to study the migration and settlement law of proppant in complex fracture, computational fluid dynamics (CFD) is used to simulate the migration process of proppant in fracture, and drag and lift models are added in the simulation process. The proppant migration and settlement law under different impact factors is analyzed from three dimensions: sand dune length ratio, maximum height ratio of sand dune and sand dune area ratio. The simulation results show that the proppant tends to enter and settle in the branch fracture close to the entrance of the main fracture. When the main-branch fracture angle is smaller, the proppant is more likely to be carried into the branch fracture for settlement. When the fracture is inclined downward, the sand dune pattern in the main fracture moves to the depth of the fracture as a whole, which is not conducive to the effective filling of proppant near the wellbore. Increasing the viscosity and injection rate of fracturing fluid, the proppant migration trends in the main fracture and branch fracture are different. The numerical simulation results and conclusions will provide reference for the optimization of fracturing construction in the field.

1. INTRODUCTION

Unconventional oil and gas resources have gradually become the focus of development (Liao et al., 2022). The unconventional oil and gas resources are mainly shale oil and gas, tight sandstone oil and super heavy oil (Meng et al., 2023; Ardakani et al., 2022). The rock properties of shale and sandstone make sand fracturing technology as the preferred measure for reservoir stimulation (Hou et al. 2014; Zhao et al., 2012). Successful sand fracturing requires effective fluid flow channels in the fracture (Yao et al., 2022). At the same time, a large amount of proppant is injected into the fracture to support the fracture and prevent it from closing.The migration and settlement trend of proppant directly affects the hydrofracture morphology and conductivity after fracturing. Therefore, it is necessary to study the impact of different factors on proppant placement during fracturing.
Mechanical Responses and Dynamic Seepage Evolution Laws of Rock Mass Containing Inclined Rough Fracture Under Three-Dimensional Stress
Authors: Tang J. Z. , Zhao G. M. , Chang J. C. , Yin Z. Q. , Duan M. K. , Wang T.
Published: August 2024

The stress-seepage tests are conducted on sandstone samples that containing inclined rough fracture under three-dimensional stress conditions. The surface scanners are utilized to scan and analyze the morphology of the fractures before and after the tests, studying the relationship between morphology and mechanical and seepage characteristics. The research results demonstrate that: (i) Confining pressure has a significantly strengthening effect on the peak stress and crack damage threshold, while pore pressure has the opposite effect; (ii) As the confining pressure increases, the JRC in vertical (JRCV) and absolute difference between JRC in vertical and horizontal (|JRCV−JRCH|) nonlinear decreases, this trend can be described by power function properly; (iii) A revised permeability calculation formula for a rough single fracture was proposed by introducing the fractal dimension D. The negative correlation between the fractal dimension of fracture surface and permeability, which confirms the validity of the revised formula; (iv) Based on gray threshold segmentation methods, the seepage channels have been identified and segmented, the ratio of the area of seepage channels to the entire fracture surface area, defined as connectivity ratio Ф, exhibited a basic positive correlation with permeability. During the compression, the permeability dynamic evolution can be divided into two modes.

1. INTRODUCTION

In rock engineering fields such as water conservancy and underground resource exploitation, the coupling effect between the seepage field and stress field of fractured rock masses has a significant impact on the response of rock masses under engineering actions (Snow 1969, Tsang and Tsang 1987). The hydraulic conductivity of fractures is much greater than the intact rock matrix, fractures are the main pathways for water movement in rock masses. The complex fracture network system is composed of interconnected individual fractures, and the study of the groundwater movement mechanism within individual fractures forms the foundation of research on complex fracture network systems (Ju et al. 2013, Zhao et al. 2017, 2021, Khanal et al. 2019, Li et al. 2022).
Time-Dependent Fracturing in Granite Under Uniaxial Compression
Authors: Zafar Sana , Hedayat Ahmadreza , Moradian Omid
Published: August 2024

