Spallation Drilling Research Articles

Thermal spalling failure mechanism of heterogeneous granite under moving heat source

The conventional mechanical methods employed for rock breaking encounter substantial challenges in deep, hard formation drilling, characterized by low penetration rates and exorbitant drilling costs. Consequently, there exists an urgent necessity to explore novel drilling methods and technologies. Thermal spallation drilling, as a non-contact rock breaking technique, offers high efficiency in rock fragmentation and substantially reduces drilling costs. However, the mechanism underlying rock fragmentation induced by thermal spallation remains unclear. Therefore, this study establishes a thermo-mechanical coupling model based on heterogeneous granite subjected to a high-temperature moving heat source. The model comprehensively considers the geometric shape, contact, and strength heterogeneity of rock minerals, and analyzes the effects of heating source speed, radius, mineral composition, heterogeneity index, and grain size on the thermal spallation mechanism of rock. The research findings indicate that lower heating source speeds and larger radii result in higher surface temperatures and greater damage volumes, whereas increasing speed and reducing radius enhance fragmentation efficiency, with speed exerting a more significant influence. Thermal spallation primarily results in tensile damage to rocks, with the surface damage and depth of damage being respectively influenced by the heating source speed and radius. Particle size exhibits minimal impact on thermal spallation, while increased quartz content enhances the rock's thermal properties, resulting in significant thermal spallation effects. Additionally, the location of rock damage is closely correlated with thermal parameters. These findings significantly contribute to a better understanding of the thermal spallation fracture mechanism in granite and facilitate the development of technical solutions for thermal spallation drilling methods.

Thermal spallation of dry rocks induced by flame parallel or normal to layering: Effect of anisotropy

Thermal spallation drilling is a prospective technique in many strategic projects involving energy. To investigate the effect of rock anisotropy on thermal spallation and provide reference for thermal rock-drilling methods, we conducted flame jet experiments on dry shale samples and finite element modelling. Thermal spallation is produced by the growth of pre-existed cracks under compressive thermal stress. These cracks separate thin layers from the bulk of the rock; the layers buckle under compressive stress producing the spalls. Simultaneously, tensile fractures are formed that can inhibit the thermal spallation process. Rock anisotropy can change the thermal spallation zone making it larger (over seven times) or smaller compared to the isotropic case, or even exclude spallation. Comparison with experimentally observed spallation zone in shale samples with different bedding orientations suggests that bedding planes do not induce anisotropy sufficient to affect thermal spallation. Strong anisotropy of rocks can generate thermal tensile fractures directly from the heating surface, the situation impossible in isotropic rocks. During progressive thermal spallation rock anisotropy can make the spallation zone larger or remain unchanged. The results can help understanding thermal spallation of rocks and are instrumental in designing thermal spallation-based drilling technique for strongly anisotropic rocks.

Теоретическая и практическая реализация перспективной технологии термического бурения горных пород

The article presents the results of theoretical and experimental studies of the conversional use of a rocket engine gas generator to create a device for thermal spallation drilling of solid rocks. The main difference between the studied device and traditional thermal drills is the relatively low temperature of the working fluid. It has been experimentally shown that this level of temperature of the working fluid provides a stable process of thermal spallation of the rock and at the same time a long life of the structure. The developed mathematical apparatus allows to select device parameters for various types of rocks. The concept of a jet drilling rig for drilling blast holes in open pit mines is proposed. This unit is mobile, does not require a heavy basement and preparation of the drilling site; it is charcterized with a high utilization factor and a long service life, and uses widely available components (air, water, kerosene, diesel fuel) as fuel components.

RETRACTED ARTICLE: Numerical simulation of temperature field and temperature stress of thermal jet for water measurement

ABSTRACT Thermal-jet for water Measurement spallation is a rock removal process that utilizes induced thermal stress to fracture the rock into small fragments before melting of the rock occurs. Because of the low conductivity, thermal stress will be created on the rock surface. When the thermal stress exceeds the strength of the rock, cracks will form and expand. Eventually, rocks disintegrating into small fragments. The simulation result obtained by the Crank–Nicolson method of the temperature field and the distribution of thermal stress has been analyzed. The result indicates that during thermal spallation drilling, the temperature of the surface exposed to the thermal-jet for water Measurement rises rapidly and create temperature gradients in both radial and axial directions. Because of volume expansion, the heated part is subjected to compressive stress in radial direction and shear stress in axial direction, respectively. The result is very helpful to the field application.

