bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification Method of Carbonate Acid Erosion Fracture Propagation Morphology Based on Impulse Analysis
Authors: Zhan Jiahao
Published: September 2022

At present, it is difficult to identify the communication between artificial acid erosion fractures and natural fractures and caves, which is an important problem faced by acidizing fracturing in carbonate reservoirs. Based on the pump pressure data from the physical simulation experiment of acidizing fracturing in carbonate rock, this paper analyzes the acid erosion fracture propagation modes corresponding to different pump pressure curve fluctuation characteristics. Based on the analysis results, a new identification method of acid erosion fracture propagation modes in carbonate rock reservoir is proposed. This method can identify and evaluate fracture communication in acidizing fracturing.

1 INTRODUCTION

In the physical simulation experiment of rock acidification fracturing, fractures generated by internal fracture of rock samples during fracturing cannot be observed directly, so the sample fracture situation needs to be analyzed by acoustic emission feedback diagram and well-head fracturing curve. By analyzing the acoustic emission intensity of the acoustic emission feedback graph, the variation time of the material’s microscopic deformation characteristics can be obtained. This method is called energy analysis method [1]. There are many high and middle frequencies of ae signals when rock is broken, and there are existing characterization models for deformation mechanism, strain rate and other materials and the energy of AE signals. The energy of AE signals is proportional to the area of wave shape and does not depend on the selection of threshold value. When analyzing the physical simulation results of rock acidification fracturing, the large fluctuation of acoustic emission signal intensity indicates fracture initiation or communication fractures: the strong signal indicates fracture matrix expansion, a small number of weak signals indicates natural fractures restart, and a large number of weak signals indicates communication to the pores. In the wellhead pressure curve, the first pressure drop point is the fracture pressure of the rock sample. The sudden drop of wellhead pressure indicates fracture propagation, and the fracture pressure that communicates with or extends along the natural fracture will be lower than the initial fracture initiation pressure [2]. However, this method still has the problem of artificial comparison operation error and low efficiency when identifying the propagation mode of fracture.
Competition Between Biotite Weathering and Regional Stresses: Answers from Homogenization and Finite Element Simulation
Authors: Xu Tingting , Shen Xianda , Reed Miles , West Nicole , Ferrier Ken L. , Arson Chloé
Published: September 2022

This paper presents a micro-mechanical model of crack propagation in rock subject to mineral weathering. We focus on the example of biotite, which expands as it weathers and generates stresses on the surrounding rock matrix. Our approach is to model a rock Representative Elementary Volume as a composite made of a homogeneous matrix that contains ellipsoidal cracks of various orientations and ellipsoidal mineral inclusions that represent biotites. The mechanical properties of the bulk rock are obtained by two-step Mori-Tanaka homogenization. This model captures the propagation of anisotropic damage as a result of biotite weathering under various stress conditions, showing that weathering eigenstress dominates the propagation of micro-cracks over the deformation energy of the bedrock. Finite element simulations further show that biotite weathering changes the distribution and intensity of stress in the rock mass beyond the depth of the weathered zone. Simulations conducted for a sinusoidal topography with a sinusoidal weathering gradient out-of-phase with the topography show that the gradient of damage always follows the weathering gradient, regardless of biotite orientations. Simulations also show that the intensity of damage in each direction of space depends on the orientation of the biotite minerals, with higher damage for vertical bioites.

1 INTRODUCTION

The expansion of biotite in granitic rock and regolith due to iron oxidation by meteoric waters is thought to generate enough stress to cause a breakdown in rock strength and initiate chemical weathering (Fletcher et al., 2006; Buss et al., 2008; Goodfellow et al., 2016). Goodfellow et al. (2016) showed evidence for distributed microcracking of the rock matrix surrounding biotite minerals upon oxidation and a coincident reduction in rock strength. How landscape-scale patterns of immobile regolith properties emerge from this microcracking is unknown. Since biotite only expands along the basal cleavage plane (Dong et al., 1998), rock with a dominant biotite orientation could exhibit different behavior and lead to immobile regolith with anisotropic properties (Leone et al., 2020).
Characterization of Mechanical Discontinuities Using Triaxial Shear Tests Under High Confining Pressure
Authors: Casagrande C. , Pique T. M. , Gómez J. L. , Epele B. , Sánchez M. , Manzanal D.
Published: September 2022

Mechanical discontinuities are defined as any surface through which some property of the rock mass is not continuous. Usually, the properties of the bulk rock separated by a discontinuity are used to predict the behavior of a formation. However, an analysis of the discontinuities as such is uncommon. The main property of a discontinuity is its shear strength. The shear strength has frictional parameters, such as basic friction angle and roughness, and may have a cohesion parameter. In this work, we evaluated the mechanical properties of artificial mechanical discontinuities. The artificial discontinuities were numerically designed. The mechanical performance of these discontinuities was initially assessed using published failure criteria that depend on the properties of the rock and the discontinuity morphology obtained by visual and 3D image analysis. Finally, triaxial direct shear tests under high confining pressure were performed to validate the criteria and assess their validity under different stress field conditions, properties of rock, and discontinuity morphology.

