bluuebunny/crossref_metadata_2025 huggingface dataset:
Rheological and Mechanical Properties of Cement Grouts for Geothermal Well Cementation with Seashell Powder as Full Metakaolin Replacement
Authors: Wilson A. , Grégoire D.
Published: September 2023

The different conditions associated with geothermal wells call for cement grouts that are lightweight, flexible and same time, possess good mechanical properties that can improve well integrity. This paper presents an experimental study of formulating lightweight and resilient cement grouts by a full replacement of metakaolin with crushed seashell wastes in a classical slurry formulation. Seashell powders made from Crassostrea gigas (Oyster), Pinctada maxima, Pecten maximus (scallop) and Crepidula fornicata were used to formulate the low-density slurries of approximately 1.2 specific gravities. Without using any chemical additives, the rheological behavior of the slurries, thermal properties and the mechanical properties of the set cements containing seashell powders were examined in comparison with classical samples containing metakaolin. The results showed an improved rheological behaviour in the context of geothermal well applications. The mechanical property investigations also revealed an improvement in elastic and flexural properties which may improve the global well integrity.

INTRODUCTION

The net-zero decarbonisation of the global energy sector will require an increase in the exploitation of renewable energy resources like geothermal energy. Geothermal energy is estimated to be capable of producing up to 8.3% of the world's electricity (WEC, 2019). Due to the high cost involved in the building and repair of geothermal wells, it is important to design them capable of producing energy for up to about 30 years. Cements used for zonal isolation in geothermal wells face conditions of high temperature and pressure (HTHP) that may affect the well integrity.

A greater number of cement failures are reported to have been due to tensile loading rather than compressive (Lullo and Phil, 2000). This makes it important to optimize flexibility of the cement sheath during slurry design. Low density cements with lower Young's modulus are more flexible and are capable of providing higher expansions to accommodate the cyclic casing expansions due to temperature variations (Sugden et al., 2012). Another importance of lowering slurry density is in the avoidance of loss circulation which could occur if the hydrostatic pressure of the slurry exceeds the fracture strength of the surrounding formation (Sugama, 2007). Some methods of achieving low density slurries include foamed cement technologies (Harms and Febus, 1985; Sugama, 2007) and hollow microspheres (Abdullah et al., 2013).
Chemical Treatment for Sustainable Acid Fracture Conductivity of Weak Carbonates
Authors: Desouky M. , Aljawad M. S. , Amao A. , AlShehri D. A. , AlTammar M. J. , Alruwaili K. M.
Published: September 2023

Acid fracturing is an essential stimulation technology to increase the production of carbonate reservoirs. However, acid usually softens the rock, and the stability of the fracture hydraulic conductivity is hardly maintained. Therefore, rock strength after an acid treatment is crucial in controlling the emerging hydraulic conductivity. Consolidation agents like diammonium phosphate (DAP) could enhance the mechanical properties of damaged or weak carbonate rocks. This study investigates the DAP consolidation effect on weak carbonate fracture hydraulic conductivity. Weak Austin chalk samples were soaked in 1 M DAP for 72 hrs. under high pressure and temperature. The surface hardness of weak Austin chalk samples was evaluated nondestructively using impulse hammering. The sample's surface hardness was greatly improved upon the DAP treatment. The treated samples’ hydraulic fracture conductivity and compaction were measured and compared with similar untreated samples. The treated samples exhibited higher and sustained hydraulic conductivity and less compaction under higher stress. The proposed solution could be particularly useful in soft carbonate formations, where induced acid softening is a common problem. Boosting the fracture surface hardness with DAP can improve the efficiency of the acid stimulation process and increase the amount of hydrocarbons that can be extracted.

INTRODUCTION

While acid fracturing manifests itself as one of the most deployed stimulation methods in carbonate reservoirs, its success is limited by the carbonate strength after the operation. The asperities that keep the fracture open are more vulnerable to failure because acidizing decreases the compressive strength of the carbonate (Gong et al., 1999). Likewise, (Nierode & Kruk, 1973) in Equations 1&2 related the hydraulic conductivity (kfwi) of the fracture to the rock embedment strength (SRE), e.g., the lower the embedment strength, the lower the hydraulic conductivity. Moreover, the rock embedment strength determines the rate at which the conductivity falls under stress (σC) (Gomaa & Nasr-El-Din, 2009). Also, soft carbonates like chalk exhibit more time-dependent deformation (creep) than harder carbonates like dolomite (Abass et al., 2006).

