bluuebunny/crossref_metadata_2025 huggingface dataset:
Study on the Damage Mechanism of Rock Around Wells Subjected to Impact Loading
Authors: Huang T. , Liu Q. Y. , Zhu H. Y. , Dai X. W. , Ji R. , He X. , She C. Y. , Zheng M. J.
Published: August 2024

To investigate the rock damage characteristics in the vicinity of a wellbore under mid-to-high temperature reservoirs with varying impact parameters, this study conducts bit impact rock-breaking experiments under high-temperature and high-pressure conditions. Standard core samples are drilled at different positions on the wellbore wall to test and obtain the distribution pattern of microcracks in the surrounding rock and to characterize their microdamage attributes. Numerical simulations are then carried out to further analyze the damage patterns to rocks near the wellbore caused by impact drilling. The results demonstrate that impact drilling can cause damage to rocks near the wellbore, with the damage becoming more severe and the microcracks more densely packed as one approaches the wellbore wall. The temperature of the reservoir significantly influences the effectiveness of impact drilling. Different phases of impact also affect the stress state and damage characteristics of the rocks, particularly during the unloading phase, where the bottomhole and surrounding rocks generate more fissures.

1. INTRODUCTION

As an important clean and renewable energy source, geothermal energy has the characteristics of low carbon, environmental protection, stability and high efficiency. It is one of the most competitive renewable and clean energy sources (Yu et al., 2018). Geothermal resources are mainly divided into open heat extraction and closed heat extraction according to different development methods. Studies have shown that the former is superior to the latter in terms of economy and heat extraction efficiency (Wang et al., 2019). For open heat extraction, formation permeability is an important factor to determine the efficiency of heat extraction. On the one hand, the higher the formation permeability, the greater the flow rate of the fluid, the more heat it can carry, and the higher the convective heat transfer intensity, so the thermal efficiency is significantly increased. On the other hand, the currently under construction and completed water-intaking geothermal systems require tail water recharge to ensure the sustainability of heat extraction and prevent environmental pollution and land subsidence. It can be seen that improving the formation permeability has become a key link to strengthen the heat extraction effect.
Numerical Investigation on the Rock Breaking Performance of Special-Shaped PDC Cutters Combination
Authors: Jin Ming , Chen Xuyue , Li Gengchen , Cao Xiaohua , Wang Rong , Zhu Mingchi
Published: August 2024

The special-shaped polycrystalline diamond compact (PDC) cutter has good rock breaking performance. This paper adopts the finite element method to conduct research on the rock-breaking capabilities of special-shaped PDC cutters. The results show that: with the increase of the spacing between the front row of cutters, the cutting force on the back row of cutters gradually increases and then tends to be stable. The optimum spacing of cutters in the combination of ‘axe-shaped and triple-ridged cutters' is 11mm, the optimum spacing of cutters in the combination of ‘axe-shaped and conical cutters’ is 14mm, the optimum spacing of cutters in the combination of ‘conical and triple-ridged cutters’ is 13mm. In transverse vibration, the combination type of ‘conical and triple-ridged cutters’ has the smallest vibration condition. In the axial vibration, the combination of ‘axe-shaped and conical cutters’ has the lowest vibration. The results of the study can provide a basis for the selection of cutter combinations and the optimal design of drilling tools for PDC drills.

1. INTRODUCTION

With the increasing global demand for energy, the exploitation of deep formation oil and gas resources has become one of the focuses of attention in the energy industry (Sun et al., 2013). Deep formation oil and gas resources are rich in reserves and high in quality, however, the characteristics of deep formations such as high rock hardness, strong heterogeneity of formations and high geostress severely limit the drilling efficiency in deep formations (Chen et al., 2024; Guo et al., 2019; Wu et al., 2020). How to efficiently crush rocks in deep hard-to-drill formations and improve rate of penetration has become a key problem to be solved. Conventional rock-breaking techniques are often ineffective when facing deep formations, and it is difficult to meet the industrial demand for efficient, safe, and low-cost drilling (K. Abbas, 2018). Special-shaped Polycrystalline Diamond Compact (PDC) cutter combination rock-breaking technology, as an innovative tool and method, offers a novel solution to overcome the limitations of traditional techniques.

Special-shaped PDC cutters not only efficiently crush a wide range of rock types, but also remain stable and wear-resistant over long periods of high-load operation (Che & Ehmann, 2014). In the combined rock-breaking technology of special-shaped PDC cutters, cutters of different shapes and sizes work in concert with each other to maximize crushing efficiency and reduce energy waste.
Characteristic Analysis of Flow Field and Cuttings Removal of Straight-Swirling Integrated Jet During Tubing Cleaning
Authors: Xiao Yongjun , Zheng Jian , Wu Chunlin , Chen Zhi , Luo Xin , Liu Bingxiao , Ren Yingqiao , Huang Haochen
Published: August 2024

