Core Bit Research Articles

Development and Application of a Rope Core Hydraulic Closed-Core Mechanism for Loose and Broken Strata

When employing the wire-line core drilling technology in loose strata, the core recovery rate is often low due to fragmented rock masses and the erosive effect of flushing fluids, severely affecting core quality and drilling efficiency. To address this, the paper designs a hydraulic closed-core cutting mechanism and water-sealed protective core bit compatible with traditional S75 wire-line core drilling tools. Comprehensive analysis and optimization of the design mechanism’s force, flow field patterns, and operational mechanisms have been conducted through numerical simulations, laboratory tests, and field applications. Studies indicate (1) the hydraulic cutting mechanism can smoothly close under flushing fluid action, with stress and strain within safe limits and the sealing area of the core barrel exceeding 75%; (2) post-optimization, the vortex flow within the flushing fluid flow control valve is significantly reduced, with the flow rate at the core after passing through the water-sealed protective core bit being less than 0.6 m/s, effectively protecting the core from being eroded by flushing fluids; (3) the hydraulic cutting mechanism has proven feasible through laboratory testing, with a recommended standard core claw with a 95% protective rate. In actual loose strata, the average core recovery rate reached 97.83%, an 88.13% increase compared to traditional drilling tools, demonstrating high engineering promotion value.

Modeling of Cutting Mechanics Analysis of Diamond Core Bits

Diamond core bits are essential tools for coring operations in deep formations during oil and gas drilling projects. By establishing a cutting mechanics calculation model for diamond core bits, this study investigates the changes in bit load and torque under the influence of key factors such as bit cutting structure, rock strength, and diamond distribution density. This research is significant for optimizing bit structure, enhancing coring drilling efficiency, and prolonging bit lifespan. This paper conducts modeling analysis of the cutting element loads for various shaped diamond particles, including rectangular, triangular, cylindrical, and spherical shapes. Based on this, a cutting mechanics calculation model for axial and tangential forces of granular spherical diamonds is established using the fundamental principles of elastoplastic mechanics. Consequently, a set of methods for calculating and analyzing the working loads of diamond core bits is developed. The findings of this paper provide a theoretical basis for studying the rock-breaking mechanism of diamond core bits, evaluating drilling efficiency, and optimizing tooth arrangement structures.

Generation of cutting heat and simulation of core tube wall temperature during coring in primary structure coal seam

When the drilling core method is used to determine the coalbed gas content, the cutting heat generated by the core bit cutting coal will increase the core tube temperature, and the excessively high core tube temperature will have an heating effect on the coal core, which will accelerate the coal core gas desorption rate and increase the gas loss amount. The generation of cutting heat of core bit and the measurement of core tube temperature are the basis for grasping the gas desorption law of coal core and projecting the amount of gas loss. Firstly, the self-developed core tube temperature measurement device was used to conduct on-site core temperature measurement experiments at different cutting speeds. Then, the cutting temperature of core bit was solved by establishing thermodynamic model for cutting coal and heat transfer model of cutting edge. Finally, based on the thermal conductivity characteristics of the core tube, the core tube temperature at different cutting speeds was simulated, and the simulated temperature was compared with the on-site measured temperature to verify the reliability of the model. The results show that when coring in primary structural coal, the temperature change trend of core tube wall temperature measurement point at different cutting speeds is basically consistent, the temperature measurement point at the front end of the core tube mainly goes through a relatively stable period in the drilling process, a sharp rising period in the cutting process, a slow rise and cooling period in the withdrawal process. However, the temperature measurement point at the back end of the core tube wall mainly goes through a relatively stable phase and a slowly increasing phase. The temperature rise of the core bit and the core tube wall are significantly positively correlated with the cutting speed. When coring in hard coal seam and the core depth is not large, the cutting heat generated by the core bit and the coal body is the dominant factor for the temperature rise of the core tube. The core tube wall temperature calculated using the model matches well with the field measured temperature, and the error is small, which fully shows that the coring thermodynamic model is feasible. This study provides a basis for further research on the dynamic distribution characteristic of coal core temperature during coring, which is of profound significance to calculate the gas loss and coalbed gas content.

