bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of Geomechanics on Hydrothermal Fluid Pathways in Enhanced Geothermal Systems: A Review
Authors: Ngoma M. C. , Wamba Kenfack A. , Kolawole O. , Cavalcante L.
Published: September 2023

Geothermal resources in low-permeability and medium-to-high temperature reservoirs in sedimentary sequence require hydraulic stimulation. However, hydrothermal fluid migration for geothermal energy use in enhanced geothermal systems (EGS) is dependent on the in-situ flow pathway network created by induced hydraulic fractures. Also, there is still no comprehensive knowledge on the impact of geomechanical alterations on the hydrothermal fluid migration in EGS. This study presents a systematic review of the effects of changes in mechanical properties on the fracture behavior and fracture pathway created for efficient extraction of heat in EGS reservoir. This review will contribute to the state-of-knowledge on the impact of geomechanics on fractomechanical behavior in EGS.

INTRODUCTION

Among several sustainable energy alternatives, geothermal energy has been widely recognized due to its low carbon footprint (Homand-Etienne et al., 1989; Olasolo et al., 2016; Kolawole et al., 2021; Zhang et al., 2021; Asai et al., 2022). However, some high-temperature geothermal resources have low permeability which makes the extraction of heat difficult. Therefore, hydraulic fracturing is used to stimulate low-permeable geo-resources to induce fractures and extract fluid from their reservoirs (Kolawole and Ispas, 2020; Moska et al., 2021). In enhanced geothermal systems (EGS), hydraulic fracturing is adopted as a stimulation technique to induce fractures for improved heat extraction (Olasolo et al., 2016; Gong et al, 2020; Kolawole et al., 2021). In EGS, cold fluid injected through a well in a hot rock becomes a mobile hot fluid and flows through the artificially induced fractures which permits easy extraction of heat from a second interconnected well (production well). The continuous interaction between the cold injected fluids and the hot rocks in EGS can alter the mechanical properties of geothermal reservoirs (especially in EGS) which could affect the efficiency of heat exploitation. Hence, the mechanical properties of geothermal resources at certain conditions are important to better understand the potential implication of geothermal energy extraction over time.
Direct Carbon Dioxide Capture at Atmospheric Conditions via Adsorption Desorption Hysteresis Using Shape-Dependent Prozzolanic Material
Authors: Fakher Sherif , Hassanin Abdulla , Tamer Sara , Eltohamy Mohamed , Abdelaty Bassel , Alsakkaf Aseel , Eldakar Shams
Published: September 2023

One of the main solutions for carbon dioxide (CO2) emissions is carbon capture and storage. Carbon capture technologies, although abundant, are very costly and therefore their application is limited. This research introduces a novel technique for carbon capture that is extremely low in cost and applicable at atmospheric conditions with no pressurization requirements. This method is designed based on the utilization of waste material for sustainability and reduction in environmental impact. The selection of the material was the first step into determining the CO2 storage potential. The material selected was a fly ash produced as a byproduct of clinker manufacturing. The fly ash was then used in three different shapes including powder, cube, and hemi-spherical shape. Following this, the CO2 storage capacity was quantified based on the CO2 adsorption to the material. The shape was one of the most significant findings in this project as changing the shape and size of the fly ash resulted in a significant alteration of the adsorption capacity. The cycles of adsorption and desorption had no significant impact on the fly ash itself which indicated that it does not need to be replaced. This research introduces a small working model for a new design that can assist in carbon capture at ambient conditions.

INTRODUCTION

The carbon capture, utilization, and storage (CCUS) initiative highlighted to importance of reducing greenhouse gas emissions by capturing the CO2 from different sources, utilizing the captured CO2 in different industries, and the storing the excess CO2 in different forms (Fakher, S. et al., 2020; 2021; 2022). Although CCUS is a widely researched topic, there are still some major challenges associated with this topic especially carbon capture. Although many technologies have been introduced for carbon capture, many of which are in pilot stage, the majority of these methods are extremely costly (Fakher, S., 2020). This is the main limitation that has resulted in the low level of global implementation. It is therefore important to find advancements that can reduce the overall cost of carbon capture in order to globalize the CCUS initiative in the near future.
Fracture Geometries and Breakdown Pressures of Multi-Branch Radial Borehole Fracturing Influenced by Horizontal Stress Difference
Authors: Yong Yuning , Guo Zhaoquan , Tian Shouceng , Wang Tianyu , Sheng Mao , Liao Lulu
Published: September 2023

Multi-branch radial boreholes can be drilled and combined with hydraulic fracturing to efficiently stimulate tight gas reservoirs. This paper experimentally investigates how horizontal stress difference affects fracture geometry and breakdown pressure in multi-branch radial borehole fracturing by a true triaxial fracturing device. Six artificial rocks (395 mm × 395 mm × 395 mm) were cast for the experiments, which are divided into two groups by the azimuth of the radial borehole. The results show that fracture geometries are controlled by the rectification of a single radial borehole, the extrusion between the adjacent radial borehole row, and the deflection of the maximum horizontal stress. Fracture propagation is affected by the dynamic variation of the three effects. Decreasing the stress difference is favorable for fracture propagation along the radial borehole axis, while it lifts the breakdown pressure. Decreasing the azimuth lifts fracture extension distance parallel to the radial borehole axis and reduces the breakdown pressure. This research sheds light on the feasibility of radial borehole fracturing for reservoirs under various stress differences.