(Equation)

(Equation)

In carbonate gas wells post-acid fracturing jobs, a severe production loss of more than 60% within 6 months was reported (Bartko et al., 2003). The reasons behind the sharp decline should be understood before considering restimulation jobs which are not usually successful (Abass et al., 2006). However, the best scenario would be to have a sustainable acid fracture at the first trial.
Analysis of the Influence of Deep/Ultra-Deep Rock Mechanical Properties Difference on Rock Breaking Efficiency of PDC Bit and Research on Cutter Shape Optimization-Taking Shunbei Oilfield as an Example
Authors: Zhuoxin Dong , Hui Zhang , Kerou Liu , Yufei Chen
Published: September 2023

Polycrystalline diamond compact (PDC) is widely used in petroleum engineering drilling due to its strong wear resistance, fast penetration rate and high bit footage. But with the increase of formation depth, the formation rocks become dense, high strength and difficult to be destroyed under the action of geostress, which makes the drilling speed in deep/ultra-deep layers slow. In this paper, the characteristic law of rock strength characteristics changing with well depth is analyzed, and the numerical simulation model of single cutter rock breaking based on the Cohesive element is established. According to the numerical simulation results and the change of confining pressure with depth in Shunbei Oilfield, the suitable depth range of each cutters is recommended by layers. The results show that the rock strength characteristics gradually increase with the depth, and the impact on the rock breaking of the cutting teeth is mainly the increase of the weight on bit and the cutting force. By analyzing the numerical simulation results, the circular cutter is suitable for shallow formation, the curved cutter is suitable for medium and deep well section, and the triangular cutter is suitable for ultra deep formation.

INTRODUCTION

Oil and natural gas are the main raw materials for obtaining energy in the world today. With the improvement of the development level of oil and natural gas, the depth of recoverable oil reservoirs is getting deeper and deeper, and the depth and number of deep/ultra-deep wells are also increasing. (Wang HG, Huang HC, Bi WX, et al. 2021). For these deep/ultra-deep wells, the traditional drilling methods cannot effectively drill, and it is difficult to meet the needs of rapid deployment of well patterns in the oilfield. (Tian S, Huang Z, Niu J, et al. 2009). Therefore, it is quite necessary to carry out the research on rock breaking law of PDC cutters for deep/ultra-deep layers.
A Comparison of Empirically Estimated Rock Mass Modulus Values at a Soft-Rock Site
Authors: Lindenbach E. J. , Bearce R. G. , Foran J. F.
Published: September 2023

The rock mass modulus, or deformation modulus, of an underlying rock mass is an important input variable for any load-deformation analysis of a foundation, such as numeric modeling of a dam. The deformation modulus can be determined empirically using a variety of different relationships that incorporate parameters readily determined from geologic logs and laboratory testing. Relying on one value can be fraught, though, as a low rock mass modulus can improve damping of seismic waves while a high rock mass modulus can increase static stability. Therefore, developing a realistic range of values is of paramount importance. In this paper, a probabilistic approach incorporating distributions for input parameters and ten empirical relationships was used to determine a range of anticipated foundation rock mass modulus values for a proposed spillway, thereby allowing for a range of model outcomes to be simulated. The analysis also found that the relationships to determine rock mass modulus that do not incorporate the elastic modulus as determined during laboratory testing all produced significantly higher rock mass modulus values than those that incorporated the laboratory data; relationships using only RQD and/or RMR appear to break down where the rock mass is relatively fracture free, but the intact rock is soft, as is the case for this site.

INTRODUCTION

Accurately modeling the response of a structure to static or seismic loads is of paramount importance when evaluating critical infrastructure such as dams. Key to the development of accurate models is developing and implementing realistic ranges of input parameters that facilitate the portrayal of a range of possible outcomes. One of the parameters needed in the modeling efforts is the deformation modulus or rock mass modulus. The terms deformation modulus and rock mass modulus are interchangeable, and both imply load-deformations of the rock mass in the elastic and inelastic ranges (Ulusay and Hudson, 2006).
Instrumented Pull Test Results in Grouted Split Sets, and Its Comparison with Non-Grouted Test Results in an Underground Manganese Mine – Corumbá/MS - Brazil
Authors: Silva P. H. , Ribas G. P. , Gontijo A. A. , Saliba G. S. , Lopez J. , Abduani T. C.
Published: September 2023

Rock mass friction stabilizers, commonly called Split Sets, were introduced in the metal mines of the United States and Canada in the early 70s. Split Sets can provide support due to friction between the anchor and the rock surface of the boreholes. As Split Sets are inserted in boreholes with slightly smaller diameter than the diameter of the retainers, the radial force is generated by the compression of the anchor by the drill wall, ensuring its efficiency by friction in this contact interface. In order to increase the maximum load capacity of Split Sets, some mining companies opt for grout injection after the installation of the anchors. The present study presents the results of instrumented pull tests performed in Grouted Split Sets and its comparison with the result of tests performed in Split Sets without the application of cement, in an underground manganese mine. A total of 72 tests were performed in 3 different mine locations, with distinct roof geological characteristics, and the results showed an increase of up to four times the initial strength without the application of grout, considering the best scenario.

INTRODUCTION

Split Sets can provide pressure support due to friction between the anchor and the rock surface of the holes. These were the first types of friction anchors to be used and still are the most widely type operated in underground mining. Split Sets consist of steel tubes sectioned longitudinally, and a steel ring fixed at its lower end, able to apply force to a steel plate that transfers the load to the surface of the excavation. As the Split Sets are inserted in boreholes with slightly smaller diameter than the diameter of the retainers, the radial force is generated by the compression of the tube over the drill wall, ensuring its efficiency by friction in this contact interface (Komurlu and Demir, 2019).