1. INTRODUCTION

The deployment of underground fluid injection and storage projects, such as CO2 underground storage, can only succeed if the fluid permanence on the targeted formation is warranted. This could only be achieved if the formation’s characteristics are understood. The main risks associated with fluid migration are related to discontinuities in the reservoir or caprock (Damen et al., 2006), therefore, these must be carefully identified, characterized, and included in the rock mechanics models. There are many definitions of discontinuities in literature, as Hencher and Richards (2014) summarize in their work. A definition adopted in the cited work is that discontinuities consist of relatively weak planes in rock volumes. Since in this work we mainly focus on the rock mechanics, we will refer to them as mechanical discontinuities, considering any surface through which some mechanical property of the rock volume is not continuous.
Geometrical Controls on Thermal Short-Circuiting in Multi-Fracture Enhanced Geothermal Systems
Authors: McLean Matthew L. , Espinoza D. Nicolas
Published: September 2022

Thermal short-circuiting in Enhanced Geothermal Systems (EGS) is a reservoir-scale phenomenon, which arises from coupled thermal-hydraulic-mechanical feedback in response to heat drainage. Coupled feedback on the hydraulic aperture may increase fracture transmissivity drastically and decrease the production fluid temperature lowering the economically recoverable heat energy. This paper identifies the geometrical controls on short circuiting for multi-fracture enhanced geothermal systems to optimize the recoverable heat energy including: fracture spacing, number of parallel factures, wellbore diameter, injector-producer spacing, initial hydraulic aperture, hydraulic aperture distribution and thermo-hydro-mechanical fracture response. We use a combination of simple 1D hydraulic network simulations and fully coupled thermo-hydro-mechanical 3D models solved through FEM. Results show that thermal short-circuiting is more severe in formations (1) with initial or late high hydraulic apertures, in the order of tens of millimeters; (2) when fluid pressure exceeds the minimum total stress, causing large fracture re-opening; and (3) with a large number of fractures in a small rock volume, causing severe thermal and mechanical fracture interaction. Our modeling work suggests that thermal short-circuiting can be mitigated with an appropriate fracture design and operation.

1. INTRODUCTION

Heat recovery in Enhanced Geothermal Systems (EGS) requires transmissive fractures connecting injection and production wellbores to increase hydraulic diffusivity of the surrounding rock (Brown et al., 1972; MIT, 2006; US Department of Energy, 2020). High surface area is desirable for large heat recovery because heat conduction in EGS reservoirs occurs at small rates (Fu et al., 2015; Gee et al., 2021). Yet, localized thermo-elastic contraction of the surrounding rock can increase fracture permeability and localize fluid flow, causing thermal short-circuiting (Armstead and Tester, 1986; Tsang and Neretnieks, 1998; Guo et al., 2016). Furthermore, fluid flow localization in EGS reservoirs can be caused by two processes: (1) fluid localization to preferred paths within a single fracture (flow-channeling) and (2) fluid localization to a few fractures within a hydraulic network of several fractures (plane-channeling). The latter is caused by distinct initial fracture geometries and viscous pressure loss between fractures (Shiozawa and McClure, 2014; Asai et al, 2018). These processes can form a thermal-hydraulic-mechanical (THM) coupled feedback loop because permeability increase within a single fracture tends to perturb the hydraulic network such that it accepts more working fluid. Hence, a coupled solution of fluid flow through wellbores and fractures with fluid flow and deformation of the surrounding rock is required to model the full transient behavior of multi-fracture EGS.
Remarks on Linear and Nonlinear Failure Criteria
Authors: Labuz J. F. , Asem P. , Fuselier H.
Published: September 2022

To explore the nature of stress states at failure, published data are analyzed for three rock types – Dunnville sandstone, Red Wildmoor sandstone, and Taiwan siltstone - within the framework of three stress invariants. Data include a series of axisymmetric (conventional triaxial) compression, axisymmetric (conventional triaxial) extension, and multiaxial strength tests. The data are fitted to linear and nonlinear failure criteria: Paul-Mohr-Coulomb (PMC), a generalized linear criterion containing all three principal stresses; Hoek-Brown, a popular nonlinear criterion containing major and minor principal stresses; and a simplified version of PMC, where the friction angle in extension is assumed to be equal to the friction angle in compression. The results show that both versions of PMC provide a better approximation of the test data and successfully capture, in a piecewise-linear manner, the well-known nonlinear nature of the failure surface in the meridian and equipressure planes.

1. INTRODUCTION

Mohr-Coulomb (MC) failure criterion considers principal stresses σI and σIII with a linear form:

(Equation)

where ϕ = friction angle, Co = uniaxial compressive strength, and σI, σIII are major, minor principal stresses. The sign convention is compression positive. Note that the friction angle is independent of axisymmetric compression (σII = σIII) or axisymmetric extension (σII = σI) stress paths, where σII is the intermediate principal stress; i.e. in the framework of MC, the friction angles in compression and extension are equal.

