bluuebunny/crossref_metadata_2025 huggingface dataset:
Casing Force Analysis During MPC Injection Considering the Coupling Effect of Temperature and Pressure
Authors: Liu Jinlu , Li Jun , Li Hui , Yang Hongwei , Liu Gonghui
Published: September 2023

The problem of casing force and deformation during MPC (managed pressure cementing) injection pose great challenges to cementing in deep and ultra-deep Wells. According to the combination structure size of casing, the properties of multi-density and variable volume fluid in casing and annulus, the calculation method of multi-density fluid position change with injection time in cementing process is presented. Combined with causes of casing force during MPC injection stage, a temperature-pressure coupled mathematical model was established to calculate the temperature and pressure of casing caused by injection fluid flow in cementing. The calculation formula of U-tube effect was used to correct the parameters in the model. The energy conservation equation of casing and annulus is given, and wellbore pressure is obtain based on the theory of fluid mechanics. Taking a well A as an example, the model has been well used in pressure prediction and casing force analysis during MPC injection stage. The results show that the casing force is complicated during the injection process. The optimization of displacement can effectively alleviate casing deformation and make the bottom hole pressure in a safe state. The research results provide a theoretical basis for the optimization of construction parameters during MPC injection stage.

INTRODUCTION

The exploitation of ultra-deep oil and gas resources has promoted the continuous breakthrough of drilling technology. At present, there are more than 30 Wells in the world with a depth of over 9000m, which is of great significance for improving self-sufficiency in natural gas, increasing energy supply, optimizing and adjusting the energy structure, and improving people's livelihood. However, increasing well depth has brought great challenges to cementing. Many new problems have emerged (Rostami et al.; Thibodeaux et al., 2018), such as high cement slurry density, high circulating resistance, high liquid column pressure, high surface pump pressure for cementing, and high temperature and pressure formation. These problems have a serious impact on casing deformation during MPC injection stage.
Energized Fracturing with CO2: A Numerical Simulation of the Effects of Thermodynamic Properties of CO2
Authors: Xiao Feng , Salimzadeh Saeed , Zhang Qianbing
Published: September 2023

Energized fracturing with CO2 is considered as an alternative method to conventional water-based hydraulic fracturing, which brings advantages in water conservation and environment protection as well as in hot enhanced geothermal systems (EGS). However, the varying thermodynamic and transport properties of CO2 could be problematic in energized fracturing. To investigate the effects of CO2 pressure-temperature-dependant properties on fracture propagation, an extensive numerical simulation is performed using a robust fracturing simulator developed on Complex Systems Modelling Platform (CSMP-HF). Based on the Span-Wagner equation of state, the thermodynamic and transport properties of CO2 in varied in-situ conditions (i.e., pressure and temperature) during energized fracturing process are calculated. The simulation results show that in-situ stress conditions affect the fracture growth speed under constant CO2 mass rate. The additional storage volume brought by injection system contribute significantly to the hydraulic fracturing speed. Higher propagation speed may result in dynamic fracture propagation and thus resulting in branching, a subject for further research.

INTRODUCTION

Hydraulic fracturing technology plays a key role in promoting efficient energy extraction. Not only hydraulic fractures can enhance the oil and gas production from unconventional reservoirs, but also de-risk coal mining by inducing and controlling the time of goaf events (Adachi et al., 2007; Hou et al., 2021; Jeffrey & Mills, 2000; Middleton et al., 2015). According to the background review commissioned by the Department of the Environment (Commonwealth of Australia, 2014), the estimate in 2012 of Australian economic demonstrated resources of CSG was 35,905 Petajoules, which is equivalent to nearly 10 times the total yearly energy use. Besides, for the future mining cave extensions to massive and deep orebodies in Australia, such as Northparkes mine (Webster et al., 2020), Ridgeway Deeps (Cuello & Newcombe, 2018), and Cadia East (Orrego et al., 2020), hydraulic fracturing is proposed as a profitable and sustainable preconditioning technique to manage and enhance the caving process (Catalan, 2015).
Casing Stress Analysis Under Dynamic Change of In-Situ Stress
Authors: Lu Z. Y. , Zhang W. , Li J. , Xie S. Y. , Wang X. G. , Zhang H. , Song L. , Lian W.
Published: September 2023

During hydraulic fracturing of shale oil wells, fracturing fluid enters the formation causing dynamic changes in in-situ stress in the near-wellbore region, which in turn affects the wellbore stress. This paper takes into account the change of in-situ stress caused by fracturing fluid pumped into the formation, derives the analytical solution of wellbore stress under non-uniform in-situ stress conditions, and analyzes the effects of surface pumping pressure, delivery rate, casing wall thickness, formation Young's modulus and Poisson's ratio on casing stress. The results show that: with the increase of pump pressure and Poisson's ratio of the formation, the Mises stress increase of the casing inner wall increases, and the non-uniformity of stress increases; with the increase of delivery rate and Young's modulus of the formation, Mises stress of the casing inner wall increases, and the non-uniformity of stress decreases sharply; With the increase of the casing wall thickness, the Mises stress in the inner wall of the casing decreases, but the non-uniformity increases slightly; When the Young's modulus and Poisson's ratio of the formation is high, the pumping pressure and displacement should be reasonably controlled and the wall thickness of the casing should be increased to reduce the risk of casing change.