According to Komurlu and Demir (2019) the length of the Anchor is an important parameter for load capacities of support with Split Sets. Load capacity increases with bigger length of anchor. In addition to the length of the Split Set, other parameters have a great influence on the efficiency of this type of anchoring. A factor of great impact on the maximum load supported by the Split Set is the drilling diameter. When anchor of the same characteristics and dimensions are used in different diameters of perforation, we realize that there is a tendency to obtain higher maximum loads using smaller diameters, as shown in figure 1 (Tompson et al. 1999).
The Design of a Downhole Source Tomography Experiment for the Detection of CO2 Plumes in the Subsurface
Authors: McCormack K. L. , Edelman E. , Moodie N. , McPherson B. J. , Paulsson B. , He R.
Published: September 2023

We developed designs to test a new method of detecting supercritical carbon dioxide plumes, which uses a form of reverse vertical seismic profile. The design focuses on two blocks of sandstone, collectively measuring 1m × 1m × 1m, separated by an epoxy seal that simulates the caprock. To simulate leakage through a legacy wellbore, a perforation will be made in the seal at an offset location from an injection well in the center of the sample, and supercritical carbon dioxide will be injected at a rate of 0.01 kg/s into the central well. The experiment is expected to last approximately four hours according to numerical results. While the plume in the reservoir block is likely to be successfully imaged tomographically, there is uncertainty about the ability to image the leak. In addition to the tomography, acoustic emissions will be passively sensed, allowing for an independent assessment of the plume's location.

INTRODUCTION

Detecting and monitoring the migration of a supercritical carbon dioxide plume in a geologic carbon sequestration site, whether in a deep saline or EOR setting, is crucial for the safe use of sequestration technologies. The significance of detection and monitoring is twofold. Firstly, it is essential to determine whether the plume is behaving as expected during site characterization. If the plume migrates in an unexpected direction or to a greater extent due to uncertainties in the injection interval properties, it is critical to assess whether injection operations can continue or if remediation is necessary. Secondly, avoiding faults, subsurface features, and political boundaries is crucial to the success of carbon sequestration operations. For example, in the San Juan Basin CarbonSAFE Phase III project, the injection is attempting to avoid both the Hogback monocline and the Colorado border, which can pose complications. At this stage of the project, simulation is the only method available for plume forecasting. However, when injection commences, collecting data on the plume extent would be immensely valuable.
Productivity Prediction and Production Parameter Optimization of Indirect Fracturing Coalbed Methane Wells
Authors: Cai Xianlu , Wang Zhiming , Huang Tianhao , Li Shengyi
Published: September 2023

The fractures formed in hydraulic fracturing in tectonic coal are relatively short. The indirect fracturing technology simultaneously fractures coal seam and its roof layer, which can significantly increase the fracture extension length. However, compared with conventional coalbed methane fracturing wells, the gas-water flow process in the reservoir becomes more complex. In this paper, a numerical simulation method for productivity prediction of indirect fracturing wells for coalbed methane is developed based on the isothermal adsorption equation, quasi-steady state diffusion equation, and seepage equation. The reliability of the seepage model is verified by the seepage simulation experiment of indirect fracturing wells. The pressure distribution, desorption area distribution, and gas production rate change during coal mining is analyzed. A method of optimizing the bottom hole flowing pressure by stages is developed, which optimizes the bottom hole flowing pressure in water drainage stage, pressure build-up stage, and pressure control stage, respectively. The average relative error between experimental and numerical results is 4.9%. In the production process, the desorption area can be divided into fast desorption area, slow desorption area, and non-desorption area according to the desorption rate. The whole rapid desorption region has a relatively high desorption rate. However, the desorption rate is relatively higher in the region near the roof layer in the slow desorption area. The roof layer provides a high permeability seepage channel for the fluid in the coal seam, and the fluid in the coal seam flows vertically driven by the pore pressure difference between the layers. Fluid flows through the pores of the roof layer and into the fractures. In the stable production stage, the gas production rate of roof layer can reach 40%. This study benefits productivity prediction and analysis of indirect fracturing coalbed methane wells. It can provide theoretical guidance for optimizing the strategy of coalbed methane extraction.
The Effect of the Heavyweight Hematite, Magnetite, and Micromax on Class G Oil Well Cement Properties
Authors: Alvayed Dandi , Mahmoud Ahmed Abdulhamid , Elkatatny Salaheldin , Alafnan Saad
Published: September 2023