Conventional triaxial compression tests show that the failure envelope is nonlinear when considering a large range of mean stress (Mogi 1971; Labuz et al. 2018; Hoek and Brown 2019). Many investigators have proposed nonlinear failure criteria, one being Hoek and Brown (1980):

(Equation)

where m and Co are material constants determined through fitting.

Because the axisymmetric tests cannot simulate general stress states, multiaxial or true-triaxial devices have been developed for investigating failure. Results of true-triaxial tests suggest that failure is affected by three principal stresses or another three stress invariants (Mogi 1971; Wawersik et al. 1997; Zeng et al. 2019).
Thermal-Hydrological-Mechanical Characterization of the Ghareb Formation at Conditions of High-Level Nuclear Waste Disposal
Authors: Kibikas W. , Bauer S. , Choens C. , Shalev E. , Lyakhovsky V.
Published: September 2022

The Ghareb Formation in the Yasmin Plain of Israel is under investigation as a potential disposal rock for nuclear waste disposal. Triaxial deformation tests and hydrostatic water-permeability tests were conducted with samples of the Ghareb to assess relevant thermal, hydrological, and mechanical properties. Axial deformation tests were performed on dry and water-saturated samples at effective pressures ranging from 0.7 to 19.6 MPa and temperatures of 23 °C and 100 °C, while permeability tests were conducted at ambient temperatures and effective pressures ranging from 0.7 to 20 MPa. Strength and elastic moduli increase with increasing effective pressure for the triaxial tests. Dry room temperature tests are generally the strongest, while the samples deformed at 100 °C exhibit large permanent compaction even at low effective pressures. Water permeability decreases by 1-2 orders of magnitude under hydrostatic conditions while experiencing permanent volume loss of 4-5%. Permeability loss is retained after unloading, resulting from permanent compaction. A 3-D compaction model was used to demonstrate that compaction in one direction is associated with de-compaction in the orthogonal directions. The model accurately reproduces the measured axial and transverse strain components. The experimentally constrained deformational properties of the Ghareb will be used for 3-D thermal-hydrological-mechanical modelling of borehole stability.

1. INTRODUCTION

The disposal of high-level nuclear waste is a top concern worldwide (Faybishenko et al., 2017). With increasing interest in nuclear power as an alternative to carbon-based fuels, the problem of safely storing nuclear waste is expected to grow (Kim et al., 2011). One solution for disposal is geological disposal of nuclear waste, where waste is isolated within subsurface geologic formations to be contained and isolated from the surrounding environment (Birkholzer et al., 2012). Given the risks associated with nuclear waste, any potential disposal site must be demonstrably capable of isolating the material from the surface environment for large time scales (Krauskopf, 1988).
Optimization of Well Configuration for EGS Performance with Multi-Dimensional Two-Phase Flow Modelling
Authors: Zhou Dejian , Tatomir Alexandru , Sauter Martin , Tomac Ingrid
Published: September 2022

Geothermal energy is a type of renewable energy with significant potential of contributing to reduce greenhouse gas emissions. Utilization of geothermal energy has become more and more attractive with the developments of the Enhanced Geothermal Systems (EGS) technology. Because of its low viscosity and density and therefore lower energy requirement for circulation supercritical carbon dioxide (scCO2) can be considered as a superior working fluid for EGS, compared to thermal brine. In this study, we developed a multi-dimensional thermo-hydraulic coupled model employing the open-source software DuMuX to model brine phase displacement and heat transport in a fractured porous reservoir. This work investigates well configuration effects with respect to three criteria of EGS performance, i.e., energy performance, economic performance, and environmental performance. Results show that the two-well configuration performs better in energy performance and environmental performance, but worse in economic performance. Additionally, for the three-well configuration, the scenario with two injection wells performs better than that with only one injection well with respect to reservoir heat production rate and sequestrated scCO2 mass.

1. INTRODUCTION

This paper investigates performance optimization of scCO2-H2O-combined Enhanced Geothermal Systems (EGS) reservoirs for different injection and production well configurations based on cumulatively produced energy, cost, CO2 storage, production temperature, and scCO2 breakthrough time. Geothermal energy, as a renewable energy source, has become attractive due to its constant baseload energy provision, the considerable potential to reduce greenhouse gas emissions and to decrease the reliance on fossil fuels (Jacobson et al., 2018; Teske, 2019). EGS are advanced geothermal reservoirs independent of the availability of an apriori minimal permeability field by generating/re-opening new/existing fractures through hydraulic or supercritical carbon dioxide (scCO2) fracturing at depths between 3 km and 10 km below the ground surface (Huang et al., 2020).