INTRODUCTION

In recent years, more and more shale oil resources have been discovered with the continuous development of China's economy and the continuous updating of exploration technology (Xu et al., 2021; Wang et al., 2018; Ning et al., 2021). The typical terrestrial mixed-phase shale oil developed in the Lucaogou Formation of the Jimsar region has great exploration potential, and the first national terrestrial shale oil demonstration region was built in China (Kuang et al., 2012; Li et al., 2021; Wang et al., 2020; Li et al., 2021). The shale oil formation conditions in the Lucaogou Formation of the Jimsar region are complex, the reservoir is non-homogeneous, the field has a poor ability to sustain stable production, and the economic benefits of preliminary development are poor (Shen et al., 2021; Xie et al., 2020; Zhang et al., 2020; Wu et al., 2019).
The Hydration Expansion Effect on Wellbore Stability in Water-Sensitive Shales Subjected to Heterogeneous In-Situ Stress
Authors: Peng Cheng , Deng Jingen , Gao Jiajia
Published: September 2023

Wellbore instability is a complex problem in water-sensitive shale formation and increases the cost of drilling. The hydration expansion effect changes the mechanical and strength properties of shale and causes the stresses to redistribute around a borehole drilling through the water-sensitive shale formations. the aforementioned changes are time-dependent and location-dependent. This paper provides a new predicting model for stresses around a wellbore subjected to a non-hydrostatic stress field in water-sensitive shales. With the convenience to solve the differential equations introduced by the new considered, one adopts the finite difference method and decomposes the boundary conditions into axisymmetric and deviatoric loadings. In addition, one further considers the influence of the water content on the elastic modulus, Poisson's ratio, cohesion, and angle of internal friction. The Mogi-Coulomb failure criterion is used to predict the collapse pressure. The main results show that the collapse pressure due to the hydration expansion effect is higher than drilling in the conventional formation. Two aspects are responsible for this phenomenon. On the one hand, the increasing water content decreases the elastic modulus and cohesion of water-sensitive shales, and thus decreasing the rock strength causes it to wellbore more easily to collapse. On the other hand, the hydration expansion effect causes the time-dependent safe mud pressure window to narrow. But, the conventional wellbore stress-predicting model tends to predict a wider and more conservative mud density required for borehole stability in the water-sensitive shale formation. The new model enables the drilling engineer to obtain a more accurate and safe mud density window to better deal with borehole instability.

INTRODUCTION

When drilling in a water-sensitive shale formation with water-based drilling fluid, the difference of hydraulic pressure and chemical potential will drive drilling fluid into the formation. Due to the water-sensitive clay minerals in shales, the rock around the well will swell. This phenomenon has come to be known as hydration expansion effect (Chenevert, 1969; Chenevert et al., 1970; Yew and Chenevert, 1990; Yew, 1993), which can cause the generation of hydration expansion stress and change the wellbore stress distribution. Moreover, the hydration expansion effect changes the mechanical and strength properties of shale. The aforementioned changes are time-dependent and location-dependent.
Work Hardening and Stress Memory for Granite Under Repetitious Triaxial Loading Cycles
Authors: Green A. , Benge M. , Lu Y. , Kelley M. , Bunger A. P.
Published: September 2023

Granitic rock subjected to multiple load/unload/reload cycles is observed to have a much lower stiffness on the first loading step compared to subsequent cycles. This apparent work hardening has been proposed to provide unique insight into a rock's stress history, making this mechanism the basis for the core-based in-situ stress estimation method known as Deformation Rate Analysis (DRA). This paper presents cyclic triaxial testing of quarried granite (Cold Spring Charcoal Granite). The basic unit of the experiments is a load/unload/load/unload (LULU) cycle, where the sample is at a constant confining level and the maximum and minimum axial loads are predetermined. For multiple cycles, the peak axial stress for each cycle increases. The experiments show work hardening wherein the slope of the stress-strain relationship is significantly smaller on the first loading cycle (L) compared to the subsequent cycles. When a subsequent cycle is run to a higher peak, the strain difference increases with stress up to the prior value of the peak stress, after which it decreases, exhibiting an apparent memory of the prior peak stress. However, for these short loading cycles, the stress memory is temporary, mostly dissipating after brief unloading to atmospheric conditions and completely dissipating after 24 hours.