A high-density cement slurry is required to adequately balance the high-pressure formations in deep oil wells. The slurry's density could be increased by adding a variety of heavy substances which affect cement slurry and sheath properties. This study investigated the impact of three weighing materials of hematite, magnetite, and micromax on the density fluctuation, compressive strength, and rheology test of Saudi Class G cement slurry of 18 ppg. The results of this work indicated that using a weighting material with large particles can cause vertical heterogeneity, the density variation shows that the hematite cement sample had the largest density variation, with an 8.7% difference between the top and bottom of 4 inches in length sample, led by the magnetite cement sample with a density variation of 7.3%. In the Micromax-weighted sample, the density fluctuation had a vertical variation of 4% and was far less severe than others. Compressive strength measurement shows that the magnetite-weighted cement sample exhibited the highest compressive strength of 65.1 MPa which is 28% greater than the hematite sample. The magnetite slurry had the highest plastic viscosity of 113 cP, followed by 112 cP in the hematite cement. At 56 cP, the plastic viscosity of the micromax-containing heavyweight system was the lowest (50.8 percent less than that of the hematite cement). The smallest yield point was 10 lbf/100 ft2 for the micromax-based cement. Hematite cement had a yield point of 25 lbf/100 ft2, while magnetite-based cement exhibited the highest yield point of 41 lbf/100 ft2. With a 10-sec gel of 14 lbf/100 ft2 and a 10-min gel of 21 lbf/100 ft2, the micromax cement had the lowest gel strength. To avoid problems of segregation and solid abrasion, it is recommended that the particle size of weighing materials be reduced in further investigations.
Numerical Investigation of Geothermal Storage and Extraction Using Huff-N-Puff Wells
Authors: Wu Yuxing , Salehi Saeed , Karbalaeisaleh Fatemeh
Published: September 2023

The subsurface geothermal energy storage system uses underground temperatures for energy extraction and storage. The energy is extracted by an injection-production-shut in procedure. Working fluids (i.e., water) are injected into the underground and heated by the subsurface high-temperature reservoir. Then, the fluids are extracted to the surface for thermal purposes, such as electricity generation and direct-use applications. This study establishes a parametric study using numerical simulation methods to comprehensively understand the influence of reservoir properties and operational factors on the geothermal energy storage system. A huff-n-puff well is modeled to simulate energy extraction and storage purposes. Water is injected for 8 hours, produced for 10 hours, and shut in for 6 hours. The reservoir is assumed to be a pure water reservoir (the only fluid in the reservoir is water). The total time for one cycle is 24 hours (a day). The performance of the well in 30 days is simulated. Effects of several reservoir properties and operational parameters such as formation temperature, heat capacity and thermal conductivity of formation, permeability, porosity, injected, and produced rates are evaluated. The results present that as formation porosity increases from 5% to 30%, energy in place grows 26.1%. The water recovery factor decreases from 0.52 to 0.087. Compared with vertical permeability, horizontal permeability has a much more significant influence. The formation temperature increases from 60 °C to 210 °C resulting in a 434% increase in the energy in place. Water recovery factor rises from −0.27 to 0.17 as the temperature from 60 °C to 90 °C, then remains constant (the negative value means under this condition the injected rate of water from the surface facilities is higher than its produced rate). Thermal conductivity has no important changes in energy extraction, while heat capacity affects the energy in place. When heat capacity is accelerated from 1.3×106 to 2.5×106 J/(m·day·°C), the total reservoir energy increases by 60%. Operation factors significantly influence the recovery factor and reservoir energy by changing the injected and produced rates.
Modeling Perforation Orientation and Hole Size Effects on Hydraulic Fracture Distribution in Horizontal Wells
Authors: Sinkov K. , Kresse O. , Hobbs B. , Shrivastava K.
Published: September 2023

A modeling study was conducted with the objective of investigating the effects of perforation design and perforation size consistency on treatment allocation in multicluster limited entry treatments. The numerical model couples an advanced wellbore and perforation flow simulator and a well-established fracture simulator, which enables realistic modeling of complex perforation designs and pumping schedules. The study demonstrates that low variability of entry hole dimension among perforations is an important prerequisite for optimization of multicluster treatment uniformity. Variation of initial diameter due to noncentralized perforation gun positioning is shown to cause preferential slurry distribution to larger perforations, faster erosion, and further shift of distribution to the most eroded perforations. Erosion of perforations receiving a major fraction of proppant reduces the pressure drop for the entire cluster and thus eliminates the positive effect of higher friction on proppant distribution along the stage. Perforation designs with consistent initial perforation sizes, e.g., achieved using zero-phasing perforation guns, are shown to result in more uniform treatment distribution among clusters.

INTRODUCTION & MODEL DESCRIPTION

The completion efficiency and production of horizontal multistage hydraulically fractured wells are strongly affected by the uniformity of slurry and proppant distribution among multiple simultaneously stimulated perforation clusters within a stage. Depending on the fracturing treatment design, various factors related to either reservoir properties and their variability (Kresse et al., 2023) or peculiarities of proppant transport in a well and perforation tunnels can determine treatment distribution regularity. In the case of limited entry designs, commonly employed to improve treatment distribution among multiple intervals, perforation friction is used to mitigate variation of slurry flow rates to different clusters. High values of perforation friction are usually achieved by reducing the number of perforations (Somanchi et al., 2017) and, accordingly, are more sensitive to inconsistencies in the dimensions of the fewer entry holes. Moreover, the reduction in the number of holes leads to an increased amount of proppant pumped through the individual perforation and facilitates erosion, which can diminish the positive effect of initially high friction and aggravate treatment distribution nonuniformity (Thiessen et al., 2021).
Characterization of Fracture Propagation in Granite Under Uniaxial Compression Test Through Digital Image Processing and Discrete Element Method
Authors: He Changdi , Mishra Brijes
Published: September 2023

