Academic Journal of Science and Technology 
ISSN: 2771-3032 | Vol. 13, No. 1, 2024 

 

15 

Analysis	of	Cuttings	Backflow	and	Flow	Path	Erosion	of	
Different	Size	Core	Tools	
Kunpeng Yao, Bin Liu, Jianlin Yao 

Chuanqing Drilling Engineering Company Limited Technical Research Institute, Deyang, China 

 

Abstract: In the process of rock coring drilling, poor rock-carrying ability and short bit life are the main reasons leading to 
low drilling efficiency, among which the cuttings deposition at the bottom of the annulus and the abrasive cuttings wear the 
drilling teeth are the main factors. Therefore, taking an ultra-deep well in the northwest of Sichuan Basin as the research object, 
a three-dimensional fluid-structure coupling numerical model of coring tool was established to analyze the flow velocity law of 
bottom hole and annulus under different sizes, and the influence of injection parameters on the flow channel erosion rate was 
investigated. The results show that: (1) There is a linear positive correlation between bottom hole flow rate and annulus flow rate 
with different sizes of coring tools. The attenuation rate of flow rate from bottom hole to annulus is about 90%, and both can 
meet the minimum rock carrying rate. (2) The inner wall of the outlet of the throat is in the area of sudden shrinkage of the fluid 
channel, and the erosion wear is the most serious; secondly, the fluid channel of the inlet of the throat decreases but does not 
decrease sharply, and the erosion phenomenon is more serious but relatively weak; (3) The comparison and analysis of Φ140, 
Φ203 and Φ172 core tools showed that the erosion rate in the erosion area decreased from 10-3 to 10-4 and 10-5 under the same 
injection displacement, showing an order of magnitude change; However, when the injection rate increases to 35 L/s, the erosion 
rate in the two areas is of the same order of 10-4. It is concluded that in the process of rock coring, it is necessary to select a larger 
size coring tool, reasonably increase the injection displacement, optimize the inlet and outlet flow channels and wall structure, 
avoid excessive increase in erosion rate, and reduce the impact of erosion effect on the fatigue life of drill teeth and wall. The 
research results can provide technical support for solving practical drilling problems on site. 

Keywords: Rock coring; Rock carrying capacity; Erosion rate; Annular velocity; Bottom-hole velocity. 
 

1. Introduction 
Global oil and gas exploration and development continue 

to move towards deep resources, which has become a further 
important direction for the oil industry. At present, the total 
proved amount of deep-earth oil and gas resources in China 
can reach 671×108 t, and the exploration of 10,000-meter 
deep Wells is an opportunity and challenge for China's future 
oil and gas exploration and development [1]. In the process of 
oil and gas well exploration, the core samples extracted from 
the target formation are the basic basis for analyzing the 
characteristics of the oil reservoir and the physical properties 
of the fluid, which is of great significance for the oil and gas 
development after exploration. 

The target oil and gas well is mainly drilled in dolomite 
formation, which is highly abrasive, some strata are loose and 
broken, and the drilling expansion phenomenon is obvious [2]. 
Therefore, in the process of drilling coring with PDC bits, the 
annular flow rate is generally lower than the average, the 
annular chip removal ability is poor, the bottom debris 
deposition, the strong abrasive rock particles repeatedly wear 
the drill teeth at the bottom of the hole, resulting in a sharp 
shortening of the service life of the bit, and seriously reducing 
the drilling and coring efficiency. Tang et al. established an 
annulus expanding model, and found in theoretical study 
combined with numerical simulation that annulus flow rate 
presents a curve downward trend with the increase of hole 
diameter expanding rate [3]. Zhao et al. studied the effects of 
drilling parameters on the particle size of cuttings and 
upreturn efficiency, and the results show that the increase of 
bit weight will increase the particle size of cuttings within a 
certain range, but beyond the critical value, the particle size 
of cuttings will decrease and may affect the bit life [4]. Yang et 

al. studied the erosion damage behavior of manifold in shale 
gas fracturing and concluded that the impact Angle and flow 
velocity will seriously affect the erosion wear degree, and the 
erosion rate increases first and then decreases with the 
increase of impact Angle [5]. In summary, the annular radius 
and upturn efficiency of coring tools of different sizes can not 
be ignored on the wear of tool runner and drill bit. 

