Hydraulic structure design and downhole flow field optimization of geophysical drill bits in a limestone stratum

Abstract

The hydraulic structure of conventional geophysical drill bit is designed for the general stratum. When conventional geophysical drill bit pierces into a limestone stratum, the shape of cuttings is large because of the high brittleness of limestone. The cuttings are ground repeatedly; this phenomenon can reduce drilling efficiency and increase drilling costs. According to the characteristics of limestone cuttings, the numerical simulation method is used to research downhole flow field characteristics of conventional geophysical drill bit. First, the influence of key hydraulic structure parameters on cuttings removal performance is found. Then, the hydraulic structure is optimized. The flow field characteristics of the hydraulic structure of the geophysical drill bit before and after optimization in the flow path is analyzed, at the bottom of the bit and the annulus area of the shaft lining. The optimized downhole crossflow area increased from 50% to 98%. No vortex was observed at the exit of the flow path and cuttings groove. The downhole pressure gradient increased from 0.12 Mpa to 0.15 Mpa. The cutting removal space in the annulus area of the shaft lining is fully utilized. Field tests show that the cutting removal and drilling performance of optimized geophysical drill bit has improved and the drilling speed increases by 20.6%.

Introduction

Increasing the exploration of oil and gas resources is the main melody of the development of the petroleum industry. With the deepening of exploration work, the exploration difficulty is also increasing. Especially in the face of some complicated strata that are difficult to drill (such as the limestone stratum in Sichuan and Chongqing regions of China), drilling efficiency is low and drilling tools are damaged seriously, which cannot meet the development requirements of speeding up and increasing efficiency.^1–5 Therefore, geophysical exploration technology needs to be vigorously developed. In the process of geophysical drilling, the geophysical drill bit is in direct contact with the rock and breaks the rock under the combined action of bit pressure, impact load, and driving speed. Compressed air is transported to the bottom of the well through the hollow region inside the drill pipe and the hydraulic structure of the geophysical drill bit. Cuttings are returned to the surface through the annulus between the drill pipe and the shaft wall, as shown in Figure 1.

Figure 1.

Drilling process of a geophysical drill bit.

If the downhole cleaning is not timely and thorough, the accumulated cuttings will be secondarily broken and cause premature damage to the geophysical drill bit, which is bound to reduce drilling efficiency and increase drilling cost. Therefore, it is necessary to design the hydraulic structure of the geophysical drill bit reasonably, and the characteristics of the downhole flow field are the basis for the structural design.

Until now, a large number of scholars have carried out relevant research on the downhole flow field of the geophysical drill bit. The research methods they adopted mainly include experiment and numerical simulation. Experimental aspects include the following: Knowlton et al. ⁶ proposed to change the traditional circular nozzle into a nozzle with a rectangular outlet; the research results showed that the improved outlet section shape of the nozzle could distribute hydraulic energy more reasonably. Xie et al. ⁷ conducted a visualization experiment on the downhole flow field structure of polycrystalline diamond compact(PDC) bit using the silk line method and high-speed photographic particle tracing method. The results showed that the downhole vortex area and stagnant area are mainly caused by the structure of the bit body and the shape of the flow path. Wells et al. ⁸ studied the causes of mudding of PDC bit through multiple flow experiments and demonstrated that the shape of the flow path of PDC bit and the width of the pinch point between the blades would affect the flow of cuttings. However, the experimental methods are often limited by the experimental conditions and instruments and cannot provide a complete description of the downhole flow field. Besides, the research of the hydraulic structure of bit by the experimental method should go through the cycle process of “structural design, manufacturing, experiment-structural, adjustment, remanufacturing, and re-experiment.” The hydraulic structure adjustment of the bit is not flexible, with a long cycle, high cost, and low efficiency. Therefore, the numerical simulation method to study the downhole flow field of geophysical drill bit is favored by researchers.

In 2005, Yang et al.^9,10 used NUMECA to conduct numerical simulation research on the downhole flow field, and the results showed that under the same displacement, the smaller the nozzle aperture, the better the rock cleaning effect. Increasing the flow rate is conducive to the cleaning of downhole cuttings, but too large flow rate will increase the risk of erosion and damage to the bit. In 2006, Shan et al^11,12 studied the influence of the placement position of the nozzle of the geophysical drill bit on the downhole flow field through numerical simulation. The results showed that the nozzle position is close to the center of the bit, which is helpful to improve the rock cleaning ability of the downhole flow field. In 2008, Huang et al. ¹³ used FLUENT to study the influence of layout of down the hole (DTH) bits cuttings groove on the downhole flow field. The results showed that the layout of DTH bits cuttings groove had a great influence on the vortex and played an important role in the distribution of the downhole flow field and the cutting-removing ability of the fluid. In 2012, Song et al. ¹⁴ used Computational Fluid Dynamics (CFD) to simulate the downhole condition of the bit and optimized the bit structure design by evaluating downhole rock particles and airflow path. In 2014, Moslemi et al. ¹⁵ assumed that cuttings were shot into the flow field from the bottom of the well and used the Deformable Part Model (DPM) model in FLUENT to track cuttings and observe their movement. In 2019, Zhao ¹⁶ of Jilin University analyzed the influence of two parameters of the internal flow path diameter and inclination angle of the air hammer bit on the downhole flow field through FLUENT. The results showed that the smaller the diameter of the flow path, the faster the velocity of flooding but the higher the pressure of the downhole flow field, the higher the power requirement of the mud pump. At the same time, too large or too small inclination of the flow path is not conducive to the migration and cleaning of cuttings.