Time-dependent processes in rock govern its long-term behavior; hence, understanding these processes is very important, especially for long-lasting surface and underground structures. A series of laboratory-based uniaxial compression tests coupled with acoustic emission (AE) monitoring was conducted to explore the influence of control mode and loading rate on AE characteristics and the damage processes of granite. The experiments were carried out under axial displacement controlled and load controlled modes at varying displacement and loading rates. The stress-strain behavior, strength, AE characteristics, and failure modes were analyzed. The loading and displacement rate in the two control modes significantly affected the mechanical properties of the rock specimen. Results indicated a decrease in rock strength by 15% when the displacement rate reduced from 1μm/s to 0.05 μm/s, and in load-controlled mode, the strength reduced by 20% as the loading rate decreased from 0.55 kN/s to 0.02 kN/s. With the increase in the loading and displacement rates, a noticeable decrease in the time of failure was observed. Acoustic emission (AE) results showed that initially, the loading rates had minimal effect on the damage source parameters, whereas during crack propagation, their influence became substantial. Explosive or tensile events were obtained as the dominant failure mechanism. Under a high displacement rate, the proportion of the explosive events was higher; as the displacement rate reduced, the proportion of the explosive events reduced, and shear events increased. Load-controlled experiments showed insignificant variation in the proportions of the different fracturing mechanisms by changing the loading rate. Decreasing b-value indicated the evolution of large-magnitude events under high rates of loading. This study offers fundamental insights into how brittle rocks undergo damage evolution under various loading and displacement rates, whether in load-controlled or displacement-controlled conditions.

1. INTRODUCTION

Understanding the mechanisms of microcracking in rocks over time is crucial for assessing the long-term stability of various rock engineering structures, including tunnels, mining chambers, rock slopes, and nuclear waste repositories. This understanding is essential for 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). Furthermore, time-dependent factors also significantly influence the exploration and utilization of resources such as oil and gas reservoirs and enhanced geothermal systems (EGS), as noted by Demarest (1976) and Cornet (2007).
Measuring Hydraulic Fracture Propagation and Closure in Sandstone
Authors: Talebkeikhah M. , Momeni S. , Lecampion B.
Published: August 2024

This study investigates experimentally hydraulic fracture propagation, arrest and closure in permeable rocks. Through a controlled laboratory experiment on a Molasse sandstone block under a true triaxial state of stress, hydraulic fracture propagation and closure was monitored via the measurements of fluid pressure, fracture opening at the wellbore and continuous recording of passive acoustic at sixteen sensors. The combination of acoustic data, fluid pressure, and fracture opening measurements offer an interesting dataset to better understand fracture closure. Both the width and pressure behavior upon closure are consistent with recent theoretical estimate. This notably justify the use of pressure derivatives with respect to the square-root of shut-in time to pick fracture closure. The scaled opening versus reverse time and scaled pressure over the square root of reverse time exhibits a linear behavior, in line with the expected theoretical behavior. As a result, the width evolution near closure can be used to estimate the leakoff coefficient. Interestingly, our measurements also reveal the existence of a irreversible/residual opening at the wellbore even after complete fracture closure, whose value is of the order of the Molasse sandstone grain size. This may be attributed to a small amount of shear slip/fracture mixed modality, potentially causing non-alignement of asperities of the fracture surfaces at closure.

1 INTRODUCTION

Hydraulic fracturing is a technique widely employed to enhance well productivity. It is also a reliable method to measure in-situ stresses at great depths from wellbores. The hydraulic fracturing process involves injecting a fluid at pressure sufficiently large to propagate a tensile fracture in the rock. Upon cessation of the injection, the fracture eventually closes due to the combined effects of the confining in-situ stress and the depressurization of the fluid associated with its permeation into the surrounding permeable rock matrix. The behavior of hydraulic fracture once the fluid injection has stopped depends on the dominant propagation regime. Indeed, in cases where toughness dominates, the fracture immediately arrests, and then close due to fluid leak-off. Conversely, if fluid viscous dissipation is dominating when the injection stops, the fracture continues to grow for a while driven by the energy elastically stored in the solid during the propagation phase (Möri and Lecampion (2021)). Following fracture arrest, fracture recession and closure starts. Its dynamic is mostly governed by fluid leak-off. Although a large number of numerical and theoretical models have been investigated (Nolte (1979a); Desroches and Thiercelin (1993); Papanastasiou (2000); Mohammadnejad and Andrade (2016); Peirce (2022); Peirce and Detournay (2022b,a)), experimental investigation with precise measurements of fracture arrest and closure are limited (De Pater et al. (1996)).
Poroelastic Stress Cycling and Microseismic Activity at the San Emidio Geothermal Field, NV (USA)
Authors: Wang H. F. , Sone H. , Cardiff M. , Guo H. , Thurber C. , Luo X. , Feigl K. L. , Sherman C.
Published: August 2024