INTRODUCTION

Horizontal wells combining multi-stage hydraulic fracturing technology are commonly used to extract tight gas reservoirs. However, it still has the challenge that is the insufficient stimulation. Radial borehole fracturing is proposed recently, which combines the horizontal radial borehole with hydraulic fracturing (Huang et al., 2020a; Huang et al., 2020b; Kamel, 2016; Landers, 1998). This technology has the following procedure. Firstly, several radial boreholes, with diameters of no more than 50 mm, are drilled perpendicular to the axis of the vertical well. The fracturing fluid is then injected into the radial boreholes via the main well. After fracturing, the complex fracture network is generated in the reservoir finally. It has been applied in many fields with good effect (Bruni et al., 2007; Cinelli and Kamel, 2013; Cirigliano and Blacutt, 2007; Li et al., 2000; Maut et al., 2017; Ragab and Kamel, 2013). The multi-branch radial boreholes are favorable to communicating several sweet spots in the tight gas reservoir, which is also more economical than horizontal wells. So, radial borehole fracturing is anticipated to efficiently develop tight gas reservoirs or other unconventional reservoirs.
Elasto-Plastic Damage and Dilation Analysis of Fibre-Reinforced Thin Spray-On Liner Coated Concrete and Coal Specimens Under Compressive Loading
Authors: Dondapati Gopi Krishna , Deb Debasis , Porter Ian , Karekal Shivakumar
Published: September 2023

Skin support provides resistance to the shearing and spalling of exposed rock layers/blocks, as well as protecting the surface from open atmospheric contact. Thin spray-on liner (TSL) is one such skin support that can be applied to the roof or sidewalls of an excavation to enhance load-deformability behaviour. The failure behaviour of concrete and coal blocks when coated with a thin layer of polymeric liner was studied experimentally and numerically. The experimental results show a distinct "slight strain-softening zone" in the stress-strain curves of cubical concrete and coal samples when coated with a 5 mm thick polymeric liner. A micro-plane based coupled-damage-plasticity material model with the Drucker-Prager yield criterion is also implemented for determining the damage and dilation behaviour in the concrete and coal samples with and without liner. The results show that liner application reduces the magnitude of damage by nearly 23% in both concrete and coal blocks. On the other hand, the dilation angle increases rapidly to its peak value with an increase in plastic shear strain (γp).

INTRODUCTION

The design of structures in underground rock mass poses a unique problem to engineers from two fronts: i) before excavation, the rock is subjected to in-situ stresses, and ii) after excavation, protecting the skin of the exposed rock mass. Once the excavation is complete, the final stress state is a combination of in-situ and induced stresses (Brady & Brown, 2004). Loss of ground confinement results in softening of the skin of the excavation. It allows excessive displacements in radial and tangential directions, and causes considerable dilation. The situation worsens if the rock mass contains pre-existing geological discontinuities, material diversity, and water saturation. Lack of skin support may be one of the main factors for the key-block instability problem at the initial excavation stages. According to the key-block theory, if all potentially unstable wedges are supported, the remainder of the rock mass will remain stable due to its inability to move (Goodman, 1995). Potvin et al. (2001) suggest that liners can improve the control of small pieces of freshly exposed rocks near an active face. Skin support prevents the shearing and spalling of exposed rock layers/blocks and protects the surface from open atmospheric contact. According to Tannant (2001), small rock displacements at interactions between rock blocks are sufficient for the liner to start functioning.
Developing a Velocity Model for an Underground Coal Mine Using a Compressed Load Column Seismic Source
Authors: Boltz S. , Chambers D. , Sbai S. , Janson P.
Published: September 2023

Induced seismicity is common in underground mining, particularly in mines that cave by design, such as longwall coal mining. Seismic monitoring is one of the few remote sensing technologies that provide an understanding of a mine's response to extraction. However, its effectiveness usually depends on understanding event locations in relation to the mine workings. A significant source of uncertainty in event locations is the velocity model: a description of the speed at which seismic energy propagates. A nondestructive, repeatable compressed load column seismic source (CLCSS) was developed for velocity model calibration in underground coal mines. This paper describes the CLCSS and its application at a longwall coal mine. Signals from the CLCSS were detected up to 950 m epicentrally (1,100 m hypocentrally) from the source. Using the ground-truthed signals from the seismic source, we estimate a velocity model for locating events from a surface geophone array. Model performance is evaluated by relocating mining-induced events with well-constrained locations. A three-layer model with P-wave velocities ranging from 3.6 km/s to 4.4 km/s and a VP/VS ratio of 2.1 performed best with location errors of approximately 100 m, which is sufficient for many applications of seismic monitoring in coal mines.

