bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Binding Material on the Acoustic, Elastic and Strength Properties of Artificial Granular Rocks
Authors: Pothana Prasad , Gokapai Vasanth , Ifrene Ghoulem , Ling Kegang
Published: September 2023

Artificial sandstone-like rocks fabricated in the laboratory offer great advantages in controlling their grain size distribution, grain morphology, porosity, and cement content. Synthetic granular rocks offer an efficient way to validate existing analytical models, calibrate numerical models, and perform sensitivity analyses on specific physical parameters. Synthetic sand-stone rocks are manufactured by gluing grains together with a binding material. In this study, we investigated the effect of the three binding materials typically used in synthetic granular rock preparation on mechanical properties and acoustic wave velocities. We used ceramic spherical particles with sizes of 250 μm − 425 μm as grains bonded together by three binding materials, namely i) Epoxy resin, ii) Sodium Silicate with Kaolinite, and iii) Portland cement. While fabricating the core plugs, in each case we kept the relative volume proportion of the binding material with respect to the grains constant. These manufactured sandstone-like dry core plugs were then tested using Autolab-1500 triaxial testing equipment to measure the static mechanical properties and elastic wave velocities at a fixed confining pressure and temperature. Although grain size and distribution, confining pressure, etc., affect the mechanical behavior of rocks, our study focuses on the influence of the binding material alone. Despite the volume fraction of each binding material being the same, they influence the mechanical properties of the rock differently. The sample prepared with epoxy resin exhibited the highest values for Young's modulus, Poisson's ratio, and unconfined compressive strength (UCS) among the three samples. Our findings demonstrate that the selection of binding material plays an important role in controlling the mechanical properties of the rock. Two main reasons account for this behavior: i) the heterogeneous distribution of the binding material within the rock specimen and ii) the effect of the binding material itself on the contact behavior between the grains.

INTRODUCTION

Thermal-Hydro-Mechanical (THM) coupling processes are essential to various engineering and geoscience problems, such as geothermal energy extraction, geological storage of CO2, and nuclear waste disposal. Understanding the interplay between these processes is crucial for predicting the behavior of the Earth's subsurface systems. However, the natural heterogeneity, inhomogeneity, and non-uniformity of geological materials make these processes inherently complex and difficult to model accurately (Tsang, 1991; Rutqvist and Tsang, 2005). To address this challenge, numerical modeling has become a popular tool for simulating THM processes. Nevertheless, robust calibration of numerical models remains a significant obstacle due to the natural variability of rocks (Yoon, 2007).
Optimization and Evaluation of a Novel Stabilizer Applied to the Strong Water-Sensitive Conglomerate Reservoir
Authors: Zhang Kun , Yao Er-dong
Published: September 2023

Mahu Oilfield is the world's largest conglomerate oil field with 1.24 billion tons of petroleum reserves discovered so far. It is mainly a tight conglomerate reservoir with low porosity and low permeability. In the early stage of field development, it usually has characteristics of high flowback rate and low oil production rate. This may be attributed to strong water-rock interactions during hydraulic fracture operation. When water-sensitive minerals in rocks encounter water, clay expansion and particle migration occur, resulting in reduction of rock strength and fracture collapse. In order to choose a suitable anti-swelling agent for Mahu Oilfield, XRD, zeta potential, anti-swelling rate, immersion, Brazil splitting and other test means are used. Then, we evaluated the performances of inorganic salts and inorganic cationic polymers on clay swelling control and rock stability, and their combination and optimal ratio were also studied. The experimental results show that the clay content of the rock is up to 37.3%, of which 88% is the expandable montmorillonite mineral. The mixture of inorganic salt and inorganic cationic polymer can effectively inhibit the water-rock reaction by reducing the clay expansion and compressing the double electric layer on the rock surface. Through the test of swelling rate, it is found that the swelling rate of optimal reagent for montmorillonite can reach 92%, and the swelling rate of the reservoir rock powder can reach over 70%. The results of the rock immersion experiments show that the optimal stabilizer can ensure the strong water-sensitive conglomerate remains stable after soaking for 24h without dispersion. Compared with the KCl solution commonly used in oilfields, it can increase rock strength by more than 10% without increasing cost. The results of this study provide a solution for hydraulic fracturing in highly water-sensitive reservoirs.