From the aforementioned literature review, it can be seen that the numerical simulation method can better reflect the downhole flow field. However, most previous studies focused on the analysis of single-factor hydraulic structure parameters, without considering the interaction between various hydraulic structure parameters and how to match to achieve the optimal effect of cuttings removal. At the same time, almost all of the studies focus on the movement of downhole fluid and cuttings, without considering the situation of fluid in the flow path and the annulus between the drill pipe and the shaft wall.

Therefore, this study takes the limestone stratum in Sichuan and Chongqing as the object of geophysical exploration, using FLUENT to analyze the downhole flow field of a conventional geophysical drill bit and find key hydraulic structural parameters that affect the downhole flow field. Orthogonal tests are performed on key hydraulic structural parameters to optimize the geophysical drill bit suitable for this stratum. The downhole flow field of the conventional geophysical drill bit and the optimized limestone stratum geophysical drill bit are compared from three aspects of the flow path, the bottom of the bit, and the annulus area of the shaft lining. The results reflect the rationality of the design optimization of the bit. The field drilling test also proved that the optimized geophysical drill bit in the limestone stratum has better cuttings removal effect and faster drilling speed.

Research on a conventional geophysical drill bit downhole flow field

When drilling into a limestone stratum, the conventional geophysical drill bit should not only bear the high-frequency impact of the impactor but also bear the rock-breaking reaction force. The shear reaction of the rock must be sustained when the rock is scraped and ground at high speed under the action of rotating torque. On the other hand, the brittleness of limestone is high, and cuttings after fragmentation are massive. Owing to the limited space at the bottom of the well, the bit will break the broken limestone cuttings repeatedly during the working process, so there is a large number of relative motion between the abrasive particles formed after the rock is broken and the bit body. If the cuttings are not discharged in time, the drilling efficiency and life of the bit will be greatly reduced. In the case of serious accidents, such as sticking, it will increase drilling costs. These problems are closely related to the distribution of the downhole flow field. By studying the downhole flow field of the conventional geophysical drill bit, the problem of hydraulic structure can be reflected. The influence of key parameters of hydraulic structure on chip removal performance is found, and then the hydraulic structure parameters reasonably adjusted to enhance the cuttings removal and drilling efficiency.

Theoretical model

$$\mathrm{Mass}\mathrm{equation}:\frac{\partial \rho }{\partial t}+\frac{\partial \left(\rho u_i\right)}{\partial x_i}=0$$ (1)

$$\mathrm{Momentum}\mathrm{equation}:\frac{\partial u_i}{\partial t}+\frac{\partial \left(u_i{u}_j\right)}{\partial x_j}=f_i-\frac{1}{\rho }\frac{\partial p}{\partial x_i}+\nu \frac{{\partial }^2{u}_i}{\partial x_i{x}_j}-\frac{\partial }{\partial x_j}(\overline{{u\prime }_i{u\prime }_j})$$ (2)

$$\mathrm{Energy}\mathrm{equation}:\frac{\mathrm{De}}{\mathrm{Dt}}=-\frac{p}{\rho }\nabla •V+\mathrm{\Phi }+\frac{1}{\rho }\frac{\partial }{\partial x_i}\left(\lambda \frac{\partial T}{\partial x_i}\right)$$ (3)

$$\mathrm{State}\mathrm{equation}:p=\rho \mathrm{RT}$$ (4)

$$e=c_{v}T$$ (5)

$$\mathrm{Turbulence}\mathrm{model}:\rho \frac{\partial k}{\partial t}+\rho {\overline{u}}_j\frac{\partial k}{\partial x_j}=\frac{\partial }{\partial x_j}[(\mu +\frac{{\mu }_t}{{\sigma }_1})\frac{\partial k}{\partial x_j}]+{\mu }_t(\frac{\partial {\overline{u}}_i}{\partial x_j}+\frac{\partial {\overline{u}}_j}{\partial x_i})\frac{\partial {\overline{u}}_i}{\partial x_j}-\rho \epsilon$$ (6)

$$\rho \frac{\partial \epsilon }{\partial t}+\rho {\overline{u}}_k\frac{\partial \epsilon }{\partial x_k}=\frac{\partial }{\partial x_k}[(\mu +\frac{{\mu }_t}{{\sigma }_2})\frac{\partial \epsilon }{\partial x_k}]+\frac{C_1\epsilon }{k}{\mu }_t(\frac{\partial {\overline{u}}_i}{\partial x_j}+\frac{\partial {\overline{u}}_j}{\partial x_i})\frac{\partial {\overline{u}}_i}{\partial x_j}-C_2\rho \frac{{\epsilon }^2}{k}$$ (7)

