Borehole Shape Research Articles

Variability and loss of uniqueness of numerical solutions in FEM×DEM modeling with second gradient enhancement

AbstractIn the last decade, a new multi‐scale FEM×DEM approach has been developed using Finite Element Method (FEM) coupled with Discrete Element Method (DEM) as a constitutive law to account for the specificities of the mechanical behavior of granular materials. In FEM×DEM model, a DEM calculation is performed on a particle assembly (volume element—VE) at each Gauss point. Recent publications have demonstrated that FEM×DEM approach naturally captures the discrete and anisotropic nature of granular materials. Despite its advantages, FEM×DEM with classical FEM, suffers from mesh dependency, especially when material enters softening phase and exhibits strain localization. To overcome this limitation, FEM×DEM model has been enriched by incorporating a local second gradient model. Nevertheless, the existence of multiple possible solutions is observed. In this paper, we study the variability and loss of uniqueness of numerical solutions to a boundary value problem. Different VEs with equivalent mechanical properties are generated and used to model the pressuremeter tests by means of FEM×DEM. The modeling results show a great variability of the numerical results, both in shape of borehole and in different modes of shear bands. For the same VE, the loss of uniqueness of numerical solutions is evidenced by a slight modification of loading history at the level of the internal pressure applied to the borehole. Finally, we show that when a certain heterogeneity is introduced by using different VEs within the same BVP, even if the uniqueness of the solution is not guaranteed, the set of possible solutions seems more restrained.

Mechanical properties and energy evolution characteristics of borehole-containing specimens under disturbances of cyclic incremental stress with different lower limits

Drilling gas drainage boreholes in coal mines is a major measure to prevent gas disasters in coal mines. However, coal mining disruptions can affect the stability and shape of gas drainage boreholes. Therefore, multi-level cyclic loading and unloading tests with a constant lower limit and constant-amplitude multi-level cyclic loading and unloading tests were conducted on hole-containing specimens. The results show that cyclic loading and unloading enlarges the deformation modulus of specimens with holes. The stress change during cyclic loading affects the deformation modulus of the specimens, rendering them more brittle and vulnerable to damage under high stress-induced disturbances. Additionally, the dissipated energy per unit volume of the specimens containing holes increases exponentially during multi-level cyclic loading and unloading. The cumulative dissipated energy per unit volume of the hole-containing specimens exhibits logarithmic growth during constant-amplitude multi-level cyclic loading and unloading. Furthermore, a discontinuity in the curves is observed as the stress level rises, which promotes brittle failure around the hole. The research results expound the deformation and energy evolution characteristics of hole-containing specimens during cyclic loading and unloading, and provide a reference for clarifying the failure modes of boreholes in the gas extraction process.

A Review on Recent Achievements and Challenges in Electrochemical Machining of Tungsten Carbide

Electrochemical machining (ECM) is a modern, unconventional manufacturing technique that uses the electrochemical dissolution of metal to produce pieces with a predetermined shape, dimensions, and surface polish. Tungsten carbide (WC) is used in many fields and applications due to its high wear resistance and hardness. WC is used in cutting tools, mining and drilling equipment, wear parts, industrial machinery, aerospace and defence components, and medical equipment. It gives a workpiece a particular shape and number of boreholes, removes a damaged outer coat, and eliminates surface roughness. Though the ECM of WC is fraught with difficulties and obstacles, altering the electrolytes and refining the technological equipment and process of the ECM of WC can help improve machining accuracy and stability. In the current study, we examined the ECM of WC and compiled a reliable database from reliable sources using the keywords "electrochemical machining" and "tungsten carbide". Research papers constitute the majority of these publications. It was discussed how WC behaves anodically as it applies to ECM, which experiments were conducted with various electrolytes, and how they performed. Several ECM modes have been considered, and each mode has been evaluated in detail for challenges and gaps.