INTRODUCTION

Mahu Oilfield is the world's largest conglomerate oil field with 1.24 billion tons of petroleum reserves discovered so far (Li et al., 2020). However, the reservoir is rich in water-sensitive minerals such as montmorillonite (Uddin, 2008). To effectively develop such tight reservoirs, hydraulic fracturing is commonly used (Li et al., 2015). Water injected during hydraulic fracturing interacts with reservoir rock, resulting in a decrease in fracture conductivity and well productivity (Ferguson et al., 2013).
Observation of Brazilian Tensile Failure of Rock Simulant by Digital Image Correlation Method
Authors: Tanaka S. , Nakashima S.
Published: September 2023

The purpose of this study was to investigate the applicability of the digital image correlation method, which uses image processing to obtain areal strain distributions, for the observation of rock fractures. In the results of the Brazilian tensile test, the strain increase process just prior to crack initiation was captured when the images were taken at 8,000 fps. The tensile testing of a rubber sheet coated with irregular spots confirmed that the accuracy of the analysis was sufficient, indicating that digital image correlation can be applied for the observation of crack propagation in rocks. By simulating the Brazilian tensile test with a manual mechanical jack, the experiment was conducted on gypsum specimens, and high-resolution images of the experiment were taken. The results of the analysis showed that the cracks started at the points of increased strain by comparing the normal strain distribution in the X direction and the shear strain distribution with the actual crack locations.

INTRODUCTION

It is important to evaluate the initiation and propagation of cracks in a rock mass during the cavity excavation. In the case of hard rock, the fracture initiation occurs in a short time, making it difficult to observe the process. However, recent advances in digital imaging devices have made it possible to capture high-speed, high-resolution images of rock fracture experiments. If the strain distribution in an area can be measured by image processing, it will be possible to track the initiation and progress of the fracture.

Strain gauges have usually been used to measure the strain generated in structures. However, the strain value obtained from a strain gauge is the average strain at one point in one direction; and thus, many strain gauges need to be installed for multipoint measuring. This involves complicated wiring to the measurement device, which causes problems in terms of labor and cost. In addition, as cracks develop or crack widths increase, the strain gauges may break off, making measurement impossible. On the other hand, digital image correlation can obtain the strain distribution over a wider range than strain gauges. It is a simple method requiring only the image of the object to be captured. Digital image correlation (DIC) can measure complex cracks and large deformations. DIC uses a camera to capture the speckle pattern (irregular dots) on a sample surface and measures the displacement and strain distribution from the correlation of images before and after deformation.
Mechanics of Tunnel Intersections: Closed-Form Approximations for Deformation Modeling and Failure Prediction
Authors: Michael Andreas , Gutierrez Marte
Published: September 2023

This study aims at improving our understanding on the stress field dominating the intersection between two tunnels of different diameter and hence provide insights on the stability against shear failures in this vicinity. Theoretical modeling of the deformation of rocks at the intersection point of two tunnels leads to prediction of possible localized shear instabilities. The general state of stress underground is that in which three principal compressive stresses acting in mutually-orthogonal directions. Subsurface stress analysis methods established in geomechanics have been applied to underground tunnels. Modifications have been necessary for the vicinity around the tunnel intersection because of stress interferences coming from each tunnel, accomplished using an analytical-approximation technique established in petroleum engineering for modeling the intersections of a well with its perforation tunnels. The orthogonal intersection between a "Tunnel A" and a smaller "Tunnel B" is considered and closed-form expressions are derived for evaluating the deformation in the vicinity as a function of the in-situ stresses and assess occurrence of shear failure induced by the application of these compressive stress loads using the Mohr-Coulomb failure criterion. Non-orthogonal intersections are also discussed. The validity of the derived closed-form approximations can be further investigated through numerical modeling, and laboratory or field-scale experimentation.

INTRODUCTION

The thrust of this paper is the improvement of our understanding on the stress field dominating the intersection between two underground tunnels, along with insights on the stability against shear failures (induced by compression), in this vicinity. This objective is achieved by analytically-approximating the behavior (stress-displacement relations) of the rock in the intersection plane between two cylindrical or elliptical excavations of different sizes ("Tunnel A" intersected orthogonally in its side by a smaller "Tunnel B"). Subsequently, closed-form approximations can be derived for deformation modeling and shear failure prediction considering the Mohr-Coulomb failure criterion.
Investigating the Impacts of Porosity on the Rock Mechanical Properties of Sandstone
Authors: Emad M. Z. , Waqas M. , Khan M. U.
Published: September 2023

Laboratory testing of rocks and developing their interrelationships is a good way for assessment of rock behavior for mining and construction industries. A detailed testing program was envisaged to establish possible correlations between porosity and mechanical properties of sedimentary rocks from Salt range Punjab, Pakistan. Sandstone samples were collected from different formations of the salt range area. Sample preparation was carried out at the rock mechanics laboratory at UET Lahore as per suggested methods of ISRM. Samples with partings or defects were removed from the testing program. Acceptable samples were tested for porosity, density, UCS, BTS, and other properties. The test results obtained were used for statistical analysis to find possible correlations using MS Excel. The analysis showed that the rocks can be divided into two groups. The predictive relationships were determined between porosity and static mechanical properties of rocks, and between porosity and dynamic mechanical properties.