The above equations constitute the closed equations for solving the distribution of the flow field in the downhole of the geophysical drill bit. The definite solution of the equations is formed as per the actual working conditions, corresponding boundary conditions, and initial conditions. Because the equations are nonlinear, the numerical method is used to solve it discretely in engineering. In this article, FLUENT is adopted to solve the nonlinear equations, and the finite volume method is adopted to discrete the solution domain. Before solving, the corresponding physical model of the bit’s downhole flow field should be established.

Physical model

Figure 2(a) shows the geometric model of the conventional geophysical drill bit. The fluid calculation domain of the conventional geophysical drill bit is extracted in a normal working state, as shown in Figure 2(b).

Figure 2.

The physical model of conventional geophysical drill bit: (a) Geometric model. (b) Fluid calculation domain.

Meshing

Based on the fluid computational domain model of the conventional geophysical drill bit, owing to the complexity of its geometric model, ANSYS Meshing software is used for unstructured mesh division. Considering the division of boundary layer meshes, the honeycomb hexahedron mesh is directly used to discrete the meshes. This solution uses Proximity and Curvature function encryption methods for local grid refinement. The number of meshes in the fluid computational domain of conventional geophysical drill bit is 1297665, and the number of nodes is 4066909. The mesh of the conventional geophysical drill bit is shown in Figure 3.

Figure 3.

The mesh of the fluid calculation domain of the conventional geophysical drill bit.

Boundary conditions

According to the field conditions of geophysical exploration, the boundary conditions of the model are as follows:^17–20

• 1. Computational domain setting: In the steady-state calculation, owing to the rotation effect of the geophysical drill bit, the rotating inertia force is taken into account, and the rotating speed of the fluid calculation field is set as 120 r/min;

• 2. Entrance boundary condition: Since the compressibility of air needs to be considered, select the pressure inlet boundary and set the relative inlet pressure as 0.5 Mpa (set the ambient pressure to one atmosphere). Inlet turbulence parameters are set as inlet turbulent kinetic energy k and turbulent dissipation rate $\epsilon$ , which are given by the empirical formula

$$k=\frac{3}{2}(\overline{u}I{)}^2$$ (8)

$$\epsilon =C_{\mu }^{3/4}\frac{k^{\frac{3}{2}}}{l}$$ (9)

In the formula, turbulence intensity $I=0.16{\mathrm{Re}}^{-\frac{1}{8}}$ , Re is the Reynolds number, u is the average velocity at the inlet, the length of the turbulence $l=0.07L$ , and the hydraulic diameter is desirable for fully developed turbulence, $C^{\mu }=0.09$ .

• 3. Outlet boundary conditions: To set the pressure outlet boundary, the outlet pressure is 0 Mpa (set the ambient pressure to one atmosphere); at the same time, outlet turbulent kinetic energy k and turbulence dissipation rate $\epsilon$ are specified;

• 4. Wall boundary conditions: Fixed wall, the no-slip condition is adopted for the wall face. The relative velocity of the geophysical bit (in wall) is 0 r/min, that is, the velocity of the bit is 120 r/min. The absolute shaft surface (out wall) velocity is 0 r/min, that is, shaft lining is static.

Composition and analysis of downhole flow field of the geophysical drill bit

To obtain the velocity vector, the central section nephogram of the fluid calculation domain of the geophysical drill bit is intercepted. By observing the velocity vector, the flow field composition inside the bit and at the bottom of the hole is obtained, as shown in Figure 4:

Figure 4.

A schematic showing the flow field composition inside the bit and at the bottom of the hole.

1. High-speed jet region: This region is close to the flow path and is less affected by the downhole wall surface. Most of the jets maintain the velocity of the flow path exit, similar to the isokinetic region in the free jet;

2. Jet impinging region: This region is formed by the impact of the nozzle jet on the downhole wall surface. The velocity of the fluid in this area decreases rapidly, while the fluid pressure increases. Pressure gradients occur in this area, forming pressure waves that hit the bottom of the well;

3. Radial sheet flood region: The fluid flow in this region is affected by the surface of the downhole wall, making the flow direction parallel to the downhole. The hydraulic energy in this region plays a major role in the flow field of the downhole washing and debris carrying capacity;

4. Backflow region: The jet of the nozzle is restricted by the surrounding shaft lining space and forced to change direction so that the fluid flow direction is opposite to that of the jet of the nozzle;

5. Whirlpool region: Because of the action of jet entrainment, a vortex takes up space in this area, which blocks the discharge channel of fluid carrying debris and consumes energy and damages the life of drill bit.