The stress relief correction method considering irregular borehole shapes

In situ stress is an essential parameter that directly affects all engineering construction and scientific research in deep rock masses. The shapes of deep borehole are often highly irregular, so that the traditional in situ stress measurement methods based on the assumption of circular or cylindrical boreholes are no longer applicable. We propose a stress relief correction method considering irregular borehole shape using the complex function method of elasticity. The conformal mapping function is introduced to project the irregular shape into a unit circle and the relationship between the strains at the borehole wall and the far-field in situ stress components are solved by the Cauchy integral methods. The results show the following: (1) The stress results from the stress relief correction method are consistent with the theoretical solutions and numerical simulation results for circular and square boreholes. (2) The irregular borehole shape significantly changes the distribution of stress magnitude and the direction of strain measurement on the borehole wall. The stress relief correction method considering irregular borehole shapes provides an essential theoretical basis for deep rock mass engineering construction and deep oil and gas field exploration.

A deep learning workflow for weak reflection extraction in pitch-catch measurements in the cased hole

As a key technology to evaluate cement bonds in the cased hole, an advanced ultrasonic logging tool combines pulse-echo and pitch-catch measurements in which the latter one provides reflections from the cement-formation interface (called third-interface-echo [TIE]) to evaluate the bond condition and determine casing eccentering as well as cement velocity. However, the TIE would be weak and not easy to pick due to the eccentered tool and casing and it would overlap with the strong multiple reflections between the casing inner surface and the transducer-housing tool. We have developed a deep learning workflow to extract weak TIE from noisy data and to preserve its amplitude at the same time. First, we use synthetic waveforms from thousands of finite-difference simulations as initial training data sets to train a deep learning network, which is modified from a network in speech separation. Then, the trained model is used to predict the field data through an active-learning strategy. The improved network is further used to extract the weak TIEs, which are not easy to pick in the initial deep learning model. Finally, the TIE waves image is converted to a pseudovelocity image to obtain the minimum traveltime path by solving the eikonal equation. The shortest traveltime path is used as the TIE arrival time. In addition, a 3D visualization is used to display the borehole shape from the picked arrival time. The applications in synthetic data and data set from a calibration well illustrate a good performance of our workflow in which the weakest TIE extracted from the network can reach 50 dB compared to the maximum amplitude in the full waveform. The picked arrival times can be used to reconstruct a borehole shape.

In situ stress measurement method of deep borehole based on multi-array ultrasonic scanning technology

In order to improve the accuracy and efficiency of deep in-situ stress measurement, based on the wall collapse in deep borehole, a deep borehole in-situ stress measurement method based on multi-array ultrasonic scanning technology is proposed in this paper. The solution idea of using the combination of multiple elements for borehole contour scanning is put forward, and a multi-array ultrasonic scanning technology suitable for fine horizontal section scanning of deep geological borehole contour is formed. Through the analysis of the calculation principle of hole wall caving method and the derivation of multi-array element ultrasonic scanning solution algorithm, a multi-array ultrasonic scanning device is developed by using the effective fusion of multi-array ultrasonic full waveform scanning signals and the reconstruction of horizontal section contour. The measurement method of in-situ stress in deep borehole combined with hole wall caving method is studied. Finally, the feasibility and accuracy of this method are verified by physical experiments. The results show that: the effective fusion of multi-array ultrasonic full waveform scanning signals and horizontal section contour reconstruction can break through the limitations of conventional in borehole probe placement and in borehole medium sound velocity calibration, and improve the fine reconstruction and accurate measurement of 360° wall shape. The multi-array ultrasonic scanning technology can obtain the borehole shape data at any borehole depth, and then obtain the three-dimensional shape measurement of borehole wall, which can provide a new technical means for the three-dimensional detection of borehole wall and in-situ stress measurement.

Semi-analytical Method for Modeling Wellbore Breakout Development

Borehole breakout initiation, progression, and stabilization are modeled using a semi-analytical method based on Melentiev’s graphical conformal mapping procedure, and Kolosov–Muskhelishvili complex stress potentials. The only input data required are the elastic moduli of the rock, the Coulomb strength parameters (cohesion and angle of internal friction), and the far-field stresses. The stresses around the borehole wall are computed, the region in which the rock has failed is then “removed”, creating a new borehole shape. This process is iterated until a shape is obtained for which the breakout will progress no further, and a stable state has been reached. This modeling shows that stresses around the flank of the breakout evolve so as to reduce the propensity for shear failure, which helps to explain why the breakout width remains relatively constant throughout the process, even as the breakout region deepens radially. The failed area around the borehole becomes smaller and more localized, as the breakout tip sharpens and deepens. Using the Mogi–Coulomb failure criterion, a good match is obtained between the modeled breakout geometry and the geometry observed by Herrick and Haimson in laboratory experiments on an Alabama limestone. The new method leads to a correlation between breakout geometry, rock strength properties, and in situ stress. The paper ends with a critical discussion of the possibility of inferring the in situ stress state from observed breakout geometries.

