Research on Cuttings Carrying Principle of New Aluminum Alloy Drill Pipe and Numerical Simulation Analysis

Abstract

In order to improve the cuttings transport ability and well hole purification effect of horizontal shale gas wells in the Sichuan and Chongqing area, a new type of aluminum alloy drill pipe is put forward, and the floatability is validated by theoretical analysis and actual parameter calculation. According to the cuttings migration mechanism, the cuttings cleaning simulation model of the new aluminum alloy drill pipe and the traditional steel drill pipe was established by Fluent, and the hexahedral mesh is used to divide the model. Thus, the mesh independence and convergence of the two models are verified. Then, the simulation model is verified by comparing the calculation results of the two simulation models with the experimental data of the indoor cuttings migration device. Finally, the rock-cleaning ability of the two drill pipes is analyzed under the conditions of changing the cuttings particle size, well inclination angle, displacement, and mechanical speed. Compared with the traditional steel drill pipe, the new aluminum alloy drill pipe can improve the borehole purification capacity by 13% on average. This research result is of great significance in reducing the quality of cuttings in the annulus and improving the borehole purification effect.

1. Introduction

With the deepening of oil and gas exploration and development resources, there are deeper and ultra-deep wells. Rock hardness, drillability level, and grinding ability are becoming higher and higher, which seriously affects the mechanical drilling speed and exploration and development cost of deep hard formation. The formation is hard and poor drillability and low mechanical drilling speed problems will lead to difficult debris return, poor drilling, blocking card, and other serious phenomena. Therefore, it is urgent to study the principle of cut bearing in the drilling process.

As clean unconventional fossil energy, shale gas has gradually drawn people’s attention due to its long exploitation life and low pollution degree [1]. It has been proved that the shale gas reserves in the Sichuan-Chongqing area have reached 100 billion cubic meters [2,3] due to the emergence of new geological equipment such as aluminum alloy drill pipe [4] (after calculation, the drilling depth can reach 50,000 m). The drilling depth in the Sichuan-Chongqing region has gradually advanced from a deep well (4500–6000 m) to an ultra-deep well (6000–9000 m) [5]. With the increase in drilling depth, shale gas wells are facing problems such as high pressure, long string, large well diameter, and difficult migration of cuttings [6]. According to statistics, many stuck drilling accidents are caused by insufficient hole cleanliness [7]. Therefore, reducing the cuttings settlement in the hole, improving the cuttings migration speed, and reducing the formation of the cuttings bed are still urgent problems to be solved in drilling engineering [8].

Fully understanding the mechanism of cuttings migration and improving the cuttings carrying capacity of tools are the basis for ensuring underground safe operation. Therefore, researchers at home and abroad have done a lot of research. In terms of the study of cuttings migration mechanism, Wang et al. [9] studied cuttings migration in an eccentric annulus of a horizontal well from the microscopic point of view. When the annulus fluid reaction velocity is different, cuttings will move in the form of contact mass, saltation mass, layer mass, and suspended mass. Sorgun et al. [10] imported the cuttings migration model into Fluent to solve the flow field and concluded that the rotation of the drill pipe did not have a significant impact on the pressure gradient of Newtonian fluid in the eccentric annulus but could reduce the pressure gradient of non-Newtonian fluid. Amanna et al. [11] used Computational Fluid Dynamics (CFD) and experiments to study the effects of different flow rates, different drill pipe speeds, different cutting sizes, and inclination angles on cuttings migration and found that cuttings are the most difficult to clean when the well inclination angle is between 45° and 60°. In the research of cuttings cleaning device, Wu et al. [12] compared and analyzed the performance of two kinds of cuttings cleaning tools and concluded that the “V” type tool is suitable for cleaning cuttings with small particle size, while the spiral type tool is suitable for cleaning cuttings with large particle size. Puymbroeck et al. [13] proposed a borehole cleaning tool with a composite blade and verified that the tool could effectively improve cuttings transport efficiency through experiments. Pang et al. [14] studied the impact of pulsed jet drilling on cuttings migration. By analyzing the amplitude and frequency changes of drilling fluid inlet velocity, they compared the simulation results with traditional drilling and concluded that pulsed jet drilling was beneficial to improving cuttings migration velocity. In terms of the study of liquid–solid two-phase flow, Gao et al. [15], based on the gas–solid two-phase flow model, used CFD to simulate the migration characteristics of single-particle cuttings and cuttings particle groups under different working conditions from the perspective of microscopic movement of cuttings particles. It is found that the movement of single particle cuttings is mainly saltation, while the movement of particle groups is mainly creep and saltation. Mohammadreza et al. [16] used experiments and the Euler particle method to simulate the cuttings migration process of coiled tubing technology (CTT) and studied the influence of different parameters on cuttings migration. To sum up, the existing cuttings cleaning tools (pipe) with spiral wing structures mainly adopt mechanical cleaning and use a device around the rotation of the enhancement that is fluent, through stirring send cuttings at the bottom of the annular ring high-speed air flow area, but quite a number of debris particles did not reach the area to fall to the bottom hole, often needing tools to clean up many times. In the current simulation analysis of cuttings movement, most of the drilling fluid and cuttings particles in the annulus are considered as a liquid–solid two-phase flow, and the fluid force of drilling fluid on cuttings particles is ignored.