INTRODUCTION

Determination of geotechnical data for any engineering project is a key aspect of design and research. Laboratory investigation of rock involves carefully testing of rock samples after preparation in accordance with the ISRM or ASTM standards. Some examples of useful rock properties include but are not limited to uniaxial compressive strength (UCS), Young's modulus, Poisson's ratio, tensile strength, and sonic wave velocities through rocks. Some non-destructive tests like sonic velocity (P and S waves), porosity (η), and density (ρ) can be performed on same sample before the destructive tests like Youngs modulus (E), Poisson's ratio (υ), compressive and tensile strength (σt). Non-destructive tests are very popular in rock engineering as these are cheaper than the destructive tests and require almost no sample preparation. Rock mechanical tests are also used for geotechnical investigation for a project in rocky areas. The most important factors that affect the sonic velocity in rocks are type of rock, mineral arrangement, density, porosity, anisotropy, pore-water, confining pressure, temperature, weathering, texture, structure, grain size, and grain shape. Generally, the rock behavior under stress is governed by the factors like interlocking between grains, grain size, texture, composition, pores, arrangement of minerals and pores, and external factors like weathering, groundwater, nature of applied stress, etc. Rock mechanic professionals and researchers have produced good literature to determine and explain rock behavior through correlations (D’Andrea. 1965; Chang et al. 2006; Chary et al., 2006; Hashemi et al., 2010; Kahraman and Yeken, 2008; Khandelwal and Singh, 2011; Norouzi et al., 2013; Sabatakakis et al., 2008; Shalabi et al., 2007).
Rock Salt Block Extraction Using Conventional Rock Mining Methods
Authors: Emad M. Z. , Majeed Y. , Waqas M. , Rehman G.
Published: September 2023

Rock salt is produced in large quantities in Pakistan's Salt Range region. Salt tiles, blocks, and lamps are the primary rock salt exports, whilst household usage and the chemical sector are the main local consumers. The size, shape and quality of the block play a role in how profitably salt may be produced for value addition. When the blocks have very few cracks, adding value to the salt is simpler. The current method of extracting salt includes drilling and blasting that causes unintended cracks in the blocks. The extraction of rock salt blocks using traditional cuboid-shaped block mining techniques is the main topic of this research. The methods include blasting with low-yield explosives, expansion agents, and wedges and feathers. The in-situ experiments resulted in wedges and feathers approach being the most effective way to mine rock salt blocks of the correct size and form, while reducing cracking. Using core drilling at the block's edge, the quality of the blocks for the various procedures was evaluated.

INTRODUCTION

Rock salt is mined using traditional drill and blast methods in both underground mining and quarrying operations. In general, automated mining processes make it possible to recover minerals using mechanical excavators, brine pumping, and evaporation techniques. Nonetheless, the room-and-pillar method of conventional underground mining of large deposits is the most time-tested and widely used method for extracting salt. The salt recovery is about 40%, rest is left in the mine to support the roof (Hustrulid, 2001).

With an annual production of roughly 2.16 Mt, Pakistan is a country that produces rock salt (Department of Mines & Minerals, 2015). Just 15% of salt produced is thought to be export quality, with the remaining salt being of low value because of cracks created during mining. Globally, Himalayan rock salt is a very well-liked product. There is high demand for a variety of products, including salt blocks, salt lamps, tiles, salt block slabs, and materials for medical slabs (Naz and Haleem, 2010; Rashleigh et al., 2014).
Numerical Modeling of Deep Borehole Heat Exchanger in Southern California - Feasibility Study
Authors: Chen Haohua , Tomac Ingrid
Published: September 2023

This paper presents an investigation into the feasibility of low thermal-gradient deep Borehole Heat Exchanger (BHE) applications on campus at the University of California San Diego (UCSD). With a review of primary source data for various formations and well logs around the UCSD campus, the stratigraphy and thermo-physical properties of the formation were evaluated. Based on the collected information, a numerical model for the BHE application was developed. Circulation of water through a closed coaxial loop system considers the variation of several parameters in order to develop the feasibility study and obtain guidelines for the future UCSD geothermal resource. For example, multiple layers of the formation and thus the variance of ground thermal conductivities along the vertical direction were considered, and fluid velocity and borehole depth were varied. To do that, a numerical scheme that employs finite element schemes for modeling Navier-Stokes fluid flow and heat flow and transport in the BHE system constructed the numerical model. The numerical model verification was first performed via comparisons with a field test on BHE from the literature. Then the effects of the thermal properties, volumetric flow rate, and other relevant parameters were systematically analyzed by using the developed numerical modeling. The proposed model can be used to evaluate the suitability of BHE applications with low thermal-gradient in southern California.