This study utilized digital image processing (DIP) and discrete element method (DEM) to explore the correlation between mineralogy and fracture propagation in granite. The study concludes that an increase in the content of quartz and feldspar leads to a gradual increase in the uniaxial compressive strength (UCS) value of granite. However, the UCS value of granite undergoes a significant decline with increasing mica content. Additionally, the presence of quartz intensifies the brittleness of granite, while feldspar enhances the ductility of granite. In the isotropic model, fractures tend to initiate from stress concentration points and propagate during the UCS test, causing further stress concentration due to the creation of high-stress zones by the crack tip. In the anisotropic model, fractures usually occur in areas where quartz and feldspar are more concentrated because of a higher likelihood of stress concentration in those regions. However, mica agglomerations are not susceptible to stress concentrations due to their low strength and high toughness, which serve as a buffer.

INTRODUCTION

Granite is commonly utilized as a host rock for various underground engineering projects such as radioactive waste disposal, deep mining, and enhanced geothermal systems (EGS)(Zhu et al., 2021; Kumari et al., 2017). Nonetheless, the presence of fractures in granite can have varying effects on these projects. In the case of radioactive waste disposal, fractures can compromise the integrity of the repository by providing pathways for radioactive material leakage into the surrounding environment (Jia et al., 2022; Zhao et al., 2022a). Moreover, in deep mining, fractures can pose a significant hazard to miners as they can destabilize the rock mass, leading to rockfalls and collapses that may endanger their lives (Zhao et al., 2022b). Conversely, fractures in granite play a crucial role in EGS, as they facilitate fluid flow and heat transfer. The distribution and connectivity of fractures can impact the efficiency of EGS by affecting the flow of fluid through the reservoir (Zhong et al., 2022).
Development of Multiphase Flow Simulation Method in DEM Under a Fixed-Grain Condition
Authors: Dai Q. , Meng Y. , Duan K. , Kwok C. Y.
Published: September 2023

Understanding multiphase flow mechanisms in porous media is vital for many engineering practices, such as geo-logical carbon sequestration. Existing numerical models that use the explicit pressure-solving scheme can only advance multiphase flow with a small timestep. None of them manages fast fluid transport due to a stability issue limiting timestep selection. This paper focuses on developing a fluid flow model that can quickly and efficiently capture fluid–fluid displacement patterns. We incorporate the implicit finite volume approach that is unconditionally stable to transport fluid with a remarkable timestep. To enhance interface–motion capture under various capillary number and viscosity ratio combinations, we set a flow front advancement criterion to reduce timestep when the injected fluid invades beyond front–line pores. Additionally, we update the flow front, pore pressure, and capillary entry pressure at every timestep to reveal more flow pattern details. We validate the model through a Darcy flow test and a fluid injection test based on existing Hele–Shaw tests. Numerical results agree well with the analytical solutions and experimental observations, confirming that the developed model is reliable for analyzing fluid migration problems and evaluating dynamic multiphase flow interactions in porous media.

INTRODUCTION

Geological CO2 sequestration is a crucial strategy for achieving carbon neutrality that stores greenhouse gas in geological formations, such as deep saline aquifers and depleted oil/gas reservoirs (Bradshaw et al., 2007; Bachu, 2008). Understanding how the injected CO2 migrates in geological media is vital to ensure successful CO2 geological storage. The injected CO2 will build up pore pressure and replace the existing pore fluid, remarkably altering the material properties (Sun et al., 2016) and pore structure (Yu et al., 2019). The pressure build-up during the injection process may trigger disasters, such as fault reactivation (White et al., 2014) and even earthquakes (Zoback and Gorelick, 2012), leading to CO2 leakage. To optimize CO2 injectivity and storage safety, it is thus essential to study multiphase flow mechanisms of fluid injection in porous media.
Three-Dimensional Salt-Pillar Equations and Their Applications to Industry
Authors: Belzer B. E. , Gupta N.
Published: September 2023

Salt-pillar equations were developed over 25 years ago to assist with mine design. Pillar design is important for salt mines because the pillar size influences convergence of underground drifts with equipment clearance and headroom constraints, subsidence at the surface, and functional life of ground support. Current methodologies for salt-pillar design include using numerical-modeling techniques and salt-pillar-design equations as a method for estimating stresses and deformation rates. In this paper, three-dimensional simulations of square and rectangular salt pillars were performed, and the results were compared to the estimates of stress conditions using the salt-pillar-design equations. The results were used to investigate if the same salt-pillar-design equations are still valid following advancements in numerical-modeling software and the constitutive behavior of salt. This paper (1) reviews the salt-pillar-design equations, (2) compares the estimated stress conditions using numerical-modeling methods with the salt-pillar-design equations, and (3) discusses applications for the use of salt-pillar-design equations to salt mine design.