Shale gas is a significant alternative resource to conventional gas. During the development of shale gas in Changning, the energy attenuation of the extracted shale gas leads to insufficient water discharge from the stratum. This ultimately leads to the gradual accumulation of sand from the stratum, which then clogs up both the tubing and casing over time, resulting in reduced or even halted productivity of horizontal wells. To improve the productivity of aging shale gas wells in Changning, a straight-swirling integrated jet (SSIJ) tool is designed. Computational fluid dynamics (CFD) method is utilized to investigate the velocity field and cuttings removal characteristics of the SSIJ tool, with comparisons made against the convergent-divergent jet (CDJ) tool. The findings suggest that: 1) Compared to the CDJ tool, the SSIJ tool's jet exhibits higher axial and tangential velocity extremes, stronger cavitation initiation capacity and cavitation erosion effect. 2) The SSIJ tool shows a greater cuttings-carrying capacity. However, this ability decreases with larger cuttings sizes, and increasing the viscosity of the liquid can greatly enhance the tool's cuttings-carrying efficiency. This research offers valuable insights for the design and optimization of downhole tubing and casing cleaning tools.

1. INTRODUCTION

Shale gas production has experienced a significant increase in the last ten years, largely due to advancements in horizontal drilling and hydraulic fracturing technologies(Dong et al., 2024). Shale gas accounts for a substantial share of China's natural gas production, with an estimated annual production of 30×109 m3 by 2025. The Changning shale gas field, situated at the southwest edge of the southern Sichuan Basin, has proven reserves amounting to 440×109 m3. In 2020, it reached a production capacity of 5×109 m3/a for shale gas (Ailin et al., 2020; Ma, 2021; Zou et al., 2016). However, while extracting shale gas in Changning, sedimentation may occur at the bottom of the well (Liu, 2019). When fracturing bridge blockage are milled, this can result in the accumulation of cuttings at the bottom of the well, showed in Fig.1. This situation has the potential to cause tubing and casing blockage, leading to decreased production or even complete shutdown of the gas well. Therefore, it is important to promptly remove the sand buildup in the tubing and casing during production.
Inversion of Creep Rate of Salt Rock Using Finite Element Method Based on Engineering Data
Authors: Li Ji , Lu Yunhu , Wei Shiming , Huang Chenxing , Li Hongda , Zhan Qing
Published: August 2024

The Carboniferous salt layer is buried deeply in the Tarim Basin. Affected by high temperature and high stress, the creep rate of salt rock is fast, which leads to a series of stuck bit and casing failure accidents. It is difficult and costly to obtain salt rock core from underground, and the outcrop core is difficult to represent the properties of deep strata, so it is difficult to determine salt rock creep rate by conventional rock mechanics testing. In this paper, the microstructure characteristics of salt rock were clarified by SEM analysis. The information on the stuck bit has been collected, and the relationship between the well diameter and time of the salt layer is mastered. On this basis, the finite element analysis model considering the creep of the salt layer is established, and the creep parameters of the salt layer are inverted based on the relationship of borehole diameter with time. This study solves the problem that the creep rate of salt rock is challenging to be quantitatively analyzed under difficult core-taking conditions and can provide technical support for drilling and completion of the salt layer.

1. INTRODUCTION

Salt layers are often encountered in both onshore and offshore oil and gas drilling. In the process of salt layer drilling, the complicated mechanical properties of salt layers lead to drilling stuck, casing stuck, casing failure, and other accidents (Al-Saeedi et al., 2011, Liu et al., 2021, Chengxin et al., 2023). Casing failure occurred in the carboniferous salt layer in many wells in Tarim Basin, China, resulting in frequent side drilling, which severely restricted the safe and efficient exploitation of oil and gas resources and brought additional economic losses. It is of great significance to determine the creep rate of the salt layer.

A series of studies have been carried out to determine the creep rate of salt rock. The most common method to obtain creep rate is to process salt rock into a standard sample of a certain size, and load it under constant pressure under uniaxial (Berest et al., 2005 and 2014) or triaxial (Maia et al., 2005, Moosavi et al., 2009, Chengxin et al., 2023) conditions, and measure its average strain change over a certain period. With the deepening of research in this aspect, some scholars began to introduce image recognition technology (Blaber et al., 2015) into creep rate test, which can obtain the change of strain with time at any point on the sample surface during loading (Ju et al., 2022). Using the creep rate test results, the creep equation of salt rock can be fitted and the creep rate of salt rock can be determined under specific temperature and pressure conditions (Zhou et al., 2011, Wang et al., 2014, Zhang et al., 2021). At present, rock mechanics testing is still the main method to study the creep rate of salt rock, and the conventional methods to obtain the creep rate of salt rock cannot get rid of the constraints on the shape and size of salt rock samples.
Analysis of Casing Damage Mechanism of Carboniferous Salt Layer in Tarim Basin
Authors: Liang Hongjun , Huang Chenxing , Qin Hongde , Luo Renkun , Deng Qingshan , Lu Hui , Zhan Qing , Lu Yunhu , Wei Shiming
Published: August 2024