INTRODUCTION

The utilization of fossil fuels has surged significantly during the last half-century, resulting in enormous carbon dioxide (CO2) emissions and thus significant climate change. To tackle that, the United Nations, the G7 economies and many countries around the world have started or are going to promote the shift to renewable energy (UN DESA 2017; Gielen et al. 2019; G7 2021). Among different renewable energy, geothermal energy has been used for centuries and it is expected to meet 3%-5% of global demand by 2050 (Craig and Gavin 2018). Based on the depth of geothermal energy reservoirs, there are two major types of geothermal systems: shallow geothermal systems (≤400 m) and deep geothermal systems (> 400 m) according to White (1966).
A Reliability Analysis of the Dilution Graph Method for Open Stope Design: A Case Study
Authors: Adoko A. C. , Zhakipbayev A.
Published: September 2023

The equivalent linear overbreak slough (ELOS) graph is a commonly used empirical design method for open stope design. Nevertheless, this method can lead to poor design performance when the geotechnical domain data lacks confidence. This is due to the inherent variability of the design input parameters, which should be considered for more reliable stope design. Hence, in this study, the First-Order Reliability Method (FORM) is used to determine the reliability index and the probability of unplanned dilution level associated with the design for each geotechnical domain. The Maleyevsky mine, located in northeast of Kazakhstan, is used as a case study. The parameters used for the analyses are the Modified Stability Number (N’), Hydraulic Radius (HR), and the actual dilution. The results indicated that the reliability indices and the probability of unplanned dilution occurrence vary between 0.1-4.63; and 0.0001% − 46.02%, respectively, depending on the rock domains. The results were in agreement with the field observations. It is concluded that the reliability-based design method could serve as a tool in the design of the open stope in connection with minimizing unplanned dilution.

INTRODUCTION

Open stope mining is characterized by large extraction rates and often with unsupported walls, which make it susceptible to unplanned dilution. Unplanned dilution is the mixture of ore with the waste material (hosting rocks, low grade mineralization, etc.) sloughed from the stope walls outside the planned excavation boundaries. This obviously leads to poor profitability of the mining operations. Serious profitability issues due to unplanned dilution have been reported (Clark and Pakalnis, 1997; Stewart et al., 2007) by many mines. Therefore, developing design tools that facilitate the reduction of unplanned dilution, is essential for the mining industry.

Several approaches to design open stopes are currently in use, for example: numerical modelling (Henning and Mitri, 2007; Sainsbury et al., 2015; Vallejos and Díaz, 2020), analytical methods (Diederichs and Kaiser, 1999), back analysis of in situ measurements of stope overbreak (Cepuritis et al., 2010), empirical dilution graphs (Potvin, 1988; Clark, 1998; Mawdesley et al., 2001), and artificial intelligence based-design (Korigov et al., 2022). Among these methods, the empirical dilution (ELOS) graph is used in practice due to its simplicity and flexibility (Clark, 1998; Capes, 2009).
Proper Weight Distribution on Bit and Reamer Enhances the ROP of the Reaming While Drilling in Deepwater Well
Authors: Chen Haodong , Wang Shiguo , Liang Jiwen , Luo Ming , Han Xu , Jin Yan , Lu Yunhu
Published: September 2023

In the HTHP wells of the South China Sea, deepwater drilling in complex strata presents more risks and challenges. The reaming while drilling (RWD) technique is used to solve the problem of drilling minimum clearance space caused by multiple casing strings, by the synchronously drilling with both pilot bit and reamer. According to the analysis of actual drilling data from a exploration well in the South China Sea, an abnormal phenomenon is observed where the weight on bit (WOB) is increased, but the torque is lower. The reamer is in a suspended state, and the average ROP is only 6.2m/h. In this paper, a weight distribution model is established, which takes into account the compressive strength of the formation at both the reamer-rock interface and the pilot bit-rock interface. According to the model, it is suggested to adopt a structure design for the reamer with stronger aggressiveness to balance the weight distribution, or apply depth of cut control (DOCC) bits to maximize synchronization in the interbedded formation. Relatively, in a high strength homogeneous mudstone formation, a more aggressive pilot bit can be selected to enhanced the ROP.

INTRODUCTION

The Lingshui 25-1 block is located in the Qiongdongnan Basin with an average water depth of 980 meters, which is characterized by deep-water, high-temperature and high-pressure (Liu et al., 2022). In deep-water gas fields, there is an abnormal high pressure, which leads to a narrower drilling mud pressure window and increases the risks of drilling and cementing. Reaming while drilling (RWD) technology can realize enlarging the borehole as the same time as the bit drills the pilot hole by the concentric expandable reamers placed at the top of the bottom hole assembly (BHA). Using a reamer in deepwater operations can save an additional drilling trip, reduce the time that problematic formations are exposed, and result in significant cost savings in rig time. Underreaming can create a wellbore larger than the inner diameter of the upper casing at the lower part of the casing, optimizing casing sizes and providing a good safety window (Chen et al., 2022, Ho et al., 2013, Li et al., 2016).
Application of Artificial Intelligence in Prediction of Wellbore Stability Using Well Logging and Drilling Data
Authors: Wu Juntao , Liu Wei , Lin Hai , Liu Hailong , Peng Chengyong
Published: September 2023