INTRODUCTION

Rock behavior depends upon the stress history of the rock, providing the conceptual foundations for essentially all core-based methods aimed at estimating the stress conditions from which the core was extracted (e.g. Yamamoto et al. 1990, Kang et al. 2000, Villaescusa et al. 2002, Wu and Jan 2010). In the context of the Kaiser effect and Anaelastic Strain Recovery testing, there have been studies showing stress memory can deteriorate with the time since the rock was extracted from the subsurface (Barr and Hunt 1999, Jin et al. 2009). These studies deal with stress memory induced over geological time which can deteriorate over periods of months to years. However, laboratory uniaxial compression testing of rocks is also known to induce changes in the rock which can include work hardening and stress (or strain) memory (e.g. Horibe and Kobayaski 1958, Yamamoto et al. 1990). The memory of an applied stress has been found to diminish slightly with time, but to nonetheless persists for 150 hours or more after a sample is unloaded (Yamamoto 2009). However, past investigations on the topic of observance and persistence of stress memory have been performed at relatively low stress levels under uniaxial conditions. Meanwhile, it has long been known history-dependent behaviors in cyclic loading of rocks depend heavily on confining stress (Zoback and Byerlee 1975). Therefore, this paper is aimed at examining the existence and persistence of stress memory associated with work hardening from laboratory triaxial loading cycles.
Geomechanical Tool for Evaluating Casing Deformation in Storage Caverns in Salt Dome
Authors: Ross T. S. A. , Chang Kyung Won , Sobolik Steven R.
Published: September 2023

Sandia National Laboratories has conducted geomechanical analysis to evaluate the performance of the Strategic Petroleum Reserve by modeling the viscoplastic, or creep, behavior of the salt in which their oil-storage caverns reside. The operation-driven imbalance between fluid pressure within the salt cavern and in-situ stress acting on the surrounding salt can cause the salt to creep, potentially leading to a loss of the cavern volume and consequently deformation of borehole casings. Therefore, a greater understanding of salt creep's behavior on borehole casing needs to be addressed to drive cavern operations decisions. To evaluate potential casing damage mechanisms with variation in geological constraints (e.g. material characteristics of salt or caprock) or physical mechanisms of cavern leakage, we developed a generic model with a layered and domal geometry including nine caverns, rather than use a specific field-site model, to save computational costs. The geomechanical outputs, such as cavern volume changes, vertical strain along the dome and caprock above the cavern and vertical displacement at the surface or cavern top, quantifies the impact of material parameters and cavern locations as well as multiple operations in multiple caverns on an individual cavern stability.

INTRODUCTION

The US Strategic Petroleum Reserve (SPR), operated by the US Department of Energy (DOE), stores crude oil in solution-mined caverns in the salt dome formations of the Gulf Coast. There are a total of 60 storage caverns distributed across four different sites in Texas (Bryan Mound and Big Hill) and Louisiana (Bayou Choctaw and West Hackberry), along with several abandoned or decommissioned caverns. Each cavern is constructed and then operated through a nested series of wellbore steel casings, each cemented from the surface to near the top of the cavern.

Sandia National Laboratories (SNL) has served as the geotechnical advisor on the SPR to DOE since 1979. As part of that responsibility, SNL has long used computational geomechanical models to analyze the viscoplastic, or creep, behavior of the salt in which the oil storage caverns reside. Salt creep causes storage caverns to deform inward, thus losing volume. This loss of volume affects the salt above and around the caverns, puts stresses and strains on borehole casings, and creates surface subsidence which affects surface infrastructure. Therefore, accurate evaluation of salt creep behavior drives decisions about cavern operations.
Study and Application of Rock Breaking by PDC Bit in Ultra Deep In-Situ High-Temperature and High-Pressure Environment
Authors: Chen Changchang , Ji Guodong , Zhang Hongyuan , Sun Yuqi , Wu Qiang , Jiang Haichao
Published: September 2023

The finite element model of the cutting tooth rock system based on ABAQUS is established by using the grid adaptive method. The average stress of each node of the cutting tooth is solved by using the secondary development of PYTHON language, and the rock breaking efficiency, real stress distribution and the influence laws of temperature and pressure of different cutting tooth shapes and rock samples are obtained. A super deep PDC bit with special non plane teeth is optimized and designed, which has achieved good results in the southern edge of Xinjiang. The research shows that the mechanical parameters of the rock change obviously under the high temperature and high pressure environment. The micro damage structure that is conducive to rock breaking by the bit is produced under the high temperature. The rock breaking efficiency is negatively related to the temperature due to the transformation of the rock from brittleness to plasticity under the high confining pressure. The double row structure of axe shaped teeth and cone shaped teeth has the best rock breaking effect under the ultra deep high temperature and high pressure environment. The drill bit footage is the longest. The problem of bit shoulder collapse is prominent under the ultra deep in-situ environment, The design of different rail double row teeth should be adopted, the design of bit stability should be strengthened, and the drilling parameters with large WOB, low speed and large displacement should be adopted.