Numerical study on the effect of confinement on thermal spallation drilling of hard rock

This paper presents a numerical study on the effect of confining pressure (down-to-hole pressure) on the performance of the thermal jet drilling on rock. For this end, a numerical method including a viscoplastic-damage model for rock and an explicit scheme for solving the governing thermo-mechanical problem, which is assumed uncoupled. Moreover, rock heterogeneity, which facilitates the thermal spallation phenomenon, is taken into account. Numerical simulations under axisymmetric conditions demonstrate that the efficiency of thermal spallation based drilling is reduces drastically due to confinement.

Review on an Advanced Combustion Technology: Supercritical Hydrothermal Combustion

Supercritical hydrothermal combustion, a new and promising homogeneous combustion technology with a wide range of application scenarios and broad development prospects, provides creative ideas and means for the enhanced degradation of organic wastes, hydrothermal spallation drilling, thermal recovery of heavy oil, etc. This technology is elaborated upon in five parts: (1) introducing the main devices including semi-batch reactor and continuous reactor to study the hydrothermal flame in accordance with research institutions, (2) presenting the research status of related numerical simulation from the angles of reaction kinetics and flow-reaction, (3) summarizing the characteristics of hydrothermal flame and combustion by five key parameters, (4) dividing up ignition process and explaining ignition mechanism from the perspectives of critical physical properties of water and heat transfer and mixing conditions, (5) discussing and forecasting its industrial applications including hydrothermal spallation drilling, the thermal recovery of heavy oil, the clean conversion and utilization of coal-based fuel, and the harmless treatment of pollutants. By and large, this paper analyzed in detail everything from experimental equipment to industrial applications, from combustion characteristics to ignition mechanisms, and from summary conclusions to prospect prediction. In the end, herein is summarized a couple of existing paramount scientific and technical obstacles in hydrothermal combustion. Further significant studies in the future should include excellent reactors, advanced monitoring techniques, and powerful computational fluid dynamics.

Simulation of rock failure modes in thermal spallation drilling

Thermal spallation drilling is a contact-less means of borehole excavation that works by exposing a rock surface to a high-temperature jet flame. In this study, we investigate crucial factors for the success of such thermal drilling operations using numerical simulations of the thermomechanical processes leading to rock failure at the borehole surface. To that end, we integrate a model developed for spalling failure with our thermomechanical simulations. In particular, we consider the role of material heterogeneities, maximum jet-flame temperature and maximum jet-flame temperature rise time on the onset of inelastic deformation and subsequent damage. We further investigate differences in energy consumption for the studied system configurations. The simulations highlight the importance of material composition, as thermal spallation is favored in fine-grained material with strong material heterogeneity. The model is used to test the relationship between the jet-flame temperature and the onset of thermal spallation.

Field test of a Combined Thermo-Mechanical Drilling technology. Mode I: Thermal spallation drilling

Accessing hydrocarbons, geothermal energy and mineral resources requires more and more drilling to great depths and into hard rocks, as many shallow resources in soft rocks have been mined already. Drilling into hard rock to great depths, however, requires reducing the effort (i.e., energy), time (i.e., increasing the rate of penetration) and cost associated with such operations. Thus, a Combined Thermo-Mechanical Drilling (CTMD) technology is proposed, which employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) Thermal spallation drilling, investigated here as a field test and (II) Flame-assisted rotary drilling, investigated as a field test in the companion paper. The CTMD technology is expected to reduce drilling costs, especially in hard rocks, by enhancing the rock penetration rate and increasing the bit lifetime. ModeI of the CTMD technology (thermal spallation drilling) is investigated here by implementing the concept on a full-scale drilling rig to investigate its feasibility and performance under realistic field conditions. During the test, the successful thermal spallation process is monitored, employing a novel acoustic emission system. The effects of thermal spallation in the granite rock are analyzed to provide conclusions regarding the rock removal performance and the application potential of the technology. The field test shows that thermal spallation of the granitic rock can be successfully achieved even when a liquid (water) is used as the drilling fluid, as long as the heat source is appropriately shielded by compressed-air jets. Thermal damage of the surrounding rock is investigated after the spallation test, employing micro-computer tomography imaging and modeling the stability of the cracks, generated by the spallation field test. This study shows that thermally induced damage is mainly confined within a narrow region close to the rock surface, suggesting that thermal spallation only marginally affects the overall mechanical stability of the borehole. Thus, this confirms that, as part of the Combined Thermo-Mechanical Drilling (CTMD) technology, thermal spallation drilling is a promising mode that has a high potential of facilitating the drilling of deep boreholes in hard rocks.