Wellbore instability is one of the most critical challenges during drilling, often manifested as wellbore collapse, shrinkage, falling rocks, and formation fracturing, which may result in complex problems such as pipe sticking, high torque, mud loss, thus impeding the drilling progress and increasing the cost of the drilling operation. Conventional wellbore stability prediction relies on some deterministic physical models, involving some empirical coefficients which are difficult to determine and often dependent on field experience. In addition, some complex factors, such as natural fractures, cannot be explicitly and quantitatively characterized in existing wellbore stability prediction models. Artificial intelligence technique has shown unique advantages in nonlinear issues. The artificial intelligence technique is used to predict wellbore stability in this study, including artificial neural networks (ANNs) and support vector machine (SVM). The logging data and drilling data were collected from the field. According to the correlation analysis between influencing factors and wellbore enlargement rate, 16 parameters were extracted, such as mud density, formation density porosity, acoustic interval transit time, weight on bit as the input data of the models, and wellbore enlargement rate as output. Both SVM and ANNs models have exceptional performance in predicting wellbore stability. When the kernel of the SVM model is Linear, predictions perform optimally. In the ANNs model prediction results, the result performs optimally when the total number of neurons is 1024 in the hidden layer. Overall, ANNs model performs better than SVM model with a coefficient of determination (R2) of 0.991, therefore it is recommended to apply ANNs to predict wellbore stability. The present analysis supplies knowledge that can be used to predict wellbore stability problems before drilling, optimize drilling parameters, and reduce drilling accidents and costs.

INTRODUCTION

Wellbore instability refer to a series of responses resulting from mechanical, chemical and other effects of the rock around the wellbore. Wellbore instability is one of the most critical challenges during drilling, often manifested as wellbore collapse, shrinkage, falling rocks, and formation fracturing, which may result in complex problems such as pipe sticking, high torque, mud loss, thus impeding the drilling progress and increasing the cost of the drilling operation.
Study on the Mechanical Characteristics and Optimization of Composite Lining Structure Under High Geostress
Authors: Deng J. Q. , Cui W. , Zhu Y. B. , Liu Y. , Du S. L.
Published: September 2023

Aiming at the problem of high geostress faced by deep buried tunnel, the mechanical characteristics and optimization of composite lining structure are studied. The influence of consolidation grouting ring and backfill grouting measures on the mechanical characteristics of composite lining structure are studied based on the three-dimensional finite element calculation theory, so as to determine the optimal thickness of concrete lining. The maximum tensile stress of 20cm lining during operation is about 8.73MPa, and that of 120cm lining during operation is about 4.68MPa. The maximum pressure stress during the construction period of 20cm lining is about −13.47MPa, and the maximum pressure stress during the construction period of 120cm lining is about −4.95MPa. After the pre-consolidation grouting of class IV surrounding rock, the maximum tensile stress during the operation period decreases. If backfill grouting is not carried out within 120° of the top arch of the lining, the maximum tensile stress during the operation period and the maximum compressive stress during the maintenance period are significantly increased, so the backfill grouting of the lining and its grouting quality have a significant role in improving the bearing capacity of the composite lining structure.

INTRODUCTION

With the continuous construction of water conservancy and hydropower projects in China, a number of deep-buried, long and large diversion tunnels crossing high altitude and high mountain areas have emerged, which are significantly different from the diversion tunnels of conventional power stations. The lining structure of deep-buried, long and large diversion tunnel is affected by many adverse factors, such as large burial depth, long tunnel line, large section, rich groundwater, high water head and high geostress level. The design and construction of lining structure is very difficult. The geological conditions in the western mountainous areas are complex, and the construction of deep-buried large-section tunnels is very easy to cause engineering accidents such as landslides, water inrush, mud inrush, etc. The main reason is the lack of understanding of the stress of such tunnel lining structures.
Effects of Supercritical CO2-Brine/shale Interaction on Fracturing Behavior
Authors: Qiu Renyi , Zhang Guangqing , Anhai Zhong , Mingjing Lu , Feng Yang , Zhou Dawei
Published: September 2023

As a caprock for CO2 geological sequestration, the mechanical properties of shale may change significantly from the long-term CO2-fluid interaction. To study the long-term effects of supercritical CO2-brine interaction on the mechanical properties of shale, the marine LMX shale was treated with supercritical CO2-brine at temperature of 110 °C and pressure of 30 MPa for 15, 30, and 60 days, respectively. The Brazilian tests combined with Acoustic Emission (AE) monitoring were conducted on shale before and after the treatments. The results show that: (1) Compared with the dry shale, the tensile strength of shale saturated with supercritical CO2-brine for 15, 30, and 60 days decreases by 40%, 48.6%, and 68.8%, respectively. The fracture morphology changes from a straightly single fracture to a fracture band consisting of two to four curved fractures; (2) The effects of supercritical CO2-brine interaction on the initiation and damage stresses are quantitatively analyzed. Taking the initiation and damage stresses of dry shale as the reference, the initiation stresses of shale saturated with supercritical CO2-brine for 15, 30, and 60 days decrease by 17.7%, 37.8%, and 48.4%; the damage stresses decrease by 35.5%, 58.3%, and 75.8%; (3) The AE characteristics of shale are closely related to fracture modes. The tensile fracturing of shale is sudden, manifested as an obvious step in the cumulative AE energy, while the shear fracturing of shale is gradual. With the supercritical CO2-brine treatment time, the fracture mode gradually changes from tensile-dominated to tensile-shear mixed failure mode, which can be inferred from that the slight step increases significantly and the ratio of tensile to shear events decreases from 10.7:1 to 0.8:1. The mechanical properties of shale weaken with the increase of supercritical CO2-brine treatment time.