Increasing the durability of an impregnated diamond core bit for drilling hard rocks

Wear rates of impregnated diamond core bits made from two composite diamond-containing materials (CDM) are studied by spark plasma sintering (SPS) in the temperature range 20–1350 ºC at a pressure of 30 MPa for 3 minutes while drilling granite. The addition of chromium diboride (CrB2) micropowder in an amount equal to 2% of the composition of CDM 25Cdiam–70.5WC–4.5Co leads to a twofold decrease in the wear rate. The wear resistance of crowns made from CDM 25Cdiam–70.5WC–4.5Co and 25Cdiam–68.62WC–4.38Co‒2CrB2 is maximized at a rotary speed of 250 rpm and an axial load of 900 kg, and minimized at 750 rpm and 1250 kg. The increase in wear resistance of the 25Cdiam–68.62WC–4.38Co‒2CrB2 crown is due to the refinement of the grains of the main wolfram carbide (WC) phase and the formation of strong adhesion of the diamond grains to the carbide matrix. Keywords: impregnated diamond core bit; composite; tungsten carbide; cobalt; chromium diboride; durability; spark plasma sintering.

Improvement and Application of Directional Reaming While Willing Bit in Tahe Oilfield

Unlike conventional sidetracking technology, the technology of reaming while directional drilling makes it possible for us to conduct directional drilling and reaming simultaneously using dual core bit, which simplifies the construction steps and can greatly shorten the drilling cycle and reduce drilling costs. With the increasing of the tasks of sidetrack horizontal well in the old wells of Tahe Oilfield, the technology of reaming while directional drilling has become an important means for sidetrack of old wells in Tahe Oilfield. However the substratum of Bachu formation has high content of gravel with large diameter and strong consolidation, which cause severe abrasion of dual core bit, short drill life and low ROP. With the problems in the substratum of Bachu Formation, Tahe Oilfield continuously improves dual core bit based on its effect of reaming while drilling, and breaks the national records of the maxim setting depth, inclination and continuous expansion length during the operations of well TH12124CH. This paper shows the improvements of dual core bit and its application effect both in well TH10233CH and well TH12124CH, and put forward a further scheme to improve the dual core bit for the substratum of Bachu Formation of Tahe Oilfield.

Research on slim-hole drilling technology for shale gas geological survey in China

Over the last 10 years, the China Geological Survey has deployed 137 slim-hole shale gas geological exploration wells for coring entire wellbores. These wells are primarily located in new blocks and geological formations where neighboring well data are insufficient, beyond the scope of developed oil fields in China, or outside of oil and gas company mining-right areas. The drilling rig equipment, coring tools, and core drill bits of slim-hole shale gas drilling technology are different from those associated with traditional petroleum drilling. Many studies have been conducted on non-coring slim-hole drilling technology. This paper focuses on coring technology and drilling safety, summarizing a set of high-efficiency shale gas drilling equipment and technology systems based on geological drilling equipment and techniques (that can be used for solid mineral exploration). We report on: 1) an improved vertical shaft drilling rig adapted to shale gas well control safety; 2) high-efficiency core drilling techniques, focusing on coring tools, and techniques incorporating an inverted tower drilling tool combination, air circulation follow-through technology, and expanded casing technology; 3) research progress on high-efficiency core drill bits, including non-planar tooth polycrystalline diamond compact bits and impregnated diamond core bits, along with their application effects. This research provides substantial advances in drill-core technology and improvements in exploration efficiency. Moreover, it provides a reference frame for well structural design and selection of construction technology for shale gas exploration drilling projects.