With scCO2 as working fluid circulation, well operation yields a considerably larger production rate for a given pressure gradient than the traditional EGS working fluid, i.e., H2O-brine, because of the lower energy consumption to upkeep circulation due to lower viscosity and density of scCO2 (Lu, 2018). Pruess et al. (2006, 2008) first investigated the performance of scCO2 as a working fluid for EGS with increased interest in employing scCO2-based EGS during recent years. Many researchers have contributed to the development of numerical models for the simulation of long-period (decades) phase migration, heat transport, and chemical reaction in EGS reservoirs with the aim of economic optimization and energy output maximization (Borgia et al., 2013; Cui et al., 2018; Guo et al., 2019; Norouzi et al., 2021).
Simulation of Proppant Placement Efficiency at the Intersection of Induced and Natural Fractures
Authors: Merzoug A. , Mouedden N. , Rasouli V. , Damjanac B.
Published: September 2022

The intersection of the pre-existing natural fractures (NF) with the hydraulic fractures (HF) forms a complex fracture network, which allows for increased well productivity. However, the complex fracture geometry may adversely affect the proppant placement and its transportation inside the fractures. The study reports the results of the Lattice numerical simulations of proppant placement at the intersection of fractures. Sensitivity analysis was performed to investigate the effect of pre-existing fracture friction angle, angle of approach as well as differential horizontal stress on HF-NF interaction mechanisms and proppant transport and placement. The results present the effect of these parameters on proppant transport and placement in NF.

1. INTRODUCTION

Hydraulic fracturing (HF) is a commonly used stimulation technique that essentially consists of generating fluid-driven fractures in the reservoir to allow the flow of oil and gas into the wellbore with the goal of increasing reservoir conductivity (Michael J. Economides, 2000). However, this process can be more complex in naturally fractured formations (Boualam et al., 2019; Alexeyev et al., 2017; Djezzar et al., 2019, 2020; Djezzar et al., 2019; Djezzar et al., 2019b, 2019a) due to the possibility of intersection of induced fractures with the natural fractures. Crossing, arresting and opening are the three interaction mechanisms when HF intersects a natural interface (Kolawole & Ispas, 2020). The fracture network development leads to increased fracture conductivity (Djezzar & Boualam, 2020, 2021; Djezzar & Boualam, 2020), with NFs contributing to well productivity. However, intersection of HF with NF may negatively affect the propagation of the HF and the proppant placement, making pressure analysis and prediction of fracture morphology very difficult. Moreover, proppant may accumulate in the near region of the intersection by forming a proppant bridge(Kou et al., 2019), or that it is a continuous transport behavior of proppant in the natural fracture(Kou et al., 2018). Therefore, it is important to anticipate and understand intersection mechanisms when designing and executing HF treatment.
Application of Seismic Attributes on Digital Elevation Model: Fractures Detection and Reservoir Implication
Authors: Djezzar Sofiane , Boualam Aldjia , Merzoug Ahmed , Chemmakh Abderraouf , Laalam Aimen , Ouadi Habib
Published: September 2022

The Ordovician formation in the Mouydir basin is characterized by low reservoir properties where the natural fractures play a critical role in its production. The study of the Ordovician analog enhances the natural fractures that affect the area of study to lead the future exploration and drilling of potential wells in the Mouydir basin. The study of outcrop analog can be easier and more accessible compared to the subsurface study which needs specific data like 3D seismic and well logs data such as borehole imagery. The reservoir analog study can be used to model fractures at different scales, from mega-scale to microscale. The ending could be used to understand the fractures’ distribution, fractures’ connectivity, and fractures’ kinematics. The aim of this work is the proposition of an integrated workflow to characterize and model the natural fractures by using seismic curvature and illumination attributes applied on a Digital Elevation Model. The Ordovician reservoir analog in the southern edge of the Mouydir basin was chosen as a case study.

1. INTRODUCTION

The unconventional reservoirs have rapidly evolved over the last years and a significant increase in hydrocarbon discoveries where natural fractures play a significant role in the production. This has helped to enhance the understanding of the effect of fracturing on fluid flow mechanisms. During the development of the fields, the wells of the implementation strategy are determined by the spatial distribution of fractures across the field or reservoir. The geoscientists seek to intersect areas with a high density of fractures and good connectivity, where the drainage of the matrix is more effective. The orientation of the wells should be in accord with the geometrical and geomechanical parameters such as the orientation of fracture planes and the maximum horizontal stress. The study of the naturally fractured reservoirs is very challenging with the main difficulty being how to model and predict the fracture networks at different scales extending from outcrops and shallow depths to the reservoir and basin depths. Advanced 3D seismic and well data are needed to characterize and model the fractures. This process requires great effort in terms of time, data collection, and analysis. The study of the reservoir analog can be a substitute approach with more readily accessible data (Djezzar et al, 2022).
Comparative Study of Anionic and Cationic High Viscosity Friction Reducers in High-TDS Marcellus Shale Formation Water
Authors: Ge Xiaojing , Biheri Ghith , Imqam Abdulmohsin
Published: September 2022

High viscosity friction reducers (HVFRs) have been recently gaining more attention and increasing in use, not only as friction-reducing agents but also as proppant carriers. Reusing of produced water has also been driven by both environmental and economic benefits. Currently, most of friction reducers on the market are anionic friction reducers, which are fully compatible with most of produced water with low to medium level of Total Dissolved Solids (TDS) but show significant decreasing at high TDS conditions in term of their friction reduction performance for most cases. On the contrary, cationic friction reducers are believed to have better TDS tolerant and performance under high TDS conditions. However, concerns are remained about performance of using anionic and cationic HVFRs with produced water to transport proppant. The ultimate objective of this experimental study is to comparably analyze the proppant transport capabilities of HVFRs in high TDS environments. The effects of TDS, temperature, wall retardation, and particle hindering on the performance of both anionic and cationic HVFRs are investigated.