INTRODUCTION

Coal bursts—violent dynamic failures which cause damage to mine openings—were first documented in European coal mines well before World War I (Guan et al., 2009) and continue to threaten underground coal miners around the world. For example, approximately 280 significant bursts occurred in U.S. coal mines between 1983 and 2017, seven of which resulted in fatalities (Mark, 2018). In China, over 200 coal mines have reported bursts cumulatively resulting in over 1,000 injuries and 100 fatalities in the past 10 years (Rong et al., 2022). Poland's Upper Silesian Coal Basin has experienced over 100 significant events, some of which resulted in injuries and fatalities (Mutke et al., 2015). Several other countries have also reported coal mine bursts, including Japan, Australia, India, France, South Africa, Czechoslovakia, Canada, Germany, and Russia (Lama and Bodziony, 1998). Although bursting mechanisms are not well understood and vary considerably, studies have identified risk factors including depth of cover or rapid changes in topographic relief, thick brittle strata near the coal seam, inadequately designed pillars, multi-seam interactions, and a variety of other mining and geological factors (Mark and Guana, 2016).
Drilling Stuck Probability Intelligent Prediction Based on LSTM Considering Local Interpretability
Authors: Wang Jianhua , Guan Zhen , Liu Muchen , Song Xianzhi , Wang Zheng
Published: September 2023

Stuck pipe in the drilling can increase the non-productive time and lead to drilling risks such as buried tool and drilling tool fracture. Due to complex sequential feature analysis and the lag of downhole data transmission, traditional methods which are mostly used for post-drilling analyses have poor timeliness and low accuracy so that it cannot meet the requirements of field application. This study provides a data-driven prediction method for stuck pipe probability. First, the dataset is established combining drilling records and logging data from 108 wells, and 15 input features are selected. Then, the intelligent prediction model based on the Long Short Term Memory (LSTM) network is developed considering the timing characteristics of drilling parameters. Finally, Shapley Additive Explanations (SHAP) is used to calculate the quantitative contribution of input parameters to solve the problem of the black box. The results show that the accuracy of the LSTM model is 97%. Through local interpretation and analysis, the model accords with the natural physical law. The proposed model has great timeliness, high accuracy, high reliability and quantitative risk prediction. This study provides useful insights into predicting stuck pipe risk.

INTRODUCTION

Drilling is the only way to get oil and gas which is an uncertain undertaking. Due to poor underground conditions, complex conditions (lost circulation, well collapse, stuck pipe, etc.) often occur, among which complicated conditions of stuck pipe often occur in the drilling construction process. According to Chunbo Zhao (1998), stuck drilling accidents account for 40%∼50% of the total drilling complications in western oilfields, which seriously restricts safe, economical, and efficient drilling. Therefore, the timely prediction of stuck pipe events is considered a primary necessity to assist the drilling team in the decision-making process, so that appropriate countermeasures can be put into effect before the situation slips out of hand.

Mudlog data shows that stuck pipe accidents occur in the process of tripping and drilling with varying degrees of severity. For example, in some highly deviated wells and horizontal wells, the stuck pipe accident is often accompanied by the process of tripping, but it is usually not serious. On the contrary, more attention is paid to the stuck pipe during drilling. Once the drill string is completely stuck, it can only be lifted by the explosion and a new side drilling scheme need to be redesigned, which seriously restricts the safety and efficiency of drilling. Therefore, we focus on the real-time prediction of stuck pipe in the process of drilling.
Estimating Subsurface Horizontal Stress Magnitudes in a Three-Dimensional Geocellular Model: Application to Permian Basin Unconventional Resources
Authors: Johns M. K. , Crawford B. R. , Amalokwu K. I. , Finn C. F.
Published: September 2023

In situ stress prediction is critical for applications associated with unconventional resource development including drilling, hydraulic fracturing, enhanced oil recovery and induced seismicity. A range of model-driven approaches have been developed to predict variation of subsurface stresses with rock type and depth. For example, if we assume poroelastic coupling and anisotropic symmetry, quantifying the horizontal stress magnitudes with depth requires log-derived density, acoustic velocities, mineral composition, and pore-pressure properties as input, plus additional "tectonic strain" calibration to subsurface measurements. Estimating stresses in a 3D mechanical earth model (MEM) can be difficult away from well control. We present a novel workflow for simplifying theoretical stress calculations, then calibrating a seismic-driven MEM to log-based SHMIN profile(s). The workflow employs these calibration constants to propagate stress magnitudes throughout the 3D MEM based on 3D properties, which include SOVB and PRES, as well as seismic-inversion derived elastic properties and rock facies. Finally, the 3D MEM yields SHMIN profiles, maps, and cross-sections that are used for assessing stress across a development region, with examples shown from the Permian Basin of the USA.