INTRODUCTION

The room and pillar size play a crucial role in the viability of an underground mine. Even for extensive mineral resource mines such as salt, pillar design influences the roof-to-floor convergence rate, stability of the salt back and nonsalt units, and accessibility period of mine workings. Unlike hard rock or coal mines, time-dependent deformational behavior of salt increases the pillar design complexity in underground salt mines. With technological advancement and laboratory creep tests on salt specimens, site-specific creep laws can be determined, and sophisticated numerical models can be developed to predict the room-and-pillar response over a specified period.

Van Sambeek (1996) presented that the simple pillar-design equations for salt pillars can reproduce the results of extensive numerical models, such as stress conditions in salt pillars. The premise of the Van Sambeek salt-pillar-design equations is that the average values of the vertical stress, the two horizontal stresses (as compared to their actual distributions), and the corresponding deviatoric of those average stresses are adequate to estimate the pillar's creep behavior and structural stability. Therefore, the salt-pillar-design equations relate the stress averages to the shape of the pillar in terms of its height-to-width (H:W) and height-to-length (H:L) ratios.
Study on the Generalized Zhang-Zhu Strength Criterion Considering Different Loading Paths of True Triaxial Tests
Authors: Zhang Q. , Shen Y. X. , Wang M. H. , Wang X. J. , Li X. J.
Published: September 2023

Rocks in true triaxial stress states (σ1> σ2> σ3) have received much attention with the construction of deep underground engineering. True triaxial tests accurately reflect the triaxial stress state of rocks and have been widely used in the developments and validations of the three-dimensional strength criteria. Different loading paths are arranged to assess the deformation and failure properties of rocks as the development of true triaxial tests. In this study, the loading paths of the true triaxial tests are summarized as the traditional loading path, Lode angle loading path, and mean stress loading path. The ratios of σ2 and σ3, Lode angle, and mean stress can be regarded as invariants for three loading paths, respectively. The alternative expressions of the generalized Zhang-Zhu strength criterion of different loading paths are obtained by combining the three invariants. The expressions incorporate the properties of three loading paths to represent the correlations between σ1 and σ3 in simple forms. Finally, the typical rock data of three loading paths are collected to investigate the characteristics of the triaxial test data and the alternative expressions. The results show that the alternative expressions accurately predict the rock strength for different loading paths and reflect the effect of invariants of the loading path on rock strength. Adoption of three loading paths is discussed, and it is noted that the appropriate loading paths should be selected according to laboratory conditions and research priorities.

INTRODUCTION

With the extensive utilization of underground space, mineral, and environmental resources, the construction of deep underground engineering has been rapidly developed (Xie et al., 2017). Deep underground engineering is confronted with a complex environment such as high ground stress, high ground temperature, and strong disturbance (He et al., 2005; Qian and Zhou, 2018; Zhu et al., 2019). The mechanical behavior of deep rock is significantly different from that of shallow rock. Numerous studies indicated that the stress of deep rock presents a three-dimensional (3D) unequal characteristic (σ1>σ2>σ3) (Haimson and Rudnicki, 2010; Zhu et al., 2013; Zhu et al., 2017; Cai et al., 2021), which is defined as true triaxial stress state. Therefore, the strength characteristics of the rock cannot be effectively reflected by conventional triaxial tests (σ1>σ2=σ3) because the effect of σ2 is neglected (Zhang et al., 2013; Ma et al., 2017; Lu et al., 2020). It is important to study the mechanical properties of rock under true triaxial stress states.
A Semi-Analytical Model for Poro-Thermo-Elastic Stresses by Injection in a Finite-Height Layer
Authors: Fokker P. A. , Buijze L. , Wassing B. B. T.
Published: September 2023

The injection of fluids in geothermal energy, as well as in CCS, is accompanied by changes in stresses due to the reduced temperatures and elevated pressures. This can result in unwanted hydraulic fracturing and fault reactivation. We have developed a fast, semi-analytic tool to evaluate these changes and their consequences in terms of risk measures like fault reactivation probability, fracturing potential, and their spatial distribution. The tool has been designed for radially symmetric scenarios, which facilitated the use of an analytical approach. Pressure and temperature are calculated based on advection in the injection layers and diffusion in the bounding layers. The induced stresses are calculated based on these fields, using expressions for finite cylinders with elevated temperatures or pressures. Together with the virgin stress field, this provides input for the risk measures. With the semi-analytic approach, calculation times are limited and stochastic analyses are possible. We show how the shape of the disturbed zone in the injection layer and the amount of temperature and pressure diffusion into the bounding layers critically affect the stress field and the associated risk measures. We demonstrate how the tool can be applied to a geothermal injection well and to a CCS prospect, and how it helps in deciding what to target with data gathering.

INTRODUCTION

The transition to a sustainable energy economy poses multiple new challenges. Many of them are related to new ways of using the subsurface. As an example, the utilization of geothermal energy through an injection – production well doublet can only be safeguarded if the produced water is injected in the reservoir from where it was produced and does not leak away. For the storage of CO2 in CCS (Carbon Capture and Storage), it is also of prime importance that the carbon remains within the intended reservoir. Both applications thus require that the integrity of bounding layers is not breached.