1. INTRODUCTION

During the last few years, the industry has been accelerating the adoption of HVFRs to replace traditional fracturing fluids, such as slickwater and guar, in hydraulic fracturing (Hu et al. 2018; Johnson et al. 2018). This is mainly due to several operational and economic advantages of HVFRs including significantly cutting operational cost with less process and equipment required, exhibiting a higher retained conductivity in the fractured formation, and yielding ideal production improvements in fields (Johnson et al. 2018; Ba Geri et al. 2019; Biheri and Imqam 2020, 2021). Besides these benefits listed above, the most critical consideration behind using HVFRs in place of traditional fracturing fluids is their proppant-carrying and transporting capability. For traditional fracturing fluids treatment, because the viscosity is too low to suspend the proppant for a sufficient time, the dominant proppant transporting mechanism occurs at a high injection rate. However, even with a significantly high pumping rate, only small and light proppant (40/70 sand, resin coated sand [RCS], and 40/80 lightweight ceramic [LWC] are the most popular) are pumped with slickwater in the early stages (Palisch et al. 2008). Then, larger and heavier proppant is typically pumped with linear or crosslinked gels, which can better carry and transport this proppant deep into the fractured formation.
A Screening Methodology Using Fuzzy Logic to Improve the Well Stimulation Candidate Selection
Authors: Mouedden N. , Laalam A. , Rabiei M. , Merzoug A. , Ouadi H. , Boualam A. , Djezzar S. , Mahmoud M.
Published: September 2022

Stimulation techniques are performed to improve the well productivity in fields where a large number of wells is present. However, the selection of candidate wells for stimulation treatments, in particular acidizing and hydraulic fracturing, is a challenging task. The conventional candidate selection technique that is based on analyzing manually a large volume of data is not efficient and cannot capture the impact of all parameters correctly. This study was carried out with the aim of developing a decision-making tool (FIScreT tool) based on Fuzzy logic that would allow the selection of candidate wells for stimulation to be automated rather than manually examining the performance of each well. The FIScreT tool uses a simple and efficient workflow that initially consists of eliminating problems related to completion status, reservoir characteristics and well integrity. Subsequently, a selection is made through two fuzzy interference systems using accessible production and reservoir data. Finally, the production gain is estimated in order to assess the wells’ productivity after treatment.

1. INTRODUCTION

Well stimulation refers to any treatment carried out to restore or improve the productivity of an oil/gas well. Its main objective is to improve the economics of oil fields through faster and greater deliverability of hydrocarbons without major investment (Michael J. Economides, 2000). In addition to reservoir performance and the quality of completion, the effectiveness of the treatment depends on three essential, interrelated elements: the selection of the candidate well, the treatment design, and the execution of the field operation (Zoveidavianpoor et al., 2012)(See Fig. 1.)

Applying the best treatment design and effective field operation to the wrong well will result in the failure of the entire operation. This implies that selecting the most appropriate wells for stimulation is one of the greatest challenges affecting reservoir production and field economic development.
Numerical Implementation of a Cohesive Zone Element Coupled with Continuum Damage Mechanics Volume Elements
Authors: He Haozhou , Arson Chloé
Published: September 2022

The objective of this paper is to develop a two-dimensional Cohesive Element Model (CZM) coupled with a Continuum Damage Mechanics (CDM) model. The novel cohesive element model allows expressing the separation criterion in terms of the damage tensor used in the CDM model implemented in the neighboring volume elements. Above a critical damage value, debonding triggers. The residual tensile and shearing strength of the CZM follow a linear softening law. The proposed model was calibrated against published triaxial compression tests conducted on shale. Single-cohesive zone tests highlight that the proposed approach allows simulation of hardening during damage propagation followed by softening and failure. The model predicts different strengths in tension and compression and different stiffnesses and strengths for different confinements. A boundary-value problem was solved by using a mesh with embedded cohesive zones at all interfaces. It is shown that the 2D model can predict realistic stress/strain curves, stress and strain fields, and fracture patterns for biaxial compression tests. The advantage of the proposed approach is that it can be used to simulate diffuse damage followed by localized fracture with a smooth transfer of field variables between the finite and cohesive elements.