INTRODUCTION

As one of major greenhouse gases, excessive CO2 exacerbates global climate change. CO2 geological sequestration is one of the important means to mitigate the greenhouse effect by injecting large amounts of CO2 into deep formation (Bachu et al., 2007; Wood, 2015; Jia et al., 2019; Nie et al., 2022). The deep shale formation has low porosity, low permeability, with beddings, which can effectively prevent and isolate the upward migration and leakage of CO2, which is suitable for the target formation of CO2 geological sequestration (Kalantari-Dahaghi, 2010; Lyu et al., 2021; Xie and Economides, 2009; Liu et al., 2013; Chen et al., 2015; Chen et al., 2023; Zhang et al., 2021). CO2-shale-fluid interaction will significantly affect the mechanical properties of shale under the condition of high temperature and high pressure for long-term CO2 geological sequestration (Zou et al., 2018; Al-Ameri et al., 2016; Choi and Song, 2012; Zhou et al., 2022).
Effects of Rapid Cooling and CO2 Depressurization on the Compressive Strength of Limestone
Authors: Verberne B. A. , van der Linden A. J. , El Azouzi K. , Wolterbeek T. K. T. , van Schalm R. , Coorn A.
Published: September 2023

CO2-injection into depleted reservoirs can lead to rapid cooling and a CO2 phase change in the near-wellbore region, potentially affecting rock mechanical integrity. We report unconfined compressive strength (UCS) and thick-walled cylinder (TWC) collapse strength tests on dry and brine-saturated limestones subjected to cooling (ΔT) or CO2 depressurization (ΔP). Plug samples were prepared from Indiana limestone and a limestone from the Green River Formation, outcropping in Sanpete Valley, Utah, USA. The ΔT-treatment consisted of a thermal shock from 60°C to −78°C, while the ΔP-treatment consisted of three cycles of rapid depressurization from ∼9 MPa CO2 pressure to atmosphere. The ΔP-treatment was performed at room temperature and at 60°C. The results show that dry samples are generally stronger than their brine-saturated counterparts, while effects from ΔT- or ΔP-treatment on rock strength or stiffness are indiscernible. Based on our findings on the Indiana and Green River Formation limestones, we suggest that CO2-injection-induced cooling or depressurization in limestone reservoirs can lead to strengthening of desiccated regions with negligible negative impact on rock mechanical integrity.

INTRODUCTION

Carbon capture and storage (CCS) is a key strategy on the way to a net-zero CO2-emissions industry, including in carbonate reservoirs, which constitute vast potential storage volumes (e.g., Bonto et al., 2021). However, large-scale implementation of CCS will only be viable if operational risks are understood and mitigated. One of the important geomechanical concerns for geological CO2 sequestration are thermal stress effects due to injection of relatively cold CO2 (Rutqvist, 2012; Roy et al., 2018). Especially in cases where the reservoir (residual) pore pressure is substantially lower than the intended CO2 supply pressure (typically >7 MPa), adiabatic, isenthalpic expansion of CO2 in the near-wellbore region can lead to sharp temperature drops, possibly even reaching below freezing conditions (Oldenburg, 2007; Mathias et al., 2010). Rapid temperature changes can result in thermal strains, fracturing, and, potentially, loss of containment (Pašić et al., 2007; Wang et al., 2022).
Long-Term Development of Shale Barrier Efficiency: Laboratory Tests on an Outcrop Shale
Authors: Fjær E. , Bakheim S. , Stenebråten J. F.
Published: September 2023

The annulus outside the casing of a deep well is often left open after completion, however it is observed in numerous shale formations that the shale creeps in and closes the annulus, thus forming a "shale barrier". This is a gradual process which implies that the sealing efficiency of the barrier is also developing gradually. This paper describes small scale laboratory tests on an outcrop shale, where long-term development of shale barrier efficiency is studied. The test conditions are such that creep is likely to be the dominating process during the tests. The results show that the sealing efficiency of the shale barrier improves with time. The magnitude of the improvement depends on the stress conditions and the conditions prior to the initiation of the barrier. The final value of the sealing efficiency appears to depend primarily on the net stress which corresponds to the in-situ stress minus the in-situ pore pressure. Impacts of rock properties and annulus gap size were not investigated in this study. Theoretical considerations suggest that the seal improvement process can be significantly faster even at moderately higher formation temperatures.

