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

Authors: Zhenxi You; Ling Chen; Cong Li; Guikang Liu; Banghua Ye; Xiaojun Shi
Journal: Geofluids
 (2022)

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## Abstract

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.


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## Body

## 1. Introduction

Energy is an important material basis for human survival and development [1–6], and coal has played a dominant role in China’s energy consumption structure for a long time [7–10]. Among the proven reserves of China’s three major fossil energy and mineral resources, coal accounts for more than 94%, while oil and natural gas account for only approximately 6%. Such resource divisions indicate that China’s energy consumption structure is still dominated by coal [11–13]. However, in the process of coal production, production accidents caused by gas leakage occur frequently. From 2013 to 2018, the number of deaths caused by gas disasters in China was as high as 1200, accounting for 31.33% of the total number of deaths in coal mine accidents during the same period [14] (Figure 1). The gas content measured by traditional open gas exploration technology is theoretically lower than the actual gas content of the reservoir, which severely affects the scientific design and formulation of gas control schemes [15–17]. Therefore, accurately measuring the content of coal seam gas has become the key to the prevention and control of coal seam gas disasters and the efficient development and utilization of coalbed gas.Figure 1
Categorization of victims of coal mine accidents from 2013 to 2018.At present, the coring technology of open gas measurement is widely used forcoal samples, but this technology suffers from low core efficiency, easy loss of gas, and poor timeliness of measurement, so it is difficult to ensure the accuracy and effectiveness of gas content and pressure data and to effectively evaluate the risk characteristics of coal seams to guide engineering practice [18–20]. Compared to open gas measurement, the use of pressure- and gas-maintaining coring equipment can maintain the original gas pressure of the collected coal seam coal core in the process of withdrawing the bit and get accurate gas content [21].The core is the main source of data [22–27], so it must be of high quality. Coring bits are the key tool for drilling formations and forming cores. The core formation quality and speed directly affect the coring rate and thus the core quality. Therefore, it is very important to optimize the hydraulic structure of the coring bit.The bit bottom-hole flow field is complicated and variable, so it is difficult to carry out field experiments. However, with the development of numerical simulation technology, computational fluid dynamics (CFD) is often used to analyze the bottom-hole flow field of drill bits to guide the optimization of the drill-bit hydraulic structure [28–30]. For example, Chen et al. [31] improved the hydraulic structure of a coring bit with fluid simulation software, reduced the disturbance of drilling fluid to the core, and thus improved the core quality. Wu et al. [32] optimized the nozzle structure of the drill bit through fluid simulation to improve the cutting transport capacity of the drill bit and reduce bit wear. Meng [33] studied the flow field of coring bits with different hydraulic structures through numerical simulation and improved the core coring rate through the increase of bit’s chip removal ability and the reduction in drilling fluid disturbance to the core. The above research applies only to the coring bits in the specific application scenarios mentioned in this paper and is not applicable to other application scenarios.In conclusion, the coring rate can be improved by improving the cutting transport ability and reducing the disturbance of drilling fluid to the core. However, the structure of the coring bit is complex and diverse, and slight changes in the form of the bit structure lead to changes in the structure of the bottom-hole flow field. Therefore, the research results of one kind of coring bit cannot be applied to the flow field analysis of other coring bits [34]. In this paper, numerical simulation using the fluid simulation software FLUENT is carried out on the bottom-hole flow field of the coring bit used by a pressure and gas sealed coring device [35] (Figure 2) developed by Sichuan University. The influence of hydraulic structure parameters, such as the azimuth angle of the bit nozzle, diameter, and shape of the inner flow passage and sluice channel of the water barrier ring, on cutting movement and core disturbance was investigated. Hydraulic structure optimization of pressure and gas coring bits is proposed.Figure 2
Structure diagram of the core sampler.