To sum up, a new type of aluminum alloy drill rod is proposed in this paper. The new structure drill pipe and the traditional steel drill pipe were simulated. Considering the interaction of drilling fluid and cuttings particles, the cuttings migration process was accurately simulated by using CFD-DEM (discrete element method) coupling. Finally, the rock-carrying (cuttings migration ratio) effect of the two drill pipes was compared. Consequently, the cuttings transport capacity and hole-cleaning effect of the aluminum alloy drill pipe of the drill pipe is improved.

2. Models and Equations

2.1. Structure and Rock-Carrying Principle of a New Aluminum Alloy Drill Pipe

2.1.1. Structure of a New Aluminum Alloy Drill Pipe

The new aluminum alloy drill pipe is a new rock-carrying tool (first proposed in this paper). Its structure is shown in Figure 1. The drill pipe is composed of a male joint, drill pipe body, hollow pipe (the hollow pipe is threaded with the chuck welded inside the male joint and female joint, respectively, during assembly), and female joint. The new aluminum alloy drill pipe is made of aluminum alloy as a whole, so its weight is relatively light. In order to prove the suspension of the drill pipe, the mechanical analysis of the new aluminum alloy drill pipe was carried out in the vertical direction:

$$F=F_{mjt}+F_{zg}+F_{gjt}+F_{kxg}$$

(1)

$$G=G_{mjt}+G_{zg}+G_{gjt}+G_{kxg}+G_{zjy}$$

(2)

To make the total gravity of the new aluminum alloy drill pipe less than the buoyancy of the drilling fluid, simply:

$$F>G$$

(3)

In the formula, F is the total buoyancy of the new aluminum alloy drill pipe, kN; $F_{mjt}$ is the buoyancy of the female joint in the drilling fluid, kN; $F_{zg}$ is the buoyancy of the drill pipe body in the drilling fluid, kN; $F_{gjt}$ is the buoyancy of the joint in drilling fluid, kN; $F_{kxg}$ is the buoyancy of the hollow rod in drilling fluid (the fluid inside the drill pipe), kN; G is the total gravity of the suspended aluminum alloy drill pipe, kN; $G_{mjt}$ is the self-gravity of the female joint, kN; $G_{zg}$ is the self-gravity of the drill pipe body, kN; $G_{gjt}$ is the self-gravity of the joint, kN; $G_{kxg}$ is the self-gravity of the hollow tube, kN; $G_{zjy}$ is the gravity of drilling fluid inside the drill pipe, kN.

Through the above theoretical analysis and actual parameter values (as shown in

Table 1. Actual parameters of new aluminum alloy drill pipe.

The Structural Parameters	Unit	Numerical
Outer diameter of drill pipe	mm	140
Inner diameter of drill pipe	mm	114
Drill pipe length	m	8.7
Outer diameter of hollow pipe	mm	70
Inner diameter of hollow tube	mm	60
Length of hollow tube	m	9.1
Aluminum alloy density	kg/${\mathrm{m}}^3$	2780
Drilling fluid density	kg/${\mathrm{m}}^3$	2200
Male joint weight	kN	0.102
Female joint weight	kN	0.111

), it is seen that the gravity of the new aluminum alloy drill pipe is less than the buoyancy of the drilling fluid.