Therefore, taking an ultra-deep well in the northwest of 
Sichuan Basin as the research object, a three-dimensional 
fluid-structure coupling numerical model of coring tool was 
established, the flow rate laws of bottom hole and annulus 
under different bit sizes were analyzed, and the influence of 
injection parameters on the flow channel erosion rate was 
explored, hoping that the research results will have guiding 
significance for solving the actual drilling core problem. 

2. Mathematical Model 

2.1. Mathematical model of flow 
The core tool bottom hole and annular flow rate follows the 

conservation of mass and momentum, and the differential 
equations are as follows: 

Mass conservation means that the increase of mass in the 
fluid microelement per unit time is equal to the net mass 
flowing into the microelement at the same time interval. The 
circulating medium of drilling fluid in the coring tool is a 
transient three-dimensional incompressible fluid, that is, the 
fluid density is constant, then the corresponding mass 
conservation equation is as follows [6,7] : 

0V
t

 
  


 

Momentum conservation means that the rate of change of 
momentum of the fluid in the micro body with respect to time 



 

16 

is equal to the sum of all external forces acting on the micro 
body, then the momentum conservation equation for the three 
coordinate directions of x, y and z is as follows: 

 
( )

: ( V)

( )
: ( V)

( )
: ( V)

yxxx zx
x

xy yy zy
y

yzxz zz
z

u p
x u f

t x x y z

u p
y u f

t y x y z

u p
z u f

t y x y z

   

    

   

   
             

             
    

            
    

 

 
ρ is the fluid density, kg/m3; u is the fluid velocity, m/s, p 

is the isotropic pressure of the fluid,MPa; f is the volume force, 
N; τ is the shear stress related to fluid viscosity, MPa. 

2.2. Erosion mathematical model 
The particle erosion wear model in FLUENT is in the form 

of [8,9] : 
 

p(u )

p

1

(d ) F( )u
ER

particles
bN

p p

p face

m C

A





   

 
Where, C(dp) is a function related to particle diameter; b(up) 

is a function related to particle collision velocity. Aface is the 
area of the wall calculation unit, m2; Nparticles are the number 
of particles colliding on the unit area. ER is the erosion rate 
in kg/(m2·s). 

2.3. Calculation model 
From the perspective of numerical simulation, the 

influence laws of different sizes of coring tools on flow rate 
and erosion were analyzed. Based on the field drilling 
structure, the fluid simulation software Fluent was used to 
analyze the flow rate distribution at the bottom of the well and 
in the annulus, and the flow channel erosion laws avoided by 
the inlet were shouted out. The three dimensional models of 
core tools with drill size Φ140, Φ172 and Φ203 are 
established, and the mesh is divided. 

 

 
Figure 1. 3D model of coring tool (Taking Φ172 as an example) 

 

 
Figure 2. 3D model of coring tool grid model (Taking Φ172 as an example) 

 

3. Results and Discussion 

3.1. Rules of cuttings backflow and flow path 
erosion with Φ140 bit size 

According to the actual working conditions on site, the 
drilling fluid injection capacity of Φ140 coring tool in the 
process of drilling coring is 10 L/s, 15 L/s, 20 L/s and 25 L/s 
respectively. Fig 3 and 4 show the distribution of bottomhole 
flow rate and annulus flow rate of Φ140 bits with different 
injection rates. According to Bernoulli's principle, when high-

speed drilling fluid is injected into the flow channel direction, 
a higher return rate will be formed in the return annulus and 
core hole, resulting in a negative pressure cavity. Meanwhile, 
the higher the injection rate, the stronger the suction capacity 
and the stronger the migration capacity of cuttings. As can be 
seen from Fig 5, as the injection rate increases, the bottomhole 
flow rate of the Φ140 coring tool basically increases linearly, 
from 18.4 m/s to 46.9m /s, and the annular flow rate increases 
from 1.75 m/s to 4.39 m/s, with a attenuation rate of about 
90%. 