Guo et al. (2023) presented a hypothetical scenario of Thermal-Hydrologic-Mechanical (THM) processes that included a microearthquake cycle associated with annual plant shutdowns at the San Emidio Geothermal Field in northwestern Nevada (USA). Their scenario focused on the relative magnitudes of fluid pressure, Pp, and critical pore fluid pressure, Pcrit, which is the magnitude of pore pressure above which Coulomb failure triggers seismicity. The scenario was based on three sets of observations and inferences associated with the 20-hour shutdown in December 2016 during which all pumping operations stopped: 1) Simulation of fluid-pressure changes, 2) Observation of microseismic events before, during, and after the shutdown, and 3) Stress inversion of focal mechanisms. We examine the THM coupling between stress, pore fluid pressure, and temperature in terms of the critical value Pcrit using first-order estimates for failure on small slip patches in a normal-fault zone.

1. INTRODUCTION

Extraction of heat from geothermal reservoirs requires a working fluid circulating in hot rock. Efficiently managing the resource must consider how fundamental physical variables of temperature and fluid pressure control reservoir stresses. Simulating thermal-hydrologic-mechanical (THM) processes in heterogeneous rock is typically approached using numerical models calibrated and validated from field observations. THM models are based on conceptual models and idealized physics. The WHOLESCALE (Water and Hole Observations Leverage Effective Stress Calculations and Lessen Expenses) project at the San Emidio Geothermal Field uses Lawrence Livermore's GEOS code to model the three-dimensional stress field through time (Feigl et al., 2024; Luo et al., 2024). The code allows for geometric flexibility, heterogeneity, and coupling between thermal (T), hydrologic (H), and mechanical (M) processes as well as limited chemical (C) but not biological (B) processes. Although THM codes are indispensable tools for understanding reservoir behavior for reservoir management, their complexity and detail can obscure the controlling processes. Another difficulty is choosing an appropriate mesh and time step that are numerically reasonable because observations may span many orders of magnitude in space and time, e.g., deformation over months and years associated with InSAR and rapid microseismic events too small to be captured by InSAR.
Cracking Behaviour of Mineralised Fractures Under Direct-Shear
Authors: Kong Lie , Shang Junlong
Published: August 2024

Mineral-filled fractures (rock veins) are common in the Earth's upper crust. Understanding the cracking and deformation behaviour of the mineral-filled fractures is of fundamental value for wide applications such as earthquakes, landslides, and the stability of underground infrastructure. In this study, we simulated the cracking behaviour of mineralised fractures under a direct-shear load using the discrete element method (DEM). A synthetic rectangular numerical specimen sized 5 cm by 5 cm was considered. An analogous vein was embedded along the middle line of the specimen. We calibrated the mechanical properties of the vein and host rock materials based on the experimental tests of a veined rock from the Cornwall area in England, UK which is a potential area for constructing the geological disposal facility (GDF) for nuclear waste. We analysed results including cracking dynamics, loading curves, and crack partitions in/along different material phases (i.e., host material, vein, and host-vein interface). We envisage that this study could provide valuable information regarding the activation and deformation of mineralized fractures especially from a micro-scale perspective.

1. INTRODUCTION

The mineral vein (mineral-filled fracture) is a unique geological structure that widely exists in geoformations. Generally, the mineral veins are created within the pre-existing fractures that were earlier formed by geo-stresses and crustal movement. The existence of fractures in the geo formations is very important for wide geoengineering and geomechanical applications such as safety and stability of underground construction, and prediction of earthquakes and landslides. In the literature, many works have investigated the effect of fractures or weak interfaces on the mechanical and hydromechanical behaviours of rock formations [1-6]. Consensus has been reached that the fracture is critical in geomechanics because it often dominates the mechanical and hydro-mechanical behaviour of rock.

