bluuebunny/crossref_metadata_2025_split huggingface dataset:
Experiment Investigation on the Fracture Initiation Characteristics of Shale Saturated with CO2 and Brine
Authors: Tian G. , Wang H. , Li G. , Yang B. , Zhao C. , Zheng Y. 
Published: September 2022

ABSTRACT:
Supercritical CO2 (SC-CO2) is regarded as a promising fracturing medium to exploit unconventional oil and gas resources. However, the reactions between CO2-brine-rock would change the mechanical properties and microstructure of rocks during SC-CO2 fracturing. To explore how the reactions affect the hydraulic fracture initiation and propagation, a series of fracturing experiments on shale saturated with CO2 or/and brine are carried out in this study. Experimental results show that the initiation pressures of rocks was reduced by 11.2 MPa after CO2 and brine saturation. Compared with dry shale and samples saturated with brine only, the initiation pressures of specimens after SC-CO2 and brine saturation reduce more significantly owing to the deterioration of mechanical properties by SC-CO2. And the fracture pattern of shale after treatment is more complex. Both tension and shear fractures are present and the fractures intersect and deflect.Moreover, fractures induced by the SC-CO2 fracturing of the untreated shale are more complex and rough than those in saturated brine and CO2. The arithmetic mean height, root-mean-square height, the maximum height and the fractal dimension of the crack surface of shale saturated with brine and CO2 decreased by 59.2%, 57.6%, 52.5% and 7.8%, respectively. The experimental results can provide laboratory evidence for the implementation of well-soaking before SC-CO2 fracturing.

1. INTRODUCTION
With the rapid development of the world economy, it is difficult for conventional oil and gas resources to meet the increasing energy demand (Hosseini et al., 2019). Countries have turned their attention to unconventional oil and gas development. Due to the dense matrix and poor physical properties of unconventional reservoirs, horizontal wells and large-scale hydraulic fracturing are required to realize efficient development (Lampe & Stolz, 2015; Soliman & Kabir, 2012). The application of clear water fracturing and slickwater fracturing in North America has successfully advanced the revolution in shale oil and gas development (Le, 2018). However, traditional hydraulic fracturing has a series of shortcomings, such as reservoir pollution, high cost, waste of water resources, difficulty in the treatment of flow-back fluid, and so on (DiStefano et al., 2019). As a result, some countries have begun to ban the use of hydraulic fracturing. For example, the French government has passed legislation prohibiting the use of hydraulic fracturing for shale gas production (Chailleux et al., 2018). Aiming at the environmental protection concept of sustainable development, it is urgent to explore clean and efficient fracturing technology. SC-CO2 fracturing can avoid the above problems and realize CO2 geological storage while efficiently exploiting oil and gas, attracting a lot of attention (Wang et al., 2012, Zhou et al., 2019).
Experimental and Numerical Evaluation of the Integrity of Cement and Geopolymer Under Low and Elevated Temperature Well Conditions
Authors: Ogienagbon A. , Khalifeh M. , Wu Y. , Salehi S. 
Published: September 2022

ABSTRACT:
Wellbores constructed in high-temperature environments require more reliable cement systems to overcome complicated near-borehole loads. In this study, we numerically evaluate the effect of temperature on the interfacial bond integrity of two cementitious materials - an industrial class expansive cement and a rock-based geopolymer based on their experimental triaxial test properties. The results from the triaxial test indicate that the test temperature did not have a significant impact on the mechanical properties of both types of materials except for the confined compressive strength. Geopolymer showed higher flexibility, higher Poisson’s ratio, and lower compressive strength than the industrial class expansive cement at both temperatures. From the numerical analysis, the results show that the most likely cement failure location is at 90° to the maximum in-situ stress on the casing-cement interface. Without considering pore pressure effects, cement crushing resulting from excessive compressive stress is the main reason for the inner interfacial debonding. The test temperature has a significant influence on hoop stress. High test temperature results 13 – 45% increase in the hoop stress. Radial stress was insignificantly influenced by the test temperature. When test temperature changes from 30 to 90°C, geopolymer experiences an 80% increase in hoop stress while the industrial class expansive cement only experiences a 1% increase.