Research on Fidelity Performance of Coring Bits during Drilling and Cutting in Deep Extreme Environments

Deep rock formations in extreme environments are characterized by complex working conditions, various structures, high hardness, and high resistance to compression. However, existing coring techniques leave the cores of deep rock formations vulnerable to residual stresses, resulting in poor fidelity during deep coring. This paper develops a rock-breaking model for the structural parameters of drill bits. It proposes that a drill bit’s structural parameters in terms of back-rake and side-rake angles will affect the core’s fidelity performance. In addition, the core’s mechanical specific energy and maximum stress will reflect the fidelity effect. The accuracy of the theoretical model was verified via simulation analysis. The simulation results show that the tool’s average cutting force and Standard deviation of cutting force increase as the drill bit’s back-rake and side-rake angles vary. This leads to increased shear friction on the core, which affects the maximum stress and mechanical specific energy, and, subsequently, the fidelity of the core. The back-rake angles ranged from 15° to 25°, with the optimum back-rake angle of 21° producing a maximum stress and a mechanical specific energy that were 0.69 and 0.85 times higher than the highest point, respectively. The side-rake angles range from 5° to 15°, with the optimum side-rake angle of 10° producing a maximum stress and a mechanical specific energy that were 0.76 and 0.96 times higher than the highest point, respectively. The finite element method error was 1.21%. This work’s main results will help reveal the fidelity mechanisms of the drilling process and contribute to the development of fidelity drill bits for complex surface drilling processes.

Design and Experimental Study of Core Bit for Hard Rock Drilling in Deep-Sea

The hard rock stratum will seriously affect the efficiency of deep-sea drilling and greatly increase the cost of drilling operations due to its complexity. In ocean drilling operations, the design of the bit is critical. Therefore, the core bit should be able to drill in a variety of formations, especially in hard rock stratum, with the function of reducing the need to change the bit due to formation changes and improving the drilling efficiency. In this paper, three different core bits were designed according to the hard rock stratum of the seabed: the roller bit, diamond bit and bionic bit. According to the test results, it was observed that: (1) It is fully proven that the roller core bit has the characteristics of stable drilling in hard strata, small torque and high drilling efficiency. The same method can realize the core in shallow soft strata through the setting of drilling parameters. (2) For the diamond bit, the field test data show that the new formula diamond bit has a good adaptability to the changing hard rock strata in this area, with an average rate of penetration (ROP) of 4.4 m/h and a bit life of 137.75 m. (3) For the bionic bit, field experiment data showed that drilling in the formation lithology was gray green tuff, which had a core recovery up to 100%. The average ROP was 1.35 m/h, and the highest ROP was 1.88 m/h. The design and experiment of three kinds of drill bits have effectively solved the technical problem of efficient drilling in deep-sea hard-rock stratum.

Experimental study and smoothed particle hydrodynamics simulation of synthetic diamond grit scratching on steel

Superabrasives like synthetic diamonds are indispensable in today’s construction industry, particularly for core drilling applications in reinforced concrete. The increase of the cutting efficiency of such core bits strongly depends on the material removal behavior of single diamond grains placed in a diamond segment acting as cutting elements. Therefore, this paper presents a newly developed experimental setup for a 3-mm single diamond grain scratch test, as well as the application of a Smoothed Particle Hydrodynamics (SPH) method to model single diamond grains scratching a rebar base material at operational parameters. Diamonds in two main different crystallographic and geometric orientations are tested and investigated with respect to their forces and temperatures at the cutting edge. Furthermore, the chip formation and different material removal mechanisms are analyzed for each orientation and compared with the model output. Validation tests and sensitivity analyses are performed in 3D and high resolution, thanks to the runtime acceleration of SPH through parallel computing on the graphics card. The results indicate: 1) A significantly different material removal behavior as a result of changing the diamond orientation; 2) The capability and high efficiency of SPH for modeling single grain scratch tests.

Design and flow field analysis of impregnated diamond bit for seafloor drill in soft-hard interlaced strata

According to the seafloor drill working conditions and the complex formation characteristics of the seafloor, this paper aims to improve the adaptability of the diamond bit to the formation and the coring quality. The cutter tooth design scheme of the impregnated diamond bit is proposed, and Fluent analyzes the flow field of drilling fluid at the bottom hole. The results show that the cone cutting tooth bit with the primary and assistant nozzles can directly avoid 85.33% drilling fluid flushing core and reduce the disturbance of drilling fluid to the core samples. The water passage structure of the bit is reasonable, and the upward return velocity of the drilling fluid can be stabilized between 0.7 and 1.8 m/s, which meets the requirements of the upward return of cuttings in seabed strata, and has a good hole wall protection performance. Based on the bottom-jet diamond bit, the assistant nozzles are added. The drilling fluid of the assistant nozzles can better cover the bit crown, conductive to cooling the bit crown. The drilling fluid of the main nozzles can timely up-return along the outer annulus hole wall, conducive to the up-return of the cuttings with drilling fluid. This study can extend existing designs of a seafloor coring bits and bottom hole flow field analysis methods.