2.1.2. Rock-Carrying Principle of a New Aluminum Alloy Drill Pipe

Because the cuttings particle density is always greater than the drilling fluid density, the cuttings particles will always settle toward the bottom of the annulus. Because the conventional steel drill pipe is tilted lower in the annulus due to gravity, the high-velocity zone is located above the annulus, the low-velocity zone is located below the annulus, and the cuttings bed is located below the annulus, resulting in cuttings moving in three layers (contact, saltation, layering, and suspension). The new aluminum alloy drill pipe, due to buoyancy, is inclined to the upper part of the annulus, causing a relative shift in the position of the high-speed and low-speed fluid zones. This allows the high-speed fluid zone to be located in the lower part of the annulus, almost at the same height as the cuttings bed. In this case, the high-speed drilling fluid can easily carry cuttings toward the wellhead.

Table 2. Rock-carrying principle of new aluminum alloy drill pipe and traditional steel drill pipe.

Drill Pipe	Cloud Image of Axial Velocity of Drilling Fluid	Cloud Image of Debris Volume Fraction
Conventional steel drill pipe
New aluminum alloy drill pipe

shows that in the high angle and horizontal well section, the new aluminum pipe under the relatively high-density drilling fluid, always in a state of suspension, under the effect of gravity, the deposited debris particles remain the fluid speed zone; we can see that cuttings particles suspended load movement, do not need an additional disturbed flow device, and at the same time can reduce friction, and reduce drilling hydraulic loss.

2.2. Installation Position of New Aluminum Alloy Drill Pipe

In long horizontal sections, the traditional steel pipe is generally installed in the screw assembly, and after a bit, because the gravity of the traditional steel pipe is always greater than drilling fluid buoyancy, so under gravity, the axis of the traditional steel pipe to the lower part of the annulus, the high-speed area is located in the upper annulus fluid, and at the bottom of the low-speed region is located in the annulus, coupled with the cuttings particles (The orange dot in Figure 2) greater than the density of drilling fluid (The blue arrow in Figure 2) density. This causes debris to settle to the bottom of the annulus. This makes cuttings easy to settle and difficult to move. In order to improve the ability of cuttings migration in the horizontal section, replace the traditional steel pipe along the horizontal section with the proposed new aluminum pipe, as shown in Figure 2, a new type of aluminum alloy pipe, because it has the flotability of making the drill pipe in the drilling fluid to the upper wellbore annulus, at this point in the annulus fluid the high-speed area will change (high-speed area moves to the bottom of the annulus fluid). Cuttings settling at the bottom are affected by the high-velocity drilling fluid, which increases the axial velocity and improves cuttings transport. This, combined with the turbulence generated by the drill pipe rotation, further reduces cuttings settling at the bottom of the annulus.

Because of the buoyancy of the new aluminum alloy drill pipe, the drill pipe is in the upper annulus, but it does not contact the upper borehole wall. This greatly reduces the wear of the drill pipe and also reduces the drilling hydraulic consumption, and avoids the supporting pressure. After calculating the maximum theoretical drilling depth of the new aluminum alloy drill pipe and the traditional steel drill pipe, respectively, it is seen that the maximum drilling depth of the traditional steel drill pipe is 6000 m, and the maximum drilling depth of the new aluminum alloy drill pipe is 3.3 times that of the traditional rigid drill pipe. Furthermore, the new aluminum alloy drill pipe has good extensibility.

2.3. Strength Check of New Aluminum Alloy Drill Pipe

As the new aluminum alloy drill pipe is lowered to the horizontal well section in actual working conditions, and in the process of drilling, the load on the drill pipe mainly includes dead weight of the drill pipe, rotary torque, static pressure of mud, buoyancy of drilling fluid, weight on bit, etc.; the stress on the drill pipe is shown in Figure 3. We assume that the pipe has been suspended in a certain position on the upper annulus, ignore the drilling fluids and drill pipe flow internal and external forces, the rod end joint as a fixed end constraint, drill pipe suspension buoyancy cavity provides buoyancy, its buoyancy cavity, drill pipe inner cavity, and drill pipe outside surface by the fluid pressure, buoyancy cavity is regarded as by upward buoyancy load and surface pressure load, the inner and outer surfaces of the drill pipe can be considered as surface pressure loads. The distribution of force load is shown in

Table 3. Loading type and size of new aluminum alloy drill pipe.