Figure 5 is the downhole velocity vector of the conventional geophysical drill bit in the working state. It can be seen from the figure that the outlet of the flow path of the conventional geophysical drill bit is too close to the edge of the bit, resulting in a small effective flooding erosion area at the downhole, with a large fluid stagnation area at the center of the bit. Therefore, cuttings tend to gather toward the center of the bit, which is not conducive to cuttings removal. At the same time, the whirlpool region appears near the cuttings groove in the figure. In this region, a large vortex that takes up space is formed because of the entrainment of the jet, which blocks the fluid-carrying discharge channel. The broken limestone cuttings loop repeatedly in the downhole, which consumes a lot of energy and damages the teeth and the bit life. Therefore, it is urgent to design the hydraulic structure of the geophysical drill bit for the limestone stratum.

Figure 5.

A schematic showing the velocity of downhole in the working state of the conventional geophysical drill bit.

Optimization design of hydraulic structure of the geophysical drill bit in a limestone stratum

Limestone’s hardness and brittleness are high; the volume of cuttings is large. The hydraulic structure design of conventional geophysical drill bit is suitable for general strata, but not for limestone stratum. By increasing the effective flooding area at the downhole, the velocity of crossflow, downhole pressure gradient, and improving cuttings migration stability can discharge cuttings quickly and efficiently.

Determine key hydraulic structure parameters

Downhole flooding area, transverse crossflow velocity, and pressure gradient are related to the setting of flow path inclination and diameter, while the stability and efficiency of cuttings migration are closely related to the length of the cutting groove. ¹² Therefore, the influence of hydraulic structure parameters such as the flow path inclination, length of the cuttings groove, and flow path diameter on the downhole flow field is mainly studied. The specific definition of hydraulic structure parameters is shown in Figure 6. $\alpha$ represents flow path inclination, L represents cuttings groove length, and Φ represents flow path diameters.

Figure 6.

The specific definition of hydraulic structure parameters.

Optimization of key hydraulic structure parameters

According to the three factors of flow path inclination, cuttings groove length and flow path diameter, through literature research,^12–14 take three levels. The orthogonal test is carried out; Table 1 is the factor level table.

Table 1.

Three hydraulic structure parameters and corresponding levels.

Level	Factor
	$\alpha$ Flow path inclination (°)	L Cuttings groove length (mm)	Φ Flow path diameter (mm)
1	50	30	10
2	60	42	12.5
3	70	54	15

According to the factors of the test and the corresponding number of levels, the L₉(3⁴) orthogonal table is selected. The test arrangements are shown in Table 2.

Table 2.

Nine different combinations of hydraulic structure parameters.

Test number	Factor
	$\alpha$ Flow path inclination (°)	L Cuttings groove length (mm)	Φ Flow path diameter (mm)
1	50	30	10
2	50	42	12.5
3	50	54	15
4	60	30	10
5	60	42	12.5
6	60	54	15
7	70	30	10
8	70	42	12.5
9	70	54	15

Physical modeling and numerical simulation for the selected 9 sets of data are carried out. The gas velocity at the outlet of the cuttings groove is used to indirectly reflect the cuttings removal efficiency. The test results are shown in Table 3.

Table 3.

The cuttings removal velocity corresponding to different hydraulic structures.

Test number	$\alpha$	L	Φ	Cuttings removal velocity (m/s)
1	1	1	1	189.87
2	1	2	2	180.22
3	1	3	3	169.47
4	2	1	2	208.43
5	2	2	3	213.76
6	2	3	1	218.63
7	3	1	3	197
8	3	2	1	208.87
9	3	3	2	228.74

Calculated from the test value of cuttings removal velocity:

$$K_1^{\alpha }=539.56K_2^{\alpha }=640.82K_3^{\alpha }=634.61$$

$$K_1^L=595.3K_2^L=602.85K_3^L=616.84$$

$$K_1^{\varphi }=617.37K_2^{\varphi }=617.39K_3^{\varphi }=580.23$$

Furthermore, calculate the average cuttings removal velocity:

$$k_1^{\alpha }=179.85k_2^{\alpha }=213.61k_3^{\alpha }=211.54$$

$$k_1^L=198.43k_2^L=200.95k_3^L=205.61$$

$$k_1^{\varphi }=205.79k_2^{\varphi }=205.8k_3^{\varphi }=193.41$$

The factor $\alpha$ represents the flow path inclination, and the actual level is 50°, 60°, and 70°. One drawing is made with the actual level as abscissas and average cuttings removal velocity as ordinate. Plot the same for factors L and Φ, as shown in Figure 7(a)–(c).