Autonomous and ultrasonically assisted drilling in a range of rocks and ice

Drilling in extreme environments may require reductions in weight-on-bit, applied torque, or energy use, without compromising rate-of-progress. This paper examines the use of ultrasonic vibration, directly superimposed onto an augering coring bit, to achieve this goal in aircrete, limestone, marble, tuff, and ice.Compared to traditional rotary drilling processes using the same tool, the ultrasonically assisted drilling processes demonstrated improved rate-of-progress (∼400%) in all materials studied. In aircrete and limestone, there were also modest but consistent reductions in torque power demand and, at optimum vibration amplitudes, total energy consumption (∼25%). The other materials gave more mixed results: ultrasonically assisted drill cycles in marble were energy intensive, those in tuff were unpredictable due to the inconsistencies in that material, and those in ice led to the failure of the tooth bonding.

Cutting Migration and Core Disturbance Caused by the Hydraulic Structure of Pressure- and Gas-Maintaining Coring Bits

Gas disaster prevention and control is essential to coal mine safety production, with accurate measurement of gas pressure and content being the challenging key aspect. Pressure- and gas-maintaining coring devices can characterize cores in situ to achieve this accurate measurement. As an important part of these devices, the hydraulic structure of the coring bit plays a decisive role in the migration, coring rate, and core disturbance of the bottom-hole cuttings, and studying this structure is thus of great significance. In this paper, the fluid simulation software FLUENT is used to carry out numerical simulation of the bottom-hole flow field to study the cutting movement and core disturbance caused by coring bits with different hydraulic structures. The results show that (1) the best nozzle azimuth angle of 75° is beneficial to bottom-hole cutting discharge and reduces the scouring effect of the drilling fluid on the core. (2) The larger the diameter of the nozzle of the coring bit is, the weaker the cutting removal ability of the drill bit and the smaller the cutting scouring effect of the drilling fluid. The optimal nozzle diameter is 12 mm. (3) Adding a sluice channel inside the water barrier ring can reduce the erosion of drilling fluid on the core. (4) Bits with a rectangular inner passage promote cutting migration. Field application results show that the designed coring bit performs well, and the research results can provide a reference for the structure optimization of coring bits for pressure and gas retention.

Temperature Variation Law of Core Tube Wall during Coring in Different Strength Coal Seams: Experiment and Modelling

Temperature is the primary factor affecting the law of coal gas desorption. When the core method is used to measure the coal seam gas content (CSGS), the temperature of the coal core sample (CCS) will increase. The heat generated by the core bit cutting and rubbing the coal during coring is transferred to the CCS through the core tube, resulting in the temperature rising of the CCS. Because the CCS entering the core tube during coring is a dynamic process, the temperature of CCS is difficult to measure. To solve this problem, the temperatures of the core tube wall during coring in the Jiulishan coal mine (JLS), Guhanshan coal mine (GHS) and Zhaogu coal mine (ZG) at the core depth of 20 m were measured by the self-designed temperature measuring device. The thermodynamic models of the core bit and the core tube during coring were established. The reliability of the model was verified by comparing the numerical simulation results with the field measurement results. The verified model was used to predict the temperature changes of the core tube wall during coring in different strength coal seams and different core depths. The results show that the temperature change of the core tube wall was divided into a slowly temperature rising stage Ⅰ, a fast temperature rising stage Ⅱ and a slowly temperature rising and slowly temperature falling stage Ⅲ, which correspond to the process of pushing the core tube, drilling the CCS and early stage and later stage of withdrawing the core tube, respectively. The maximum temperature of the core tube wall appears in the first 3 min of withdrawing the core tube, and increases with the core depth increasing. The temperature of the measuring point at the end of drilling the CCS and the maximum temperature during coring linearly increase with the core depth. The temperature heating rate of the core tube is negatively linear, with the coal seam strength during pushing the core tube wall process. However, the temperature heating rate of the core tube wall is positively linear with the coal seam strength during drilling the CCS process. This study can provide a basis for further research on the dynamic distribution characteristics of temperature in the CCS during coring, which is of profound significance to calculate the gas loss amount and CSGC.