Number	Location	Constraint Type	Load Type	Size
L1	End face of female joint	Fixed	Fixed Support	--
L2	Outer surface of drill pipe	Extrusion	Pressure	30 MPa
L3	Outer surface of the buoyancy chamber	Extrusion	Pressure	30 MPa
L4	Drill pipe inner cavity surface	Extrusion	Pressure	30 MPa
L5	Drill rod torque	Gravity	Acceleration	9.81 m/s2
L6	Buoyancy cavity	Buoyancy	Force	Buoyancy
L7	Drill pipe	Torque	Moment	5000 N·m
L8	Male joint face	Bit pressure	Force	100 KN

. L1 is the fixed end face of the female joint; L2, L3, and L4 are the internal pressure on the external surface of the drill pipe, the surface of the buoyancy chamber, and the internal cavity surface of the drill pipe, respectively. L5 is the gravity on the drill pipe, L6 is the buoyancy on the drill pipe, L7 is the torque on the drill pipe, and L8 is the bit pressure on the end face of the male joint.

As shown in Figure 4, ANASYS 21.0 software was used for the mechanical analysis of the new aluminum alloy drill pipe. The model material was aluminum alloy material provided by the software. Mises equivalent stress and equivalent strain of the new aluminum alloy drill pipe were obtained under various composite loads. Finite element analysis shows that the maximum equivalent stress of the male and female joint is located at the joint between the stiffener and the buoyancy chamber, and the maximum equivalent stress of the buoyancy chamber is located in the middle part, where there is a large stress concentration, and compared with other parts, the maximum equivalent stress is received. This design strength check is based on the shape change specific energy theory (the fourth strength theory); shape change specific energy $v_s$ is the main cause of material yield, no matter in what stress state, as long as the shape change specific energy $v_s$ at the dangerous point reaches the limit $v_{s\mathrm{u}}$ related to the material property, the material will yield. For plastic materials, such as steel, aluminum, copper, etc., this theory agrees with the experimental results, and the fourth strength theory is more consistent with the experimental results than the third strength theory. The yield stress of Al-Zn-Mg material ${\sigma }_s$ = 350 MPa, and the allowable stress of al-Zn-Mg material [σ] = 291.67 MPa if the safety factor is set as 1.2. The maximum stress in the buoyancy chamber of the new aluminum alloy drill pipe is 228.87 MPa, which is less than the allowable stress [σ], so the strength of the modified aluminum alloy drill pipe can meet the requirements.

2.4. Calculation Method

2.4.1. Ordinary Rock-Carrying Drill Pipe

When the drilling fluid carries cuttings (generated by bit rotation) into the borehole annulus, the single liquid phase (drilling fluid) in the annulus becomes liquid–solid two-phase (drilling fluid and cuttings). The Eulerian multiphase flow model in Fluent is selected for solid–liquid two-phase flow simulation, and the annulus flow field is regarded as an incompressible turbulent flow field. The flow follows the Navier–Stokes equation (in fluid mechanics), so the following equation can be established in the Euler coordinate system [17].

Continuity equation:

$$\frac{\partial ({\alpha }_l{\rho }_l)}{{\partial }_t}+\nabla \cdot ({\alpha }_l{\rho }_l{v}_l)=0$$

(4)

Momentum equation:

$$\frac{\partial ({\alpha }_l{\rho }_l{v}_l)}{{\partial }_t}+\nabla \cdot ({\alpha }_l{\rho }_l{v}_l)=\nabla \cdot ({\alpha }_l{\tau }_l){\alpha }_l{\nabla }_p+{\alpha }_l{\rho }_{l}g-f_{drag}$$

(5)

In the above formula, the interaction force between the volume fraction of drilling fluid and fluid particles is:

$${\alpha }_l=1-{\displaystyle \sum _{i=1}^m\frac{V_i}{∆v}}$$

(6)

$$f_{drag}=\frac{1}{∆v}{\displaystyle \sum _{i=1}^m{F}_{drag,i}}$$

(7)

In the formula, ${\rho }_l$ is the drilling fluid density, kg/m³; ${\alpha }_l$ is drilling fluid volume fraction, %; $v_l$ is drilling fluid velocity, m/s; ${\tau }_l$ is shear stress of drilling fluid, Pa; $f_{drag}$ is the interaction force of fluid particles, N; g is the acceleration of gravity, m/s²; $V_l$ is the volume of the i particle, m³; ${∆}_v$ is the volume of the cell, m³; $F_{drag}$ is the drag force of a single particle, N; m is the total number of particles in the cell.