Figure 7.

The range diagram of the influence of three parameters on cuttings removal velocity: (a) Flow path inclination (°). (b) Cuttings slot length (mm). (c) Flow path diameter (mm).

It can be seen from the figure that the range of cuttings removal velocity in Figure 7(a) is 33.76, that in Figure 7(b) is 7.18, and that in Figure 7(c) is 12.39. Therefore, flow path inclination $\alpha$ had the greatest influence on cuttings removal velocity, the influence of the flow path diameter Φ second, and the influence of cuttings groove length L is minimal. Through the intuitive analysis method, it can be concluded that the cuttings removal velocity of test no. A3B3C2 is the fastest. Specific parameters are flow path inclination 70°, cuttings groove length 54 mm, and flow path diameter 12.5 mm. Considering the strength of the bit body, when the flow path inclination is 70°, the flow path is too concentrated in the center of the bit and the life of the bit is difficult to guarantee. Therefore, the hydraulic structure design scheme of the limestone stratum geophysical drill bit is flow path inclination 60°, cuttings groove length 54 mm, and flow path diameter 12.5 mm.

Comparison of key hydraulic structure parameters before and after optimization

The key hydraulic structure parameters of conventional geophysical drill bit are flow path inclination 50°, cuttings groove length 30 mm, and flow path diameter 10 mm; the specific structure is shown in Figure 8(a). The key hydraulic structure parameters of the optimized limestone stratum geophysical drill bit are flow path inclination 60°, cuttings groove length 54 mm, and flow path diameter 12.5 mm; the specific structure is shown in Figure 8(b).

Figure 8.

The hydraulic structure diagram of two kinds of geophysical drill bits: (a) Conventional geophysical drill bit. (b) Limestone stratum geophysical drill bit.

Comparative analysis of downhole flow field before and after hydraulic structure improvement

During the geophysical drill bit operation, the gas flows through the internal flow path of the bit and then spurts out from the outlet of the flow path to impact the bottom of the hole. The gas carries cuttings and discharges it from the annulus area of the shaft lining. The whole process constitutes the downhole flow field of the geophysical drill bit. Compare and analyze the flow field distribution in the flow path of the bit, the bottom surface of the bit and the annulus area of the shaft lining. We can understand the cuttings removal process more comprehensively, thereby optimizing the downhole flow field and improving the cuttings removal efficiency of the bit.

Comparative analysis of gas velocity in the flow path of the bit

Before the gas jet reaches the bottom of the well, it must first pass through the flow path inside the bit. By analyzing the deceleration and backflow of the airflow in the flow path, we can understand the influence of the inclination and the diameter of the flow path on the flow field inside the flow path of the bit.

Figure 9(a) and (b), respectively, show the velocity vectors of the center section of the conventional geophysical drill bit and the optimized limestone stratum geophysical drill bit. It can be seen from the comparison in Figure 9, when the airflow from top to bottom impacts into the inclined flow path, the deceleration is serious owing to the small flow path inclination of the conventional geophysical drill bit. The jet velocity at the circle in Figure 9 is 400.2 m/s. However, after the hydraulic structure is optimized, the flow path inclination of the limestone stratum geophysical drill bit is set to 60°, and the airflow impact resistance is reduced. The jet velocity at the circle in Figure 9(b) is 555.3 m/s, and the flow velocity is increased by 38.7%.

Figure 9.

The velocity vector diagram of the center section of two geophysical drill bits: (a) Conventional geophysical drill bit. (b) Limestone stratum geophysical drill bit.

The backflow region appeared in the flow path before and after the optimization of the hydraulic structure. On the left side of the dotted frame, the backflow region occupies two-thirds of the whole flow path space., which seriously affects the gas flow rate, causes energy loss, increases local resistance, and weakens export energy. Backflow even brings the broken cuttings into the flow path, causing the flow path to be blocked, while the large size of limestone cuttings may cause a higher risk of blockage. However, the backflow region in the right dotted frame only exists on one side of the outlet of the flow path. The small area of the backflow has little effect on the gas flow velocity in the entire bit flow path. On the other hand, the increased flow path diameter can reduce the risk of debris blocking the flow path.

Figure 10 is the comparison curve of air velocity in the flow path of two geophysical drill bits. As shown in the figure, the average velocity of air in the flow path of conventional geophysical drill bit is 357.3 m/s, while the limestone geophysical drill bit is 448.5 m/s, with an average velocity increase of 25.4%. When air is ejected from the flow path of the bit, the air velocity of the conventional geophysical drill bit drops obviously from about 350 m/s to about 120 m/s. The air velocity of the limestone geophysical drill bit remains above 300 m/s at the exit of the flow path, which ensures the velocity of air reaching the bottom of the well and the velocity of the crossflow after the air hits the bottom of the well.