INTRODUCTION

Onshore deep and extra deep layers where 39% of the remaining oil and 57% of the remaining natural gas are distributed [1-3]. The ultra deep layer in the south border region of Junggar basin is an important oil and gas reservoir and production increasing block in Xinjiang Oilfield. The reservoir in this block has deeply buried depth with high temperature and pressure, low ROP and long drilling cycle. The average well depth of the three risk exploration wells drilled in 2020 has reached 7205m, and the estimated well depth of well Matan 1 to be newly deployed in 2021 reaches even 8200m. There are extraordinary high pressure system and extremely thick salt gypsum layer in the deep well section, resulting in many complex situations, and long drilling cycle with the per-well average drilling cycle being 335d, and the average ROP of deep formation being less than 2m / h. Improving the ROP and reducing the complexity of accidents are the urgent needs and great challenges for the deep layer drilling in the south border region. Under the ultra-deep high temperature and high pressure environment in the south border region, the crustal stress increases. Affected by high temperature and high pressure, the rock crushing mechanism is extremely complex, which puts forward higher requirements for the design of bit [4].
Fracture Behavior of Jointed Rocks Subjected to Static Compressive Loads: An Overview
Authors: Li Peng , Cai Meifeng , Ren Fenhua
Published: September 2023

Joints widely distributed in rock masses can possess a noticeable influence on, or even dominate, the mechanical response and fracture behavior of the rock mass. Consequently, considerable efforts have been made to explain the mechanisms involved. This article provides an overview of the mechanical properties, fracturing processes, and failure patterns of jointed rocks, which have been investigated through experimental and numerical tests. The results of these tests indicate that, under the action of static compressive load, the existence of flaws changes the stress distribution mode in jointed rocks, resulting in noticeable anisotropic characteristics. The crack evolution is mainly caused by the redistribution of stress due to the inclusion of joints. Furthermore, the fracturing process of jointed rocks often occurs at the tips of pre-existing microcracks and joints, which can lead to different failure modes. Additionally, the mechanical parameters, fracture process, and failure modes vary depending on the geometry and configuration (such as dip angle, length, and layout) of the joints. Finally, some prospects for studying the fracture behavior of jointed rocks are proposed. This work is expected to contribute to disaster prevention and mitigation in underground projects.

INTRODUCTION

The rock mass is the primary construction object for underground projects such as mining, nuclear waste treatment, carbon sequestration, and oil and gas reservoir development. The natural rock mass is a complex and inhomogeneous geological body existing in nature (Sagong et al., 2011). After long-term geological tectonic evolution, the crust is randomly distributed with different scales and directions of structural surfaces or discontinuities (such as faults, bedding planes, joints, splits, slits, soft interlayers, and holes), which intersect in the rock mass and form a special structure of discontinuous body (Li et al., 2021; Li and Cai, 2021). They will have a great effect on the mechanical behavior of the rock mass. Numerous studies indicated that many instabilities of jointed rocks are caused by nucleation, expansion, and coalescence of joints, resulting in a large number of deaths and huge losses of material wealth. For example, the typical wedge-shaped destruction along the fault on the steep slope of the Three Gorges Project and the step-type destruction of the rock slope at the Xiaowan Hydropower Station in China are all related to the joints in the rock mass (Huang et al., 2018). These disasters have seriously restricted the safe and stable development of rock engineering.
Integrating a Boundary Element Method Based Hydraulic Fracturing Simulator with a Compositional Reservoir Simulator for Well Lifecycle Simulations
Authors: Cao Meng , Zheng Shuang , Sharma Mukul M.
Published: September 2023

In this paper we present an integrated DDM-based hydraulic fracturing-compositional reservoir simulator that can simulate each phase in the lifecycle of a hydraulically fractured well from hydraulic fracturing to shut-in, to flowback/production and gas huff-n-puff improved oil recovery. The new model is first bench marked against our in-house fully 3-D model of a penny-shaped propagating fracture by comparing the width distribution and fracture geometry. Then, the model is used to simulate production from complex fracture networks in 3-D. The results show that the reservoir drainage area is constrained by the geometry of the fracture network. The impact of pre-existing natural fractures is investigated by propagating a hydraulic fracture with and without natural fractures and conducting production from the propagated fracture networks using the compositional simulator. The results show that: (1) natural fractures can enhance well productivity from a more compact reservoir region closer to the production well, and (2) it is important to use realistic natural fracture geometry models for history matching well production. Finally, to illustrate the model capabilities, the model is used to investigate well drawdown strategies. Results show that aggressive drawdown strategies can be used in rocks with low clay content.

INTRODUCTION

Hydraulic fracturing and horizontal drilling have been employed to unlock oil and gas resources from unconventional reservoirs, including naturally fractured formations. To achieve the maximum oil and gas recovery from the subsurface, a model is needed to provide guidance on fracture treatment design and production for a well lifecycle. However, most current fracturing simulators and reservoir simulators are separate from each other, which results in restrictions for how they can be used to simulate different phases in the life of a well. In this paper, a novel fracturing-reservoir simulator is presented by integrating the displacement discontinuity method (DDM)-based hydraulic fracturing simulator with a compositional simulator for a naturally fractured formation.