Temperature of the Core Tube Wall during Coring in Coal Seam: Experiment and Modeling.

Temperature is the primary factor affecting the law of coal gas desorption. When the core method is used to measure the coal seam gas content (CSGC), the temperature of the coal core sample (CCS) will increase because the heat generated by the core bit cutting and rubbing the coal is transferred to the CCS through the core tube. To solve the above problems, the temperature of the core tube wall during coring at core depths of 10, 20, and 30 m was measured by a self-designed temperature measuring device. The thermodynamic models of the core bit and the core tube during coring were established. The thermal flux of the system at different stages was inverted numerically by the dichotomy method. The reliability of the model was verified by comparing the numerical simulation results with the field measurement results. The main influencing factors during coring were studied by numerical simulations. The results show that the temperature change of the core tube wall goes through four stages: slowly rising, fast rising, slowly rising, and slowly falling, which correspond to the process of pushing the core tube, drilling the CCS, and the early stage and later stage of withdrawing the core tube, respectively. The maximum temperature of the core tube wall appears in the first 5 min of withdrawing the core tube and increases with the increase of core depth. When the core depth is 30 m, the maximum temperature of the core tube wall reaches 105.17 °C. The temperature of the measuring point at the end of drilling the CCS and the maximum temperature during coring linearly increase with the core depth, friction heat generated while pushing the core tube, and coal strength. This study can provide a basis for further research on the dynamic distribution characteristics of temperature in the CCS during coring, which is of profound significance to calculate the gas loss amount and CSGC.

Study on the wear mechanisms and rock destruction of core surface set bit in drilling for solid mineral exploration

In this paper, the authors present some research results of wearing process and rock destruction by diamond grit of core bit depending on drilling regime, rock hardness, diamond grit strength, etc. through the simulation of the interaction effect between diamond grit attached to the core bit and the rock. The relationship of the wear rate and rate of penetration of diamond core bit to the rotation per minute has been tested. On the basis of the research results, the authors have proposed solutions to select the appropriate technology for diamond drilling to improve the efficiency of solid mineral exploration in Vietnam.

Core Jamming in Unconsolidated Formations

In coring operations, core jamming is a permanent concern, taking place when the rock core sample becomes stuck within the inner core barrel, thus preventing further acquisition. This is undesirable in hard formations, for it stops the penetration of the drill bit and results in waste of time and money, especially if it occurs at the beginning of the core run. In unconsolidated formations, there are even more disadvantages, as the jammed sample may rotate along with the core bit, leading the formation to be flushed instead of acquired. This phenomenon is unnoticeable on the surface, and, as a result, the drillers are given no indication of it, thus hindering them from making the decision to pull out of the hole prematurely. Consequently, the characterization of the reservoir rock will be permanently incomplete due to the unrecovered material that was flushed. Despite its importance, the topic of core jamming has not been studied at length. This paper presents an analytical and experimental study in order to contribute for the understanding of jamming processes in unconsolidated or poorly consolidated formations. The study comprises the development of an analytical model, experiments and numerical analysis of the problem. Results are presented and implications for field applications are discussed.