Figure 10.

Comparison curve of air velocity in the flow path of two geophysical drill bits.

Comparative analysis of the flow field at the bottom of the bit

Guan et al. ²¹ indicates that the area of crossflow, the velocity of crossflow, and the pressure gradient of the downhole flow field are important parameters to characterize the downhole cuttings clearance capacity. The area of crossflow reflects the effective area of downhole that can be washed by the gas flow, the velocity of crossflow represents the velocity of cuttings migration, and the existence of pressure gradient enables cuttings to be pushed to the pressure trough under the action of the downhole pressure. To clearly explain the parameter changes of the downhole flow field, the fluid parameters were taken from the horizontal surface 2 mm away from the bottom of the well for comparative analysis.

The area of crossflow

A cross-section of 2 mm above the bottom of the well in the fluid calculation domain of the two geophysical drill bits was intercepted, and get the velocity vector of the plane. Figure 11(a) and (b), respectively, show the velocity vectors of downhole 2 mm cross-section of the conventional geophysical drill bit and the limestone stratum geophysical drill bit.

Figure 11.

The velocity vectors of downhole 2 mm cross-section of two geophysical drill bits: (a) Conventional geophysical drill-bit. (b) Limestone stratum geophysical drill bit.

It can be seen from Figure 11 that after the gas jet impinges on the bottom of the well, three crossflow regions are formed at the bottom of the well. The flow in each region is non-invasive. At the junction of the center of the well and each fan region, the downhole crossflow of each jet collides with each other, forming a low-speed stagnation region at the center of the well. The low-speed stagnation region of conventional drill bit is large, as shown in the dotted region of Figure 11(a), which takes up about 50% of the area at the bottom of the well. The crossflow velocity in the stagnation region is significantly lower than that in the surrounding region, which is easy to cause cuttings to accumulate in the center of downhole, increase the wear of the bit, and reduce cuttings removal efficiency and drilling efficiency. The optimized limestone stratum geophysical drill bit has almost no low-speed stagnation region, and the crossflow area reaches 98% of the downhole area, which greatly improves the cuttings migration efficiency and removal efficiency.

The whirlpool region is near the teeth of the conventional geophysical drill bit. According to the strong vortex conservation theorem, the vortex tube cannot begin or end with fluid, but it can only become annular, begin or end with boundary, or extend to infinity, form a closed “swivel” at the bottom of the well. “Swivel” will not only consume jet energy but also block the downhole space, causing cuttings in the vortex region to fall back down again. These phenomena are not conducive to cutting removal and seriously reduce the bit of service life. However, the optimized limestone stratum geophysical drill bit will downhole flow field without the vortex region.

Most of the velocity vectors in the downhole crossflow region of the optimized limestone stratum geophysical drill bit point to the cuttings groove, which is conducive to carrying the broken cuttings to the annular area of the downhole edge in time and discharging out.

To sum up, the flow field at the bottom of the limestone stratum geophysical drill bit after the hydraulic structure improvement is optimized, the crossflow area increases greatly, reaching 98%, and the influence of the vortex region is reduced.

The velocity of crossflow

The downhole radial crossflow velocity of two geophysical drill bits are intercepted, as shown in Figure 12. The velocity of downhole radial crossflow can reflect the migration velocity of cuttings. As can be seen from the figure, near the horizontal axis of −0.03 m is the exit location of the flow path. The maximum value of radial flooding velocity at the exit of the flow path of the limestone stratum geophysical drill bit is significantly higher than that of the conventional geophysical drill bit. The high crossflow velocity is conducive to the separation of cuttings from the original crushing pit and bedrock, thus improving the cuttings-carrying efficiency of gas. In the figure, the average crossflow velocity of the conventional geophysical drill bit is 65.3 m/s, and the average crossflow velocity of the limestone stratum geophysical drill bit is 76.7 m/s. Compared with the conventional geophysical drill bit, the speed is increased by 17.5%, which greatly improves the migration velocity of cuttings and makes the cuttings removal process smoother.

Figure 12.

Comparison curve of the downhole radial crossflow velocity of two geophysical drill bits.

The pressure of crossflow

The downhole pressure gradient is an important parameter to characterize the cuttings clearance capacity of the downhole flow field. The higher the pressure gradient, the greater the pressure value of the cuttings under unbalanced pressure, which can not only strengthen the migration capacity of the transverse flow but also improve the cuttings removal capacity. It is conducive to the migration of cuttings and the purification of the downhole environment. Figures 13 and 14 are the downhole pressure contours of conventional geophysical drill bit and limestone stratum geophysical drill bit, respectively. As can be seen from the figure, the exit of the flow path location is the peak of downhole pressure. The conventional geophysical drill bit downhole pressure peak value is about 0.21 Mpa, and the limestone stratum geophysical drill bit downhole pressure peak value is about 0.29 Mpa, about 38% higher. High pressure can ensure the exit of flow path position cuttings be lined around quickly, preventing the broken limestone rock debris plug flow path, which causes the difficulty in cuttings removal. At the same time, a high-pressure peak will bring a greater pressure gradient. The unbalanced pressure value borne by cuttings will also increase. It is more conducive to cuttings leaving the original crushing pit and separating from bedrock, improving rock-breaking efficiency and cuttings removal efficiency.