The Carboniferous strata in Tarim Basin, China developed salt beds. Casing damage occurred in the drilling process, which led to frequent sticking and even forced sidetracking, which seriously affected the development of oil and gas in this block and caused great economic losses. To solve the problem of casing damage in the Carboniferous salt layer, the mechanism of casing damage is analyzed in this paper. Based on the statistical analysis of geological and engineering data, the casing damage mechanism of bending and stretching deformation is deduced, and the above casing damage mechanism is confirmed by establishing the finite element model of casing strength checking. The effects of borehole enlargement rate, casing center degree and casing program on the bending and stretching yield of casing are also analyzed. On the whole, the casing damage risk is positively correlated with borehole enlargement rate and axial tension, and negatively correlated with arm of force and casing center degree. In the casing program design, the salt layer should be located at the bottom of a certain run, and the anhydrite should be destroyed by reaming multiple times in the salt layer to avoid the formation of "hard fulcrum" during cementing, which can effectively reduce the risk of casing damage.

1. INTRODUCTION

There are a lot of oil and gas buried in salt layers around the world, such as the United States Gulf of Mexico, offshore Brazil, West Africa, and the North Sea (Greenhalgh et al., 2012; Chitale et al., 2014). Casing damage in the salt layer is a problem that has been discovered for a long time (Cheatham et al., 1964). This problem is still happening today, resulting in oil and gas wells being forced to sidetrack or even be abandoned, which not only wastes much money but also seriously restricts oil and gas production.

Rock salt can creep and deform, and creep begins the moment it is drilled into the salt layer (Wang et al., 2016). Even after the cementing operation is carried out, the cementing quality is often difficult to guarantee, so the rock salt creep will still exert force on the casing (Willson et al., 2002). At present, the damage modes of casing in the salt layer are mainly extrusion deformation and bending deformation (Clegg et al., 1971; Taheri et al., 2020).
The Slipping Characteristics of Bedding Interface Under Shearing Stress in Unconventional Reservoirs
Authors: Chen Lei , Li Fengxia , Huang Zhiwen , Zhou Tong , Chen Shijing , Zhang Min , Zhang Guangqing
Published: August 2024

The bedding interfaces with different cohesions usually exist in unconventional reservoirs. During reservoir stimulation, the slips induced by hydraulic fractures are related to the fracturing effects. In this paper, the rock specimens with cemented and uncemented bedding interfaces were selected to conduct direct shearing experiments, and the slipping behaviors were analyzed using the digital image correlation (DIC) method and acoustic emission (AE) technique. The results show that the slip weakening mode exists in cemented bedding interface during slipping, and the corresponding variation of AE b-value is small. The entire uniform slippage will appear in the uncemented bedding interface during slipping, accompanied by the larger variation of AE b-value. The effects of the interlayer cohesion on the slip weakening zone length, AE magnitude, and the difference between static and kinetic friction were discussed. It is found that the opening feature is not significant during slip weakening, but obvious during cohesionless slipping. And a relationship model was established to describe the opening displacement affected by slipping. This study can help to understand the slip mechanisms during fracturing in unconventional reservoirs.

1. INTRODUCTION

Unconventional reservoirs, such as shale, have been the key areas in oil and gas production (Gao et al., 2022). Unlike conventional reservoir rocks, unconventional reservoirs are usually characterized by bedding development. Therefore, investigating the slip of bedding plane in unconventional reservoirs during fracturing is one of the focuses in rock fracture mechanics (Zhao & Gray, 2021).

Microseismic monitoring results of in-situ fracturing demonstrate that hydraulic fractures can induce interlayer slip (Rutledge et al., 2016; Tan & Engelder, 2016a). The enhancement of hydraulic conductivity by slip (Zhao et al., 2013; Ye & Ghassemi, 2018) is a crucial factor for promoting shale oil and gas production (Urbancic & Baig, 2016; Wang et al., 2019; Zoback et al., 2012). In field hydraulic fracturing, the slip fractures are usually monitored by microseismic moment tensor analysis (Tan & Engelder, 2016b) and micro-resistivity scanning imaging technology (Zoback & Lund, 2019). Laboratory experiments have also demonstrated that hydraulic fractures can induce interlayer slip at the bedding plane (Chen et al., 2023). Some researchers think that the shear slips of interlaminations during fracturing are part of complex fracture networks (Bunger et al., 2013; Huang et al., 2013; Min et al., 2010; Ye & Ghassemi, 2018).
Gas Production Prediction Model and Parametric Analyses for Fractured Horizontal Well in Deep Coalbed Methane Reservoirs
Authors: Jia Yanran , Shi Juntai , Ji Changjiang , Xiong Xianyue , Huang Hongxing , Chen Ming , Wang Wei
Published: August 2024

To ensure the long-term, efficient, and stable development of deep coalbed methane, it's urgent to conduct research on gas production prediction and influencing factors analyses. In this study, based on the three zones (stimulated zone with proppant filled, stimulated zone without proppant filled, and un-stimulated zone) distribution in deep coalbed methane reservoirs developed with fractured horizontal wells, composite pseudo-steady-state gas-water productivity equations are established. Subsequently, coupling this productivity equation with pressure propagation models considering well shut-in, dynamic permeability models, and the material balance equations, a composite gas production prediction model for fractured horizontal wells in deep coalbed methane reservoirs is proposed and solved. Finally, the validations using commercial numerical simulation software and history matching with field case are conducted, and the influencing factors on gas production in deep coalbed methane reservoirs are analyzed. Results show good agreements between the predicted and actual results are achieved for both synesthetic case and field case. The sensitive influencing factors on gas production are ranked as follows: number of fractures, free gas saturation, fracture half-length, permeability of Zone I, Zone I size, degree of coal fines blockage, permeability of Zone II, Zone II size, permeability of Zone III, and horizontal well spacing.