Analysis of Rock Dynamic Stresses During the Drilling by Polycrystalline Diamond Compact Bits

In view of shortening the development period of polycrystalline diamond compact (PDC) bits, the finite element method was adopted to simulate the dynamic stress of rocks. By employing drilling related theories, the three dynamic principal stresses of rock were analysed and the dynamic rock-breaking criterion was established. Second, the drilling model of PDC core bit was constructed, and the stress was simulated and calculated. Finally, laboratory tests were carried out to verify the simulation results. The analytical results demonstrate that the two obvious stages in the rock-breaking process are the initial rock-breaking stage and the normal one. The dynamic rock-breaking stress in the normal drilling stage varies from 66.3 to 99.6 MPa, which is lower than 278.4 MPa in the initial rock-breaking stage. During spud drilling, the axial force and the tangential force are 1.85 and 1.60 kN, respectively. During normal drilling, the axial force ranges from 0.2 to 0.9 kN, and the tangential force from 0.15 to 0.6 kN. The load of normal drilling is lower than the spudding load, and the bit is more likely to be damaged during spudding. The bit is normally worn during normal drilling.

Apparent moduli and strength differences between cylindrical and prism specimens caused by edge confinement effects

Rock samples obtained from excavation and fault damage zones are friable, and it is challenging to obtain cylindrical specimens from such rocks using coring bits in the lab. On the other hand, preparation of prismatic specimens may be less destructive because it does not exert excessive rotational torque around the sides of the rock as in the preparation of cylindrical specimens. Therefore, prism-shaped specimens could become a more favorable sample geometry when working with highly fragile rock samples. However, prism specimens may experience shape-specific boundary effects during mechanical deformation due to the presences of corners and edges, which might cause differences in the measured rock strength and elastic properties compared to a cylindrical specimen. This study aims to investigate how the corners and edges of prism specimens affect the apparent strength and moduli of samples via comparison of laboratory data and finite element modelling results. We find that the prismatic samples can have higher apparent elastic moduli due to the enhanced confinement at the sample edges, which in turn also increases the measured compressional strengths of prism samples compared to cylindrical samples. Whether this apparent strengthening effect influences laboratory data interpretation will also depend on the degree of sample variability and how well specimen edge boundary conditions are controlled.

Effect of thrust force, torque, and induced temperature on Kevlar reinforced composites during drilling process

The present study aims at finding optimum drilling process parameters to reduce thrust Force and torque. Heat induces in drilling zone during drilling process has analysed by thermal image camera. The kevlar 49 reinforced epoxy composite specimen has utilized for experiments. Drill bits utilized for the experiments namely, Twist, Core, step cone, and candle stick. According to the experiment results feed rate is most influencing on thrust force followed by torque. A core and candlestick drill bits has produced lesser thrust force compared to other drill bits. Heat generated in drilling zone during drilling process has higher for step cone and standard twist drill bits compared to glass transition temperature of matrix. Hence, matrix has not able to providing necessary support to fiber in composite. Candle stick and Core drill bit has been providing lesser thrust force, torque and defect free machining at lower spindle speed and feed rate.

Energy analysis-based core drilling method for the prediction of rock uniaxial compressive strength

The uniaxial compressive strength (UCS) of rock is a basic parameter in underground engineering design. The disadvantages of this commonly employed laboratory testing method are untimely testing, difficulty in performing core testing of broken rock mass and long and complicated onsite testing processes. Therefore, the development of a fast and simple in situ rock UCS testing method for field use is urgent. In this study, a multi-function digital rock drilling and testing system and a digital core bit dedicated to the system are independently developed and employed in digital drilling tests on rock specimens with different strengths. The energy analysis is performed during rock cutting to estimate the energy consumed by the drill bit to remove a unit volume of rock. Two quantitative relationship models of energy analysis-based core drilling parameters (ECD) and rock UCS (ECD-UCS models) are established in this manuscript by the methods of regression analysis and support vector machine (SVM). The predictive abilities of the two models are comparatively analysed. The results show that the mean value of relative difference between the predicted rock UCS values and the UCS values measured by the laboratory uniaxial compression test in the prediction set are 3.76 MPa and 4.30 MPa, respectively, and the standard deviations are 2.08 MPa and 4.14 MPa, respectively. The regression analysis-based ECD-UCS model has a more stable predictive ability. The energy analysis-based rock drilling method for the prediction of UCS is proposed. This method realized the quick and convenient in situ test of rock UCS.