In order to improve the calculation accuracy of the flow field, the SST K-ω turbulence model (which has a good application effect in the rotating flow field) is selected, and its transport equation is as follows [11]:

$$\frac{\partial ({\rho }_{l}k)}{{\partial }_t}+\frac{\partial }{\partial x_i}(k{\rho }_l{v}_l)=\frac{\partial }{\partial x_i}\left[\left(\mu +\frac{{\mu }_t}{{\sigma }_k}\right)\frac{{\partial }_k}{\partial x_j}\right]+G_k-Y_k+S_k$$

(8)

$$\frac{\partial ({\rho }_l\omega )}{{\partial }_t}+\frac{\partial }{\partial x_i}(\omega {\rho }_l{v}_l)=\frac{\partial }{\partial x_i}\left[\left(\mu +\frac{{\mu }_t}{{\sigma }_{\omega }}\right)\frac{{\partial }_{\epsilon }}{\partial x_j}\right]+G_{\omega }+Y_{\omega }+D_{\omega }+S_{\omega }$$

(9)

In the formula, k is turbulent kinetic energy, m²/s²; $\omega$ is the dissipation rate of turbulence, m²/s²; $\mu$ is the dynamic viscosity, kg/(m∙s); $G_k$ is the turbulent kinetic energy generated by laminar flow velocity gradient, J; $G_w$ is the turbulent kinetic energy generated by ω equation, J; $Y_k$ and $Y_w$ are turbulence generated by diffusion, $D_{\omega }$ are orthogonal divergence terms, ${\sigma }_k$ and ${\sigma }_{\omega }$ are turbulence Prandtl numbers of k equation and ω equation. $S_k$ and $S_{\omega }$ are user-defined source entries.

Considering that the fluid in the rotating flow field meets the non-Newtonian rheological characteristics, the power-law fluid model is selected, and the rheological equation of the power-law fluid is:

$$\eta =k{\gamma }^n$$

(10)

In the formula, $\eta$ is shear stress, Pa; k is consistency coefficient, Pa∙s; $\gamma$ is shear strain rate, s⁻¹; n is the flow index.

2.4.2. DEM Governing Equation

The discrete element method is used to study the cuttings transport, which can improve the calculation accuracy of the rock-carrying flow field. Each moving cuttings particle is a discrete element. The corresponding momentum conservation equation is [18]:

$$m_i\frac{d^2}{d_t^2}{x}_i=F_{fluid}+m_{i}g\left(1-\frac{{\rho }_l}{{\rho }_p}\right)+{\displaystyle \sum _q{F}_c}$$

(11)

In the formula, $m_i$ is the mass of cuttings, kg; g is the acceleration of gravity, m/s³; ${\rho }_p$ is the cuttings density, kg/m³; $F_c$ is the contact force between cuttings particles (particles and wall surface); $F_{fluid}$ refers to the interaction between a fluid and a particle.

The angular momentum conservation equation of debris particles is:

$$\frac{d}{d_t}{I}_i{\omega }_i={\displaystyle \sum _q(T_{t,j}^i+T_{n,j}^i)+T_D^i}$$

(12)

In the formula, $T_{t,j}^i$ and $T_{n,j}^i$ are tangential and normal torques of cuttings particle I, respectively; $I_i$ and ${\omega }_i$ are the inertia tensor and rotational moment of cuttings particle, respectively. $T_D^i$ is the torque of the drag force in rotational motion.

The HertzMindlin (non-slip elastic contact model) model is selected in EDEM software, which can effectively simulate a series of processes such as collision, extrusion, and deformation between cuttings particles and wall surface so as to improve the accuracy of simulation results. In order to ensure the stability of the solution, the CFD time step is set to 20 times of DEM time step.

2.4.3. Cuttings Migration Rate Equation

The cuttings migration rate (CMR) is defined as the ratio of the total cuttings left in the annulus (the total cuttings stabilized in the annulus minus the remaining cuttings cleaned) to the total cuttings stabilized in the annulus.

$$CMR=\frac{m_{\omega }-m_s}{m_{\omega }}\cdot 100\%$$

(13)

In the formula, $m_w$ is the mass of cuttings particle after stabilization in the annulus, kg; $m_s$ is the residual cuttings mass after well cleaning, kg.