Many techniques have indicated the existence of pre-existing natural fractures, such as fiber optic-based sensing, microseismic evaluation, pressure interference analysis, geochemistry analysis, and slant core wells (Cao and Sharma, 2022a; Ciezobka, 2021; Zhang et al., 2022). The presence of natural fractures can result in the formation of complex fracture networks, such as at Hydraulic Fracturing Test Site #1 (HFTS #1) and HFTS #2 (Bessa et al., 2021; Cao et al., 2023; Cao and Sharma, 2023a; Gale et al., 2021; Pudugramam et al., 2021; Shrivastava et al., 2018). Since these complex fractures can have a direct impact on the productivity of an unconventional reservoir, it is important to accurately capture the formation of propagated fracture networks with pre-existing natural fractures.
Combining Machine Learning and Physics Modelling to Determine the Natural Cave Property with Fracturing Curves
Authors: Qiao Jiangmei , Gao Fei , Wu Di , Tang Xuhai
Published: September 2023

Currently, it is challenging to determine the geological parameters of natural caves in deep reservoirs accurately. By inheriting the advantages of Machine Learning (ML) method and physics modelling, a novel ML-Physics method is developed to determine the geological parameters of natural caves based on the data obtained during Hydraulic Fracturing (HF) operation. The process of ML-Physics method is divided into preparation-stage and operation-stage. The preparation-stage happens before HF operation without the limitation of computational time, during which the implicit relationship between cave property and fracturing curve is generated. The computational time of the operation-stage is limited because the geological parameters of natural caves should be determined in real time during HF operation. During the operation-stage, the physical modelling based inverse analysis method is carried out, in which the initial value is chosen based on the results of preparation-stage. Results show that, with the same target error, the iteration step required by ML-Physics method is much less than that of traditional inverse analysis method. With the same iteration steps, the error of ML-Physics method is lower than that of the traditional inverse analysis method. The ML-Physics method is potentially useful to optimize the HF design in real time.

INTRODUCTION

The exploitation of oil and gas resources is very important for maintaining world energy security and keeping economic development. It is necessary to determine the geometry and geological parameters of natural caves to optimize the hydraulic fracturing design (Ma et al., 2018; Ali et al., 2019). Currently, seismic tomography is widely used to detect the distribution of natural fractures and caves (Chalikakis et al., 2011; Xu et al., 2016). With the rapid development of Internet of Things (IoT) technology, large quantities of monitoring data can be obtained in real time (Wang et al., 2022). Therefore, it is critical to develop an available method to determine the geological parameters according to the evolution of fracturing curves.
Influence of Sampling Window Geometry on the Variation of Fracture Trace Density
Authors: Choi J. , Kim J. , Song J.-J.
Published: September 2023

Sampling windows are essential tools for characterizing fracture distribution from rock exposures, yet few studies have focused on their geometry and how it affects the obtained fracture trace information. In this study, we investigate the influence of window dimension and orientation on the variation of trace density, derived by the end-point estimator, using analytic and numerical approaches. We employ rectangular, square, and circular windows for sampling the trace density of fractures with various dip angles using Monte Carlo simulations. We find that, regardless of window shape, the variation of trace density decreases as window size increases. Additionally, for rectangular and square windows, fracture dip angle affects the variation of trace density, with more impact on smaller windows. The variation is also impacted by the fracture density and fracture size. Our results illustrate that window geometry and fracture characteristics should be considered when conducting window surveys to acquire fracture data with reduced variance.

INTRODUCTION

Fracture characterization is an essential prerequisite for the analysis of rock mass behavior in both mechanical and hydraulic aspects. To describe the fracture distribution characteristics, including fracture spacing, density, size and orientation, rock exposures are investigated through window surveys by installing sampling windows upon the rock exposure and collecting data within the window.

Different sampling windows have been suggested for the purpose of acquiring various fracture data. In an early study by Kulatilake and Wu (1984), a rectangular window was used to estimate the mean number of trace midpoints per unit area. Later on, Zhang and Einstein (1998) introduced circular windows to estimate mean trace length without orientation bias, and Mauldon (1998) proposed another method for estimating mean trace length and density using convex windows. The proposed methods were applied to an outcrop in Yingxiu, China, and their results were compared (Wu et al., 2011). Song and Lee (2001) also applied rectangular and circular windows to estimate trace length distributions, and reported comparisons between the two types of windows.
The High Temperature-Mine Thermal Energy Storage (HT-MTES) Project in Bochum, Germany
Authors: Hahn F. , Klein S. , Güldenhaupt J. , Seidel T. , König T.
Published: September 2023

The aim of the German HEATSTORE sub-project has been the development of a mine thermal energy storage (MTES) pilot plant for the energetic reuse of an abandoned small colliery below the premises of the Fraunhofer IEG in Bochum, Germany. In the summer 2020 three wells have been drilled into existing open mine voids from the IEG drilling site. During the winter of 2020/2021, a first MTES test operation has been carried out. The underground temperature has been recorded before, throughout and after a test operation by Distributed Temperature Sensing (DTS) as well as groundwater temperature and pressure loggers, which had been installed into the MTES wells. In spring 2021, a local district heat network (DHN) has been built up at the case study site. In summer 2021, a Concentrated Solar Power plant (CSP) as the future renewable heat source has been successfully erected at the demo site and was connected to the MTES via a common heat exchanger.