1. INTRODUCTION

As a type of unconventional resource, coalbed methane is a very clean natural gas resource. Its development and utilization not only increase the rare low-carbon clean energy, but also serve as a practical carbon neutral industrial path, which can significantly reduce methane emissions during coal mining and reduce the greenhouse gas effect of methane. The shallow coalbed methane resources in China are 30.1 trillion cubic meters, and the preliminary estimate of deep coalbed methane resources can reach 40 trillion cubic meters. Obviously, deep coalbed methane resources are richer than those in the middle and shallow areas. Regarding the depth limit of deep coalbed methane, American scholars defined it as coalbed methane with a burial depth greater than 1,524 meters in the coal reservoir; Based on the progress of coalbed methane in recent years, Xu has proposed that deep coalbed methane specifically refers to those with a burial depth greater than 1500 meters in coal reservoirs. The industry gradually agrees that coalbed methane with a burial depth greater than 1500 meters is called deep coalbed methane.
Geomechanical Effects of Cyclic Loading on Rocksalt
Authors: Houben M. E. , van der Linden A. J. , El Azouzi K. , Marcelis F. H. M. , Coorn A. H. , van Schalm R.
Published: August 2024

Hydrogen-based solutions are gaining importance as a renewable clean energy source. A geomechanical challenge when hydrogen storage in salt caverns is considered is the cyclic loading and unloading of the host rock (rocksalt). As hydrogen is stored and removed based on fluctuating demand throughout the year the cavern wall will be exposed to pressure cycles, where in times of high demands the hydrogen is supposed to be readily available and in times of low demand, the hydrogen will be stored in the cavern. Ideally these pressure cycles do not influence the integrity of the cavern as a storage site. To investigate the effect of cyclic loading on rocksalt, several cyclic loading experiments have been performed in the lab using two different very pure rocksalt samples from Germany (Asse Speizesalz and Z3 Leine Rocksalt (both > 98% NaCl). Loading stresses, number of cyclic loading event and length of high versus low differential stress have been varied to investigate the effect of cyclic loading. Daily loading cycles versus weekly loading cycles showed very similar strains on the sample. When reaching stresses above the dilatancy boundary of the rocksalt though the samples kept deforming.

1. INTRODUCTION

Hydrogen storage is one of the key necessary steps in the energy transition, where hydrogen-based energy solutions are gaining importance as a renewable clean energy (e.g.: Abdalla et al., 2018; Gulcin Caglayan et al., 2020; Lankof et al., 2020; Tarkowski, 2019; Zhang et al., 2016; Zivar et al., 2020). With energy consumption varying daily as well as seasonally, and the hydrogen supply-demand chain persistently being out-of-phase, flexible hydrogen storage solutions are critical. Specifically, in times of low energy demand excess hydrogen must be temporarily stored (Zivar et al., 2020), while in times of high demand the hydrogen must be easily accessible and readily available. A well-known hydrogen storage option is geologic sequestration in salt caverns (e.g.: Tarkowski, 2019; Heinemann et al., 2021). Salt is known as a natural barrier with general low permeability, frequently acting as a seal for hydrocarbons and widely considered as possible host rock for the storage of radioactive waste (e.g.: Arson et al., 2012; Gulcin Caglayan et al., 2020). In the past 30 years, research has been performed on the mechanical behavior of rocksalt (A selection of references: Schutjens, 1991; Peach and Spiers, 1996; van den Bogert et al., 1998; Hunsche and Hampel, 1999; ter Heege et al., 2005; Alkan et al., 2007), but less work has been performed on the cyclic loading of rocksalt.
Fracturing Breakthrough and Comprehensive Evaluation of CP1 Well in Yingxiongling Shale Oil Reservoir, Qaidam Basin
Authors: Y. Liu, Y. , G. Xian, C. , Y. Liu, , X. Chen, , Y. Zhang, M. , Cao W. , Wan Y. Y.
Published: August 2024

The Yingxiongling shale oil reservoir in the Qaidam Basin poses unique challenges due to its high stress differentials, steep stress gradients, abundant limy dolomite content, strong heterogeneity, and underdeveloped natural fractures. This paper conducts a comprehensive geoscience-to-engineering analysis of the CP1 well to offer insights for reservoir stimulation and production optimization. Results indicate that the "single well proactive interference and control" fracturing plan effectively manages fracture propagation. Comparison of liquid production across lithofacies reveals laminated limy-dolomitic shale's strong liquid supply capacity and layered limy-dolomitic shale's high oil production capacity. The spatial relationship between these lithofacies creates a distinctive enrichment and seepage pattern, forming the geological basis for maximizing the oil production potential of the Yingxiongling continental shale oil reservoir. Additionally, this configuration regulates post-fracturing oil and water production dynamics, emphasizing the significance of the interbed combination of laminated limy-dolomitic shale and layered limy-dolomitic shale as favorable lithofacies.