However, in the current geomechanics and rock mechanics studies, the fracture is often considered as a linear interface. The presence of mineral veins in fractures was often ignored or less investigated. While is revealed that the existence of the mineral vein will alter the mechanical and hydraulic properties of the host rock by changing the mechanical properties (strength, modulus, friction etc) and hydraulic properties (void/apertures, etc) of the host fractures [7], how the mineral veins affect the activation and deformation behaviours of pre-existing fracture is still unclear. It is important to understand the activation and deformation of the mineralised fractures which is fundamentally important for rock and geological engineering projects. The knowledge obtained through the proposed works is beneficial because it will allow us to make use of the mechanisms that could contribute to techniques that could either improve efficiency (e.g., enhancing the stability) or migrate the unwanted risks (e.g. severe collapse/leakage of tunnels and caverns).
Evolution of Hydraulic Fracture Permeability in EGS Considering Natural Fracture Compressibility and Strength of the Surrounding Rock
Authors: McLean M. L. , Espinoza D. N. , Ahmmed B.
Published: August 2024

The long-term success of an Enhanced Geothermal System (EGS) project requires distributed fluid flow in created fractures, ideally each with uniform and moderate permeability to avoid early thermal breakthrough. Yet, thermal depletion causes fracture opening, increasing the likelihood of flow channeling in areas with high fracture permeability. Furthermore, the effective reservoir rock stiffness (including natural fracture compliance) has a first-order impact on thermally induced stress changes, and thus fracture permeability. The objective of this work is to explore the role of thermal depletion on hydraulic fracture permeability considering a non-linear elastoplastic geothermal reservoir response. We utilize three-dimensional numerical simulations based on effective medium theory of fractured rocks to implicitly account for natural fracture compressibility and strength. Results demonstrate that a portion of the thermal strain-induced by cooling- is absorbed by natural fracture compressibility, which reduces the overall stress change, and tends to attenuate hydraulic fracture opening. Critically stressed natural fractures can yield during operation and decrease the likelihood of flow channeling. Lastly, the modeling results indicate that linear elastic models tend to overpredict fracture opening compared to models that account for effective properties of fractured rock masses.

1 INTRODUCTION

Predictions of recoverable heat energy from Enhanced Geothermal Systems (EGS) reservoirs with models that neglect stress-dependent and non-linear fracture permeability are conservative estimates (Kohl et al., 1995). Flow channeling and thermal short-circuiting caused by thermo-poroelastic coupled feedback is often observed early on in field tests and would limit the installed capacity unless efforts were made to improve the flow distribution (MIT, 2006). Localized fracture opening increases injectivity and decreases the geothermal effective reservoir volume by localizing injected flow (Hicks et al., 1996). Hence, reservoir stresses (in-situ and any changes during operation) play a significant role in the distribution of EGS circulation fluid and evolution of fracture permeability (McLean and Espinoza, 2023). Spatial heterogeneity in the initial fracture aperture may further decrease reservoir performance from the beginning because preferential flow paths may exist prior to injection (Guo et al., 2016).
High-Precision Triple Multiscale Model Characterizes the Complex Pore-Fracture System of Continental Shale Reservoir
Authors: Li Haoyu , Wang Wendong , Su Yuliang , Deng Yuxuan , Li Dongxia , Li Lei , Hao Yongmao
Published: August 2024

Continental shale reservoirs display distinct features, including elevated organic-rich clay content, intricate migration mechanisms, and multiscale porosity. Traditional reservoir flow and transport theories are unsuitable for the porous structure of shale oil reservoirs. This study employs a dual-medium model to depict both organic and inorganic porosity within shale reservoirs. Furthermore, an embedded discrete fracture model characterizes hydraulic fractures, establishing a multi-medium model encompassing organic porosity, inorganic porosity, and fractures. Additionally, this investigation comprehensively addresses the micro-scale flow mechanisms of shale oil, precisely delineating the multiphase flow characteristics in the multiscale media of continental shale. This methodology significantly improves the accuracy of numerical simulations for continental shale oil reservoirs. The exploration of different flow mechanisms' impact on shale oil production reveals a consistent decline in cumulative oil production as the stress sensitivity coefficient increases. The analysis of shale oil reservoir production variation, considering adsorption, indicates that organic matter adsorption contributes approximately 10% to the total shale oil production.