Figure 13.

Downhole pressure contours of the conventional geophysical drill bit

Figure 14.

Downhole pressure contours of the limestone stratum geophysical drill bit

The radial pressure distribution at the bottom of the well is intercepted, as shown in Figure 15. The horizontal axis X represents the radial position of the bottom of the well. The bottom center is represented by 0 . The downhole radial pressure value of the conventional geophysical drill bit is about 0.047 Mpa and that of the limestone stratum is about 0.13 Mpa. As seen from the overall trend of the curve, the pressure of the conventional geophysical drill bit is lower in the center area of the bottom hole, while the pressure is higher at the outlet of the flow path and the corresponding cuttings discharge groove, which results in the debris accumulation in the center of the bottom hole and difficult to discharge. However, the pressure of the limestone stratum geophysical drill bit reaches the peak at the outlet of the flow path. As it is far away from the outlet of the flow path, the pressure gradually decreases, forming a pressure gradient, which is conducive to the migration of cuttings.

Figure 15.

Comparison curve of radial pressure distribution at the bottom of the well.

Comparative analysis of flow field in the annulus area of the shaft lining

Figure 16(a) represents the conventional geophysical drill bit cuttings migration streamline diagram and (b) shows the limestone stratum geophysical drill bit cuttings migration streamline diagram. To some extent, the streamline diagram reflects the track of cuttings carried by the airflow. When the airflow passes through the flow path and reaches the impact zone, the airflow will drive the movement of breaking cuttings. The higher the velocity of the airflow, the stronger the ability to carry cuttings upward, reducing the residence time of cuttings at the bottom of the well and delaying the generation of cuttings bed. At the same time, the high velocity of the airflow will also produce a scouring effect on the cuttings bed that has been formed, making the deposited cuttings move again and return to the annulus.

Figure 16.

Cuttings migration streamline diagram of two geophysical drill bits: (a) Conventional geophysical drill bit. (b) Limestone stratum geophysical drill bit.

It can be seen from Figure 16(a), the streamline diagram on the top of conventional geophysical drill bit is skewed toward one side of the bit, while the other side has almost no streamlines. This suggests that during cuttings migration, a considerable part of the annular space has not been used effectively but accumulated at the one side. This phenomenon causes slow migration or even stagnation, which is also the main reason for the phenomenon of sticking in the field operation. On contrast, the streamline diagram of the limestone stratum geophysical drill bit in Figure 16(b) shows that the overall streamline rule is consistent and the annulus area above the bit is fully utilized. The average flow velocity in the annulus area of the shaft lining is measured; the conventional geophysical drill bit is 153.7 m/s, and the limestone stratum geophysical drill bit is 211.3 m/s, increasing the speed by about 37.4%. This ensures that cuttings could be discharged from the borehole regularly and rapidly during geophysical exploration, improving cuttings removal efficiency and drilling efficiency.

The field test

The field test is used to test the drilling and the cuttings removal effect of the limestone stratum geophysical drill bit and conventional geophysical drill bit. The Huaying mountain area in Guang An city, Sichuan province, is selected for the field test. A large area of Huaying mountain is exposed to carbonate rocks represented by limestone. At the same time, there are also mixed and intersected other complicated and difficult drilling stratum. The high mechanical strength of this stratum makes drilling difficult. The Polodyakonov coefficient f of limestone is about 8. Figure 17 is the comparison diagram of the bits, where Figure 17(a) is the conventional geophysical drill bit and Figure 17(b) is the limestone stratum geophysical drill bit. The diameter of the drill bit is 80 mm, and the rotation speed is 120 r/min. The bit weight is 10 KN.

Figure 17.

The comparison diagram of the bits: (a) Conventional geophysical drill bit. (b) Limestone stratum geophysical drill bit.