Field test of a Combined Thermo-Mechanical Drilling technology. Mode II: Flame-assisted rotary drilling

To enhance the drilling performance in deep hard rocks and reduce overall drilling efforts, this work proposes a Combined Thermo-Mechanical Drilling (CTMD) technology. This technology employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) thermal spallation drilling, investigated in the companion paper and (II) flame-assisted rotary drilling, investigated here as a field test. The CTMD technology is expected to reduce drilling efforts, especially in hard rocks, enhancing the rock penetration rate and increasing the bit lifetime, all of which reduces the drilling costs. The present work investigates Mode II (flame-assisted rotary drilling) of the CTMD technology by implementing the concept in an existing drilling rig and testing the technology under relevant process conditions. This contribution studies the underlying rock removal mechanism of CTMD and demonstrates its drilling performance, compared to conventional rotary drilling methods. Acoustic emission monitoring, and analysis of the collected drill cuttings provide multiple evidences for thermal-cracking-enhanced rock removal during the flame-assisted rotary drilling. This removal mechanism appears to represent an optimal compromise to minimize rock fragmentation and cutting transport efforts during drilling, compared to a less efficient mechanical scraping of the hard granite rock, observed during the standalone-mechanical drill test. The drilling performance, in terms of removal and wear rates, are evaluated for the flame-assisted rotary drilling. This shows that the proposed drilling approach is capable of enhancing the removal process in hard granite rock, by a factor of 2.5, compared to standalone-mechanical drilling. The implementation of this drilling approach into a conventional drilling system shows that integration of thermal assistance to conventional rotary drilling requires marginal technical efforts. Additionally, this technology can profit from established knowledge in conventional mechanical drilling, facilitating its implementation to improve drilling performance in hard rocks. Hence, this study demonstrates that the Combined Thermo-Mechanical Drilling method is feasible and concludes that this technology constitutes a promising approach to improve the drilling process, thereby increasing the viability of accessing deep geo-resources in hard rocks.

Experiment investigation of granite damage under the high-temperature and high-pressure supercritical water condition

Geothermal energy has the potential to produce significant amounts of electricity. Hot dry rock, stored abundant geothermal energy, is mainly made up of granite and hard to drill. Hydrothermal spallation drilling (HSD) is suitable to drill in hard granite formation. The granite breakage in HSD means the granite is induced to spall under high-temperature and high-pressure supercritical water condition, which is a key problem in HSD application. In this paper, we investigated the effect of supercritical water on granite damage by experiment. First, the granite samples were heated under the supercritical water condition. Meanwhile, another granite samples were also heated by air for comparison. After that, abundant post processing tests were conducted to compare the three type of granite samples: samples heated by supercritical water, samples heated by air and original samples. Specifically, the mineral analysis, uniaxial tensile test, uniaxial compression test, acoustic wave test was conducted to quantitatively investigate the granite damage. Meanwhile, thin section observation and scanning electron microscope observation were conducted to investigate the fracture distribution in the samples after damage. This study can provide useful insight into the granite damage heated by supercritical water, which can help to optimize the design of HSD in field application.

An analytical method to determine rock spallation temperature and degree of heterogeneity in thermal spallation drilling for geothermal energy

Thermal spallation drilling is an alternative technology expected to be suitable for exploitation of geothermal energy in hard, polycrystalline rocks. The rock spallation temperature and degree of heterogeneity are two important parameters for spallation studies. The rock spallation temperature can offer guidance to provide a suitable temperature environment for different formations, and the degree of heterogeneity plays an important role in determining the spallability of rock. However, it is difficult to measure the rock’s surface temperature at the onset of initial spallation and there is no universal method to determine the degree of heterogeneity of rock. This paper intends to provide a convenient analytical approach to approximate the rock spallation temperature and degree of heterogeneity for field applications. Based on the Weibull statistical theory of tensile failure, the rock temperature at spallation can be obtained by solving a set of over-determined equations. All other rock properties can be calculated based on the temperature, and then the degree of heterogeneity of rock can be determined. Compared with experimental data, the calculated rock spallation temperature and degree of heterogeneity are all within acceptable ranges. All results in this paper can provide implications for further study on thermal spallation drilling in geothermal reservoirs.