INTRODUCTION

The fundamental transformation of energy supply systems in Germany by 2050 represents an immense technological and economic challenge. The German government is focusing on the increased use of renewable energies and also the efficient use of energy. Based on this strategy, thermal energy storage systems represent an important foundation for the increased use of renewable energies. The transition to a sustainable energy supply requires the provision of large electrical as well as thermal storage capacities. The potential of volatile renewable energy sources can only be fully exploited through flexible management of the electricity and heat supply networks and a wide range of different storage technologies.

The aim of this research project was to establish a pilot plant for seasonal heat storage within the former and no longer accessible IEG small colliery, which is located under the drilling site of the Fraunhofer Institute for Energy Infrastructures and Geothermal Energy (IEG) in Bochum, Germany. For this purpose, a numerical model of the former IEG small colliery was created within the HEATSTORE project and calibrated and validated by drilling three wells (MP1, MO1 and MI1) inside the mine workings of the IEG small colliery down to a depth of 64 meters. Within the scope of the project the wells were coupled with an above-ground solar thermal system.
Numerical Modeling of Cracking Behaviors of Coal Reservoirs Subjected to Cryogenic Shock
Authors: Han Songcai , Yan Xinchuang , Li Lile , Wang Lei , Yang Liang , Wang Rui , Gao Qi , Yang Junchao , Shi Xian
Published: September 2023

Thermally-induced cracking has attracted extensive attention in improving reservoir permeability. In this paper, a thermo-elastic coupling model incorporating the strain-based elastic-brittle damage theory is used to analyze the cracking behaviors of the coal reservoir subjected to cryogenic liquid nitrogen shock. The evolution of temperature and thermally-induced stress with damage is analyzed. The effect of different factors including in-situ stress difference, elastic modulus, thermal expansion coefficient, thermal conductivity and quenching temperature on the induced crack morphology is investigated. It is found that the failure mechanism of the coal rock during cryogenic shock is mainly dominated by elastic brittle tensile damage. The induced fracture morphology is more sensitive to elastic modulus and thermal expansion coefficient relative to in-situ stress difference and thermal conductivity. The increases in elastic modulus and thermal expansion coefficient will bring about more fractures with greater complexity. The higher in-situ stress difference or lower thermal conductivity can generate more short thermal fractures. The critical quenching temperature for inducing thermal cracks around the wellbore is between −110 °C and −105 °C. The results of this study can provide some guidelines for cryogenic fracturing in coal reservoirs.

INTRODUCTION

Coalbed methane (CBM) reservoirs have the characteristics of low permeability and low porosity. At present, hydraulic fracturing is still the most widely used stimulation technology to improve the overall permeability of unconventional oil and gas reservoirs, including CBM, shale gas and tight gas (Montgomery et al., 2010; Kumari et al., 2018; Ma et al., 2021). However, hydraulic fracturing will bring about a series of thorny problems, such as high fracture initiation pressure in hard formation, pore plugging and water locking effect in water sensitive formation, serious waste of water resources, treatment of flowback fluid and environmental pollution. Compared with hydraulic fracturing, cryogenic liquid nitrogen (LN2) waterless fracturing can overcome these negative problems (Gregory et al., 2011; Rozell et al., 2012, Hung et al., 2020). The representative advantage of LN2 fracturing is that the low temperature LN2 with a boiling point of −195.8 °C produces a huge temperature difference in the rock (Jacobsen et al., 1986), resulting in tens of MPa of thermal stress. In addition, hundreds of times of the gas-liquid ratio of nitrogen also plays an important role in rock cracking. However, because of its high risk of vaporization, it is necessary to control the pressure in the wellbore through the safety valve, thus weakening the effect of nitrogen expansion-induced fracturing (Cha et al., 2018).
Review of Recent Research Related to Ultrasonic Technologies for Well Productivity Enhancement
Authors: Mierez Jesus , AlTammar Murtadha J. , Alruwaili Khalid M.
Published: September 2023

In the last two decades, ultrasonic (US) technologies research has increasingly earned attention for applications in the oil and gas industry. Numerous laboratory and field studies have proven ultrasonics as an efficient, sustainable, and cost-effective technology for improving well productivity. This paper pursues the elaboration of a review of the most recent research related to ultrasonic technologies for applications in the oil and gas industry. The huge gap between numerical and field studies in comparison with laboratory studies deems it necessary to increase efforts on developing numerical models and field tests of the ultrasonic effect. A review of the US mechanisms for enhanced oil recovery (EOR) and emulsification/demulsification was conducted. Cavitation and thermal effects on wellbore fluid and formation rock have been widely accepted as two of the most influencing mechanisms. Most authors agreed that ultrasonics is a highly efficient method for EOR and emulsion treatment if the optimal conditions are identified and achieved. Treatment with ultrasound waves has shown improvement of oil recovery efficiency rates of over 90% and viscosity reduction values of over 80%. The most efficient results were observed when in combination with another conventional EOR method, where ultrasound boosts the recovery efficiency.