## 2. Numerical Simulation Modeling and Evaluation Index Establishment

### 2.1. Simulation Model of Bottom-Hole Flow Field Drilling with a Coring Bit

The numerical simulation of the bottom-hole flow field of a coring bit relies on a bottom-hole flow field model based on the N–S equation (Equation (1)) of the computational fluid mechanics model.
(1)ρDuDt=−∂ρ∂x+∂τxx∂x+∂τzx∂y+∂τyx∂z+ρfx,ρDuDt=−∂ρ∂y+∂τxy∂x+∂τyy∂y+∂τzy∂z+ρfy,ρDuDt=−∂ρ∂z+∂τxz∂x+∂τyz∂y+∂τzz∂z+ρfz.Estimation of the Reynolds number (Re) of the drilling fluid flow indicates that the bottom-hole flow state of the coring bit is turbulent. Thek-ε turbulence model is used to simulate the flow of drilling fluid, and the KTGF model is used to simulate the movement of cuttings. Because the bottom-hole flow field movement involves the movement of the granular phase (cuttings) and continuous phase (drilling fluid), a discrete phase model based on the Euler–Lagrange method is used to simulate the coupled solid–liquid two-phase movement. The discrete phase model considers fluid as a continuous phase medium and particles as a discrete phase distributed in fluid and considers the interaction between particles and fluid and particles.
(2)Re=ρD‐Dpvμ,where ρ is the drilling fluid density, v is the average flow rate of the drilling fluid in the pipe, Dp is the outer diameter of the drill string, D is the inner diameter of the drill string, and μ is the viscosity of the drilling fluid.In the discrete phase model, the interaction between particles and fluid can be described by the force of particles in the fluid. The forces of particles in fluid can be divided into the following two categories: the forces unrelated to the relative movement between particles and fluid (inertia force, gravity, buoyancy, differential pressure force, thermos swimming force, electrophoresis force, etc.) and the forces (Basset force, drag force, additional mass force, Magnus force, Saffmans force, etc.) generated by the relative motion between fluid and particles [36–39].The interaction between particles can be calculated by the KTGF model (hard ball model) in FLUENT. The calculation process of the hard sphere model [40] is completely driven by the collision “events” between particles, that is, a series of two-body collisions drive changes in particle positions. The collision between two particles is an elastic collision, and the friction between particles obeys Coulomb’s law. The collision process is shown in Figure 3, and the momentum equation is as follows:
(3)m1v1−v10=J,m2v2−v20=−J,I1w1−w10=r1n×J,I2w2−w20=r2n×J,where J is the impulse exerted by particle 2 on particle 1 and n is the unit normal vector pointing from particle 1 to particle 2. Subscripts 1 and 2 represent colliding particles 1 and 2, respectively; superscript 0 represents the state before the collision; m is the particle mass; r is the particle radius; I is the moment of inertia.Figure 3
Particle collision diagram.


### 2.2. Coring Bit Mesh Division and Solution Setting

The SolidWorks 3D modeling software is used to establish the geometric model (Figure4). The geometric model is a fully water-isolated PDC coring bit designed for the pressure and gas coring device of Sichuan University. The maximum outer diameter of the core bit is 114 mm, the inner diameter is 104 mm, and the maximum outer diameter of the associated coring device is 89 mm. There are four nozzles and four blades on the coring bit, and any two adjacent blade surfaces and the bit base surface form a flow channel.Figure 4
Geometric model of the coring bit.The 3D model of the coring bit is imported into the preprocessing module DM of ANSYS, and the fluid computing domain is extracted by the filling command of DM. After obtaining the fluid computing domain, the fluid computing domain is imported into the mesh module of the ANSYS meshing tool, and tetrahedron was used as the meshing unit. The number of grid elements is approximately 190,000, and the number of grid nodes is approximately 40,000. The grid after partition is shown in Figure5.Figure 5
Meshing diagram of the coring bit.The basic parameters of the numerical simulation are shown in Table1.Table 1
Numerical simulation calculation parameters.
Numerical simulation conditionsModelParameterTurbulence modelEulerian/Maximum number of particle tracking steps/50000Residual/1e-5DPM physical modelsSaffman force, virtual mass force, pressure gradient forceFlow field outlet conditionsPressure-outlet3.5 MpaBit speedSRF120 r/minInlet conditions of flow fieldMass-flow-inlet5 L/sDrilling fluid density/1300 kg/m3Viscosity of drilling fluid/0.03 kg/m-sDebris particlesDDPMDiameter: 0.6 mmDensity: 1400 kg/m3


