bluuebunny/crossref_metadata_2025_split huggingface dataset:
Solving Coupled Partial Differential Equations in Porous/Fractured Geomaterials
Authors: Cui X. 
Published: August 2024

ABSTRACT:
Exploring efficient and robust algorithms to solve the simultaneous Partial Differential Equations (PDEs) is essential to model the prevalent multiphysics processes in deep rock engineering activities, such as the thermo-hydro-mechanical coupling in nuclear waste disposal and geothermal exploitation. In this study, the staggered and monolithic solution schemes are developed in the context of poroelastic and fractured geomaterials, and the applicability of the two solution schemes is analyzed in detail. It is found that the degree of coupling between primary variables plays a pivotal role in determining the performance of the staggered and monolithic solution schemes. Two-way coupling can only be tackled by the monolithic solution scheme, while the staggered solution scheme is very robust and efficient to deal with one-way coupling. This study provides an overarching principle to solve simultaneous PDEs: Understand the degree of coupling between primary variables, and use the staggered and monolithic solution schemes to address one-way and two-way couplings, respectively.

1. INTRODUCTION
Rock mechanics, as an applied geoscience, has its roots deeply entangled with various rock engineering activities. The pioneers and early generations of rock engineering practitioners established the framework and theory of rock mechanics mainly from on/near ground surface projects, such as rock foundations, rock slopes, and shallowly buried tunnels. In the last two decades, rock engineering activities started to go deeper and deeper to serve increasingly complex engineering purposes. Examples include nuclear waste disposal, hydraulic fracturing, geothermal exploitation, CO2 sequestration, and injection-induced earthquakes. The above new rock engineering activities pose great challenges to the existing rock mechanics theory, but at the same time, they provide a fertile ground for rock mechanics to extend and prosper. Future rock mechanics will interact more closely with computer science, seismology, petroleum engineering and many other disciplines to further highlight its interdisciplinary attributes.
A common feature of rock engineering at a great depth is the potential coexistence of in-situ stress, high temperature, fluid pressure in pores/fractures, and chemical reactions. These factors are coupled together, and each of them plays a considerable role in the entire system. Therefore, deep rock engineering is a typical multiphysics system. It is not trivial to replicate such a complex working environment in the laboratory, neither can the field tests be readily conducted at such a depth. Numerical simulation, however, provides an insightful, economical and effective approach to understand the multiphysics system.
Effects of Interactions Between Produced Formation Fluid and Rock Matrix on Pore Structure of Caney Shale, Southern Oklahoma
Authors: Xiong Fengyang , Awejori Gabriel , Radonjic Mileva 
Published: August 2024

ABSTRACT:
Rock-fluid interactions change properties of shales during exploitation. To investigate effects of rock-fluid interactions on pore structure of shales matrix after hydraulic fracturing, powder samples from two late Mississippian Caney Shale cores in the Ardmore Basin, southern Oklahoma, were used to react with formation produced fluid from the field in the batch reactor analysis. X-ray diffraction for mineralogy and Low-pressure nitrogen adsorption isotherms for pore structure were measured for original, after-7days, and after-30days samples. Results show that the samples consist mainly of quartz, followed by clay minerals, carbonates, and feldspar. The pore sizes of micropore (&lt;2 nm) and mesopore (2-50 nm) increase 14%-233% due to dissolution of pyrite, feldspar, and carbonates after 7 days. Due to the transformation from smectite to illite and the increase of pore size, the specific surface area (SSA) decreases after 7-days interactions. After 30-days interactions, the micropore volume slightly increases and the mesopore and macropore volume decreases. Due to the decrease of pore size, the SSA of 30-days reacted samples increases correspondingly and is lower (for the clay-rich sample) or higher (for the calcareous sample) than that of the unreacted samples. Findings improve our understanding of dynamic alteration of shale properties during production.

1. INTRODUCTION
Energy demand will continuously grow owing to the increasing global population as well as energy consumption (EIA, 2023). On the other hand, shale gas and oil reshaped the energy market in the United States, enabling the United States to become a net-export of natural gas country in 2017 (EIA, 2023). However, shale reservoirs are challenging tight formations that are still poorly understood in the extraction and production of hydrocarbons (Ross and Bustin, 2009; Curtis et al., 2012; Xiong et al., 2015, 2021a; Li Y. et al., 2016; Gong et al., 2019a; Benge et al., 2021; Awejori et al., 2022; Huang et al., 2022). One of the most challenging topics is the rock-fluid interactions post hydraulic fracturing and its subsequent impacts on the pore structures of fractured formation matrix.
Experimental Study of Fluid Rheological Properties on the Friction of Cuttings Bed in the Horizontal Well
Authors: Zhou Mengmeng , Wang Huanhao , Xiao Hao , Song Xianzhi , Qi Yiqun , Wang Haobo , Han Liang , Duan Shiming , Xu Zhengming , Zhu Zhaopeng 
Published: August 2024