INTRODUCTION

Knowledge of the in-situ state of stress is fundamental to all subsurface geomechanics/rock engineering endeavors. The prediction of layer-to-layer variation of horizontal principal stress magnitudes is vital for a wide range of energy-related applications including the following: drilling (wellbore stability); completions (hydraulic fracturing); mechanical seal capacity/caprock integrity (underground gas storage, carbon capture, utilization, and storage), in-fill drilling and field development (parent-child well interactions, depletion planning) and abandonment (annular barriers).

The magnitude and orientations of principal stresses can be directly measured. The magnitude of the minimum principal horizontal stress "SHMIN" is measured in the subsurface through identification of the closure pressure after shut-in of a "DFIT" diagnostic fracture injection test, which involves the pumping of a relatively small volume of water into a short wellbore interval over a brief period, also referred to as a minifrac test. The magnitude of the maximum principal horizontal stress "SHMAX" can be inferred from image log analysis of compressional (breakouts) and tensile (drilling induced tensile fractures) failures of the borehole wall (Zoback et al., 2003). Yet neither of these measurements is conducted at sufficiently high frequency or fine scale to enable adequate characterization of horizontal stress magnitude variation with depth, nor with lateral frequency, as required for practical engineering applications across a region.
Numerical Analysis on Characteristics of Cracks and ROP Enhancement Performance in Hard Rocks Subjected to Axial-Torsional Coupled Percussion
Authors: Sun Zhaowei , Wu Xiaoguang , Yang Rui , Huang Zhongwei , Shi Huaizhong , Mu Zongjie , Yu Huiyong
Published: September 2023

Axial-torsional coupled percussive drilling (ATCPD) is a practical technique for creating volumetric breaking patterns and increasing rate of penetration (ROP) of hard rocks. Our paper focus on numerical investigations on ATCPD process. We employed a continuous-discrete coupling method based on cohesive elements to characterize the rock failure subjected to different percussion (axial, torsional and coupled percussion). Based on this model, the penetration-cutting process of the single cutter at different percussion modes was simulated, respectively. The corresponding cracks characteristics, penetration/cutting force and torque were analyzed. Rock scratch experiments were further conducted on granites to validate the model. The results show that the complex cracks propagation zone and the larger tensile cracks around the cutter are developed subjected to coupled percussion, resulting in larger failure elements number and cracks volume. That is, more significant macro damage is thought to form in coupled percussion, thereby increasing the penetration and cutting force of single cutter. It is worth noting that the coupled percussion mode can also generate stable fluctuation torque, which improves the ROP enhancement performance. Our results are expected to provide some guidance for efficient drilling in hard rocks.

INTRODUCTION

Increasing oil, gas, and heat reserves in deep formations will be critical in addressing the worldwide energy supply (Santos et al., 2000; Moore and Simmons, 2013; Hu et al., 2013). The construction of wells to exploit deeper energy in more challenging environments requires large investments, which are mainly related to the involved drilling operations (Abdo and Haneef, 2013; Diaz et al., 2017). For example, the reliance on drilling costs is especially high in the development of enhanced geothermal system (EGS), primarily because this method of obtaining geothermal energy necessitates the challenge of drilling deep holes in hard crystalline basement rocks, such as granites (Song et al., 2018; Anderson and Rezaie, 2019). These limiting factors pose a major challenge to our traditional drilling technologies (e.g. rotary drilling), whose poor performance in hard rocks, in terms of wearing of the drill bits and low penetration rates (Diaz et al., 2017; Rossi et al., 2020). As a result, the development of drilling techniques to increase the ROP enhancement effect in deep reservoir hard rocks has attracted more and more interest in the engineering field.
Size Effects on Crystalline Rocks Under Uniaxial Compression and Brazilian Tests
Authors: Sun Y. , Kwok C. Y. , Duan K.
Published: September 2023

The tensile strength of crystalline rocks in Brazilian tests (BT) shows a more significant size dependency than the uniaxial compression strength in uniaxial compression tests (UCT), while the micro-mechanism is still unclear. This study seeks to elucidate the micro-mechanism by comparing different microcracking processes under UCT and BT. We adopt a grain-based model in the discrete element method (DEM) to reproduce the interlocked microstructure of crystalline rocks and perform UCT and BT with different sizes ranging from 0.5 to 3 times the standard sample size. It was found that peak stresses in UCT are nearly insensitive to sizes, which is consistent with experimental results measured at very low loading rates. Due to the homogenous stress distribution, no clear cracking paths are observed; instead, randomly distributed microcracks appear at failure. In contrast, numerical results indicate that the tensile strength measured in BT decreases as size increases and continuous cracking paths are observed at failure. Larger specimens have a higher possibility to include weaker cracking paths, hence tensile strength shows stronger size dependency. The findings of this work offer microscopic explanations for different size effects observed in UCT and BT, which can bridge the gap between laboratory-scale strength data and field-scale applications.