### 2.3. Establishment of Cutting Migration Ability and Core Disturbance Evaluation Index

The bottom-hole flow of the drilling fluid is shown in Figure6. The drilling fluid flows out of the nozzle of the coring bit and the passage between the barrier ring and the core. The largest proportion of drilling fluid flow is from the nozzle, which mainly plays a role in chip removal and cooling of external PDC cutters. The flow of drilling fluid from the passage between the riser and the core is small, which mainly lubricates the core and prevents core blockage but also causes core erosion. To improve the coring rate of the pressure and gas retaining core bit, this paper takes improving the cutting movement ability of the coring bit and reducing the scouring effect of the drilling fluid on the core as the optimization objectives.Figure 6
Drilling fluid flow diagram.Good cutting removal relies on the ability of the drilling fluid to remove cuttings from the bottom hole quickly and prevent bit balling. In this paper, cutting retention is used as an index to measure the cutting removal ability. The residual mass of cuttings refers to the mass of cuttings remaining in the annulus at the bottom of the coring bit. The maximum velocity of the drilling fluid near the core is taken as an index to measure the disturbance of the drilling fluid to the core.The flow of drilling fluid near the coring bit is turbulent. In the turbulent flow state, the normal velocity near the wall has a very large velocity gradient. At different distances along the wall normal, the fluid flow can be divided into the near-wall region and the core region. The flow in the core region is completely turbulent, and the fluid flow velocity is the maximum in this region. In the near-wall region, fluid movement is affected by wall flow conditions and can be divided into three subregions: the viscous region, buffer region, and log-law region (Figure7). As shown in Figure 7, the horizontal axis represents the wall surface, and the y-axis is the distance between the fluid and the wall surface.Figure 7
Division of the flow area near the wall surface in the turbulent flow state.By solving Equation (4), the normal distance y between the core areas where drilling fluid flows near the core and the core wall surface can be calculated. The region where the normal distance from the core wall greater than y is the region with the maximum fluid flow velocity. By solving the maximum velocity of a section whose normal distance from the core is greater than y, we can obtain the maximum velocity of the drilling fluid near the core.
(4)y+=yμρτw,where τw is the shear stress on the wall, y is the normal distance between the fluid and the wall, ρ is the density of the fluid, μ is the dynamic viscosity of the fluid, and y+ is a dimensionless number. When y+>300, this region is the turbulent core region.


## 2.1. Simulation Model of Bottom-Hole Flow Field Drilling with a Coring Bit

The numerical simulation of the bottom-hole flow field of a coring bit relies on a bottom-hole flow field model based on the N–S equation (Equation (1)) of the computational fluid mechanics model.
(1)ρDuDt=−∂ρ∂x+∂τxx∂x+∂τzx∂y+∂τyx∂z+ρfx,ρDuDt=−∂ρ∂y+∂τxy∂x+∂τyy∂y+∂τzy∂z+ρfy,ρDuDt=−∂ρ∂z+∂τxz∂x+∂τyz∂y+∂τzz∂z+ρfz.Estimation of the Reynolds number (Re) of the drilling fluid flow indicates that the bottom-hole flow state of the coring bit is turbulent. Thek-ε turbulence model is used to simulate the flow of drilling fluid, and the KTGF model is used to simulate the movement of cuttings. Because the bottom-hole flow field movement involves the movement of the granular phase (cuttings) and continuous phase (drilling fluid), a discrete phase model based on the Euler–Lagrange method is used to simulate the coupled solid–liquid two-phase movement. The discrete phase model considers fluid as a continuous phase medium and particles as a discrete phase distributed in fluid and considers the interaction between particles and fluid and particles.
(2)Re=ρD‐Dpvμ,where ρ is the drilling fluid density, v is the average flow rate of the drilling fluid in the pipe, Dp is the outer diameter of the drill string, D is the inner diameter of the drill string, and μ is the viscosity of the drilling fluid.In the discrete phase model, the interaction between particles and fluid can be described by the force of particles in the fluid. The forces of particles in fluid can be divided into the following two categories: the forces unrelated to the relative movement between particles and fluid (inertia force, gravity, buoyancy, differential pressure force, thermos swimming force, electrophoresis force, etc.) and the forces (Basset force, drag force, additional mass force, Magnus force, Saffmans force, etc.) generated by the relative motion between fluid and particles [36–39].The interaction between particles can be calculated by the KTGF model (hard ball model) in FLUENT. The calculation process of the hard sphere model [40] is completely driven by the collision “events” between particles, that is, a series of two-body collisions drive changes in particle positions. The collision between two particles is an elastic collision, and the friction between particles obeys Coulomb’s law. The collision process is shown in Figure 3, and the momentum equation is as follows:
(3)m1v1−v10=J,m2v2−v20=−J,I1w1−w10=r1n×J,I2w2−w20=r2n×J,where J is the impulse exerted by particle 2 on particle 1 and n is the unit normal vector pointing from particle 1 to particle 2. Subscripts 1 and 2 represent colliding particles 1 and 2, respectively; superscript 0 represents the state before the collision; m is the particle mass; r is the particle radius; I is the moment of inertia.Figure 3
Particle collision diagram.