1 INTRODUCTION

Simulating fracture propagation from the inception of microscopic cracks to the localization of macroscopic fractures remains a challenge. To date, finite element modeling approaches based on Continuum Damage Mechanics (CDM) and Cohesive Zone Models (CZM) emerge as the most widely used approaches. The goal of this study is to couple both, in order to take advantage of the benefits that each technique can bring.

In CDM, sets micro-cracks are represented by a damage tensor, which is modeled as an internal variable within the framework of thermodynamic irreversible processes. Stiffness and strength decrease as damage accumulates. CDM models can be formulated based on phenomenological principles (Xu and Arson, 2014) or micromechanics principles (Jin and Arson, 2017). Exhaustive reviews of damage rock mechanics are available in (Arson and Gatmiri, 2008; Arson, 2020).
Modeling of In-Situ Indentation Tests in the Framework of Material Point Method
Authors: Hu L. , Bhide R. , Nguyen W. , Butt D. , McLennan J. , Gadikota G.
Published: September 2022

This work presents numerical simulations of a series of In-situ TEM (Transmission Electron Microscopy) indentation tests using the framework of the Material Point Method (MPM). The selection of MPM is based on its robust capacity for handling large deformation problems which could otherwise be an arduous challenge for mesh-based numerical methods such as the finite element method due to issues with mesh tangling. In this work, the nanoparticles were represented with a collection of material points which carry all the information relevant to the material. The mechanical bonds between particles were simulated with cohesive elements. For simplicity, our current model is a 2D simulation and 3D modeling is pending. Furthermore, up-scaling was necessary to avoid excessively small-time steps. Comparison between the experimental data and the numerical results shows that the numerical modeling with MPM successfully simulates the failure processes manifested by the test specimens including both tensile and shear failure. Our simulation experience also suggests that using an implicit scheme in MPM could be a suitable and beneficial way for extending the application of this powerful numerical technique.

1. INTRODUCTION

Nanoindentation is a common and convenient experimental protocol to investigate near-surface mechanical properties, to depths of a few micrometers (Schuh, 2006; Oyen and Cook, 2009; Wheeler, 2015; Ma et al., 2020). The complementary application of in-situ transmission electron microscopy (TEM) enables the observation of deformation and creation of cracks at a micro scale (Minor et al., 2002; Sun et al., 2009; Wang et al., 2017). Unlike the considerable progress and extended applications of indentation tests for revealing the material behavior at nano/micro scale, numerical modeling of this technique in particulate agglomerates - at a micron scale - is relatively rare (Sajjad et al., 2020;). The biggest challenge arises from the large deformation relative to the particle size and the complicated interaction/movement between the particles. Commonly used mesh-based numerical methods such as the finite element method will fail due to mesh tangling if sophisticated and time-consuming mesh treatments are not employed. In this work, we simulated experimental in-situ TEM nanoindentation tests using the framework of the Material Point Method (MPM).
Transparent True-Triaxial Apparatus for Investigation of Hydraulic Fracture Branching
Authors: Li Wenfeng , Frash Luke P. , Carey J. William , Welch Nathan J. , Meng Meng , Viswanathan Hari S.
Published: September 2022

Hydraulic fracture branching has been observed in the laboratory and in the field. Our previous studies demonstrate that hydraulic fractures can branch into multiple directions for weak anisotropic in-situ stresses when intermedium injection rate and fluid viscosity are used. However, it is quantitatively unclear how the in-situ stress anisotropy affects hydraulic fracture branching. Understanding this question is important for the optimization of hydraulic fracture design for real-world applications because reservoirs are subject to varying degrees of in-situ stress anisotropy. To address this problem, we designed a transparent true triaxial (TTT) cell that allows us to investigate hydraulic fracture branching under different anisotropic stress conditions with distinct injection parameters and sample heterogeneity. This self-designed TTT cell has a transparent bottom window for us to record hydraulic fracture propagation in real-time. We completed a few benchmark experiments to verify the capability of the TTT cell for hydraulic fracturing tests. The most striking experimental observation is that hydraulic fractures can grow and curve in the direction inclined to the minimum principal stress. This crack growth can stop at some critical points, after which new hydraulic fractures are initiated in the direction normal to the minimum principal stress. The curvature-stop-reinitiation cycle repeats during hydraulic fracturing and generates crack strands. Our experimental finding agrees with the observations at the hydraulic fracturing test site. Therefore, the TTT cell offers unique capabilities for us to experimentally understand complex hydraulic fracture branching and stranding behavior in subsurface conditions.