INTRODUCTION

Rock engineering applications often require assessing the strength and failure characteristics of rock masses. These rock masses are composed of intact blocks across various scales. However, the strength and cracking mechanism of these field-scale intact blocks are usually indirectly measured and studied through laboratory-scale tests on intact specimens (Duan et al., 2017; Duan and Kwok, 2015; Fei et al., 2021; Mahabadi et al., 2014; Martin and Chandler, 1994; Wong et al., 1996).

Uniaxial compression test (UCT) and Brazilian tests (BT) are standard tests that measure the uniaxial compression strength (σf) and tensile strength (σt) of rocks (Eberhardt et al., 1999; Li and Wong, 2013; Martin, 1994). The sizes of UCT and BT specimens in the laboratory usually vary from 5-10 cm in diameter, much smaller than in fields. However, a dependence of strength on size is widely known in rocks (Bažant, 1984; Choo et al., 2023; Paterson & Wong, 2005). The strength determined by these laboratory-scale tests may overestimate the loading capacity in fields, and the cracking process observed in the laboratory-scale specimens may also not accurately manifest the failure mechanism in the field scale. Therefore, to apply laboratory-scale experimental data to practical applications, it is vital to investigate the influence of specimen size on strength and cracking patterns.
Prediction of In-Situ Stresses by Using Machine Learning and Intelligent Optimization Algorithms
Authors: Zhang Haoze , Wu Bisheng , Nie Yuanxun , Zhang Xi , Chen Zhaowei
Published: September 2023

In-situ stresses play an important role in affecting many geological processes such as hydraulic fracturing and CO2 storage, and a good understanding of the magnitude and direction of in-situ stresses is very important for deep energy exploitation. In this study, a machine learning model consisting of generative adversarial networks (GAN), particle swarm optimization (PSO) and support vector regression machine (SVRM) is proposed to predict the minimum in-situ horizontal principal stress (Shmin) from a series of existing experimental breakout data. First, the GAN and PSO are used to improve the quantity and quality of training data and to optimize the hyperparameters of the SVRM. Second, the enhanced training data are used to train the SVRM that predicts the Shmin based on the wellbore breakout geometries. In order to examine the reliability of this technique, the Shmin predicted from the proposed model is compared against the experimental data and it is found that the proposed model has a high accuracy with an average relative error of less than 10%. In addition, the proposed model requires only a few seconds to run on a laptop computer, thus providing a useful tool for accurate and efficient prediction of the Shmin.

INTRODUCTION

In-situ stresses play an important role in affecting many geological processes such as hydraulic fracturing and CO2 storage, and a good understanding of the magnitude and direction of in-situ stresses is very important for deep energy exploitation. During exploiting unconventional petroleum resources, drilling of a well and its stability depend on accurate measurement of in-situ stresses. Once the stress concentration at the borehole wall exceeds the rock strength, borehole failure occurs. In addition, in-situ stresses are closely related to earthquake faulting, volcanic eruptions and other geotechnical or geological processes. As a result, accurate measurement of in-situ stresses is of great significance.
Fracture Roughness Characterization from 360 Unrolled Core Images in a Sandstone Reservoir, Case Study, Algeria, Hassi Messaoud
Authors: Ifrene Ghoulem E. H. , Irofti Doina , Khouissat Abdelhakim , Pothana Prasad , Aihar Aimene , Li Bo
Published: September 2023

Various existing indirect measurement methods and techniques in the petroleum industry are used to estimate some of the fracture parameters, such as sonic and electrical image logs. However, none of them can estimate the fracture roughness and the degree of mismatch between the two opposite fracture sides due to the sampling resolution. These two are important parameters to consider when dealing with geomechanics and fluid flow in tight naturally fractured reservoirs because they are important in quantifying shear strength and permeability. A quantification and detailed characterization of the joint roughness coefficient (JRC) and the degree of mismatch has been done on fractures gathered in a total of 23 wells in the Algerian Cambro -Ordovician oil-bearing formation Hassi Messaoud which consists of sandstone, clay, and some volcanic intrusions. These data have been collected from the digitalization of high-resolution unrolled 360-core photographs at the reservoir level. The results have been correlated with rock type, grain size, and fracture dip angle. Fracture roughness is ranging between 5 (low) to 19 (high) on the JRC scale and can be predicted from analysis of the general reservoir rock types (sandstone, eruptive, and clay). The results are applicable to other, especially siliciclastic reservoir lithologies, and will therefore contribute to defining a more accurate aperture and permeability estimation in building reliable Discrete Fracture Network (DFN) models.