## 2.2. Coring Bit Mesh Division and Solution Setting

The SolidWorks 3D modeling software is used to establish the geometric model (Figure4). The geometric model is a fully water-isolated PDC coring bit designed for the pressure and gas coring device of Sichuan University. The maximum outer diameter of the core bit is 114 mm, the inner diameter is 104 mm, and the maximum outer diameter of the associated coring device is 89 mm. There are four nozzles and four blades on the coring bit, and any two adjacent blade surfaces and the bit base surface form a flow channel.Figure 4
Geometric model of the coring bit.The 3D model of the coring bit is imported into the preprocessing module DM of ANSYS, and the fluid computing domain is extracted by the filling command of DM. After obtaining the fluid computing domain, the fluid computing domain is imported into the mesh module of the ANSYS meshing tool, and tetrahedron was used as the meshing unit. The number of grid elements is approximately 190,000, and the number of grid nodes is approximately 40,000. The grid after partition is shown in Figure5.Figure 5
Meshing diagram of the coring bit.The basic parameters of the numerical simulation are shown in Table1.Table 1
Numerical simulation calculation parameters.
Numerical simulation conditionsModelParameterTurbulence modelEulerian/Maximum number of particle tracking steps/50000Residual/1e-5DPM physical modelsSaffman force, virtual mass force, pressure gradient forceFlow field outlet conditionsPressure-outlet3.5 MpaBit speedSRF120 r/minInlet conditions of flow fieldMass-flow-inlet5 L/sDrilling fluid density/1300 kg/m3Viscosity of drilling fluid/0.03 kg/m-sDebris particlesDDPMDiameter: 0.6 mmDensity: 1400 kg/m3


## 2.3. Establishment of Cutting Migration Ability and Core Disturbance Evaluation Index

The bottom-hole flow of the drilling fluid is shown in Figure6. The drilling fluid flows out of the nozzle of the coring bit and the passage between the barrier ring and the core. The largest proportion of drilling fluid flow is from the nozzle, which mainly plays a role in chip removal and cooling of external PDC cutters. The flow of drilling fluid from the passage between the riser and the core is small, which mainly lubricates the core and prevents core blockage but also causes core erosion. To improve the coring rate of the pressure and gas retaining core bit, this paper takes improving the cutting movement ability of the coring bit and reducing the scouring effect of the drilling fluid on the core as the optimization objectives.Figure 6
Drilling fluid flow diagram.Good cutting removal relies on the ability of the drilling fluid to remove cuttings from the bottom hole quickly and prevent bit balling. In this paper, cutting retention is used as an index to measure the cutting removal ability. The residual mass of cuttings refers to the mass of cuttings remaining in the annulus at the bottom of the coring bit. The maximum velocity of the drilling fluid near the core is taken as an index to measure the disturbance of the drilling fluid to the core.The flow of drilling fluid near the coring bit is turbulent. In the turbulent flow state, the normal velocity near the wall has a very large velocity gradient. At different distances along the wall normal, the fluid flow can be divided into the near-wall region and the core region. The flow in the core region is completely turbulent, and the fluid flow velocity is the maximum in this region. In the near-wall region, fluid movement is affected by wall flow conditions and can be divided into three subregions: the viscous region, buffer region, and log-law region (Figure7). As shown in Figure 7, the horizontal axis represents the wall surface, and the y-axis is the distance between the fluid and the wall surface.Figure 7
Division of the flow area near the wall surface in the turbulent flow state.By solving Equation (4), the normal distance y between the core areas where drilling fluid flows near the core and the core wall surface can be calculated. The region where the normal distance from the core wall greater than y is the region with the maximum fluid flow velocity. By solving the maximum velocity of a section whose normal distance from the core is greater than y, we can obtain the maximum velocity of the drilling fluid near the core.
(4)y+=yμρτw,where τw is the shear stress on the wall, y is the normal distance between the fluid and the wall, ρ is the density of the fluid, μ is the dynamic viscosity of the fluid, and y+ is a dimensionless number. When y+>300, this region is the turbulent core region.