ABSTRACT:
Horizontal wells have been frequently used in many oil reservoirs, which has significant advantages in improving oil development efficiency. During horizontal well drilling, the cuttings can easily accumulate in the wellbore to form a cuttings bed at the bottom. The formed cuttings bed would directly increase the contact area so that wellbore friction during drilling is also increased. The increase in friction can cause serious underhole accidents. Hence it is necessary to study the parameters affecting the friction between the drillpipe and cuttings bed. The viscosity of drilling fluid and the rotation speed of drillpipe are important factors affecting the cuttings transport. In this paper, a laboratory experiment was designed to study the effects of fluid viscosity and drillpipe rotation speed on the sliding friction coefficient, then a general correlation model is built according to experimental results. Genetic programming is used here to analyze the effects accurately and optimize the structure and parameters of the model. This new way could greatly improve the model's accuracy and provide guidance for the sliding friction coefficient prediction.

1. INTRODUCTION
Horizontal wells have unique advantages in the development of these reservoirs, which is an important technology to improve the development efficiency of shale oil and gas reservoirs. The long horizontal well may greatly increase the exposed area of the reservoir in the wellbore and effectively improve the oil and gas recovery. However, horizontal wells also face some problems, such as cuttings accumulation during drilling, which will lead to out-of-control wellbore trajectory, excessive friction and torque, stuck drilling and wellbore instability. Borehole cleaning is one of the key problems affecting the safety and efficiency of drilling in horizontal wells. The distribution of cuttings bed in horizontal well has great influence on drillpipe friction, drillpipe position and drillpipe buckling. Cuttings bed accumulation is common in horizontal well, especially in inclined and horizontal well section. The cuttings bed accumulation increases the roughness of the contact surface between drillpipe and wellbore, and the friction coefficient between drillpipe and cuttings bed may increase significantly with the extension of the inclined section and horizontal section length. The excessive friction of drillpipe is easy to affect the drilling speed and wellbore trajectory, aggravate the fatigue damage of drill tool, affect the service life of drill tool, increase the probability of complex downhole conditions, and restrict the extension of horizontal section. Therefore, the frictional characteristics between drillpipe and cuttings bed during horizontal well drilling becomes to a key problem.
Experimental and Numerical Investigations on Fully Grouted Instrumented Rock Bolts Subjected to Pull Load
Authors: Modi J. V. , Deb D. 
Published: August 2024

ABSTRACT:
Rebar type rock bolts of 22 mm diameter and length of 600 mm were instrumented with four strain gauges along the length. The embedded length of the bolt was 450 mm grouted with resin inside a cement mortar cylindrical sample of diameter 250 mm. The diameter of the drill hole was varied as 32, 36 and 42 mm. The load-deformation relationships and variation in axial strains was measured with the pull load until debonding occurred. The bond strength and stiffness data revealed that hole diameter should be kept as low as possible for better performance of bolt. The axial strains at different locations along the bolt rod are analyzed and a modified equation based on Farmer (1975) is presented. Numerical studies on the axisymmetric models of the above experiments reveal the debonding phenomena between the bolt-grout and grout-rock interfaces which are evident from the experiments. A cohesion reducing constitutive model in the interfaces is applied to simulate the debonding with applied load and are explained in the paper.

1. INTRODUCTION
Rock bolt is the key reinforcement member used to provide support to rock strata in tunnels, underground mine drives/galleries and other excavations. A bolt enhances the mechanical properties of rock masses and reduces deformation and support time (Pinazzi et al., 2020). A fully grouted rock bolt generates shear stress along the interfaces of grout-bolt and grout-rock and resists the movement of rock by increasing the overall stiffness of the bolted zone (Cai et al., 2004; Deb & Das, 2010). Every year millions of rock bolts are installed worldwide to support and stabilize engineering structures (Benmokrane et al., 1995).
On other hand, the mechanics of shearing along the interfaces of bolt-grout-rock due to pull load was investigated by a few researchers. These studies showed that in the absence of rock movement, shear strain along the bolt rod varies non-linearly having the maximum value at the load side and near zero value at the other end (Farmer, 1975). Li & Stillborg, 1999 showed a similar distribution of strain development on the bolt rod considering the deformation of rock. Since then, several researchers attempted to simulate the shearing mechanism of bolt-grout and grout-rock interfaces using experimental and numerical models. A significant contribution was made to understand and evaluate the mechanism of bolt-grout interfaces (Hyett et al., 1996). Moreover, the extended finite element method (XFEM) was also proposed for analyzing the behavior of grouted rock bolts and applied for simulating reinforcement around tunnels and slopes (Deb and Gujjala, 2018). This experimental study evaluated the effects of grout, hole diameter and rate of loading on bond strength, stiffness and distribution of axial strains along the bolt rod.
Numerical Evaluation of Viscous Fingering Behavior During Underground Hydrogen Storage in Saline Aquifers
Authors: Cunqi Jia , Jinchuan Hu , Kamy Sepehrnoori 
Published: August 2024