1. INTRODUCTION

The progress in horizontal well drilling and hydraulic fracturing technology (Bowker et al., 2007) has positioned the development of shale oil reservoirs as a prospective solution to meet the growing global energy demand (Sun et al., 2021). This has, in turn, prompted the exploration and exploitation of shale oil in several nations. Advances in shale oil extraction have spurred the exploration and development of shale oil in multiple nations (Liu et al., 2022). In China, the potentially recoverable resources of low-maturity shale oil in continental shale are estimated to range from 700 to 900 million tons (Zhao et al., 2020), underscoring the considerable potential of shale oil resources. Therefore, the efficient extraction of shale oil is of paramount importance for national energy independence and security.

Continental shale oil reservoirs in China exhibit characteristics such as low porosity, low permeability, heterogeneity, and multiscale porosity media (Xu et al., 2022). The structural composition of shale includes organic matter, predominantly kerogen, along with minorganic minerals like quartz, clay minerals, and natural fractures (Liu et al., 2022). Under the action of hydraulic fracturing, a multi-scale pore-fracture network is formed in shale oil reservoirs (Deng et al., 2023). Nevertheless, current numerical simulation tools struggle to precisely depict the migration mechanism of shale oil within multi-scale media and calculate flow transitions between different media (Wang et al., 2023). This limitation has implications for the accuracy of flow simulations in shale oil reservoirs and the precise prediction of production outcomes.
Study on Fracture Propagation Law of Cement Sheath Interface During Fracturing Process
Authors: Lai Chenxi , Feng Yongcun , Su Feiyu , Gu Chenwang , Li Xiaorong
Published: August 2024

In the process of fracturing construction, the injection of high-pressure fluid will lead to the propagation of fractures at the first and second interfaces of the wellbore cement sheath, which seriously affects the integrity of the wellbore, and even cause the interlayer channeling of the fracturing fluid. In this paper, a three-dimensional finite element model of fracture propagation at cement sheath interface is established based on the Cohesive Element Method. The numerical model can quantitatively analyze the influencing factors of fracture propagation at cement sheath interface, including mechanical properties of cement sheath, horizontal in-situ stress difference, initial fracture shape, injection pressure and time of fracturing fluid. The results show that the wellbore integrity can be maintained and the interface cementation strength can be improved by optimizing the appropriate cement slurry system and the elastic modulus of cement sheath. Before fracturing, the selection of perforation location needs to fully consider the heterogeneity of the formation. Selecting the formation with relatively low horizontal in-situ stress difference will effectively slow down the fracture propagation at the cement sheath interface. When the injection pressure of the fracturing fluid exceeds the propagation pressure of the interface fracture, it will lead to the interlayer channeling of the fracturing fluid, and even fracturing failure. In summary, in order to improve the wellbore integrity in the fracturing production process, it is very important to optimize the appropriate cement slurry system and fracturing design scheme.

1. INTRODUCTION

Wellbore integrity is an important attribute of wellbore to resist structural damage and maintain wellbore function. It can ensure the safety of drilling engineering and is very important to the whole oil and gas production process (Tian et al., 2015; Song et al., 2016). The cement sheath is the core of wellbore integrity, providing interlayer isolation and support for the casing to ensure its integrity throughout the life of the wellbore (Gray et al., 2009; Feng, 2016).

At present, hydraulic fracturing technology is a major means of increasing production in the oil field. This technology is to inject the fracturing fluid (mainly water, sand or other proppants suspended with the help of thickeners) into the wellbore through a high-pressure pump to form artificial fractures in the deep strata, so that oil and gas can flow more freely in the reservoir. When the fluid pressure is removed from the well, the small particle hydraulic fracturing proppant (sand or alumina) maintains the fracture in an open state to increase the reservoir permeability (Xu et al., 2019; Biao, 2011). In the process of fracturing construction, the injection of high-pressure fluid will lead to the propagation of fractures at the first and second interfaces of the wellbore cement sheath, which seriously affects the integrity of the wellbore.