## 3. Influence Law of Coring Bits with Different Hydraulic Structures on Cutting Migration and Core Disturbance

### 3.1. Influence of Nozzle Azimuth Angle on Cutting Migration and Core Disturbance

The drilling fluid mainly flows out from the nozzle, and the nozzle angle directly affects the flow direction of the drilling fluid. Therefore, it is very important to study the influence of the nozzle angle change on the bottom-hole flow field of the coring bit. Under the condition that the diameter of the nozzle and other parameters remain unchanged, different coring bits can be obtained by adjusting the azimuth angle of the nozzle (the included angle formed by the axis line of the nozzle and the vertical line of the axis line of the coring bit). The coring bit models with different nozzle azimuth angles (Figure8) are simulated numerically.Figure 8
Coring bit model with different nozzle azimuth angles.
(a)(b)(c)(d)(e)(f)Figure9 shows the residual mass of cuttings and the maximum drilling fluid velocity near the core at the different azimuth angle of the nozzle. With increasing nozzle azimuth, the residual mass of cuttings first decreases and then increases. The lowest value of the residual mass at the nozzle azimuth angle is 75°. The maximum velocity of the drilling fluid near the core decreases with increasing nozzle azimuth.Figure 9
Residual mass of cuttings and maximum velocity of drilling fluid near core at different nozzle azimuth angles.Figure10 shows the drilling fluid flow diagram under the coring bit with different nozzle azimuth angles. When the azimuth of the nozzle is 15 degrees, there are two notable backflow zones in the bottom-hole flow field. However, with increasing nozzle azimuth, the two backflow zones decrease until the nozzle azimuth increases to more than 75°, and there is only one backflow zone. With fewer backflow zones, cuttings are more easily pushed out of the bottom hole. However, as the azimuth of the nozzle increases from 75° to 90°, the drilling fluid flowing from the nozzle washes directly to the bottom of the well, creating a large backflow zone at the bottom of the well and resulting in a decrease in the cutting removal capacity.Figure 10
Drilling fluid flow diagram of the coring bit with different nozzle azimuth angles.In conclusion, the 75° nozzle azimuth enables the coring bit to discharge cuttings well and reduce the disturbance of drilling fluid to the core. Compared with the worst nozzle azimuth (15°), the optimal nozzle azimuth (75°) improves cuttings transport by 28.6% and reduces the maximum drilling fluid velocity near the core by 5.1%. Therefore, it is reasonable to set the nozzle azimuth at 75°.