2.5. The Establishment of Simulation Model

According to the actual working conditions, the length of the drill pipe is 8.6 m, and the inner and outer diameters are 114 mm and 140 mm. Combined with the size of the drill pipe, the length of the hollow pipe is 9.1 m, and the inner and outer diameters are 60 mm and 70 mm. The aluminum alloy density was set as 2780 kg/m³; the drilling fluid density was 2200 kg/m³; the male joint weight was 0.102 kN; the female joint weight was 0.111 kN. The floatability of the new aluminum alloy drill pipe is verified by the calculation and theoretical analysis of the actual parameter values above.

Since the total gravity of the traditional steel drill pipe is greater than the buoyancy of the drilling fluid, while that of the new aluminum alloy drill pipe is less than the buoyancy of the drilling fluid, in order to more accurately simulate the borehole purification process, a borehole purification simulation model is established as shown in Figure 5. The simulation model contains a certain degree of eccentricity, and the calculation formula for eccentricity is as follows:

$$\epsilon =\frac{2e}{D_w-D_n}$$

(14)

In the formula, $\epsilon$ is eccentricity; e is the eccentric distance, mm; $D_w$ is the diameter of shaft wall, mm; $D_n$ is the outer diameter of drill pipe, mm.

2.6. Meshing and Convergence Analysis

Because the new aluminum alloy drill pipe model and the traditional steel drill pipe model are eccentric long straight pipes, in order to improve the accuracy of calculation and simulation, the hexahedral grid should be used in the selection of the mesh division method. It can be seen from Figure 6 that the convergence of the two models is verified to be independent of the grid by changing the numerical values of tangential, radial, and axial equal fractions of the two models. After changing the total number of grids, the total cuttings mass calculated by the traditional steel drill pipe model fluctuates between 7.18 kg and 7.45 kg. The total cuttings calculated with the new aluminum alloy drill pipe model fluctuated between 3.66 kg and 3.79 kg, both of which verified the mesh independence and showed good convergence. In the two enlarged images, since the total cuttings of #4 and #9 have little fluctuation, the #4 mesh method is used for the traditional steel drill pipe model, while the #9 mesh method is used for the new aluminum alloy drill pipe model.

2.7. Validation of Model

The cuttings migration experimental device shown in Figure 7 was set up indoors, which could simulate a 20 m long well section. The lifting hoist and lifting frame were used to change the inclination angle of the device so as to simulate cuttings migration in different well inclination sections. For intuitive analysis of cuttings migration, the outer layer uses thick transparent glass tubes. Since real drilling fluid easily pollutes the environment, xanthan gum and sodium formate powder are artificially added into the clean water (xanthan gum is used to increase the viscosity of the fluid, and sodium formate can improve the density of the fluid), and the mixed liquid is used to simulate the real drilling fluid, so as to improve the authenticity and accuracy of the experiment. Through the investigation of a shale gas well in the Sichuan Basin, the cuttings were sampled every 10 m to obtain the cuttings particle size distribution of the well section 4300–4500 m, and finally, it was concluded that the cuttings’ particle size mainly concentrated in 0–4 mm. As shown in Figure 8, in order to analyze the cuttings transport capacity of different particle sizes, screens with different diameters were selected to screen the gravel. After screening different gravel sizes ranging from 1 mm to 5 mm, a dryer was used to dry the gravel. Finally, an electronic scale was used to weigh the same quality of gravel for each experiment.

The device can simulate the cuttings migration process with a hole diameter of 215.9 mm, drill pipe diameter of 139.7 mm, and drill pipe speed of 80 rpm. The diameter of the nozzle (new pulse jet drill pipe) is set as 12 mm, drilling fluid density of 2200 kg/m³, displacement of 25 L/s, and mechanical speed of 5 m/h. Choose 3 mm to 4 mm gravel instead of debris particles, and take at regular intervals weighing and record the cutting quality of the tube method; the experimental results are shown in

Table 4. Experimental data simulating rock-carrying capacity of two drill pipes.

Drill Pipe Position	Simulation of the Drill Pipe	Number of Experiments	Cuttings Quality after Stabilization(kg)	Average Cuttings Mass after Stabilization(kg)
	Conventional steel drill pipe	1	4.28	4.52
		2	4.83
		3	4.45
	New aluminum alloy drill pipe	1	3.96	3.75
		2	3.82
		3	3.47

, then record the data and the results of finite element simulation, Figure 9 shows that when the tube cutting total quality gradually stabilized, the finite element simulation results and the indoor cuttings migration device recorded data close to the correctness of the simulation model are proved.