An analytical model to evaluate the heating conditions for drilling in hard rock using an innovative hydrothermal spallation method

Hydrothermal jet spallation drilling (HJSD) is a new drilling method suitable for drilling deep wells in hard rock formations. The characteristics of HJSD are that the rock surface is impinged by a high-velocity hydrothermal jet, causing rock breaking into small spalls. In this study, we develop an analytical model to evaluate the heating conditions (surface temperature and surface heat flux) in HJSD, in which the effect of jet impact on the heating conditions is first considered. Meanwhile, we conduct thermal spallation experiment using sandstone and granite samples. After that, the effects of jet impact on the heating conditions are investigated. Furthermore, the impacts of all physical parameters on the heating conditions are compared to clarify the key factors. Additionally, we compare the rate of penetration and heating conditions between sandstone and granite, which can provide direct understanding about the suitability of HJSD for sandstone and granite. Our study provides a further understanding of critical physic affecting the rate of penetration and heating conditions in HJSD.

Numerical simulation of reaction under high pressure conditions for thermal spallation drilling

Thermal spallation technology is a drilling method, which is potentially suitable for the exploitation of petroleum in hard rocks with low costs. In this paper, characteristics of reaction are investigated under relatively high pressure in thermal spallation drilling. The Peng-Robinson equation of state and eddy dissipation model are applied in the simulation. Effects of methane and oxygen flow rate, nitrogen fraction on the flow field are studied. Simulation results are validated in experiments that monitor the temperature of the flame jet using thermocouples. Results show that besides a portion of oxygen reacting with the methane, the left oxygen mainly flows towards the boundary of the reactor. The reaction between methane and oxygen mainly occurs at the central part of the reactor. Also, the temperature close to the wall of the reactor is only 1300 K, which can help to protect the reactor wall from damage due to high temperature. Increasing the methane flow rate within a certain range can help to obtain high temperature and high jet velocity simultaneously. Also, injecting mixture of oxygen and nitrogen can prevent too much excessive oxygen from existing in the wellbore during thermal spallation drilling. Results in this paper could provide guidance for field applications.

Shape factor of the flake-like particle in thermal spallation and its effects on settling and transport behavior in drilling annulus

Thermal Spallation Drilling (TSD) is widely applied in the exploration and development of oil, gas, and geothermal resources. Flake-like spalls (or particles) are often created during the TSD process. Our study in this paper focused on shape factor of flake-like spall in thermal spallation and its effects on settling and transport behavior. First, we conducted thermal spallation experiment and collected spalls. The shape factor of spall was obtained. The spalls with the shape factor in the range of 0.6 to 0.7 occupy the largest percentage. Based on that, a settling velocity model of single spall was obtained. We verified the model by comparison with the settling experiment results. After that, we investigated the effects of fluid and spalls properties on settling velocity. Finally, the effects of shape factor on spalls transport in drilling annulus were clarified by numerical simulation. In vertical section, when the shape factor is smaller, spalls can transport for longer distance and the spall distribution is more uniform. In the horizontal section, with the decrease of shape factor, the spalls have a longer transport length and lower spalls bed. All these results can be used for the optimization of drilling fluid.

Investigations on thermal spallation drilling performance using the specific energy method

Thermal spallation drilling is an alternative technology expected to be suitable for the development of oil and gas in hard rocks. In this paper, experiments on six types of rocks were performed to analyze characteristics of rock spallation and spall distribution. Additionally, a specific energy model for thermal spallation drilling was established. The analytical temperature results were compared with experimental values for validation. Finally, the specific energy of granite was compared with those of other rock breaking methods. Results show that it is important to control the lowering velocity of the coiled tubing (recommended in field application) to obtain the optimal penetration rate during spallation drilling. The degree of heterogeneity of rock may play an important role in the spall-ability. The shale has the largest average aspect ratio (diameter/thickness) of 30.97, with the following sequence being dolomite, limestone, granite and sandstone. The granite shows the highest penetration rate, while the limestone has the lowest. The specific energy values for sandstone, granite, shale and limestone are approximately 1000 MPa. Compared with other rock breaking methods, thermal spallation has advantages when drilling hard rocks (e.g., granite). All results in this paper can provide important implications for further study on the thermal spallation drilling.