1. INTRODUCTION

Hydraulic fracturing is a key technology that enables economic hydrocarbon production from low-permeability reservoirs (Smith and Montgomery, 2015). The same technology is also used to develop the Enhanced Geothermal System (EGS) for carbon-free geothermal energy extraction from the subsurface (Frash et al., 2014; Fu et al., 2021). Significant efforts have been made to advance our understanding of hydraulic fracture initiation and propagation in rock formations with anisotropic in-situ stresses (Haimson and Fairhurst, 1967; Detournay 2004; Fisher and Warpinski, 2012; Frash et al., 2015 & 2019). However, it remains unclear how do we effectively produce complex hydraulic fracture patterns in the subsurface which is very often subject to anisotropic in-situ stresses.
Study on External Water Pressure and Decompression Measures of Tunnel Concrete Lining
Authors: Deng J. Q. , Cui W. , Zhu Y. B. , Liu Y. , Du S. L.
Published: September 2022

Aiming at the problem of high external water pressure in deep buried tunnel, the maximum external water pressure of concrete lining caused by seepage field of surrounding rock is numerically studied, and the influence of consolidation grouting ring and drainage measures is studied based on the stable seepage analysis. When drainage holes are arranged in the tunnel, the underground water level around tunnel is funnel-shaped and its line is affected to some extent. The surface water level at tunnel location is about 100m lower than the original water level. The maximum pressure head outside consolidation grouting ring, shotcrete and secondary lining is about 490m, 260m and 255m, respectively. Considering the pressure reduction of drainage holes, the average external water pressure head of secondary lining is 170m. Without drainage holes, the underground water level around tunnel is funnel-shaped, but the range is small, and its line is basically not affected. The maximum pressure head outside consolidation grouting ring, shotcrete and secondary lining is about 1100m, 950m, 940m, respectively. Compared with the case with drainage holes, the maximum external water pressure head of the secondary lining increases by about 700m. The research can provide more scientific basis for designing and optimizing the lining structure and ensuring the safety of tunnel.

1. INTRODUCTION

Lining and surrounding rock are the main components of hydraulic tunnel, and the external water pressure is one of the main loads, as shown in Fig. 1. With respect to lining design, infiltration and external pore-water pressures are often one of the most important factors to be considered [1-4]. There are a lot of research on the problem of high external water to deal with the safety of lining in high external water pressure and the general treatment concept of "combination of blocking and drainage" is gradually formed. The composite lining is used to deal with the problem of high groundwater pressure and has been successfully applied in practical projects [5-11].
PDC Bit Wear Detection Based on YOLO V5 and OpenCV
Authors: Yang Donghan , Song Xianzhi , Wang Zheng , Pei Zhijun , Zhu Zhaopeng
Published: September 2022

In recent years, due to the remarkable advantage of increasing speed and reducing cost, PDC bits has occupied a dominant position in the oil bits market. In the drilling process, the bit will wear itself when breaking hard rock. When the abrasion reaches a certain degree, the drilling efficiency of the bit will be significantly reduced. Therefore, during the drilling process, it is necessary to change the bit in time to ensure the rate of penetration. The wear degree of cutters is the key factor affecting the efficiency of PDC bit. In order to quantitatively monitor the wear degree of cutters, computer image recognition is adopted to judge the bit abrasion level by analyzing the missing proportion of each cutter. The data set is made from the bit data used in Sichuan, China. We locate the position of each cutter to get coordinates based on YOLO v5, a deep learning algorithm of object detection. Then, we use OpenCV cropping the whole image with those coordinates to acquire every picture of cutters and perform noise reduction and smoothing. After that, the processed cutter images are used to calculate the missing area, and use the proportion to confirm the wear degree.

1. BACKGROUND

Nowadays, roller cone bits and PDC bits are the most commonly used bits in the field of oil drilling. With the continuous improvement of PDC bit cutter technology, PDC bit has become an important tool for drilling speed. Compared with traditional roller cone bits, PDC bits offer significant advantages in cost, rate of penetration and footage per bit when drilling formations, especially in deep and high-pressure wells.[1] Although PDC bits has high wear resistance, it’s inevitable that the PDC bits being worn out. If the bit abrasion reaches a certain degree, the drilling efficiency of the bit will be significantly reduced. Therefore, it is necessary to obtain the wear condition of the drill bit in time to ensure the drilling speed.[2]
Micro Model Establishment and Numerical Simulation of Supercritical CO2 Filtration in Porous Media of Shale Oil Reservoir
Authors: Ping Xie , Haizhu Wang
Published: September 2022

Supercritical CO2 fluid has the characteristics of low viscosity, easy diffusion, non-toxic and tasteless. It is considered as an ideal working fluid in the fracturing of unconventional reservoirs such as tight oil and gas and shale oil and gas. However, compared with water-based fracturing fluid, supercritical CO2 has very strong filtration. The filtration of supercritical CO2 in the fracturing process has an important impact on the fracturing effect and the design of surface construction parameters. In the past few years, many experimental and simulation studies have been carried out to understand the filtration of Sc-CO2 in dense and shale formations. Although great progress has been made in the macro modeling of unconventional reservoirs, there is still a lack of research on the numerical simulation of micro seepage law, especially the modeling method and numerical simulation of S c-CO2 micro filtration law in fractured shale oil reservoirs. Through python programming, a method for establishing the micro model of shale oil reservoir with discrete fracture network is proposed. Based on the established micro model, the numerical simulation is carried out by using OpenFOAM calculation software to verify the availability of the model and the feasibility of the method.