Induced seismicity can also be an unwanted result of injection operations. Several geothermal field trials have been suspended or terminated because of unwanted induced earthquakes (Grigoli et al., 2018; Vörös & Baisch, 2022; Zang et al., 2014). At the same time, fault reactivation is sometimes required to ensure adequate transmissivity, e.g. in Enhanced Geothermal Systems (Majer et al., 2007). The concept of soft stimulation was introduced as a means of optimization of these two requirements (Hofmann et al., 2019). For balanced injection into porous sandstone formations seismicity has not yet been observed. However, because the number of projects is still small and duration of operations is short, it is important to assess fault reactivation potential for these types of operations as well.
Sand Production Prediction Based on Deep Learning and BP Neural Network
Authors: Jing Hongtao , Tu Zhixiong , Zhang Bao , Zeng Nu , Yi Jun , Yu Xiaotong , Si Xiang , Liu Suo , Xu Taiping
Published: September 2023

Deep learning can better simulate highly nonlinear engineering problems through training adjustment, and has good practicability for predicting invisible data. BP Neural Network (BPNN) is a widely used deep learning algorithm, and strengthens the accuracy of developing training models through feedback adjustment to increase the accuracy of target prediction. This work considers the similar lithology characteristics in the same block, the sand production profile of a single production well is first predicted by using empirical formulas and logging data, and further the logging data and sand production profile are trained by back-propagating to continuously adjust the weights and values of BPNN. Finally, one develops a sand production prediction model based on deep learning and the BP neural network algorithm, and uses the model to predict the potential of sand production of other production wells in the same block. The prediction results (training set) of the BP neural network algorithm are in good agreement with the traditional empirical formula (testing set), and the goodness of fit reaches 0.9 or more. Therefore, it increases the confidence in the proposed sand production prediction model based on deep learning and the BPNN algorithm to improve the early warning of the risks of sand production.

INTRODUCTION

Both the poorly consolidated sandstone reservoirs with lower strength and high-pressure gas wells with a higher production speed are potential candidates for severe sand production. Therefore, accurately predicting sand production plays a role in the choice of sand control methods. The core samples are often restricted to enough obtained, and thus it fails to get a sufficient amount of test data related to the rock mechanics and strength parameters in a laboratory, such that a poor prediction of the sand production is made. However, a good choice that the logging data combines with the classical statistical empirical formulas, such as combined elastic modulus, sand production index (Chen et al.,2008), and Schlumberger sand production index (Tixier et al.,1975), can quickly provide the sand production profile of a single well, and thus sand production profile is convenient for engineers to capture the potential formation section of sand production in oil and gas wells, and helps engineers to timely adjust production allocation and optimize sand control methods.
Theoretical and Modelling Insights into Rate-Dependent Sorptive Viscoelasticity
Authors: Siddiqui M. A. Q. , Regenauer-Lieb K. , Roshan H.
Published: September 2023

Recent thermodynamics-based constitutive modelling has enabled robust formulations for complex coupled mechanical, hydraulic and chemical interactions. Despite such advances, constitutive modelling of fractured sorptive media with complex mechanical behaviour has attracted little attention. This study presents a new sorptive poro-viscoelastic model for fractured rocks specifically integrating the rate-dependent viscous flow into the coupled fracture fluid flow and matrix gas desorption processes using the two-potential framework, mixture theory, and continuum mechanics. The proposed dissipative viscoelastic strain rate evolution law is governed by the applied stress variable and the amount of adsorbed gas in the matrix pores. The model is exemplified through the simulation of gas production from a sorptive shale formation. The numerical results show that poro-viscoelasticity becomes dominant at late times of production when pore pressure and desorption fronts have progressed significantly. It is also revealed that time-dependent stress accumulation can reach high magnitudes relatively early in the production phase which can cause fracture closure risking impedance to further gas production. Neglecting viscoelastic multiphysics effects in the modelling of fractured sorptive rocks can reduce accuracy of the predicted production data. In addition, the contribution of desorption-induced viscoelasticity to bulk rock deformation may be substantial in shales and other highly adsorbing rocks and may be the key to explaining some of the complexities encountered during hydraulic fracturing operations such as shale water loss and fracture closures.