INTRODUCTION

Fracture roughness is a key parameter in characterizing the permeability and fluid flow in petroleum (Camac et al., 2006) and geothermal systems (Porlles & Jabbari, 2022). As the majority of currently used large-scale models still significantly rely on simplified smooth parallel-plate models and its related models for naturally fractured reservoirs with rough walls, it is crucial to quantify the roughness in order to enhance the performance of these models (Bodin et al., 2007).
Experimental Evaluation of Flow Heterogeneity in Enhanced Geothermal Systems (EGS)
Authors: Villaquiran Vargas A. P. , Knippel E. P. , Xiong Q. , Hampton J. C.
Published: September 2023

Enhanced Geothermal Systems (EGS) are unconventional reservoirs with significant stored thermal energy and low hydraulic connectivity e.g., Hot Dry Rocks. To take advantage of these resources as a sustainable option, it is necessary to create and/or enhance existing fractures to achieve economic production rates. Laboratory-scale tests can assess critical knowledge gaps by conducting research under controlled conditions, allowing to understand mechanisms and then extrapolate them at the field scale, especially those contributing to, and controlling, flow heterogeneity. To that end, we use novel equipment that allows drilling, casing, stimulation, and multi-physics monitoring all under in-situ stress and temperature conditions, with multiple oriented wellbores and complex fracture networks connecting them. The laboratory EGS environments are assessed through acoustic emissions, foil strain gauges, fiber optic sensing, and tracer injection/recovery, measuring changes in stress, pressure, flow, and temperature, where fast-flowing channels that may lead to early thermal breakthrough affecting the heat recovery process can be identified and characterized. The experimental approach to mitigate the effects of flow channeling, is to design and evaluate different circulation schemes that minimize short-circuiting flow paths by adjusting specific operational parameters. During this first experimental stage, only surface control will be used since other strategies such as mechanical and chemical conformance control may not be suitable for EGS conditions. This paper provides a snapshot of a work-in-progress of the experimental results showing a single wellbore stimulation and hydraulic characterization in an unconfined Dakota granite.

INTRODUCTION

Geothermal Energy is considered a renewable energy resource since the rate of consumption is smaller than the renewing process rate - i.e., Just 1% of the total heat available in the Earth's crust is necessary to meet global energy needs; therefore, geothermal resources can provide energy to the planet for 2800 years at a constant consumption rate. Considering that this resource is continuously being renewed by various forms of heat received, it is a sustainable energy (Olasolo et al., 2016).
Benchtop Experimental Studies of Stick-Slip Mitigation Methods
Authors: Hilborn H. , Jorgenson H. , Su J.-C. , Mazumdar A.
Published: September 2023

Drilling vibrations can cause inefficient drilling and accelerated damage to system components. Therefore, reducing or eliminating such vibrations is a major focus area for natural gas and geothermal drilling applications. One particularly important vibration mode is stick-slip. Stick-slip occurs when the bottom-hole angular velocity starts oscillating while the top hole angular velocity remains relatively constant. This not only causes poor drilling, it is also difficult to detect using surface sensors. In this work, we describe the development and testing of a benchtop drilling system for studying stick-slip dynamics and mitigation. We show how this system can produce stick-slip oscillations. Next, we use this data to formulate a data-driven rock-bit interaction model. This model can be combined with linear systems analysis to predict stick-slip and understand mitigation methods. We describe out instrumentation that enables closed-loop control under simulated communications constraints. We conclude by providing preliminary experimental data on bench-level stick-slip.

INTRODUCTION

Exploration via autonomous drilling processes for geothermal resources is an important focus area for drilling research. However, to fully realize the clean-energy promise of geothermal energy, key challenges still need to be resolved.

Issues arising in the drilling process often originate from a drillstring's increased susceptibility to vibrational oscillations as depths increase. Some examples of drilling vibrations include stick-slip (Navarro-Lopez and Suarez, 2004), bit-bounce (Spanos et al., 1995), and whirl (Jansen, 1991). Torsional oscillations are the focus of this work.

Torsional vibrations result in a destructive phenomenon known as stick-slip. Initiated at the bit-rock surface, the drillstring bit experiences large angular velocity oscillations not seen at the surface (Pavone and Desplans, 1994; Besselink et al., 2011; Kessai et al., 2020). Stick-slip results in premature bit wear and drillstring fracture.

Stick-slip is a fundamentally nonlinear and unpredictable phenomena. Stick-slip results from the combination of bit-rock interactions and drillstring compliance. As a result, there is a key need for experimental studies of stick-slip dynamics and mitigation.
Numerical Simulation of Hydraulic Fracturing of Hot Dry Rock Based on Coupled THM Model
Authors: Xie Zixiao , Huang Zhongwei , Wu Xiaoguang , Long Tengda , Yang Rui , Wu Sitong , Huang Pengpeng
Published: September 2023

One of the challenges of developing an enhanced geothermal system (EGS) is to create connected fracture networks by hydraulic fracturing between the injection well and the production well. However, limited work has been conducted on studying the fracture initiation and propagation of hot dry rock. To have a clear understanding of how the temperature differences between the rock and the fluid influence hydraulic fracturing, numerical simulations were conducted under different temperature conditions (100°C-300°C) based on the FDEM approach. The results indicate that the increase in the temperature difference between the rock and the injected fluid could induce the larger fracture tortuosity and fracture apertures effectively. The breakdown pressure decreased by 52% as the initial rock temperature rise from 100°C to 300°C. The present study is expected to provide an in-depth understanding of the reservoir stimulation in EGS.