Experimental analysis on characteristics of micro-structure and mineralogy changes in thermal spallation drilling

Thermal spallation drilling is a promising technology expected to exploit hydrocarbons in hard rocks. In this paper, thermal spallation experiments are conducted on six types of rocks. Comparisons are made in terms of mineral compositions and micro-structures. Spallation temperatures are obtained, and analytical results are compared with experimental data. Results show that spallation temperatures obtained by analytical method show good agreement with experimental data. Additionally, drilling performances can be continuous or intermittent for various types of rocks. Quartz expands in different rocks after thermal impact. Calcite increases and dolomite decreases due to thermal decomposition. Moreover, the induced thermal stresses in sandstone push against the micro-particle and melting may occur. For granite, micro-cracks and micro-particles exist after spallation. In contrast, there are no cracks in shale, but melt holes and micro-particles distributed randomly due to reaction. For limestone, there exist relatively large cracks, micro-cracks and micro-particles. For dolomite, no obvious micro-particles remain but only a few micro-cracks. The variations of elements correspond well with mineral changes. However, under the conditions of this paper, marble fails to break due to strong homogeneity. All results can provide guidance for the further study on the rock-breaking of thermal spallation drilling.

Numerical model of laser spallation drilling of inhomogeneous rock

Spallation of rock during laser drilling is dependent mainly on temperature on the surface, material properties and presence of discontinuities within the rock. A numerical model of laser spallation of heterogeneous granitic rock using the Finite Element(FE) method is presented. The developed model accounts for the presence of microcracks within the rock. Convective boundary conditions are also considered and the effect of propagation of these cracks on spallation is investigated. Temperature and stress profiles generated are discussed. The results indicate stress development on the surface of the rock and its effect on eventual rock fracture.

Numerical study on reaction characteristics under high pressure conditions for thermal spallation drilling

Thermal spallation technology is an alternative drilling method, which has the potential to exploit petroleum and geothermal energy with low costs. In this paper, the influence of the structure of the reactor on the characteristics of the flame jet is analyzed from aspects of outlet velocity, temperature, pressure and mole fraction. The Peng-Robinson equations of state and eddy dissipation model are applied in the simulation. An experimental setup is designed and experiments are performed to validate the simulation results. Results show that the reactor length and nozzle length can be reduced in a proper range to obtain better results. The suitable nozzle diameter may be changed for different types of rock to obtain the best rate of penetration. Also, no cooling water may be injected until the temperature of the reactor wall reaches the limitation of failure. Results could provide guidance for field applications.

Comparative numerical analysis and optimization in downhole combustion chamber of thermal spallation drilling

Thermal spallation drilling is a non-contact and efficient technology, which is suitable for hard rock formations. It uses downhole combustion to generate high temperature media to heat the bottom rock, which leads to the spallation of the rock. However, no investigation has been carried out in combustion of thermal spallation drilling. In this paper, three combustion models (eddy dissipation model, eddy dissipation concept model and non-premixed model) are applied and compared to simulate the reaction in the combustion chamber. Through modifying Magnussen constants, the eddy dissipation model is optimized and validated. Results show that under high velocity conditions, combustion models based on infinite fast reaction mechanism may not be accurate. An additional reaction step in eddy dissipation model may significantly improve the accuracy. Besides, there is a linear relationship between the average outlet temperature and Magnussen constant in eddy dissipation model. Moreover, through modifying the Magnussen constants, the accuracy of the results can be significantly improved. The optimized eddy dissipation model with two reaction steps not only has the advantages of small computational cost, but also may reflect the actual situation of the combustion reaction. Results in this paper could provide guidance in the design of combustion chamber in thermal spallation drilling.

A theory on thermal spalling of rocks with a focus on thermal spallation drilling

AbstractThermal cracking of rocks is an intensively studied topic in different research areas and for various engineering problems. In this context, we present a modeling approach which describes thermal spalling of rocks with a focus on thermal spallation drilling. This drilling technology uses high thermal loads to locally destruct the surface of the rock formation. With the presented model, the operating conditions which are required to initiate spalling of rocks can be estimated. Additionally, a Spallability number is introduced allowing a categorization of rocks according to their ability to spall. The presented model is based on linear fracture mechanics and the stress intensity concept. It evaluates if a crack with a certain geometry embedded in a rock with specific properties propagates during exposure to different heat loads and external pressures. Thereby, rapid heat transfer processes are coupled with induced thermal stresses and fracture mechanics in rocks.