INTRODUCTION

"Shale barrier" is a commonly used term for a natural barrier against uncontrollable flow outside the casing of a deep well through a shale formation. A shale barrier is formed as the shale creeps in and fills the initially open annulus between rock and casing, usually without dedicated interference by the operator (Williams et al., 2009; Kristiansen et al, 2018). Such barriers represent huge cost savings for the operator, as it may reduce or even eliminate the need for time-consuming cement jobs. It is essential however that the long-term sealing efficiency of the barriers can be trusted.

The presence of barriers outside the casing is usually detected by indirect, acoustic methods which may reveal whether the casing is in contact with a solid material or only fluid (Williams et al., 2009). The sealing efficiency of such barriers can only be tested by leak-off or pressure communication tests, however such tests are either very time consuming or rather inaccurate (Raaen and Fjær, 2020). The long-term sealing efficiency of shale barriers is therefore rarely studied in the field.
Modeling Material Fracturing with a Phase-Field Method Taking into Account Hydro-Mechanics Coupling and Material Heterogeneity
Authors: Wang Meng , Yu Zhan , Vu Minh-Ngoc , Shao Jian-Fu
Published: September 2023

We introduce a novel numerical approach for modeling cracking in heterogeneous materials by considering hydromechanical coupling. The proposed approach addresses the need for more accurate models to study cracking processes in materials with mineral heterogeneity, which are frequently encountered in geomechanics and civil engineering applications. A new phase field model is developed to describe the transition from diffuse damage to localized cracks, including both tensile and shear cracks. The model incorporates an elastic-plastic constitutive law to describe the basic mechanical behavior of the material. We account for mineral heterogeneity by considering the mineral compositions of rock-like materials, such as inclusion content, which affect the macroscopic elastic properties of the material. The proposed model is applied to analyze desiccation and mechanical tests performed on mortar. We compare the numerical results with experimental measurements to demonstrate the accuracy of the proposed model.

INTRODUCTION

Based on laboratory tests and in-situ monitoring, it has been observed that rock-like materials commonly experience failure as a result of the initiation and propagation of fractures and cracks. These materials are subjected not only to mechanical loading, but also to changes in water saturation, temperature, and chemical degradation. Hence, it is imperative to conduct an analysis of the damage mechanics of these materials by accounting for the multi-physical coupling phenomena.

Over the past few decades, numerous phenomenological damage models have been proposed for various engineering materials, including rocks and concrete. Many of these models were developed within the framework of thermodynamics and employed a scalar internal variable for isotropic damage (Jefferson and Mihai, 2015; Chen et al., 2015; He et al., 2015) or tensorial variables for anisotropic damage (Voyiadjis et al., 2008; Desmorat, 2016; Zafati and Richard, 2019). In these models, the connection between macroscopic behavior and microscopic crack evolution is not explicitly established. More recently, the phase field method has emerged as a powerful tool for modeling the transition from diffuse damage to localized cracks (Bourdin et al., 2000; Miehe et al., 2010a,b; Borden et al., 2012; Ambati et al., 2014), by approximating sharp cracks with regularized smeared cracks. Various formulations have been proposed for modeling tensile, shear, and mixed cracks in elastic materials. Additionally, several phase-field models have incorporated the coupling between plastic deformation and damage evolution (Miehe et al., 2015a; Borden et al., 2016; Areias et al., 2016; Samaniego et al., 2021; Khalil et al., 2022).
Study on Reducing Casing Shear Deformation by Using Composite Cement Slurry Column Structure
Authors: Xi Y. , Yao Y. , Chen J. , Jiang H. L. , Ren Z. K. , Liu P. L. , Wang H. T.
Published: September 2023

Shale reservoirs are usually developed by horizontal wells and staged fracturing, and severe casing deformation occurs during development. The main deformation form is shear deformation. The main reason for casing shear deformation is that during hydraulic fracturing, when the fracturing fluid enters the natural fracture or weak cementation surface, the normal stress between the fracture surface or bedding surface will be significantly reduced, and the friction coefficient between the two will also be substantially reduced, which will lead to relative sliding of the rock mass on both sides of the fracture or bedding. Once the rock mass slips, it will lead to shear deformation of the casing. Given this, a method of reducing casing shear deformation using composite cement slurry column structure is proposed: inject conventional cement slurry, elastic and ductile cement slurry, and cement slurry containing hollow glass beads into the wellbore in sections. The high porosity cement sheath formed by cement slurry containing hollow glass beads can effectively reduce the degree of casing shear deformation during formation slip, and the elastic and ductile cement slurry can effectively seal the possible gas channeling problem. During the research, the optimal rheological model of the composite cement slurry column was optimized, the change of the annulus pressure loss caused by the mixing was calculated, and the influence of the cement sheath formed by the structure solidification of the composite cement slurry column on the reduction of the shear deformation of the casing was analyzed. The research results show that the established rheological composite cement slurry column model can be well applied to the project site. Still, the influence of the cement slurry system on friction should be considered in the use process. The cement sheath formed by cement slurry containing hollow glass beads can effectively reduce the shear deformation of the casing. With the continuous increase of the content of hollow glass beads, the ability to reduce the shear deformation of the casing is continuously improved. The research results provide a new way to solve the problem of casing deformation caused by interlayer slip and shear during hydraulic fracturing.
Study on the Internal Pressure Strength of Perforated Casing in High Pressure Formation
Authors: Zou Yiqi , Tao Lin , Liu Nanxiang , Gao Deli , Li Wenlong , Chen Xuyue
Published: September 2023