ABSTRACT:
Underground hydrogen storage (UHS) is a promising option for fully realizing hydrogen potential as an alternative energy source to balance seasonal availability with irregular demand. This study is motivated to investigate and evaluate the viscous fingering behavior during underground hydrogen storage. The specific numerical simulations are implemented using a component simulator, incorporating input parameters such as density, viscosity, salinity, and relative permeability curves, all derived from experimental data. The study findings are first validated through a comparison with oil-water two-phase lab-based flat-plate displacement experiments. Numerical simulation schemes with different grid sizes are employed for comparison with experimental results, validating and optimizing the grid independence to determine the optimum fine grid size. Utilizing the same physical model, a hydrogen-aquifer water two-phase system is subsequently adopted to study the phenomenon of viscous fingering during the hydrogen injection process. The results show that in general, for the studied cases, there is no significant difference between the two mesh sizes, especially in the water recovery factor and hydrogen mass ratio. Based on the current numerical model and results, fingering behavior has a relatively minor impact on the underground hydrogen storage process.

1. INTRODUCTION
As the global climate crisis intensifies, transitioning to a low-carbon economy has become an urgent global mission (Zhao, Jia 2023, He, Liu 2024, Wang, Liu 2024). To reach such an ambitious target, a multitude of solutions are being explored and developed. Hydrogen, as a clean energy carrier, has garnered widespread attention (Zhang, Jia 2024, Bai, Jia 2024). Hydrogen not only possesses various advantages, such as high energy density and zero emissions, but it also plays a significant role in supporting renewable energy systems. When there is an overproduction of renewable energy, hydrogen can be produced through the electrolysis of water, thus forming a closed-loop system for a low-carbon transition. However, the application of hydrogen currently encounters a series of challenges, particularly with the storage of hydrogen. The interaction between hydrogen and metal ions, for example, can cause hydrogen embrittlement. In response, underground hydrogen storage technology has been developed. Utilizing the natural advantages of underground spaces, such as consistent environmental conditions, capacity for large-scale storage, and increased safety, the technology provides a promising approach to enable extensive, secure, and dependable storage of hydrogen.
Experimental Study on Fracture Propagation Characteristics of CO2 Pre-Fracturing in Shale Oil Reservoir
Authors: Wu Hua , Wang Xiaoqiong , Ge Hongkui , Song Jiaxin , Hou Shuoyang , He Jixiang , Wang Zhenyu 
Published: August 2024

ABSTRACT:
CO2 pre-fracturing offers the dual advantages of enhancing oil recovery and generating complex fractures, demonstrating positive application effects in the field. However, there is a scarcity of research on the characteristics of fracture propagation associated with CO2 pre-fracturing. This paper presents a comparative experimental study between CO2 pre-fracturing and hydraulic fracturing, utilizing a true triaxial fracturing simulation system. The study examines the fracture propagation patterns of CO2 pre-fracturing through the integration of acoustic emission (AE) monitoring and CT scanning techniques. The findings indicate that CO2 pre-injection significantly reduces the breakdown pressure in the subsequent hydraulic fracturing phase. After the pre-injection of CO2, the pressure curve during the hydraulic fracturing process exhibited a distinct zigzag pattern, with higher pressure fluctuation frequency, AE rate, and cumulative AE counts, indicating that the CO2 pre-fracturing had generated a large number of micro-fractures. CT scan results before and after the fracturing process demonstrate that the fractures formed following CO2 pre-fracturing have a larger fractal dimension, indicating a more complex fracture network. Additionally, the pre-injection of CO2 activates the beddings within the formation, which subsequently influences the direction of fracture propagation during the following hydraulic fracturing. This study confirms that CO2 pre-fracturing provides superior stimulation effects compared to conventional hydraulic fracturing methods. Consequently, the application of CO2 pre-fracturing technology is recommended for the development of shale oil.