 

 
Figure 3. Bottom hole flow rate distribution for a Φ140 coring tool 

 



 

17 

 
Figure 4. Annular flow velocity distribution for a Φ140 coring tool 

 

     
Figure 5. Bottom hole and annular flow rates of Φ140 core tool 

 
It can be seen from Fig6 that obvious erosion is easy to 

occur in two areas near the orifice surface of the inlet of the 
throat and the inner wall surface of the outlet of the throat. 
Among them, the inner wall of the outlet of the throat is in the 
area of sudden shrinkage of the fluid channel, so the erosion 
wear is the most serious. Secondly, the fluid channel at the 
entrance of the throat decreases but does not decrease sharply, 
so the erosion phenomenon is relatively serious but relatively 
weak. As can be seen from Fig 7, in all the four injection 
schemes, the annular flow rate is higher than the minimum 

rock-carrying flow rate, and the bottom hole flow rate can 
timely clean the cuttings on the surface of the cutter. However, 
when the flow rate reaches 25 L/s, the erosion amount on the 
wall of the core tool presents a nonlinear increase, and the 
erosion rate increases from 1.49×10-3 to 1.49×10-3. The 
growth rate increased from 53.6% to 231%. In summary, for 
the Φ140 coring tool, it is recommended to use a lower 
injection rate while ensuring normal drilling, so as to 
successfully complete the rock carrying process and reduce 
the erosion effect of excessive flow rate on the flow channel. 
 

 
Figure 6. Erosion distribution of a Φ140 coring tool 

 



 

18 

 
Figure 7. Erosion rule of Φ140 coring tool 

 

3.2. Rules of cuttings backflow and runner 
erosion with Φ172 bit size 

Drilling fluid injection rates for the Φ172 coring tool are 15 
L/s, 20 L/s, 25 L/s, 30 L/s, and 35 L/s. FIG8 and 9 show the 
distribution of bottomhole flow rate and annular flow rate of 
Φ172 bits at different injection rates. Compared with Fig.3, it 
can be seen that the distribution rules of bottomhole flow rate 
and annular flow rate are similar with different sizes of coring 
tools, but there is no obvious difference, and both meet the 

minimum rock carrying rate. It is worth noting that by 
comparing the bottom hole rate and annular rate of the Φ172 
core tool and the Φ140 core tool, the bottom hole flow rate of 
the Φ172 core tool is 12.4 m/s, 16.2 m/s and 20.7 m/s when 
the injection rate is 15L/s, 20L/s and 25L/s, respectively. The 
bottomhole flow rate of the Φ140 core tool was 27.7 m/s, 36.8 
m/s, and 46.9m /s, respectively. It is not difficult to see that 
the bottomhole flow rate is negatively correlated with the size 
of the core tool, and the decrease is about 55.8%. 

 

 
Figure 8. Bottom hole flow rate distribution for a Φ172 coring tool 

 

 
Figure 9. Annular flow velocity distribution for a Φ172 coring tool 

 
As can be seen from Fig.11, compared with the Φ140 

coring tool, the Φ172 coring tool has a larger annular area, 
and the erosion rate of the two areas near the hole surface at 
the entrance of the pipe and the inner wall at the outlet of the 
pipe is reduced by a single order of magnitude. When the 

injection displacement is 15L/s, 20L/s, and 25L/s, 
respectively. The erosion rates of Φ172 coring tools were 
0.88×10-4 kg/m2·s, 3.38×10-4 kg/m2·s and 5.04×10-4 kg/m2·s, 
respectively. The erosion rates of Φ140 coring tools were 
0.97×10-3 kg/m2·s, 1.49×10-3 kg/m2·s and 4.93 kg/m2·s, 



 

19 

respectively. In addition, when the injection displacement is 
greater than 25 L/s, the increase of Φ172 coring tool is 
significantly increased. To sum up, in the process of rock 
coring, if there is no special ground conditions, due to the 

selection of larger size of core tools, at the same time, for tools 
of the same size, the tool structure should be reasonably 
designed to achieve annular diameter expansion, and reduce 
the impact of erosion effect on drill teeth and wall fatigue life. 
 