Drilling process of the field test

The field test is shown in Figure 18. The SKZ-30 drilling rig is adopted in the field, as shown in Figure 18(a). Figure 18(b) shows the air compressor system, and Figure 18(c) shows the field drilling operation. Figure 19 shows the comparison of discharged cuttings. Figure 19(a) shows the cuttings drilled by conventional geophysical drill bit; Figure 19(b) shows the cuttings drilled by limestone stratum geophysical drill bit. By comparison, it can be concluded that most of the cuttings discharged by conventional geophysical drill bit is in the form of powder, with an average diameter of about 0.2 mm, which indirectly indicates that during the drilling progress of conventional geophysical drill bit, limestone cuttings is repeatedly circulated at the bottom of the well, repeatedly ground and cannot be discharged quickly. Most of the cuttings discharged by limestone stratum geophysical drill bit are large particles with an average diameter of about 0.5 mm, which indicates that the limestone cuttings are not repeatedly ground after crushing. The cuttings discharge process is smooth with great cuttings removal effect and high efficiency. At the same time, in the process of the field test using the conventional geophysical drill bit, sticking accidents occur, which was solved by applying bit pressure and pipe trip, wasting a lot of time, and the bit was damaged to some extent. However, during the whole drilling process of limestone stratum geophysical drill bit, no accidents such as sticking accidents occurred.

Figure 18.

Field drilling test: (a) SKZ-30 drilling rig. (b) Air compressor system. (c) Field drilling operation.

Figure 19.

Comparison of discharged cuttings: (a) Conventional geophysical drill bit. (b) Limestone stratum geophysical drill bit.

Test result

Record the drilling time of each drill pipe during the field test

When the new drill pipe is connected to the rig, the timing starts. When this drill pipe is fully drilled into the ground, it stops timing. This is the drilling time of a single drill pipe. Figure 20 shows the drilling time of a single drill pipe of conventional geophysical drill bit, and Figure 21 shows the drilling time of a single drill pipe of limestone stratum geophysical drill bit. According to the field test results, when drilling limestone, the average drilling time of a drill pipe by conventional geophysical drill bit is 14.47 minutes, and the average drilling time of limestone stratum geophysical drill bit is 11.5 minutes, with a speed increase of about 20.6%. When drilling with conventional geophysical drill bit, the seventh drill pipe got stuck. By applying pressure on the bit and repeatedly lifting the drill, the accident was solved, wasting a lot of time and manpower. However, the whole drilling process of the limestone stratum geophysical drill bit is very smooth, without any accidents such as sticking accidents. It indicates that the limestone stratum geophysical drill bit has higher cuttings removal efficiency and drilling efficiency and can effectively prevent accidents such as sticking accidents, more in the line with actual situation of geophysical exploration production.

Figure 20.

Drilling time of a single drill pipe of conventional geophysical drill bit.

Figure 21.

Drilling time of a single drill pipe of limestone stratum geophysical drill bit.

Conclusion

1. The flow path inclination of conventional geophysical drill bit is small, which leads to air decelerating severely in the flow path inside the bit. At the same time, the exit of the flow path is too close to the edge of the bit, which results in a small effective crossflow area at the bottom of the hole, while a large fluid stagnation area accumulates at the center of the bit. The cuttings tend to gather at the center of the bit, which is not conducive to cuttings removal.

2. Among the three factors of the flow path inclination, the flow path diameter, and the cuttings groove length, the influence of the flow path inclination and the flow path diameter on the cuttings removal effect is relatively large, while the effect of the cuttings groove length on the cuttings removal is relatively small. Therefore, to optimize the downhole flow field to achieve better cuttings removal effect, the settings of the inclination of the flow path should be considered to increase the crossflow velocity and pressure gradient of the downhole flow field, thereby improving the cuttings removal ability.

3. The specific parameters of limestone stratum geophysical drill bit after the optimization of the hydraulic structure are as follows: the inclination of the flow path is 60°, diameter of the flow path is 12.5 mm, and length of the cuttings groove is 54 mm. Such a hydraulic structure can expend the effective crossflow area at the bottom of the hole, reduce the fluid stagnation area, and increase the velocity of crossflow and discharge of limestone cuttings. At the same time, the optimized bit makes full use of the annulus area of the shaft lining, reducing sticking accident. Comprehensively, cuttings removal efficiency is improved and drilling costs are reduced.

Author biographies

Jing Zhu is a Master Student in the School of Mechanical Engineering, Southwest Petroleum University, Sichuan, China. His research focuses on the fluid dynamics.

Zhiqiang Huang is a Professor in the School of Mechanical Engineering, Southwest Petroleum University, Sichuan, China. His research focuses on the oil and gas equipment.

Yachao Ma is a Lecturer in the School of Mechanical Engineering, Southwest Petroleum University, Sichuan, China. His research focuses on the mechanism of tooth wear of PDC bit.

Dou Xie is a Doctor in the School of Mechanical Engineering, Southwest Petroleum University, Sichuan, China. His research focuses on the drill string dynamics.

Xueying Yang is a Master Student in the School of Mechanical Engineering, Southwest Petroleum University, Sichuan, China. Her research focuses on the geophysical prospecting equipment.

Cao Zhou is a Master Student in the School of Mechanical Engineering, Southwest Petroleum University, Sichuan, China. His research focuses on the bit dynamics.