INTRODUCTION

Geothermal energy is widely distributed across the world and is rapidly gaining interest globally as it shows great potential to decrease carbon consumption. Hot Dry Rock (HDR) is a kind of geothermal resource where no water or only a small amount of fluid exists inside the rock and the average temperature of HDR is typically above 180°C(Li et al., 2022). Enhanced Geothermal System (EGS) is currently considered the most efficient way to develop HDR reservoirs, which applies the hydraulic fracturing technique to create the fluid flow path, enabling the working fluid to pass through the fracture network transferring heat from underground to the surface(Xie et al., 2022; Mcclure and Horne., 2014).

The main difference in hydraulic fracturing of the hot dry rock and other types of reservoirs is that strong thermal stress would be induced by cold water contacting with the hot rock mass, which plays a crucial role in the initiation and propagation of the hydraulic fracture(Li and Tang., 2022; Li et al., 2022). It has been investigated that the mechanical properties of the granite derogated substantially after the specimens are subject to cyclic heating and cooling(Wu et al., 2019). The large temperature difference between hot rock and cold fluid leading to a larger thermal stress contributes to damaging the granite. Therefore, it is necessary to study how thermal stress influences the hydraulic fracturing of hot dry rock.
Investigation of Long-Term Casing Strength Degradation Prediction Method Under "Thermal Stress – Corrosion" Combined Effect in Cyclic Steam Injection Wells
Authors: Zhang Binqi , Deng Jingen , Li Fuli , Zhang Jingru , Yan Wei , Zhang Binqi , Lin Hai
Published: September 2023

Steam huff and puff is the main production mode of heavy oil, but the casing damage problems during the cyclic thermal injection process are very serious. The casing damage of thermal recovery wells is an important factor affecting oilfield production and economic benefits. The conventional casing strength calculation method does not consider the effect of corrosion and thermal degradation in terms of long-term casing strength designing. The establishment of a long-term casing strength prediction method to check and optimize casing strength is an important guarantee for the safe production of heavy oil thermal recovery wells. In this paper, the strength degradation characteristic of two kinds of casing steels (N80 and TP100H) are tested under the cyclic thermal effect using a High Temperature Tensile Testing Machine. The heat temperature is from 30˚C to 350˚C, and heating cycles are 1, 3, 5 respectively. The results show that the N80 casing strength decreases by 18.2% in the first thermal cycle, and the average damage of each subsequent thermal cycles are about 0.88%. However, the TP100H decreases by 13.7% in the first cycle and subsequently with 0.31% degradation averagely. The long-term corrosion rates of N80 and TP100H were measured in HTHP autoclave, which are 0.43m/a and 0.3m/a respectively. Combining the obtained experimental results with the standard casing strength calculation formulas, a newly prediction method of casing long-term strength for cyclic thermal injection well is presented. This method has been successfully used in Lvda 21-2 block of Bohai oil field CNOOC, which provides a technical basis for casing strength design in cyclic thermal injection wells.

INTRODUCTION

Casing damage in oil fields has now become a pressing problem worldwide, and many fields have encountered casing damage during development. The factors affecting casing damage are extremely complex. Steam injection is the most widely used and effective method in the exploitation of heavy oil reservoir. However, in the process of steam huff and puff, the high temperature steam injected in circulation makes the casing undergo great alternating stress, and the residual stress increases constantly, reducing the casing strength. In addition, after steam injection, thermal cracking of heavy oil produces acidic gas and the casing is in a corrosive environment. This leads to the casing strength decay and reduces the casing service life. Therefore, casing damage is a difficult problem in the development of heavy oil reservoir.
Study on the Variation Law of Rock Mechanical Properties with Depth in Deep/Ultra-Deep Layers-Taking Shunbei Oilfield as the Rresearch Object
Authors: Mingming Geng , Hui Zhang , Zhuoxin Dong , Yufei Chen , Kerou Liu
Published: September 2023

At present, the development of petroleum industry is gradually developing from shallow oil layer to deep/ultra-deep layer, but with the increase of well depth, the rock strength is also getting higher and higher, which hinders the drilling work. In this paper, based on the well logging data, the variation of rock strength characteristics with well depth is analyzed, and the uniaxial compression model is established using the global cohesive element. The reason why the rock is difficult to be destroyed in the deep layer is analyzed, and the relationship between the numerical simulation results and the confining pressure and the well depth is analyzed. The results show that the confining pressure can greatly increase the mechanical strength characteristics of rock, and the rock strength increases faster and faster with the increase of well depth.

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 necessary to further study the influence of bottom hole confining pressure on deep/ultra-deep layers.

Now, some studies have proved that confining pressure has an effect on rock mechanical properties. When the strain rate remains stable, the confining pressure can improve the compressive strength of rock. (Christensen et al, 1972;Hokka et al, 2016). Under the condition of dynamic loading, the strength characteristics of rock are still affected by the confining pressure. (Wang SM, et al., 2018).