1. INTRODUCTION

China possesses abundant continental shale oil resources, with recent years witnessing significant advancements in exploration and development, particularly in the Jungar, Ordos, Songliao, and Qaidam basins (Zhao et al., 2021; Jin et al., 2021; Fu et al., 2022; He et al., 2022). Volume fracturing technology is the key to achieving efficient development of unconventional oil and gas reservoirs. With the goal of maximizing the effective stimulated reservoir volume (SRV) and single-well control of EUR, guided by the idea of geoscience-to-engineering integration, we targeted the characteristics of high stress differentials, steep stress gradients, high limy dolomite content, strong heterogeneity, and underdeveloped natural fractures in the Yingxiongling continental shale oil reservoir (Jiang et al., 2023). Following a "technically prudent, parameter aggressive" approach, we employed a core volume fracturing technique that includes "dense cutting of multiple segments and clusters, extreme flow limiting perforation, high pumping rate, high intensity continuous sand addition with slickwater with variable viscosity". This approach resulted in the formation of a complex network of fractures, establishing an artificial seepage system to enhance single-well production.
The Thrust-Penetration Gradient: Decoding Geomechanical Dynamics and TBM Response
Authors: Schlicke M. , Nübel K. , Wannenmacher H. , Amann F. , Marcher T.
Published: August 2024

The Thrust-Penetration Gradient (TPG) is an essential tool in analyzing and predicting the performance of Tunnel Boring Machines (TBMs). It evaluates TBM efficiency, reflecting geomechanical dynamics during tunneling. The TPG effectively captures the complex relationship between thrust and penetration rates across various rock masses, drawing a parallel to the CSM model (Colorado School of Mines model) but with broader applicability. Originating from the TBM's start-up phase, the TPG offers real-time, continuous insights into machine-ground interactions, highlighting non-linear gradients. It involves isolating shield friction from total thrust, a process revealing frictional variations even in consistent geological settings. TPG also accounts for supplementary losses due to cutterhead curvature. By comparing torque and thrust per cutter, it accurately determines shield friction for each stroke, using quadratic and exponential models to correlate thrust per cutter with penetration rate. More comprehensive than the Specific Penetration metric, this approach avoids linear reduction constraints, offering a more precise platform for TBM performance evaluation. TPG's utility extends to grouping gradients based on geomechanical non-linearities through a novel matrix system, facilitating forward predictions of penetration rates irrespective of project or geomechanical conditions.

1 INTRODUCTION

A non-linear correlation between the acting thrust at the cutter head and the corresponding penetration of disccutters in a specific rock mass greatly influences the performance of a Tunnel Boring Machine. This correlation is known as the Thrust-Penetration Gradient (TPG), and it reflects the disc-cutter indentation process within a specific rock type. The TPG is a specific and sensitive metric to evaluate the TBM's performance. It is considered a ground truth parameter for performance characterization, based on the Colorado School Mines Penetration Prediction for intact rock in the laboratory. The TPG represents the required work for rock mass mining and predicts performance for various rock conditions. The TPG for rock mass resembles the Colorado School of Mines model, representing the upper limit of disc-cutter indentation on massive rock conditions. A matrix notation is introduced as best practice to align specific TPGs towards rock strength and fracturing.
Monitoring Slope Movement Using Unmanned Aerial Vehicles
Authors: Tohm Calvin , Lingwall Bret , Uy Linus
Published: August 2024

Slope movement post precipitation events are a well-documented and understood phenomenon. The increase in moisture, if able to infiltrate the hill slope, can lead to the buildup of excess pore water pressure resulting in a decrease in shear strength of the soil. This phenomenon results in surficial failures that can slowly degrade the stability of a hill slope and eventually lead to failure. Monitoring this using traditional surveying techniques can prove to be time consuming and difficult due to the sheer number of points required to accurately capture the slope movement. To overcome this, this study sought to use unmanned aerial vehicle (UAV) techniques and equipment to monitor existing slope movement and failures. A study area in Rapid City, South Dakota around the South Dakota School of Mines and Technology (SDSMT) campus, which included multiple existing slope failures, was used. To monitor slope movement, photogrammetric imagery was taken of the study area both prior to and after precipitation event. Drone imagery was used to construct structure from motion (SFM) models to monitor slope movement by comparing previous flights to one another. All areas of interest or failures documented from the flights were individually ground truthed to ensure accuracy of the imagery. From this study it was found that seepage areas were a high indicator of slope failure of exposed ground surfaces and SFM models were able to accurately capture and document slope failures that had occurred after precipitation events.