INTRODUCTION

Current oil well stimulation methods, such as acid stimulation, are in many cases ineffective and might cause additional operational issues such as emulsion formation, secondary chemical reactions, among others. These methods also involve the use of dangerous contaminants jeopardizing the facilities, personnel, and the environment. Emerging new technologies such as ultrasound stimulation present numerous advantages compared to conventional methods required by the Oil & Gas (O&G) industry to perform maintenance operations and oil well stimulations.

The present work intends to perform a comprehensive review of the available literature gathered regarding existing laboratory studies and field tests of ultrasound technology.

Previous work covered state of the art summaries for research related to ultrasound with quite specific scopes and timelines. The current work intends to continue previous researchers’ efforts with a wider focus integrating these publications, analyzing the most prominent applications for ultrasound technologies, and compiling a global summary with the latest publications and trends in ultrasonics and sonochemistry research.
Effect of the Biot Coefficient in the Dolomite Acid Stimulation Process
Authors: Cunqi Jia , Saif Alkaabi , Kamy Sepehrnoori
Published: September 2023

Carbonate reservoirs are very important for the petroleum industry. Acid stimulation is one of the practical techniques for effective development of carbonate reserves. This study is motivated to study the effect of the geomechanical process on the acid stimulation process. In this work, an improved two-scale continuum model describing the mass, momentum, and energy change is used to investigate the acid stimulation process coupling the geomechanical process. Results show that a typical acid stimulation curve of acid consumption volume can also be observed with gemechanical process, in which acid consumption volume obtains a minimum value with a certain acid injection rate. When the acid injection rate changes from this optimum acid injection rate, more acid solution is consumed for achieving a breakthrough. But as for the comparison of the acid consumption volume with and without geomechanical process, no obvious difference is found. Introduction the geomechanical process has little influence on the acid stimulation process.

INTRODUCTION

Carbonate reservoirs are important in the oil industry as they contain more than 60% of conventional oil reserves and 40% of gas reserves (Jia et al. 2021a). Carbonate rocks are composed of at least 50% carbonate minerals, usually calcite and dolomite. These rocks provide challenging characteristics such as strong heterogeneity and double porosity, which are a result of the formation process. During oil and gas production, particles eroded from the reservoir accumulate near the wellbore region and consequently reduce permeability (Jia et al. 2015, Tian et al. 2018, Tian et al. 2021, Li et al. 2022, Li et al. 2023a, Li et al. 2023b). Acid stimulation is a common technique to increase permeability near the wellbore by injecting acid (usually HCl) to dissolve and create conductive channels called wormholes (Jia et al. 2021c). These wormholes provide high permeability channels that bypass the damaged zone near the wellbore.
Methodology for Determination of Orthotropic Strength Parameters and Principal Directions from Laboratory Testing
Authors: Loken M. C. , Johnson J. C.
Published: September 2023

The Sanford Underground Research Facility (SURF, formerly known as DUSEL) is a dedicated underground scientific research facility located in Lead, South Dakota, at the former site of the Homestake gold mine. The SURF mission includes the construction of large underground excavations, at depths of about 1500 meters (m), to house large-scale physics experiments. A rock mechanics concern is the stability of these large-diameter (165 m) openings being constructed in the host formation mine rock that has been characterized with orthotropic material properties. The in-situ stress has also been measured at the mine and reported in the literature. The focus of this paper is on the orthotropic strength of the intact mine rock that is developed using laboratory strength data obtained from the literature. The objective of this paper is to present the theoretical development and implementation of the equations that define two- and three-dimensional orthotropic strength. The equations are developed using laboratory strength data on intact rock specimens. The method is validated using three numerical examples and applied to a data set of test results performed on mine rock specimens to determine the orthotropic strength parameters and the principal directions.

BACKGROUND

Loken and Johnson (2022) studied orthotropic elasticity on amphibolite and rhyolite rocks at the Homestake mine. The conclusion from the study was that the rocks were not elastically orthotropic because of the variation in each of the orthogonal directions was greater than the calculated orthotropy in the laboratory test results in the three mutually perpendicular orientations. Therefore, the mine rocks were assumed to be elastically isotropic where E = 70 GPa.