1. INTRODUCTION
Shale oil is one of the most important unconventional oil and gas resources in China, and due to its tight reservoir and poor crude oil mobility, it needs to be stimulated by large-scale fracturing to realize the effective mobilization of crude oil. Shale oil wells are developed through large-scale fracturing, but problems such as rapid depletion of formation energy and rapid decline in production often occur during the production process. Some oilfields have conducted CO2 pre-fracturing tests, effectively alleviating the above problems. The results of production data analysis show that after the implementation of CO2 pre-fracturing, the oil well pressure maintenance level was greatly improved, the production decline rate was significantly reduced, and the oil recovery rate was effectively improved.
Permeability Evolution of Fractures in Shale in the Presence of High Salinity Water
Authors: Bai T. , Melkoumian N. , Hashemi S. , Badalyan A. , Zeinijahromi A. 
Published: August 2024

ABSTRACT:
Shale reservoirs are often associated with hydraulic fracturing, which involves interactions of invading fluids and shale minerals. Formation damage caused by injection impairment is a well-known challenge in the exploration and development of unconventional reservoirs. In this study, the impact of loading history and high salinity water (HSW) on the permeability of fractured shale has been investigated via laboratory studies. The fractured shale sample was loaded and unloaded under different effective stresses with nitrogen gas injection, resulting in a decrease in fracture permeability due to inelastic compaction. Then, the shale sample was stepwise loaded with HSW injection and further decrease in its permeability was registered. This phenomenon became more expressed after extending the imbibition duration to more than ten days, suggesting that shale swelling was caused by fluid-shale interactions. Furthermore, sudden drops in the differential pressure were observed during the process, implying fracture generation. This study shows that the chemistry mismatch between treating fluids and shale can lead to permeability impairment, which can have negative impacts on well injectivity in field applications.

1. INTRODUCTION
Unconventional resource extraction methods are of great interest to the petroleum industry due to their potential in supplying abundant resources to the market. It has been estimated that oil production from tight oil reservoirs can exceed 12 million barrels per day (Aloulou and Cook, 2017). To effectively extract oil and gas resources in deeper, tighter, and more depleted conditions, hydraulic fracturing is necessary to improve well productivity (Xu et al., 2016). Shale formation in the vicinity of wellbores can experience cyclic loading during drilling, completion, and production activities. Therefore, it is crucial to gain a better understanding on how shale fracture permeability responds to repeated stress-relief process over time. Applying loading-unloading cycles under various effective normal stress conditions is a conventional method used to examine the response of shale fracture permeability and its deformation. Three factors influence this permeability changes: changes in fracture roughness, changes in the contact area, and changes in fracture aperture (Zimmerman and Bodvarsson, 1996; Witherspoon et al., 1980). The main stages of the permeability change can be described as decrease in the permeability with the increase in the axial stress and partial recovery of the permeability after the stress relaxation. Experiments conducted on fractured shale samples by Gutierrez et al. (2000) demonstrated that fracture permeability after loading was always lower than before it. Hashemi and Zoback (2021) stated that the irreversible fracture permeability is likely caused by fine-grain displacement and compliant elements such as clays and total organic carbon within shale.
Non-Darcy Coefficient in Fractured Rocks
Authors: Cabrera D. 
Published: August 2024

ABSTRACT:
In the present work, an experimental methodology was developed for measuring the high-velocity coefficient in fractured rocks. The measurements were carried out at reservoir conditions (high pressure and temperature), in four full-diameter (whole) samples of a naturally fractured gas and condensate reservoir, with an approximate depth of 6500 m and an average temperature of 160 °C. The results of calculated inertial coefficients show a dependence on the effective stress applied to the rock, due to the effect of the compaction of the pore volume of the core and the morphology change in the the rock. Also, variation effect with water and condesate are described. These results are applicable for estimating the high velocity skin factor in gas reservoirs. This work presents a new correlation for estimating the coefficient of inertial resistance from permeabilityfor fractured rocks. This correlation was compared with several published in the literature.

1. INTRODUCTION
The non-Darcy coefficient (inertial, non-laminar, turbulence, high velocity) is essential for characterizing a gas-condensate reservoir because its effects occur near the well due to the high-velocity gas flow.
In the case of naturally fractured reservoirs, the effects are amplified due to the properties of the fracture network present in the reservoir and the geomechanical impact that can be caused during reservoir exploitation.
For example, the compaction generated by the closure of fractures during the pressure drop of the reservoir is an adverse effect that produces a considerable increase in the inertial coefficient.
Given the importance of the inertial coefficient, it is proposed to experiment with reservoir conditions to evaluate said petrophysical parameter.
Four full-diameter samples were used from a terrestrial carbonate gas and condensate reservoir in southern Mexico to carry out an experiment representative of the behavior of naturally fractured reservoirs.
The samples present a natural fracture network representative of the reservoir. Furthermore, the behavior of the inertial coefficient was analyzed as a function of the effective stress applied to the rock. Nitrogen gas injection was used, and an average rock temperature of 150°C. The effective applied stress range is 500 psi – 7000 psi.
These experimental pressure and temperature conditions generate a ductile constitutive behavior in the carbonates, allowing a greater range of stress application in the rock without reaching induced fracturing.