1. INTRODUCTION

Seasonal weather cycles result in the development of anisotropic stresses within the surface layer of earthen structures, resulting in altered engineering properties and the development of an active layer of soil1. These weathering cycles include iterative shrink-swell with wetting and drying and freeze-thaw as well as times of acidic water, organic acids, and other chemical weathering processes. As the degree and severity of weathering cycles increases, this active layer propagates downward and results in near surface (upper 5-m) failures, weakening the overall integrity of geotechnical earth structures, resulting in increased erosion rates2-5, slope failure5, and compromised hydraulic conductivity in dams and levees2,7-9. These alterations in the engineering properties of earthen structures results in unforeseen or premature failure for slopes sensitive to changes in properties and conditions in the active zone (3 to 5-m), especially materials in the upper 1-m sensitive to seasonal freeze-thaw. Therefore, it is understood that seasonal weathering cycles are a key contributor to obscelense of earthworks.
Chemo-Mechanical Modeling of Anhydritic Claystones
Authors: Antonia Nousiou A. N. , Georgios Anagnostou G. A.
Published: August 2024

This paper presents a model for the swelling of anhydritic claystones – a severe problem in tunnelling. The model incorporates the effect of the chemical process of anhydrite to gypsum transformation via a strain decomposition in a mechanical (e.g., elastoplastic) part and a reaction-induced part. The latter depends on the mass change of anhydrite and considers that the volumetric chemical strain rate is the sum of the solid and pore volume change. The former is given by the chemical reaction's stoichiometry and the molar volumes of anhydrite and gypsum; while the latter may decrease or increase depending on whether new gypsum crystals precipitate within the available pore space or expand the matrix. Laboratory observations indicate that the higher the stresses and the porosity, the less the pore volume will increase during the chemical reaction. Moreover, the model assumes that the chemical strain rate is coaxial with the stress and that the strain in every principal direction depends on all three principal stresses. The predictions of the proposed model agree well with the results of test on artificial anhydrite-kaolin specimens and a high degree of correlation is achieved (R2 = 0.92).

1. INTRODUCTION

The swelling of anhydritic claystones is attributed to the water uptake by the clay minerals and the chemical process of anhydrite (CaSO4) to gypsum (CaSO4·2H2O) transformation ("AGT"). This phenomenon has caused extensive damages to a number of tunnels (e.g., tunnel floor heave, damage of the invert arch; cf., Amstad and Kovári, 2001; Anagnostou, 2007) and hence it has attracted great attention from the scientific community (e.g., Anagnostou et al., 2010; Ramon, 2014; Serafeimidis, 2014; Butscher et al., 2016; Wanninger, 2019). Nevertheless, a swelling law, capable of capturing these processes, is still missing (Anagnostou et al., 2010; Butscher et al., 2016), thus introducing uncertainties in tunnel design.

Early swelling models for tunneling (e.g., Einstein et al., 1972; Wittke and Rissler, 1976; Fröhlich, 1986) were based on Grob's (1972) "semi-logarithmic swelling law" and described swelling as a purely mechanical process; the developed strains are associated only with the stress changes. However, swelling rocks might also deform under constant stress or experience a stress increase without deformation, phenomena beyond the scope of purely mechanical models.
Kinematic Analysis of Geomechanical Displacement Fields to Assess Lifetime Integrity of Wellbores
Authors: Sathuvalli Udaya B. , Suryanarayana P. V. , Lewis David B.
Published: August 2024

Displacements in geomechanically perturbed formations induce additional stresses on wellbore tubulars. An appropriate well design basis should consider the mechanical response of the tubulars and the associated architectural implications. Traditionally, assessments of well integrity to geomechanical loading states have been conducted with large scale numerical models of reservoir, rock and the wellbore. In this context, we present a method to assess well integrity from a kinematic analysis of the geomechanical displacement field. The crux of the analysis is to regard the displacement field from the viewpoint of an observer on the wellbore centerline. Since the mechanical stiffness of a moving formation greatly exceeds that of a wellbore, the well practically follows the rock, and its shape accords with the equilibrium positions of the surrounding rock. The kinematic analysis identifies the dominant loading mode(s) on all the casings. We then apply specifically developed mechanical limit states to assess the mechanical response of the casings and cement sheaths (when applicable) to the geomechanical displacements. These limit states are used to determine well integrity, times to failure, and architectural options to prolong or optimize well life in an evolving set of geomechanical perturbations.

1. INTRODUCTION

Geomechanically induced displacements can severely test the integrity of wells. Regardless of the cause, an arbitrary point in the formation moves to a new location as a function of geomechanical displacements. Reservoir compaction, salt creep, tectonic stresses and fault movements are some of the most common sources of geomechanical loads on wellbores. Traditionally, assessments of well integrity to complex geomechanical loading states have been conducted with large scale integrated numerical models of reservoir, rock and the wellbore (Chia and Bradley, 1989; Bruno and Bovberg, 1992; Fredrich et al. (2001); Ibekwe et al. (2003); Li et al. (2003); Abou-Sayed et al. (2004)). Though these models are technically detailed and accurate, they are time consuming and inflexible, especially when parametric sensitivities are needed. During early stages of field development, when bottom hole locations, surface coordinates, trajectories, casing points and well architectures have not yet been finalized, the impact of geomechanical stresses on well integrity and well lifetimes can determine the success or failure of the project. In this light, our paper presents a method to analyze well integrity by examining the kinematic parameters of the geomechanical displacement field. Since the mechanical stiffness of the formation greatly exceeds the stiffness of the wellbore1, the geomechanical displacements are almost entirely transferred to the wellbore. The displacement deforms the wellbore centerline from its initial shape (defined by the drilling trajectory) into a three-dimensional curve characterized by its kinematic parameters.
Development of a Methodology for Predicting the Strength of Intact Bentonitic Shale and Gypsiferous Siltstone Using Fragments and Micro-Indentation Modulus Testing
Authors: Tohm Calvin , Lingwall Bret , Mitra Rudrajit
Published: August 2024