In-situ stress measurements taken at the Homestake mine was also examined by Loken and Johnson (2022). The conclusion from the study was that the in-situ stress state at Homestake mine has the major principal in-situ stress being vertical and the two horizontal principal stresses being approximately equal and somewhat less in magnitude than the vertical principal stress. The compressive (+) vertical stress is consistent with gravity loading where σz = 47 MPa and σx = σy = 38 MPa at a depth of 1500 m. Loken and Brosnahan (2010) investigated the Homestake mine orthotropic rock mass strengths based on laboratory testing by Duan (1985) and theoretical work by Pariseau (1972) where an orthotropic rock strength yield surface was given with the following equations in terms of nine rock property constants (H, F, G, U, V, W, L, M, N).
Study on the Integrity of Full-Well Cement Sheath During Hydraulic Fracturing of Shale Oil Well
Authors: Lu Z. Y. , Wang D. , Li J. , Xie S. Y. , Wang X. G. , Zhang H. , Song L. , Lian W.
Published: September 2023

During the hydraulic fracturing of shale oil wells, the cement sheath is faced with the problem of integrity failure, which easily induces sustained annular pressure and affects normal production. Therefore, taking the shale oil well in the Jimsar area of China as an example, the calculation model of the full-well cement sheath stress is derived based on the theory of elasticity. The reliability of this model was verified by the finite element method. This paper calculates the full-well cement sheath stress distribution and analyzes the cement sheath failure zones and influencing factors in hydraulic fracturing. The results show that the shallow cement sheath is primarily in tensile failure, and the deep cement sheath is primarily in compression failure. Using high Poisson's ratio cement above the surface casing shoe and low elastic modulus cement below the surface casing shoe, along with increasing the wall thickness of the production casing, will help reduce the failure length of the cement sheath. The study results have important implications for controlling wellbore integrity in shale oil wells.

INTRODUCTION

Multi-stage hydraulic fracturing technology for horizontal shale oil wells is vital to improve reservoir conditions and increase shale oil production capacity (Wang et al., 2012; Zhang et al., 2015; He et al., 2021). Multi-stage hydraulic fracturing is characterized by large discharge volume, high pressure, and long operation time, which leads to a harsh casing-cement sheath service environment and easily induces wellbore integrity problems (Lian et al., 2002; Tian et al., 2015). Of the 15,000 wells tested in the Gulf of Mexico, approximately 43% experienced cement ring integrity problems during completion, ultimately resulting in sustained annular pressure (Muehlenbachs, 2011; Vengosh et al., 2014). Similar problems existed during shale oil development in the Jimsar area of China. The cement sheath integrity problem during hydraulic fracturing severely restricts large-scale development activities in shale oil fields.

The cement sheath stress is closely related to the service environment. During the fracturing of shale oil horizontal wells, the internal casing pressure, formation properties, and in-situ stress conditions vary at different depths, which affect the cement sheath stress and failure risk (Liu et al., 2017; Xi et al., 2018). Therefore, revealing the full-well cement sheath stress state in horizontal shale oil wells during hydraulic fracturing and elucidating the main controlling factors of cement sheath integrity failure are of great significance to controlling the wellbore integrity of shale oil wells.
Subjectivity in Crack Initiation Measurements Using Conventional Stress-Strain Based Methods
Authors: Koosmen K. , Wang T. , Asche H. , Serati M.
Published: September 2023

Stress-strain based methods are frequently adopted in the literature to estimate crack initiation (CI) strength in rock. However, despite their widespread use, these methods can introduce subjective interpretations of the stress-strain curves, particularly in rocks with pre-existing microcracks or high porosity. This study aims to examine the subjectivity inherent in strain-based methods and its potential impact on consistent measurement of rock CI. A survey is then conducted using experimental data obtained from sandstone and granite specimens. Results suggest a wider scatter of CI values for sandstone samples, which aligns with expectations due to their inherent pore structure heterogeneity. The Lateral Strain Response (LSR) method demonstrates the least coefficient of variation, indicating less subjectivity. Further research and validation is recommended to improve confidence in the LSR method using parallel measurement techniques.

INTRODUCTION

In rock engineering, the crack initiation (CI) stress threshold refers to the stress level where new microcracks start to develop within the rock matrix. In practice, it has been demonstrated that the CI stress level can be used to estimate the strength of rock against spalling and rockburtst type failures in deep underground excavations (Martin, 1997; Diederichs, 2007). Further, the CI stress also provides an indication of the stress conditions where breakout can occur in boreholes and petroleum wellbores. The CI stress is usually recognised to be around 30 to 60% of the peak value measured during an Unconfined Compressive Strength (UCS) testing (Brace et al., 1966, Bieniawski 1967, Holcomb & Costin, 1986; Cai et al., 2004; Nicksiar & Martin 2013, Wen et al., 2018)

There is a wide range of methods that are available to estimate the CI stress which includes several strain-based methods (SBMs), acoustic emissions (Keshtgar & Modarres, 2013), electrical resistivity (Lataste et al., 2003), digital image correlation (Dong & Pan, 2017), optical diffraction patterns (Li et al., 2002), laser speckle interferometry (Ennos, 1975), and ultrasonic probing (Mi et al., 2006). The strain-based methods are most common due to their low cost and simplicity (Mutaz et al., 2020). With these methods, the CI stress is interpreted from a characteristic change in the stress-strain response that is measured during compression loading.