INTRODUCTION

Shale formations are among the most abundant and widely distributed geological formations on earth, with enormous potential as a source of energy. Understanding the mechanical behavior of shales is essential for optimizing the hydraulic fracturing and gas production processes and the long-term stability of shale formations (Algazlan et al., 2022; Huang & Ghassemi, 2013; Siddiqui et al., 2022). The viscoelasticity and sorption behavior of shales are critical parameters that influence the stress-strain response of shale rocks, particularly under dynamic loading conditions as experience during gas production. In this context, a constitutive model that captures the sorptive viscoelasticity of shales can provide valuable insights into the mechanical response of shale formations.
Shear and Vertical Deformation Behaviour at the Field Scale During CO2 Storage in a Depleted Chalk Reservoir (Danish North Sea)
Authors: Amour F. , Hosseinzadehsadati S. B. , Hajiabadi M. R. , Nick H. M.
Published: September 2023

CO2 storage represents one of the recommended remediation methods to reduce the emission of anthropogenic CO2 in the atmosphere. As part of the Bifrost project that aims at developing CO2 storage activities in Denmark, a series of coupled hydro-mechanical-chemical simulations is carried out in the Harald East chalk field to assess the flow and mechanical response of the storage site, assuming both continuous and intermittent injection. A total of two continuous injection scenarios during 16 years (scenario 1) and 32 years (scenario 2) and scenario 3 which assumes intermittent injection are considered. The injection rate in scenario 2 and 3 is half of that of scenario 1, and the duration of the injection in scenario 3 is increased compared to scenario 1. The rate and duration of injection are adjusted to ensure a similar volume of stored gas in each simulation run. The outcomes show that fast injection prevents significant compaction in the reservoir. Besides, the calculated shear deformation in the overburden is utilised to study wellbore stability during storage. The results indicate that the accurate characterisation of the porosity distribution in the reservoir as well as the potential mechanical weakening of chalk by CO2 are essential for reliable wellbore integrity analyses. Further studies will be conducted to assess wellbore stability in the Harald East area during CO2 storage activities.

INTRODUCTION

The capture and underground storage of anthropogenic CO2 (CCS) represent a valid technology that helps tackling climate change by reducing the amount of greenhouse gas emitted by our societies (Al Amer, 2022; ICPP, 2005). Using pre-existing infrastructures present in depleted hydrocarbon fields drastically reduces the cost associated with CCS activities compared to other sites e.g., onshore and offshore aquifers (Schmelz et al., 2020). Besides, the proven efficiency of the cap rock that contained hydrocarbon for millions of years and the current low formation pressure below hydrostatic conditions are limiting risks of leakage. Located offshore, CO2 can be either shipped to the storage site, such as planned for the Greensand project or transported by pipeline as planned for the Northern Lights project. Taking into account that the floating units used to temporary store CO2 transported by boats prior to injection have limited storage capacities, shipping gas can compel the stakeholders to carry out intermittent CO2 injection in case of bad weather conditions, workovers, and potential shortages in CO2 supplied by the industry.
FDEM Modeling on the Fracture Propagation Behavior of Hot Dry Rock Subjected to Cyclic Soft Fracturing
Authors: Wu Xiaoguang , Yang Rui , Li Gensheng , Huang Pengpeng , Huang Zhongwei , Liang Rui , Long Tengda
Published: September 2023

Conventional hydraulic fracturing (HF) is an important way to develop Enhanced Geothermal Systems (EGS), but there are some problems such as high injection pressure and having trouble to create complex fracture network. To deal with those difficulties, Cyclic soft fracturing (CSF), an emerging technology, has been proposed, which adopts circulating stress waves to break down rock and forms a volumetric fracture network achieving oil and gas stimulation. In this paper, a numerical model of FDEM and a fully coupled thermal-fluid-solid solver are used to perform CSF experiments on hot dry rock (HDR) in field scale. The influences of stress disturbance, fracture initiation and propagation and three key parameters on fracture morphology are analyzed. The research results show that CSF can indeed generate complex fracture network under parameter optimization of peak pressure and loading frequency. Compared with conventional HF, CSF forms a larger stress disturbance area and has bigger tensile stress at fracture tip, which may contribute to fracture propagation. Under large stress difference condition, with the increase of peak pressure, the fracture complexity increases first and then decreases. The results are expected to provide an insight into the stimulation mechanisms of CSF method in HDR geothermal.

INTRODUCTION

Geothermal energy is a common renewable energy in the earth crust with the advantages of great potential resource, wide distribution and cleanliness (Wang, 2012). Hot dry rock (HDR), as a typical type of geothermal resources, offers great potential in meeting the growing demand for energy and achieving low-carbon solutions. Enhanced Geothermal Systems (EGS) is the main method for developing HDR geothermal energy. Hydraulic fracturing (HF) is the most important mean of EGS reservoir reconstruction, by which the production rate increases with stimulated reservoir volume created. Considering the special characteristics of HDR (e.g. high temperature, high hardness and high density), conventional HF has the problem of high injection pressure, low injection rate and the formation of simple fracture plane. Also, induced seismicity by HF has become one of the important limiting factors for the sustainable development of EGS engineering around the world (Li et al., 2022). In order to decrease breakdown pressure, mitigate seismicity and generate a complex fracture network, new stimulation strategies have been proposed, including fatigue hydraulic fracturing (Zang et al., 2013) and cyclic soft stimulation (Hofmann et al., 2018), which adopts fluid-injection protocols of displacement control and pressure control, respectively. A large number of experiments have proved that these methods are beneficial to solving the problems existing in the development of HDR (especially induced seismicity) caused by conventional HF stimulation (Hofmann et al., 2018; Zang et al., 2019; Zhuang et al., 2019).