In many geologic and geotechnical investigations, obtaining sufficient intact rock core samples for uniaxial compression or point load testing can be difficult in weak sedimentary rock that spans into the realm of an Intermediate Geo-Material (IGM). These IGM materials have sufficient strength for infrastructure construction, but are difficult to core, sample, and test with sufficient volume to meet conventional ASTM or other test standards. Samples that are obtained, have often been fractured in the rock coring process. Thus, it is needful to develop alternative laboratory test methods to characterize the strength of these IGM soil/rock formations for engineering design. In the case of IGM bentonitic shales and gypsiferous siltstone materials of the Cretaceous Pierre Shale and Jurassic Spearfish formations of Western South Dakota, micro-indentation equipment was herein proposed as an alternative to uniaxial compression or point load testing. Fractured specimens of these materials were immobilized in polymer resin and tested using a micro-indentation apparatus to obtain the micro-indentation modulus. Complimentary intact and remolded specimens were tested in uniaxial compression to obtain parallel data. Coupling of the data from testing of fractured material and intact material and regression analyses show that a strong correlation exists between micro-indentation modulus and intact or remolded IGM strength. The method proposed in this paper is portable and extractable to other formations and validates the work of others around the world who have proposed this alternative approach for characterizing the strength of rock masses that are difficult to sample and test using conventional methods.

1. INTRODUCTION

Nearly 80% of the near surface geology west of the Missouri River and east of the Rocky Mountains in the Dakotas, Montana, Wyoming, Colorado, Nebraska and into Canada is composed of weathered residual soil, weathered rock, or weak sedimentary rock that spans from mechanically behaving as soil, Intermediate Geomaterial (IGM) and weak rock. IGM may be defined as cohesive or cohesionless geomaterial with unconfined strength ranging from 0.5 to 5-10 MPa or penetration blowcounts at 60% energy for a standard split-spoon sampler of 50-1501. The transition between soil and IGM may be noted by increased compressive strength at the 0.5 MPa threshold, while the transition between IGM and weak rock may be governed by an uncertain strength range of 5 to 10 MPa but also by a Rock Quality Designation (RQD) of 20 or less2. Highly fractured weak sedimentary rock with RQD < 20 may behave similar to IGM3 in foundation engineering.
Stress and Load Path Dependent Static and Dynamic Moduli and Ultrasonic Velocities in Granite
Authors: Knippel E. P. , Wu C. , Xiong Q. , Villaquiran Vargas A. P. , Hampton J. C.
Published: August 2024

We performed a study of ultrasonic velocity as a function of stress and stress path using axial and radial ultrasonic P and S-waves. Young's modulus and Poisson's ratio are calculated using these velocities along axial and radial directions. Static elastic moduli are also measured along the same stress and stress path protocol. Effects of pre-existing microcracks in the sample are hypothesized via directional observations of velocity changes as a function of differential stress and mean stress. Dynamic Young's modulus is observed to increase primarily as a function of mean stress, whereas static Young's modulus is mainly dependent on differential stress. Since the dynamic moduli are reliant on the accuracy of manually picked arrival times, a comparison is made between the manual picks and automated picked arrival times using AIC, STA/LTA, and CUSUM methods. The agreement between manual and automated picks is highest for P-wave arrivals found through AIC, and more varied for S-wave arrivals overall.

1 INTRODUCTION

Experimental damage mechanics provides an excellent lens for which to view the evolution of rock physical property behavior. The ability to set user-defined testing environments creates a wide variety of stress states rock samples can be subjected to. Ultrasonic velocities and strain recorded in situ can be used to measure the dynamic and static elastic properties of a material under stress (Knippel et al. (2023b)). The state of stress and stress path contribute to the magnitude and behavior of these elastic properties (Tutuncu et al. (1998a); Tutuncu et al. (1998b)). The co-evolution of stress-strain relationships and ultrasonic velocities is well-known to be influenced by the presence and accumulation of microcracks within a sample (Zoback and Byerlee (1975)). The presence of microcracks changes the intrinsic material properties that govern elasticity and compliance at the continuum scale. Cracks experiencing a positive volumetric change under stress increase pore space within the rock, decreasing acoustic wave speed which is used to calculate dynamic elastic moduli. Permeability can also be affected by the presence of microcracks, as tensile microcracks with positive volumetric change could coalesce and provide additional pathways for fluid movement (Hampton et al. (2019); Kluge et al. (2021)).
Measuring Brittle Damage Thresholds of Crystalline Rocks by Acoustic Emissions on Compressive and Tensile Laboratory Tests
Authors: Malicki E. A. , Packulak T. R. M. , Gagnon É. , Diederichs M. S. , Day J. J.
Published: August 2024