3. Results and Discussion

Taking a shale gas well as an example, its borehole diameter is 215.9 mm. The material density of the new aluminum alloy drill pipe (eccentricity is 0.5) is 2780 kg/m³, the material density of the traditional steel drill pipe (eccentricity is −0.5) is 7850 kg/m³, the outer diameter of the drill pipe is 139.7 mm, the length of the drill pipe is 20 m, the rotational speed of the drill pipe is 80 rpm, the cuttings density is 2600 kg/m³, and the drilling fluid density is close. The viscosity coefficient was 0.45, and the fluidity index was 0.55. The borehole purification ability of new aluminum alloy drill pipe and traditional steel drill pipe is compared and analyzed.

3.1. Effect of Cuttings Particle Size on Well Cleaning

Due to the settlement of cuttings with cuttings particle size is directly related to how much, by setting model of the mechanical speed of 4 m/h, angle of 0°, drilling fluid inlet displacement of 25 L/s, in the case of cuttings particle size change, the traditional just drill pipe model and a new model of the aluminum alloy pipe impact the hole cleaning; Figure 10 shows that gradually increase with the diameter of cuttings, cuttings in the annulus are also increasing in quality. When the cuttings size is the same, the cuttings growth rate and total mass of stabilized cuttings in the annulus of the new aluminum alloy drill pipe model are lower than those of the traditional steel drill pipe model. When cuttings are stopped at 150 s, the new aluminum alloy drill pipe has a better well-cleaning effect than the traditional rigid drill pipe. When the diameter of the cutting is larger than 2 mm, the well-cleaning effect of the traditional steel drill pipe is significantly worse, resulting in a large amount of cuttings in the annulus, which is easy to form a cuttings bed. Figure 11 shows that the cuttings migration rate of the new aluminum alloy drill pipe is about 11% higher than that of the traditional steel drill pipe. As shown in Figure 12, the section cloud of cuttings volume fraction extracted from 13 m away from the inlet gradually increases as the cuttings particle size increases, but the maximum cuttings volume fraction of conventional steel drill pipe is always more than two to three times that of the new aluminum alloy drill pipe.

3.2. Effect of Hole Inclination Angle on Well Cleaning

Hole inclination angle changes often affect the size of the cuttings migration ability by setting a model of the mechanical speed of 4 m/h, cuttings particle size for 4 mm, drilling fluid inlet displacement of 25 L/s, in the case of hole inclination angle change, analyzes the traditional just drill pipe model and a new model of the aluminum alloy pipe impact the hole cleaning; Figure 12a shows that as the angle increases gradually, cuttings in the annulus are also increasing in quality. The cuttings growth rate and the total mass of stabilized cuttings in the annulus of the new aluminum alloy drill pipe model are lower than those of the traditional steel drill pipe model when the well inclination is the same. When cuttings are stopped at 150 s, the new aluminum drill pipe has a better hole-cleaning effect than the traditional rigid drill pipe, while the traditional steel drill pipe has a large amount of cuttings after the hole cleaning, which is easy to form a cuttings bed. Figure 12b shows that the cuttings migration rate of the new aluminum alloy drill pipe is about 10% higher than that of the traditional steel drill pipe. As shown in Figure 12c, the sectional cloud image of the cuttings volume fraction extracted 13 m from the inlet increases gradually as the well hole inclination angle increases, but the maximum cuttings volume fraction of conventional steel drill pipe is always more than two to four times that of the new aluminum alloy drill pipe.

Figure 12. Effect of hole inclination angle on well cleaning: (a) effect of hole inclination angle on borehole cleaning; (b) effect of hole inclination angle on cuttings carrying capacity of drill pipe; (c) cross-section cloud map of cuttings volume fraction.

Figure 12. Effect of hole inclination angle on well cleaning: (a) effect of hole inclination angle on borehole cleaning; (b) effect of hole inclination angle on cuttings carrying capacity of drill pipe; (c) cross-section cloud map of cuttings volume fraction.