In order to study the influence of confining pressure on rock mechanical properties, many scholars have carried out sufficient theoretical research work from the microscopic field. From their research contents, it can be seen that confining pressure can reduce the initiation of cracks and hinder the expansion of cracks, and increase the energy required for crack expansion to cause block fragmentation. (Zhou et al., 2016; Zhou & Yang, 2007; Zhou et al., 2004).
3D Numerical Manifold Method for Modeling Heat Conduction and Thermo-Mechanical in Quasi-Brittle Materials
Authors: Tong Defu , Yi Xiongwei , Tan Fei , Han Zhenyu
Published: September 2023

Thermal fractures are common in natural rock masses, and numerous works are concentrated on this topic at present. Comprehending the failure mechanism of thermo-mechanical problems is crucial for enhancing the effectiveness and safety of engineering applications. Despite considerable endeavors to experimental results, precisely simulation 3D thermo-mechanical coupling remains a formidable challenge. This paper proposes a novel thermo-mechanical coupling model (3DNMM-TM) that integrates a heat conduction algorithm with the numerical manifold method (NMM) to effectively simulate problems related to heat conduction and thermo-mechanical coupling. The discrete equations for the system equation are derived using a variational method, and the penalty method is utilized to solve the crucial boundary conditions. The accuracy of the proposed 3DNMM-TM model is verified through numerical tests which compares with analytical solution. The results show that the proposed heat conduction model can effectively capture the effect of cracks on heat conduction as well as the sudden temperature change across the cracks. The model can be used as a robust tool to study the whole process and basic mechanism of thermodynamic behavior in quasi-brittle materials.

INTRODUCTION

The field of engineering frequently utilizes quasi-brittle materials, including but not limited to concrete, ceramics, and rock. The thermal conductivity within these solids is a crucial factor to consider (Ezekoye, 2016). Any temperature variation could potentially modify the thermal and mechanical characteristics of quasi-brittle materials (Gautam et al., 2018; Tan et al., 2022), leading to material degradation and engineering mishaps. For example, when a fire breaks out in a tunnel, its temperature can swiftly escalate to 700 °C or beyond within a brief period (Shen et al., 2023). A significant temperature gradient within concrete can result in thermal cracking (Committee, 2007). During CO2 geological sequestration, the injection of CO2 into the brine stratum at a depth ranging from 1000 to 2500 m may give rise to shrinkage or even fracturing of the adjacent saline rocks owing to the frigidity of the CO2 (Taylor & Bryant, 2014). In the subterranean repository of nuclear waste, the thermal energy generated by nuclear decay may elevate the temperature of the surrounding rock, leading to thermal cracking, and incidents involving nuclide leakage may transpire (Kim et al., 2011). Consequently, it is imperative to investigate the interdependent thermal-mechanical impact.
Influence of Local Grain Size Distribution on Damage Development in X-Ray CT Images of Boise Sandstone Deformed by Confined Compression
Authors: Louis L. , Boyd P. , Hofmann R. , Saxena N.
Published: September 2023

MicroCT imaging is performed before and after confined compression testing on a minicore of Boise sandstone in order to extract quantitative information regarding the microstructural controls on compaction initiation and distribution, with the ultimate goal of formulating compressibility predictions at in situ conditions. In contrast with traditional compaction studies, our analysis focuses on local statistics among regions of a given sample as opposed to global statistics in a sample set. The original workflow includes a segmentation strategy and the definition of a series of metrics that are evaluated throughout the entire image volumes before and after testing. The results suggest that local porosity and sorting are the main controls on local strength, whereas neighborhoods with larger mean grain size do not appear to be more prone to yielding. This is consistent with previous findings on confined comminution which emphasize the tendency of grain size distributions to become self-similar through the elimination of same size neighbors at all scales.

INTRODUCTION

The limited ability to predict the onset and development of compaction at the pore scale in high porosity reservoir rocks undergoing depletion represents a substantial barrier to the optimization of appraisal and production planning. Compaction may be potentially beneficial as a pressure support mechanism but can also be a hindrance to overall flow properties through reservoir compartmentalization and fines production/migration. This topic has been the focus of many studies both within industry and academia (Fatt, 1958; Zhang et al., 1990a; Schutjens et al., 1998; Wong and Baud, 1999; Olsson et al., 2002). Beyond conventional reservoir operations, better understanding of the mechanical behavior of confined granular aggregates is of interest in many other subsurface applications from 4D seismic interpretation to storage and subsidence, but also across industries in environmental and civil engineering domains.

As a powerful extension of traditional microscopy work, non-destructive X-ray CT imaging has been enabling investigations that were not previously possible. An example is the in-situ monitoring of 3D microstructural changes occurring in materials under active loading, with the continuous introduction of ever more sophisticated setups (e.g. Renard et al., 2016; Voltolini et al., 2019; Shahin et al., 2022; Francois et al., 2022).