VERTICALITY OF BOREHOLES

The study contains the problem of the borehole shape of during drilling. Usage of a drilling bit and tubular or screw rods is traditional and most common drilling methods. The requirement of "verticality" for boreholes in the drilling process is necessary to comply with design solutions and design schemes. At the same time, the actual implementation of this important quality is often very difficult. An analysis of the regulatory documentation is carried out to ensure the requirement during conducting geotechnical investigations, and during installing pile foundations made in the ground: bored piles and continuous flight auger piles (CFA). Data about the researchers in the field of assessing of bending of the pile axis are presented for CFA piles. The article presents the results of field studies of the borehole verticality at two investigation sites. At the first site, a borehole was drilled with a depth of 9.0 m, at the second site, ten boreholes were drilled with a depth of 25.0 m. The spatial position of the boreholes was determined by an inclinometer. The results of the measured verticality deviations of the boreholes are presented on the spatial graph for clear visibility. A conclusion is made about the influence of the borehole shape on its vertical projection during conducting of geotechnical investigations. A series of comparative numerical calculations was performed using the Midas GTS NX program to assess the effect of the verticality deviations of the boreholes during installing bored piles on their bearing capacity. A single pile action in the ground was simulated for the pressing and pulling loads. It is concluded that the shape of the borehole influences on the single pile action in the ground.

A new analysis model for borehole stability in gas drilling

Gas drilling is one of the main underbalanced drilling techniques, which has a significant effect on the ROP (rate of penetration) improvement in hard formation. However, because the circulating medium in drilling process is gas instead of liquid, the fluid column pressure of gas drilling cannot provide enough support force for the rock around borehole as that of conventional overbalanced drilling. Due to the combination influence of engineering and geological factors, the borehole rock is prone to instability during gas drilling, which can lead to rock blocks falling from the well wall and the formation of irregular borehole shape. To avoid the occurrence of complex underground accidents caused by too many falling rock blocks, such as the borehole instability and pipe-sticking, it is necessary for gas drilling to take into consideration the true triaxial stress state of the rock in the deep formation and the possibility of the borehole breakout. Based on the permitted breakout angle of borehole and true triaxial strength theory of rock, an analysis model of borehole stability has been developed for the engineering of gas drilling. The theory of linear elasticity, involved in this analysis model, is used to analyse the stress state of rock around a cylindrical hole under lower wellbore pressure. The failure of the rock around borehole is determined by the Mogi-Coulomb criterion, which considers the intermediate principal stress effect on rock material. The results show that the model is as simple and practical as the traditional model, which takes no account the effect of the intermediate principal stress or the permitted borehole breakout, and it can provide an accurate analysis for the borehole stability of gas drilling. Therefore, the results of this study, combined with the related work such as the prediction of water formation, can guide the adaptability evaluation for the engineering design of gas drilling.

Borehole multi-functional logger for geophysical high-precision monitoring in Antarctic and Greenland ice sheets and glaciers

AbstractThis article presents the main aspects of the design solutions (based on the application of sensors MEMS and cantilevers), testing and applying of the multi-functional borehole logger ANTTIC (Antarctic Thermo-barometer, Inclinometer, Caliper) for geophysical high-precision monitoring (when simultaneous registering of temperature, pressure, axis inclination angle and radii of borehole cross-sections at 12 points), which is designed specifically for ultra-low temperatures and ultra-high pressures, and to determine an elliptical borehole shape and registration anisotropy factor in deep ice boreholes in the central region of Eastern Antarctica, in the areas of dome A at the Kunlun station (China) and/or of lake Vostok at the Vostok station (Russia).