1. INTRODUCTION
Cement is one of the major barrier elements for maintaining well integrity. It is conventionally used for in primary, remedial and plug and abandonment operations. The loss of well integrity can lead to environmental risks, economic losses for the operator and even safety issues. Several factors can compromise cement integrity during its placement, these include but are not limited to poor cement displacement, fluid loss, mud contamination, etc. Despite the completion of a good cement job, fissures, cracks and microannuli may still develop long after well completion.
Mechanical Properties of Shales Mapped at the Nanometer Scale
Authors: Wang Qisheng , Wang Tianyu , Zhong Pengjun , Verdiyev Gorkhmaz , Zhang Panpan , Tian Shouceng 
Published: September 2022

ABSTRACT:
The mechanical properties of organic matter (OM) strongly affect the way shales deform and fracture. Lack of knowledge regarding mechanical properties of OM in fine grained rocks mainly because it often occupies tiny nanometer-scale voids between the mineral grains. In this study, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and atom force microscope (AFM) are used to measure the micromechanical properties of the shale. Then, the pyrite, feldspar, quartz, clay, calcite and OM are organized as the shale minerals database. The OM of the belt-like mechanical properties is found. Meanwhile, the greater the gray value (brighter) of the OM SEM image, the greater the elastic modulus of OM are. We propose a standalone software, the distribution, mean modulus, mean deformation and proportion of each minerals can be calculated by AFM In-Suit Identified software. These findings help understand the properties of organic-rich sedimentary rocks from a microscale perspective and provide the fundamentals for fracturing.

1. INTRODUCTION
Organic-rich shales have been commercially exploited with the hydraulic fracturing and the horizon wells development (Wang et al., 2014). The mechanical properties of the shale both affect the fracture propagation and the stability of the wells. However, the traditional uniaxial compressive strength (UCS) is highly dependent on the integrity of the shale. The UCS is a destructive test with poor repeatability, which reduces the core’s utilization value. Besides, the accuracy of cm-above cannot test the mechanical properties of organic matter (OM). For commercial development, the horizontal wells must pass via high total organic carbon (TOC) formation. The properties of OM deserve more attention, though they are difficult to determine.
Scanning electron microscope (SEM), energy dispersive spectrometer (EDS), nanoindentation, and atom force microscope (AFM) have been used to measure the micro properties of the shale(Wang et al., 2018). Cheng et al. (Cheng et al., 2021) classified bedding layers of the shale into fine-grained, transition and coarse-grained layers by SEM and the mechanical properties of each layer are tested by nanoindentation. Kumar et al. (Kumar et al., 2012) located the kerogen by focused ion beam and the whole kerogen was tested by one nanoindentation point. Xu et al. (Xu et al., 2020) first reported the friction behavior of the kerogen by AFM. Eliyahu et al. (Eliyahu et al., 2015) found the stiffer kerogen surround the softer bituminous by PeakForce quantitative nano-mechanical mapping (PK-QNM) AFM mode. The shapes of OM in marine and continental shales are rather different (Tian et al., 2018). However, currently, studies are focused on the properties of circular bulk OM. The properties of long-strip OM are rare to report.
Experimental Investigation of Hydraulic Fracture Growth in an Anisotropic Rock with Pre-Existing Discontinuities Under Different Propagation Regimes
Authors: Lu G. , Momeni S. , Lecampion B. 
Published: September 2022

ABSTRACT:
Laboratory experiments have been carried out to investigate the growth of hydraulic fracture (HF) in an anisotropic rock with pre-existing discontinuities such as bedding planes and veins. The experiments are designed in light of scaling relationships that enable investigation on the impact by the weak mechanical discontinuities on the HF growth under desired regimes (under either a toughness dominated or a lag-viscosity dominated regime based on the scaling analysis). According to our laboratory observations, planar HF growth is promoted in the experiments under the lag-viscosity regime, in which the radial HF is able to cross both the bedding planes and veins. In contrast, profound influence of the weak discontinuities is shown in the toughness dominated experiments, causing the HF to be diverted into, or arrested by the pre-existing weak planes. The test results suggest that the complexity of the fracture path is highly dependent on a dimensionless toughness, Km. This laboratory study highlights the importance of the hydromechanical characteristics in the HF propagation in anisotropic rocks with pre-existing weak discontinuities.

1 INTRODUCTION
Weak discontinuities, such as weakly cemented bedding planes, veins, natural fractures, and defects are often encountered in unconventional reservoirs. It has been shown by both laboratory (Suarez-Rivera et al., 2013; Burghardt et al., 2015; Lee et al., 2015; Llanos et al., 2017; Fu et al., 2018; Xing et al., 2018; Lu et al., 2021) and field-scale hydraulic fracturing experiments (Warpinski and Teufel, 1987) that the discontinuities can locally reduce the strength of the rock, and alter the propagation direction of a hydraulic fracture (HF) that is growing towards the planes formed by the discontinuities. Such interaction between the HF and discontinuities can lead to the HF crossing, diverted into, or completely arrested by the pre-existing weak planes.
Numerical Simulation of Limited-Entry Fracturing in Multi-Layer Heterogeneous Reservoir
Authors: Wang F. , Deng J. G. , Liu W. , Tan Q. , Tan Y. W. 
Published: September 2022