### 3.2. Influence of Nozzle Diameter on Cutting Migration and Core Disturbance

Similar to the nozzle azimuth angle, the change in nozzle (Figure11) diameter also has a direct influence on the flow of the drilling fluid, so it is very important to study the influence of the change in nozzle diameter on the bottom-hole flow field of the coring bit. The bottom-hole flow field of coring bits with nozzles of 9, 10, 11, 12, 13, and 14 mm is simulated with other structures unchanged.Figure 11
Schematic diagram of the nozzle of a coring bit.Figure12 shows the residual mass of the cuttings and the maximum drilling fluid velocity near the core under different nozzle diameters. The residual mass of cuttings and the maximum drilling fluid velocity near the core change differently with increasing nozzle diameter. First, with increasing nozzle diameter, the bottom-hole residual mass of the cuttings gradually increases. This is because the larger the nozzle diameter is, the smaller the initial kinetic energy of the drilling fluid as it flows out of the nozzle, and the smaller the initial kinetic energy of the drilling fluid is, the lower the cutting transport capacity. Moreover, the maximum velocity of the drilling fluid near the core decreases with increasing nozzle diameter. As the nozzle diameter increases, more drilling fluid flows out of the nozzle, resulting in a decrease in the flow rate of the drilling fluid from the vicinity of the core and a decrease in the maximum velocity of the drilling fluid near the core.Figure 12
Residual mass of cuttings and maximum drilling fluid velocity near the core at different nozzle diameters.When the nozzle diameter is 12 mm, the coring bit has good cutting transport ability and low core disturbance. At this time, the maximum velocity of the drilling fluid near the core is 17.3% lower than the maximum value in the control group, and the cutting transport capacity is 7.3% lower than the optimal value in the control group. Therefore, the nozzle diameter of 12 mm is reasonable.


### 3.3. Influence of Sluice Channels on Cutting Migration and Core Disturbance

As shown in Figure13, an inner flow channel of the drilling fluid is formed between the water barrier ring at the lower part of the coring bit and the core. The change in the inner wall structure of the water barrier ring directly affects the flow of drilling fluid near the core, which impacts the core.Figure 13
Model diagram of (a) coring bit with a sluice channel and (b) coring bit without a sluice channel.
(a)(b)Figure14 shows a cloud image of the drilling fluid velocity near the core of the coring bit with and without the sluice channel. The drilling fluid flows more smoothly with a sluice channel added to the inner wall of the coring bit’s baffle ring. The average drilling fluid velocity near the core of the coring bit with the sluice channel was 0.5 m/s lower than that of the coring bit without the sluice channel. Therefore, a water trough can be added to the barrier ring to reduce the disturbance of drilling fluid to the core.Figure 14
Cloud image of the drilling fluid velocity near the core of the coring bit (a) with a sluice channel and (b) without a sluice channel.
(a)(b)


### 3.4. Influence of Internal Flow Shape on Cutting Migration and Core Disturbance

The shape of the inner flow passage affects the flow of drilling fluid inside the coring bit. Changes in the shape of the inner flow passage directly affect the inner flow field of the coring bit and further affect the cutting migration and core. In this paper, a coring bit with cone and rectangular (Figure15) inner flow channels is designed for the coring bit. The two types of coring bits have the same geometrical parameters, such as nozzle diameter, nozzle azimuth angle, and bit diameter, except for different inner flow channels.Figure 15
Sectional drawings of (a) rectangular inner flow passage and (b) trapezoidal inner flow passage.
(a)(b)Figure16 shows the cloud diagram of drilling fluid velocity near the core of two types of coring drill bits with different inner passages. With the trapezoidal inner passage, the flow near the core is more intense, but the maximum drilling fluid velocity is smaller. However, by comparing the flow charts of rectangular and trapezoidal inner flow channels (Figure 17), it can be found that, compared with the case of trapezoidal inner flow channels, the backflow zone of rectangular inner flow channels moves down significantly. This shift makes it easier for cuttings to be carried to the hole-outside by the drilling fluid. Considering the cutting migration ability and core disturbance caused by the drilling fluid, the coring bit with rectangular inner passage is more advantageous.Figure 16
(a) Trapezoidal inner passage and (b) rectangular inner passage cloud image of drilling fluid velocity near the core.
(a)(b)Figure 17
Drilling fluid flow diagram of (a) trapezoidal inner passage section of (b) rectangular inner passage.
(a)(b)