1. INTRODUCTION

The US energy information administration estimates that the world’s total shale oil reserves are 345 billion barrels, including 75 billion barrels in Russia, 58 billion barrels in the United States and 32 billion barrels in China. Shale oil and gas will be an important field for countries to tackle key problems in the future. Since the "shale gas revolution" in the United States, the shale oil and gas industry in the United States has developed vigorously and the shale oil production has increased rapidly. As of December 2019, shale oil production in seven major production areas in the United States, including Permian , Bakken and Eagle Ford, has exceeded 9 million barrels / day, accounting for 70% of the total crude oil production. In 2019, Saudi Arabia became the world’s largest producer of oil and gas, surpassing the United States in the global oil and gas market.
Impact of the Back-Stress Effect on Horizontal Stress Values Calculation in Hydraulic Fracturing Experiments
Authors: Novikova E. V. , Trimonova M. A. , Nachev V. A. , Dubinya N. V.
Published: September 2022

The study presents an analysis of the back-stress effect on precise determination of in-situ stresses in a model samples in laboratory experiments on hydraulic fracturing. Main stage of this study included the determination of the fracture closure pressure using standard methods and comparing this pressure with the minimum loads applied to the sample. The comparison showed the inequality of these values. To eliminate the identified discrepancy, it was proposed to take into account the back-stress effect. This stress occurs due to an increase in pore pressure when fluid is filtrated into the reservoir from a hydraulic fracture. It affects the walls of the fracture, contributing to its closure. The second part of the study was devoted to analysis of back-stress effect impact on estimation of maximum horizontal stress. In this work, we evaluated the influence of this effect on the determination of the minimum and maximum stresses in the medium in laboratory experiments on hydraulic fracturing. Some assumptions about how to account for back-stress in the calculation of minimum stresses were considered. It was found that taking into account the back-stress when calculating the horizontal stresses significantly reduces the difference between the calculated values and experimental data.

1. INTRODUCTION

The problem of boundary conditions estimation emerges often when dealing with reservoir geomechanics problems (Zoback, 2007). First of all, contrary to the classical problems of mathematical physics, determination of stress-strain state of rock masses cannot be carried out with boundary conditions postulated at real boundaries of the rock mass, as there are no distinct boundaries constraining the rock mass within a single domain (Mukhamediev, 2015). Information necessary for stress estimation can be obtained only at discrete points – from seismic events analysis or wellbore data. This paper is focused on the latter – the problem of using wellbore data to understand the in-situ stresses at rock mass via estimating stresses existing in the well vicinity.
Physical and Mechanical Performance of Oil Well Cement Systems Under Simulated Shale Oil In-Situ Conversion Condition
Authors: Li Hailong , Pang Xueyu , Chen Xiaorong , Zhang Jian
Published: September 2022

The strength of Portland cement material deteriorates sharply in the high-temperature environment under the shale oil in-situ conversion condition, which can reach up to 650 C. During this study, the high-temperature resistance of oil well cement modified by different grades and dosages of silica flour was investigated. The dosages of silica studied ranges from 40 % to 70 % BWOC; the silica particle size ranges from 6 μm to 53 μm; the exposure temperature includes 380 C, 500 C, 550 C, and 650 C, respectively. The physical and mechanical properties of set cement, including permeability, compressive strength, Young’s modulus and stress-strain characteristics, were evaluated before and after high-temperature treatments. The results show that silica-modified cement can maintain reasonably high compressive strength and Young’s modulus after exposure to 550 C for 7 days. The use of ultrafine silica flour (D50 = 6 μm) at high dosage (70% BWOC) seems to be most effective, which resulted in a system with an average compressive strength of 43 MPa and a gas permeability of 0.1 mD. However, the physical and mechanical properties of all systems deteriorated dramatically when the specimens were exposed to 650 C for 7 days. Thermogravimetry analysis revealed that decomposition of carbonate hydration products is the primary reason for oil well cement strength decline exposed to high temperature in dry condition.

1. INTRODUCTION

In recent years, with the gradually increasing demands for oil and gas resources, conventional oil and gas resources have been unable to meet human needs (Zou, 2015). The exploitation of shale oil & gas has attracted widespread attention from researchers and investors alike. Shale oil with different maturity levels varies in composition content and production methods. High-maturity shale oil is normally produced by horizontal drilling and multistage hydraulic fracturing, while low-maturity shale oil cannot be produced in this way (Zhao, 2020). Considering the large potential of low-maturity shale oil reserves, their effective exploitation has become an urgent problem (Zou, 2015). Field experiments have shown that underground in-situ heating can achieve large-scale development of low-maturity shale oil and reduce environmental pollution. The in-situ conversion of shale oil requires heating the formation temperature to 500 °C or higher and maintaining it for a long time (Ma, 2017), which may have irreversible effects on the long-term stability of cement sheath. Therefore, it is necessary to conduct corresponding simulation experiments to better understand the set cement’s physical and mechanical properties under shale oil in-situ conversion conditions.