Study of the Law of Hydraulically Punched Boreholes on Effective Gas Extraction Radius under Different Coal Outputs

Hydraulic punching technology has recently developed into an effective pressure relief measure and permeability enhancement method for soft and low permeability coalbeds. Different coal outputs directly affect the shape and size of boreholes as well as the effective extraction radius. Taking the Zhongmacun mine as an example, the influence of different coal outputs and different extraction periods on effective extraction radius was analyzed and studied through field tests and numerical simulation. The results show that the increase in coal outputs from hydraulic punching can improve the effective extraction radius of the boreholes. For example, when the gas extraction reaches 90 days, with a coal output of 0.5 t/m, 1.0 t/m, and 1.5 t/m, the effective extraction radius is 3.08 m, 3.46 m, and 3.83 m, respectively. The difference in gas extraction effect of different coal output boreholes increases significantly with the extension of the extraction time, but the speed of growth gradually decreases, which is consistent with the conclusions obtained on-site. This result has important practical significance for optimizing the technical parameters of hydraulic punching, guiding the accurate layout of extraction and drilling, and enhancing the effect of gas control in mines.

Research on Calculation Model of Horizontal Wellbore Shape with Non-uniform In-situ Stress

In horizontal Wells, non-uniform ground stress is the root cause of irregular borehole shape. The failure of caliper instrument in horizontal section leads to the failure of accurate evaluation of borehole shape, which makes it difficult to calculate the frictional resistance of tubing string in the later stage. This paper studies the borehole shape calculation model from the following two aspects: (1) in the aspect of diameter expansion caused by shaft wall collapse. By establishing coulomb-Mohr criterion in cylindrical coordinate system, the well wall collapse instability is judged, and the hole diameter expansion is evaluated by establishing the calculation model of the instability range. (2) Shrinkage caused by stress concentration. Firstly, the boundary position of formation deformation is determined. Secondly, the calculation model of borehole deformation is established by analyzing the stress characteristics of formation, which reflects the time effect of borehole deformation. Therefore, the establishment of borehole shape calculation model of horizontal well provides a basis for calculating the friction between the pipe string and the horizontal section.

Hole enlargement by drillstring stabilizers

The goal of this study is to quantify the hole enlargement caused by drillstring stabilizers and the relation of this enlargement to the drilling parameters, contact forces, and rock strength. For this purpose, an extensive test campaign was conducted. In each test, a real-size stabilizer is placed within a predrilled hole in a mortar block. A side force is then applied as the stabilizer is rotated and moved downward. The change in the hole diameter is measured using a laser scanner before and after completing the experiments. The results of these experiments demonstrated the extent of the effect, which the stabilizers can have on the final borehole shape and quality. A simple implementation within an existing in-house steering prediction model is proposed to address this problem. Numerical simulations are presented to show the benefit of capturing borehole enlargement by stabilizers.

Numerical Simulation for the Dynamic Breakout of a Borehole Using Boundary Element Method

Borehole breakouts are formed by spalling of fragments in a direction parallel to the least horizontal stress. As the breakouts falling into the wellbore or being scoured by the drilling mud piece by piece, the stress concentration on the wellbore will release gradually. The wellbore is not circular or elliptical, which requires a numerical method to determine the redistributed stress around the irregular borehole. In this article, boundary element method associated with the brittle rock failure criterion is deployed to simulate the dynamic breakout of a borehole. The borehole shape is updated step by step during the time-dependent breakout. The first breakout volume is smaller than the analytical solution (Gholami et al. in J Rock Mech Geotech Eng 8:521–532, 2016). However, the total breakout volume is larger than the analytical solution because our model considers the subsequent failure of the irregular wellbore. The stress distribution around the irregular wellbore after each step of breakout is investigated and the maximum cohesive strength at which the rock will fail is figured out. These results provide new insights to understand the dynamic process of borehole breakout.