## 3.1. Influence of Nozzle Azimuth Angle on Cutting Migration and Core Disturbance

The drilling fluid mainly flows out from the nozzle, and the nozzle angle directly affects the flow direction of the drilling fluid. Therefore, it is very important to study the influence of the nozzle angle change on the bottom-hole flow field of the coring bit. Under the condition that the diameter of the nozzle and other parameters remain unchanged, different coring bits can be obtained by adjusting the azimuth angle of the nozzle (the included angle formed by the axis line of the nozzle and the vertical line of the axis line of the coring bit). The coring bit models with different nozzle azimuth angles (Figure8) are simulated numerically.Figure 8
Coring bit model with different nozzle azimuth angles.
(a)(b)(c)(d)(e)(f)Figure9 shows the residual mass of cuttings and the maximum drilling fluid velocity near the core at the different azimuth angle of the nozzle. With increasing nozzle azimuth, the residual mass of cuttings first decreases and then increases. The lowest value of the residual mass at the nozzle azimuth angle is 75°. The maximum velocity of the drilling fluid near the core decreases with increasing nozzle azimuth.Figure 9
Residual mass of cuttings and maximum velocity of drilling fluid near core at different nozzle azimuth angles.Figure10 shows the drilling fluid flow diagram under the coring bit with different nozzle azimuth angles. When the azimuth of the nozzle is 15 degrees, there are two notable backflow zones in the bottom-hole flow field. However, with increasing nozzle azimuth, the two backflow zones decrease until the nozzle azimuth increases to more than 75°, and there is only one backflow zone. With fewer backflow zones, cuttings are more easily pushed out of the bottom hole. However, as the azimuth of the nozzle increases from 75° to 90°, the drilling fluid flowing from the nozzle washes directly to the bottom of the well, creating a large backflow zone at the bottom of the well and resulting in a decrease in the cutting removal capacity.Figure 10
Drilling fluid flow diagram of the coring bit with different nozzle azimuth angles.In conclusion, the 75° nozzle azimuth enables the coring bit to discharge cuttings well and reduce the disturbance of drilling fluid to the core. Compared with the worst nozzle azimuth (15°), the optimal nozzle azimuth (75°) improves cuttings transport by 28.6% and reduces the maximum drilling fluid velocity near the core by 5.1%. Therefore, it is reasonable to set the nozzle azimuth at 75°.


## 3.2. Influence of Nozzle Diameter on Cutting Migration and Core Disturbance

Similar to the nozzle azimuth angle, the change in nozzle (Figure11) diameter also has a direct influence on the flow of the drilling fluid, so it is very important to study the influence of the change in nozzle diameter on the bottom-hole flow field of the coring bit. The bottom-hole flow field of coring bits with nozzles of 9, 10, 11, 12, 13, and 14 mm is simulated with other structures unchanged.Figure 11
Schematic diagram of the nozzle of a coring bit.Figure12 shows the residual mass of the cuttings and the maximum drilling fluid velocity near the core under different nozzle diameters. The residual mass of cuttings and the maximum drilling fluid velocity near the core change differently with increasing nozzle diameter. First, with increasing nozzle diameter, the bottom-hole residual mass of the cuttings gradually increases. This is because the larger the nozzle diameter is, the smaller the initial kinetic energy of the drilling fluid as it flows out of the nozzle, and the smaller the initial kinetic energy of the drilling fluid is, the lower the cutting transport capacity. Moreover, the maximum velocity of the drilling fluid near the core decreases with increasing nozzle diameter. As the nozzle diameter increases, more drilling fluid flows out of the nozzle, resulting in a decrease in the flow rate of the drilling fluid from the vicinity of the core and a decrease in the maximum velocity of the drilling fluid near the core.Figure 12
Residual mass of cuttings and maximum drilling fluid velocity near the core at different nozzle diameters.When the nozzle diameter is 12 mm, the coring bit has good cutting transport ability and low core disturbance. At this time, the maximum velocity of the drilling fluid near the core is 17.3% lower than the maximum value in the control group, and the cutting transport capacity is 7.3% lower than the optimal value in the control group. Therefore, the nozzle diameter of 12 mm is reasonable.