Casing damage is the main form of wellbore integrity failure, severely affecting oil and gas production. The mudstone has rheological properties under the high temperature and pressure of the formation, and creep deformation of the formation leads to increased external pressure of the wellbore. At the same time, perforation operations severely reduce the strength of the casing. Therefore, it is crucial to analyses the collapse strength of perforated casing in mudstone formations. In this paper, a three-dimensional numerical model of the formation-cement sheath-perforated casing is developed for the geological conditions of the Missan oilfield. The casing damage mechanism under long-term rheological effects is revealed. The effects of different casing sizes, casing thickness, perforation diameter and perforation density on casing collapse strength were carried out. The results show that the equivalent force on the perforated casing gradually increases with increasing operating time. For the same parameters, the smaller the casing diameter and the thicker the casing thickness, the higher the residual strength of the perforated casing. The collapse strength of the perforated casing decreases with increasing perforation diameter and perforation density. It is recommended to increase the casing thickness and optimize the perforation parameters in mudstone formations. The study provides a theoretical basis for the field application of perforation technology in mudstone reservoirs and has a good application prospect for improving the casing collapse strength.

INTRODUCTION

The Missan oil field has frequent drilling complications, among which casing damage is a serious problem. The Asmari Formation is 3000m burial depth and has a complex geological structure with extensive mudstone. Field applications and relevant statistics show that the majority of casing damage and failure occurs in the perforation holes. Perforation is an important stage of oil exploration and development, and is closely linked to subsequent production enhancement and reconstruction techniques. During the completion of an oil and gas well, the jet produced by the injection projectile will penetrate the casing, cement sheath and part of the formation. As a result, a number of perforated holes are created in the casing along the injection spiral, resulting in a reduction in the stability of the casing. During subsequent production, the perforated casing is more susceptible to strength failure due to excessive loading, which has a non-negligible impact on the wellbore integrity. At the same time, the creeping behavior of the mudstone at high in-situ stress increases the extrusion of the casing, exacerbating the potential damage to the perforated casing. Therefore, the strength analysis of the perforated casing under mudstone creep is critical to the integrity of the wellbore.
Understanding the Source of Pumps off Gas in Impermeable Rock
Authors: Thorpe D. T. , Chan A. W. , De Gennaro S. , Porter M. J. , Colgan G. , Murray D.
Published: September 2023

While drilling, an increase in background and onset of connection gas is frequently related to an increase in pore pressure within a formation. This can lead to the risk of being near balanced or underbalanced resulting in the possibility of a kick when the pore pressure is greater than the static mud weight. This work shows that in some cases this is unlikely to be the leading interpretation and reacting to the gas by increasing the mud weight can exacerbate the situation. Here, we clearly show that the pumps off gas seen from this well is a result of fault reactivation and not a result of an underbalanced situation. The techniques demonstrated in this report can be applied to other case studies where the cause of pumps off gas is unclear.

INTRODUCTION

The Penguins Cluster is located in the Northern North Sea, 150 km North East of the Shetland Islands (Thorpe, Porter, McKie, & Ritchie, 2020). The cluster is currently going through redevelopment, after the previous host facility reached end of field life in 2021. As part of the redevelopment campaign, the decision was made to build a new Sevan 400 floating, production, storage and offloading vessel and to drill an additional eight wells to supplement the seven remaining production wells. This paper discusses a pumps off gas (POG) response identified while drilling the passive Shetland Group claystone in one of the redevelopment wells.

WELL DESIGN AND STRATIGRAPHIC OVERVIEW

Three of the eight redevelopment wells use a slim well design concept, to help optimise well delivery in terms of both time and cost. The newly applied concept to the Penguins Cluster is a four-string well design, which removes a casing string from the previously carried five-string design. Rather than setting a casing point in the relatively unreactive Heather Formation to case off the reactive Kimmeridge Formation source rock, the design now sets the 9 5/8" production casing in the lower Shetland Group, with the 8 ½" section taking the well to total depth (TD) (Figure 1). The reservoir section must therefore carry a mud weight which maintains the stability of the reactive Kimmeridge Formation, while drilling to well TD.