A robust approach for determining borehole shape from six-arm caliper logs

We have developed a robust and reliable method for determining borehole shape from six-arm caliper logs. Four- and six-arm calipers are the common caliper tools used in open-hole logging. They provide information about wellbore geometry that is important in petrophysical and geomechanical analyses. The analysis procedure for four-arm caliper logs is well-established, but the analysis for six-arm caliper logs can be troublesome in complex hole environments containing breakouts or keyseats when the tool is off center. The challenge with the six-arm caliper is how to remove the effect of tool decentralization, which cannot be handled properly by using conventional correction methods, which are developed based on the assumption of circular or elliptical boreholes. To resolve this issue, we have developed a new approach for tool decentralization correction. The new method is based on an assumption that the true borehole center should be the center of a circle that fits the caliper pad positions in the least-squares sense subjected to the restriction that the circle is confined within the six pads. We first compare the new method with the conventional chord method and ellipse-fitting method through numerical modeling. We numerically investigate the general performance of these three methods by using a Monte Carlo approach to generate a large number of simulations that mimic caliper logging run in boreholes with a variety of different wellbore geometry. We then study the applicability of the new method in field data analysis by applying it to a field logging data set acquired in a well that contains the breakout and the keyseat. The modeling and field-data results indicate that the new method can yield more reliable estimates of the virgin borehole center than the other two methods and thus can give clearer delineation of the borehole shape.

Research on structure design and flow field characteristics of the novel jet bit for radial horizontal drilling

AbstractThe self‐propelled swirling jet bit was designed to ensure the rock‐breaking efficiency of radial horizontal drilling and meet the roundness of borehole diameter and borehole shape. The jet bit provides swirling jet through internal rotating impellers and increases the drilling depth by jetting through the central holes on the impeller. Based on numerical simulation method, RNG k−ε turbulence model was employed to analyze the three‐dimensional flow characteristics of flow field inside and outside of the two kinds of jet bits. Moreover, the rock‐breaking efficiency of the two jet bits was also compared and analyzed in accordance with the laboratory experiments. The results show that the simulated bottom‐hole flow field for the two jet bits can be both divided into bottom‐hole overflow area, forward jetting area, and backward jetting area. The forward jet is a jet beam which consists of a central jet and four circumferentially equispaced jets, which generates more complex overflow area at the bottom hole. The maximum jet speed and the jet impact energy of forward jetting from the combined swirling and round multijet bit are larger than that of the swirling multijet bit. The speed attenuation law of forward jet nozzle from the two kinds of jet bits is similar. They both increase slowly in the mixing zone, while increase remarkably when approach to the nozzle. Then decrease linearly in the impact zone, and decline straightly when get close to the impact area. The simulation results of flow field characteristics are in coincidence with the actual rock‐drilling features. Therefore, the simulation results of flow field characteristics can provide guidance for jet bit design and structure parameter optimization for radial horizontal drilling.

A system for measuring borehole diametric deformation based on mechanical contact and micro-optical imaging

Quantifying diametric deformations of boreholes is vital in rock engineering due to its applications in the determination of in-situ stresses using the stress relief methods. New methods are desired to overcome the harsh environment featuring high temperature and high pressure in deep boreholes. Therefore, this study develops an apparatus using rigid probes combined with microscopic camera to measure borehole diametric deformation. The structural design of the rigid probe and package of the measuring device avoid the advert impact of environmental factors. The usage of multi-probes and high resolution micro-optical imaging technique enables simultaneous collection of multi borehole diametric deformation data. Furthermore, using these data, a new method of borehole shape measurement is proposed at high speed compared with traditional methods that measure one diametric deformation at a time. The feasibility of the system is verified by conducting laboratory tests. This system can be readily used in the in-situ stress measurement based on borehole shape analysis.

Rock heterogeneity at the centimeter scale, proxies for interfacial weakness, and rock strength-stress interplay from downhole ultrasonic measurements