ABSTRACT:
Limited-Entry fracturing underwrites commercial production from multi-layer heterogeneous reservoirs. The uneven contribution of multi-layer can be adjusted by Limited-Entry Fracturing to improve the productivity of poor formations and better control the propagation of fractures at each cluster position. In this study, the fracture propagation model is combined with reservoir productivity simulation to explore the effect of perforation parameters on multi-layer fracture propagation and enhanced oil recovery in multi-layer heterogeneous reservoirs. The fracture propagation model considers the impacts of perforation parameters and reservoir properties on hydraulic fracture morphology, which provides a theoretical basis for completion optimization in limited-entry fracturing. This work also emphasizes the control of fluid partition at each cluster. Based on the geomechanical model of the PengLai oilfield, the reservoir productivity was simulated after limited entry fracturing treatment. In addition, the multiple cluster fracture morphology is analyzed to assess the performance of adjusting perforating parameters in overcoming rock heterogeneity. Numerical simulation results show that the application of adjusting perforating parameters is beneficial to multi-layer recovery balance. The fracture length and width build with the increase of perforation density, diameter, and decrease with permeability and minimum horizontal principal stress. Moreover, the effect of limiting entry fracturing is obvious, increasing cumulative oil production by 10%~20%. This paper provides good guidance for perforation parameters optimization for limited-entry fracturing in multi-layer heterogeneous reservoirs.

1. INTRODUCTION
Miller evaluated 114 wells in six U.S. shale basins (Marcellus, Haynesville, Eagal Ford, Fayetteville, Barnett, and Woodford), and found that the proportion of invalid perforation clusters was about 30%. Reservoir heterogeneity and completion quality are the key factors affecting the production of each cluster. There are few invalid perforation clusters using 2~6 cluster perforation schemes.
Ugueto et al. used optical fiber temperature and acoustic monitoring technology to analyze the fracturing of multi-cluster fractures and found that effective liquid inlet clusters accounted for 50 % ~ 70 % of the total perforation clusters. He proposed that maintaining high perforation friction is the key factor to increase the number of effective perforation clusters.
Evaluating the Influence of Parameter Inputs on Macroscopic Behavior of Bonded Block Models with Inelastic Blocks
Authors: West I. , Walton G. 
Published: September 2022

ABSTRACT:
Bonded Block Models (BBMs) are commonly used to simulate laboratory Unconfined Compressive Strength (UCS) and triaxial tests on rock. BBMs require input parameters that describe the deformation of the blocks themselves (block properties), as well as the interaction of the blocks with adjacent blocks (contact properties). The blocks that make up BBMs can be modeled either elastically or inelastically, where elastic blocks only deform elastically, whereas inelastic blocks both deform elastically and can yield. BBMs with inelastic blocks are unique in that they are capable of simulating the deformation and breakage of mineral grains, allowing them to simulate a broader range of rock behaviors. Previous studies have documented the effects of variation of contact properties and elastic block properties on the overall behavior of BBMs, but little has been written regarding the effects of inelastic block properties. This study evaluated more than 300 sets of inelastic block input parameters to identify their influences on trends in BBM macroscopic behavior. The inelastic block properties were found to only affect the peak strength and the post-peak behavior (like brittleness) of the BBM, within the range of values tested.

1. INTRODUCTION
Mechanical behavior of rock has historically been characterized in the laboratory via Unconfined Compressive Strength (UCS) and triaxial tests (Robertson, 1955; Handin and Hager, 1957). With the continued advances in computing, however, numerical models have become increasingly used to study rock behavior. Numerical models can be used to complement laboratory testing, as they require significantly less time and labor to complete. Specifically, the behavior of low porosity intact rock in the context of UCS and triaxial tests can be simulated using Bonded Block Models (BBMs) (Ghazvinian et al., 2014; Farahmand and Diederichs, 2015; Sinha and Walton, 2020).
BBMs are a subset of discrete element method (DEM) modeling where material is represented as a series of continuous elements that interact discretely with adjacent elements. The deformations of these continuous elements, and their interaction with adjacent elements, are governed by a series of input parameters (Jing, 2003). Each input parameter affects the DEM model’s macroscopic behavior uniquely. In cases where numerical models use input parameters that cannot be directly tested or estimated from field or laboratory testing, as in the case of BBMs, the macroscopic behavior of the model must be calibrated to laboratory data via an iterative process of varying the input parameters.
Identification Method of Carbonate Acid Erosion Fracture Propagation Morphology Based on Impulse Analysis
Authors: Zhan Jiahao 
Published: September 2022

ABSTRACT:
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

ABSTRACT:
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

ABSTRACT:
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.