## 3.3. Influence of Sluice Channels on Cutting Migration and Core Disturbance

As shown in Figure13, an inner flow channel of the drilling fluid is formed between the water barrier ring at the lower part of the coring bit and the core. The change in the inner wall structure of the water barrier ring directly affects the flow of drilling fluid near the core, which impacts the core.Figure 13
Model diagram of (a) coring bit with a sluice channel and (b) coring bit without a sluice channel.
(a)(b)Figure14 shows a cloud image of the drilling fluid velocity near the core of the coring bit with and without the sluice channel. The drilling fluid flows more smoothly with a sluice channel added to the inner wall of the coring bit’s baffle ring. The average drilling fluid velocity near the core of the coring bit with the sluice channel was 0.5 m/s lower than that of the coring bit without the sluice channel. Therefore, a water trough can be added to the barrier ring to reduce the disturbance of drilling fluid to the core.Figure 14
Cloud image of the drilling fluid velocity near the core of the coring bit (a) with a sluice channel and (b) without a sluice channel.
(a)(b)


## 3.4. Influence of Internal Flow Shape on Cutting Migration and Core Disturbance

The shape of the inner flow passage affects the flow of drilling fluid inside the coring bit. Changes in the shape of the inner flow passage directly affect the inner flow field of the coring bit and further affect the cutting migration and core. In this paper, a coring bit with cone and rectangular (Figure15) inner flow channels is designed for the coring bit. The two types of coring bits have the same geometrical parameters, such as nozzle diameter, nozzle azimuth angle, and bit diameter, except for different inner flow channels.Figure 15
Sectional drawings of (a) rectangular inner flow passage and (b) trapezoidal inner flow passage.
(a)(b)Figure16 shows the cloud diagram of drilling fluid velocity near the core of two types of coring drill bits with different inner passages. With the trapezoidal inner passage, the flow near the core is more intense, but the maximum drilling fluid velocity is smaller. However, by comparing the flow charts of rectangular and trapezoidal inner flow channels (Figure 17), it can be found that, compared with the case of trapezoidal inner flow channels, the backflow zone of rectangular inner flow channels moves down significantly. This shift makes it easier for cuttings to be carried to the hole-outside by the drilling fluid. Considering the cutting migration ability and core disturbance caused by the drilling fluid, the coring bit with rectangular inner passage is more advantageous.Figure 16
(a) Trapezoidal inner passage and (b) rectangular inner passage cloud image of drilling fluid velocity near the core.
(a)(b)Figure 17
Drilling fluid flow diagram of (a) trapezoidal inner passage section of (b) rectangular inner passage.
(a)(b)


## 4. Field Application

According to the simulation results, the following structural optimization is carried out for the coring bit: The azimuth of the nozzle is set as 75°; the nozzle diameter design is 12 mm; a rectangular inner flow channel structure is used; a sluice channel is added inside the baffle ring. The optimized coring bit achieves good results in the Pingdingshan coring experiment (Figure18).Figure 18
(a) Pingdingshan coring site and (b) coring device integration physical diagram.
(a)(b)


## 5. Conclusion

In this paper, the numerical simulation software FLUENT is used to optimize the structure of a pressure- and gas-maintaining coring bit, which provides a reference for the hydraulic structure design of the bit. The main research achievements are mainly reflected in the following aspects:(1)
Over a certain range, increasing the azimuth angle of the drilling fluid nozzle is beneficial to the bottom-hole cutting discharge and reduces the disturbance of the drilling fluid to the core. The optimum nozzle azimuth is 75°. Compared with the worst nozzle azimuth (15°), the optimal nozzle azimuth (75°) improves cutting transport by 28.6% and reduces the maximum drilling fluid velocity near the core by 5.1%(2)
The larger the diameter of the drilling fluid nozzle is, the better the cutting removal ability of the drill bit, and the stronger the cutting scouring effect of the drilling fluid. Considering the two aspects, the nozzle diameter of 12 mm is the best. At this time, the maximum velocity of the drilling fluid near the core decreases by 17.3% compared with the maximum value in the control group, and the cutting transport capacity decreases by only 7.3% compared with the optimal value in the control group(3)
Adding a sluice channel to the inside of the riser can reduce the scouring of the core by the drilling fluid, and the average drilling fluid velocity near the core of the coring bit with a sluice channel is 0.5 m/s lower than that of the coring bit without a sluice channel(4)
Compared with trapezoidal bit, the backflow area of drilling fluid in rectangular channel moves downward, and residual debris moves from the hole bottom to the top more easily



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*Source: 9728046-2022-05-04.xml*