Analysis of data acquired with an ultrasonic rotating device lowered inside a vertical well of a highly laminated, kerogen-rich, carbonate source formation reveals centimeter-scale rock heterogeneities. The high-frequency (50–600 kHz) measurement used consists of a pulse-echo modality that yields a high-resolution (millimeter-scale) borehole shape and a pitch-catch modality with one transmitting and two receiving transducers that provide compressional (P) and shear (S) slownesses (depth-versus-azimuth) images estimated from signals propagating in the near-wellbore region as compressional head waves and pseudo-Rayleigh surface waves. The slownesses are compared with their counterparts estimated from a lower-frequency (1–15 kHz) sonic measurement logged in the same well interval. The sonic P and S logs are seen to average the centimeter-scale slowness spatial variation between compliant and stiff laminates at a scale larger than 30 cm (1 ft). This is also accompanied by a reduction in resolution of the slowness contrast, which markedly reduces the spectrum of rock-mechanical property variations that are estimated from the ultrasonic data. Further, ultrasonic images of the P and S slownesses and borehole acoustic caliper reveal a host of features associated with rock geomechanics in the near-wellbore and that inform on a first-order basis the interplay between rock strength and local stress regime, as affected by the lamination. The features include breakouts present in the stiffer limestone layers and their arrest at the intersection with the compliant siltstone layers, as well as azimuthal P and S slowness variations indicative of azimuthal stress concentrations, but without the appearance of breakouts. The ultrasonic borehole shape data also identify compliant thin layers that retract into the formation by amounts that are commensurate with their Young’s modulus, suggesting a proxy for detecting and characterizing thin-layer weakness in situ.

A Gas–Solid–Liquid Coupling Model of Coal Seams and the Optimization of Gas Drainage Boreholes

For a gas–solid–liquid coupling model of coal seams, previous permeability models basically supposed uniaxial strains as the boundary condition for coal reservoirs without considering the deformation caused by changes in humidity. The permeability model varies under different boundary conditions. According to the true triaxial stress state of coal reservoirs, a permeability model considering the effective stress, sorption and desorption, and wet strain was established. Based on the permeability model, the continuity equation of gas and water and the stress field equation were coupled. Then, the model was incorporated in the COMSOL suite to simulate gas drainage from boreholes in floor roadways passing through seams in a coal mine. By comparing with the measured gas flow on site, the model was verified as being reliable. Moreover, the spacing and layout shape of boreholes in floor roadways were simulated. To achieve the aim of eliminating regional outburst within 180 days and decreasing the number of boreholes so as to reduce the cost, the spacing and shape of boreholes were optimized. When the superimposed effect of the boreholes was not considered, the optimal spacing of boreholes was 3 r; if the superimposed effect was taken into account, the spacing could be set to within 3 r ≤ L ≤ R, where r and R represent the effective gas drainage radius and the influence radius of gas drainage, respectively. The borehole spacing could be appropriately increased when the boreholes were arranged in rhomboidal form. To achieve the same range of outburst elimination, the rhomboidal layout can decrease the number of boreholes to reduce cost, thus realizing the objective of this optimization process.

Research status and future trends on surface pre-grouting technology in reforming wall rock of vertical shafts in coal mines in China

In the mine construction, the surface pre-grouting technology is an important method to prevent water blast in excavation process of vertical shaft when the shaft must pass through the thick, water-rich and high water-pressure bedrock aquifer. It has been nearly 60 years since the technology was used to reform wall rock of vertical shaft in coal mine in China for the first time, and the existing technology can basically meet the needs of constructing 1000m deep vertical shaft. Firstly, the article introduces that in view of Magg’s spherical seepage theory and Karol’s spherical seepage theory, Chinese scholars found that the diffusion of grout from borehole into the surrounding strata in horizontal direction is irregular through a lot of research and engineering practice of using the surface pre-grouting technology to reform wall rock of vertical shafts, and put forward the selecting principles of grout’s effective diffusion radius in one grouting engineering; Secondly, according to the shape of the grouting boreholes, surface pre-grouting technology of vertical shaft is divided into two stages: vertical borehole stage and S-type borehole stage. Thirdly, the development status of grouting materials and grouting equipment for the technology is introduced. Fourthly, grouting mode, stage height and pressure of the technology are introduced. Finally, it points out that with the increasing depth of coal mining in China, the technology of reforming wall rock of 1000~2000m deep vertical shafts will face many problems, such as grouting theory, grouting equipment, grouting finishing standard, testing and evaluation of grouting effect, and so on. And it put forward a preliminary approach to solving these problems. This paper points out future research directions of the surface pre-grouting technology in China.