The use of acoustic emission (AE) monitoring in brittle rock laboratory testing is one of the most accurate methods for measuring damage evolution and failure. In this study, AE data was collected from two separate laboratory testing programs. The first program consists of a series of unconfined compressive strength (UCS) tests completed on moderately foliated meta-sedimentary rocks from the Bathurst Mining Camp in New Brunswick, Canada, and high metamorphic grade tonalite gneiss from the Pointe du Bois pluton in Manitoba, Canada. The second program consists of a series of Brazilian tensile strength (BTS) tests completed on polymineralic homogenous and isotropic granites and amphibolite from the Pointe du Bois pluton. Using data collected by a Physical Acoustics Pocket AE system, individual waveforms are analyzed to distinguish tensile fracturing from shear fracturing events. Distinct trends and timing in the occurrences of tensile and shear fractures are observed leading to further insight into the behaviours of the materials throughout the loading process. This study provides enhanced guidelines for the collection of AE data and choice of instrumentation, filtration of measured events, and interpretation of filtered data to develop a greater understanding of the micromechanics occurring within these materials in both UCS and BTS tests.

1. INTRODUCTION

Acoustic emissions (AE) are one of the most accurate measurement tools that can be used to study damage development in rock and concrete materials. Acoustic emissions are elastic vibrations emitted when microcracks form within a brittle material. For structural and tunnel stability, acoustic emissions can be used to assess strength and monitor the development of damage including microcrack formation, growth, and spalling. There are numerous studies examining acoustic emissions and microfracturing for homogenous unconfined compressive strength (UCS) tests such as Scholz (1968), Ohnaka & Mogi (1982), Lockner (1993), and Diederichs et al. (2004). There are few recent studies regarding AE in Brazilian tensile strength (BTS) tests, which presently include Keshavarz et al. (2008), Liu et al. (2015), Wang et al. (2019), and Khadivi et al. (2023). In much of this literature, acoustic emissions are used to quantify damage development. However, as examined in Diederichs et al. (2004), Ohtsu (2008), Shiotani (2008), and Ohno & Ohtsu (2010), AE can also be used to determine the type of damage occurring and how it impacts material strength.

In this study, two (2) testing programs were completed; the first being a series of UCS tests on moderate to high grade metamorphic rocks and one crystalline rock, and the second being a series of BTS tests on low grade metamorphic rocks. Sample details are provided in Section 2, along with testing equipment specifications.
Challenges and Uniqueness of a Dual-Injection Fracture Model
Authors: Kresse O. , Hobbs B. , Pei Y.
Published: August 2024

Approximately 23% of US land operators have tried simultaneous pumping operations and reported on average more than 30% gain in efficiency. However, surface efficiency without comparable well performance is not useful—all simultaneous fracturing jobs demand compromise in completion strategy or pumping schedule. The simultaneous fracturing model has been developed as an enhancement to the existing complex fracture model describing propagation of a hydraulic fracture in conventional and unconventional formations with the capability to simulate simultaneous pumping operations. The simultaneous or timely delayed pumping could be from the same pad or even offset pads—there is no limitation. This will include proper modeling of fracture initiation, propagation, and interaction between fractures from different wells through stress shadow effects and crossing rules. The simultaneous fracturing model will allow engineers the ability to design, evaluate, and optimize the efficiency of simultaneous fracturing. The enhanced model also allows evaluation of different pumping schedules, well placement schemes, and scenarios to improve cluster efficiency. It will help to achieve the next level of surface efficiency through predictive modeling of simultaneous fracturing from different wells on the same or different pads. The new approach is bringing new challenges to the model to account for the different previously unaccounted for field related scenarios and situations, as well as will open new modeling related issues and problems. This will be the main topic of the paper.

1. INTRODUCTION

As an alternative to the widely used zipper-fracturing technique, the new completion technique of simultaneous fracturing (simul-frac) became more popular among US-shale producers (Jacobs, 2021). In simul-frac operations, one pumping crew can stimulate two horizontal wells at the same time and reduce completion times by more than half compared to the zipper-fracturing approach (Fig. 1). Some examples of successful simul-frac stimulations can be found in the papers by the Matador Completions Team (2023) and Chevron (Kim et al. 2021).

The typical simul-frac operation requires at least 25% more horsepower than the average zipper-frac spread. Simul-fracs require splitting the pumping rates between two wells, and while more pressure pumping units, i.e., an additional four stimulation pumps are required to be on location, they function at lower rates and spend less time on each pad - enhancing pumping efficiency and maximizing resource usage by significantly trimming pad completion time.

The interaction between fractures during simultaneous fracturing is, in a way, similar to the parent-child interaction, with the difference that the interaction is in real-time mode, when both jobs are still active, and both networks are still propagating.