3.3. Effect of Displacement on Well Cleaning

Based on field feedback, the increased flow rate will improve cuttings migration and reduce the risk of stuck holes. By setting the model in the mechanical speed of 4 m/h, angle 60°, cuttings particle size for 4 mm under the condition of the drilling fluid displacement change, analyzes the traditional just drill pipe model and a new model of the aluminum alloy pipe impact the hole cleaning; Figure 13a shows that as the drilling fluid displacement increases gradually, the cutting quality is gradually reduced in the annulus. At the same displacement, the cuttings growth rate and total mass of stabilized cuttings in the annulus of the new aluminum alloy drill pipe model are lower than those of the traditional steel drill pipe model. When cuttings are stopped at 150 s, the new aluminum drill pipe has a better hole-cleaning effect than the traditional rigid drill pipe, while the traditional steel drill pipe has a large amount of cuttings after the hole cleaning, which is easy to form a cuttings bed. Figure 13b shows that the cuttings migration rate of the new aluminum alloy drill pipe is about 14% higher than that of the traditional steel drill pipe. As shown in Figure 13c, the sectional cloud of cuttings volume fraction extracted 13 m from the inlet gradually decreases as the drilling fluid displacement increases, but the maximum cuttings volume fraction of conventional steel drill pipe is always more than two to four times that of the new aluminum alloy drill pipe.

3.4. Effect of Rate of Penetration on Well Cleaning

The rate of penetration is one of the key parameters of drilling engineering. Its size directly affects the borehole debris sedimentation of how many by setting the model of angle as 55°, cuttings particle size for 4 mm, drilling fluid inlet capacity of 30 L/s, under the condition of the mechanical drilling rate change, analyzes the traditional just drill pipe model and a new model of the aluminum alloy pipe impact the hole cleaning; Figure 14a shows that with the increase of rate of penetration, the cuttings quality in annulus also increases gradually. When the rate of penetration is the same, the cuttings growth rate and total mass of stabilized cuttings in the annulus of the new aluminum alloy drill pipe model are lower than those of the traditional steel drill pipe model. When cuttings are stopped at 150 s, the new aluminum drill pipe has a better hole-cleaning effect than the traditional rigid drill pipe, while the traditional steel drill pipe has a large amount of cuttings after the hole cleaning, which is easy to form a cuttings bed. Figure 14b shows that the cuttings migration rate of the new aluminum alloy drill pipe is about 15% higher than that of the traditional steel drill pipe. As shown in Figure 14c, the sectional cloud of cuttings volume fraction extracted from 13 m away from the inlet gradually increases as the rate of penetration increases, but the maximum cuttings volume fraction of conventional steel drill pipe is always more than three to four times that of the new aluminum alloy drill pipe.

4. Conclusions

In this paper, the cuttings migration process is accurately simulated by using CFD-DEM (discrete element method) coupling. The main conclusions are as follows:

1. To improve cuttings transport and wellbore cleaning, put forward the new aluminum alloy pipe to improve the structure of the aluminum alloy pipe (increase the drill pipe bottom annulus area, improve cuttings migration velocity, form a new type of structure of drill pipe), using theoretical analysis and actual parameters are calculated and proved that the new type aluminum alloy flotability of the drill pipe, and analyzes the installation location of the drill pipe and the intensity of its structure. Finally, according to the cuttings migration mechanism, the borehole purification simulation model of the new aluminum alloy drill pipe and the traditional rigid drill pipe was established in ANSYS. The hexahedral mesh division method was selected to verify the mesh independence and convergence of the two simulation models. The simulation model is verified by comparing the experimental data recorded by the indoor cuttings migration device with the finite element simulation results;

2. A just drill pipe will be a new aluminum pipe with the traditional simulation model of the import of fluent software, hole cleaning simulation analysis of the new aluminum pipe with the traditional just drill pipe to change the cutting size, angle, displacement, and mechanical speed under the factors such as hole cleaning ability, it is concluded that the new aluminum alloy pipe has a better hole-cleaning effect, compared with the traditional steel drill pipe, the cleanout capacity of the drill pipe is about 13% higher than that of the traditional steel drill pipe, which is of great significance to improve the cuttings transport capacity and the cleanout effect of the well;

3. Carbon fiber is 7 to 10 times stronger than steel, one-fourth as dense, and has high fatigue resistance and strength. With the development of new materials, a carbon fiber drill pipe may be developed. The research method of the aluminum alloy drill pipe in this paper is also applicable to the carbon fiber drill pipe. It can be inferred that the carbon fiber drill pipe will have better cuttings transport capacity and hole-cleaning effect than the aluminum alloy drill pipe.