      
Figure 10. Bottom hole and annular flow rates of Φ172 core tool 

 

 
Figure 11. Erosion distribution of a Φ172 coring tool 

 

3.3. The rule of cuttings backflow and runner 
erosion with the size of Φ203 bit 

Depending on the site conditions, the drilling fluid 
injection rate of the Φ203 coring tool was increased to 25 L/s, 
30 L/s, 35 L/s and 40 L/s to ensure rock carrying. When 
drilling fluid displacement increases, the flow rate at the 
bottom hole is continuous with the annular flow rate, except 
for the area where the flow path is reduced. The comparison 
of Fig.5, 10 and 15 shows that with the increase of the size of 
the coring tool, when the injection rate is 25 L/s, the 

bottomhole flow rate of the Φ140, Φ172 and Φ203 coring 
tools is 46.9m /s, 20.7m /s and 18.4m /s, respectively. The 
annular flow rates were 4.39 m/s, 1.92 m/s and 1.84 m/s, 
respectively. The bottom hole flow rate and annular flow rate 
are negatively correlated with the core tool size, which is 
consistent with the above rules. It is worth noting that when 
Φ140 increased to Φ172, the bottom hole flow rate and 
annular flow rate changed greatly; when Φ172 increased to 
Φ203, the bottom hole flow rate and annular flow rate 
changed slightly, with a decrease of only 11.1%, far less than 
the decrease of 55%. 

 

 
Figure 12. Erosion rule of Φ172 coring tool 



 

20 

 
Figure 13. Bottom hole flow rate distribution for a Φ203 coring tool 

 

 
Figure 14. Annular flow velocity distribution for a Φ203 coring tool 

 

 
Figure 15. Bottom hole and annular flow rates of Φ203 core tool 

 
As can be seen from Fig.16 and 17, compared with the 

Φ203 coring tool, the annular area of the Φ203 coring tool 
increases due to the influence of the tool structure, and when 
the injection displacement is 25 L/s and 30 L/s respectively, 
the erosion rate of the two areas near the inner wall of the 
throat entrance hole and the throat outlet decreases by a single 
order of magnitude, which is similar to the above rule. 
However, when the injection displacement is increased to 35 

L/s, the erosion rate of the two tools is in the same order of 
magnitude, the erosion rate of the Φ172 core tool is 5.03×10-

4 kg/m2·s, and the erosion rate of the Φ203 core tool is 
1.23×10-4 kg/m2·s. Therefore, for the Φ203 coring tool, the 
drilling fluid injection rate should be less than 35 L/s in the 
actual operation of the field application, in order to improve 
the bit life and drilling efficiency. 

 



 

21 

 
Figure 16. Erosion distribution of a Φ203 coring tool 

 

 
Figure 17. Erosion rule of Φ203 coring tool 

 

4. Conclusion 
(1) In the coring process of different sizes of coring tools 

(Φ140, Φ172, Φ203), with the increase of injection 
displacement, the bottomhole flow rate of coring tools is 
linearly positive correlated with the annular flow rate. During 
the process of drilling fluid flowing into the annulus from the 
bottom hole, the flow rate distribution is similar, and the 
attenuation rate is about 90%, and all can meet the minimum 
rock carrying rate. 

(2) Corer tools of different sizes are prone to obvious 
erosion in two areas near the hole surface of the throat 
entrance and the inner wall surface of the throat outlet. 
Among them, the inner wall of the throat outlet is in the area 
of sudden shrinkage of the fluid channel, so the erosion wear 
is the most serious. Secondly, the fluid channel of the throat 
inlet hole decreases but does not decrease sharply, so the 
erosion phenomenon is more serious but relatively weak. 
Therefore, the structural optimization of the two areas should 
be emphasized in the design of coring tools. 

(3) Compared with the Φ140 core tool, the core tool 
presents annular diameter expansion at the same injection 
displacement (15, 20 and 25L/s), and the erosion rate of the 
inlet hole and the outlet inner wall of the pipe decreases from 
10-3 to 10-4, showing a single order of magnitude change; 
Compared with the Φ172 coring tool, when the injection 
displacement of Φ203 is 25 L/s and 30 L/s, the erosion rate of 

the two regions shows the same rule, but when the injection 
displacement of 35 L/s, the erosion rate of the two regions 
changes relatively little. To sum up, in the process of rock 
coring, large size corer tools should be selected, and injection 
displacement should be reasonably increased to avoid 
excessive erosion rate increase and reduce the impact of 
erosion effect on drill teeth and wall fatigue life. 

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