Torsional Impact Tool Based on the Problem of Stick-Slip Vibration and Its Application

Abstract

PDC drill bits are widely used in deep and hard-rock formations. The stick-slip vibration causes premature damage of PDC bits, which is an important factor for the increase of the drilling cost. The model of stick-slip vibration system of PDC bits has been established to account for mechanism of analysis about stick-slip vibration. Moreover, a torsional impact tool has been developed for weakening the influence of stick-slip vibration. PDC bits optimally matched with the tool were selected by the transmission efficiency of impact energy, based on the experimental data. The laboratory experiment shows that the self-developed torsion impact tool can produce the impact frequency of 1500 r/min; the torsional impact load provided by tool can effectively inhibit the stick-slip effect of PDC bits, thereby improving the penetration rate; the ultrashort monotube bit body can help to improve performance of the torsional impact tool; when the tool is in the condition of high weight on bit (WOB) and low rotary speed, the tool can achieve the optimal speed increase effect. Furthermore, the field experiments of QG5 well in Tarim Oilfield show that the rate of penetration (ROP) after using the torsional impact tool is more than 1 times than adjacent well.

1 Introduction

PDC bits have been widely used in geological exploration and development because of its high drilling speed, long life, good stability, and flexible bit design. However, when drilling in deep competent formation, especially in multi-interbedded formation, there are generally problems such as low drilling efficiency and premature failure of PDC bits [1]. Mechanical vibration is the main reason for the premature failure of the bottom hole assembly (BHA) and the drill string. There are three types of drill string vibration: torsional vibration (stick-slip) [2], axial vibration (bit bounce) [3], and lateral vibration (whirling motion) [4]. This paper focuses on the study of the stick-slip vibration of the drill pipe. The stick-slip effect means that the top of the drill string rotates at a constant speed, while the bit speed varies between zero and six times of rotation speed of the surface. The appearance of the stick-slip effect is due to friction between the different components of the drill string and the friction between the drill string and the wellbore [5].

The stick-slip instability of the drill string is traditionally attributed to the dry friction model at the bit rock interface [6] (Stribeck effect). However, in recent years, many scholars have established many models with very fixed friction coefficients, which also show stick-slip instability. Richard [7] established a model with a single degree of freedom. Abbassian [8] established a model with two degrees of freedom. Mihajlovic [9] combined the turntable system model with two degrees of freedom model. In 2007, Richard coupled the axial and torsional motions at the drill–rock interface to analyze the stick-slip vibration [10]. A two degree of freedom model with drag bit was established, which presents a stick-slip motion by time delay.

In field test, in order to reduce the stick-slip effect, many attempts have been made. In recent years, Ulterra and Halliburton have developed torsional impactor. In 2000, the Ulterra company proposed and developed a torsional impactor to assist the rock breaking of the PDC bit, which were widely used in major oil fields. TorkBuster developed by Ulterra can drive the turbine with high-velocity drilling fluid, and then drive the impactor to strike the transmission shaft, eventually transmitting the torsional impact energy evenly to the drill bits. Rotary impact device developed by Halliburton can also drive the turbine with high-velocity drilling fluid, making the impact block hit the base in the opposite direction yet to transmit energy. The Southwest Petroleum University has developed a torsional impact tool that works in a similar way to the TorkBuster of Ulterra, which drives the impactor with the help of a screw. Therefore, the SLTIDT torsional impact tool developed by Shengli Oilfield uses the fluid pressure difference between drilling fluids to drive the internal impact hammer to circle the impact repeatedly. In 2014, Prof. Li [11] developed a near drill torsional impactor and applied it to Jilin Oilfield, Tarim Oilfield, and Daqing Oilfield, where the average speed increased to 39–145%. More important, the average lifespan of the tool was about 120 h, and the performance was stable and the effect of slanting was good. In addition, many major research institutions, such as Daqing Oilfield Drilling Engineering and Technology Research Institute, Beijing Drilling Engineering Technology Research Institute, have carried out researches on the torsion impact tools.

2 Mechanical Model of Stick-Slip Vibration System with PDC Bit

The basic structure of the rotary drilling system mainly consists of drilling rig, drill string, and drill bit. The main component of the drill string is a set of bottom hole assembly (BHA) made of heavy steel pipes and a drill pipe made of fine pipes. For the convenience of analysis, the following assumptions are set up as follows: (1) boundary conditions: a constant upward force H₀ and a constant angular velocity Ω₀ applied to the top of a drill string by a drill rig; (2) the wellbore is vertical; (3) no lateral motion of a bit; and (4) ignore the friction between the string and the wellbore and between the string and the drilling fluid.

Based on the above assumptions, a drilling system model with two degrees of freedom, including the axial and torsional vibrations of a drill string, is established, as shown in Fig. 1. U(t) and Φ(t) represent the vertical position and angle position of the bit, respectively. Particle M and moment of inertia I represent bottom hole assembly (BHA). The torsional stiffness C of the spring represents the torsional stiffness of the drill string. The angle position of the vertex of the drill string is expressed in Ω₀t.

Fig. 1.

Simplified model of drilling system

As shown in Fig. 1, when an upward pulling force H₀ and a constant angular velocity Ω₀ are applied to the upper end of the torsion spring, there is corresponding response that p(t) = {W(t), V(t), T(t), Ω(t)}, which is usually a function of time t. There are two conjugated kinetic quantities and kinematic quantities: bit drill pressure W and rate of penetration V = dU/dt, moment of rotation T and bit rotate speed Ω = dΦ/dt.

As shown in Fig. 1, the motion equation of the bit in the torsional direction and the axial direction is

$$ I\frac{{{\text{d}}^{2}\Phi }}{{{\text{d}}t^{2} }} + C\left( {\Phi -\Omega _{0} t} \right) = T_{0} - T\left( t \right) $$

(1)

$$ M\frac{{{\text{d}}^{2} U}}{{{\text{d}}t^{2} }} = W_{0} - W\left( t \right) $$

(2)

where T₀ is the driving torque provided by the drill rig and W₀ = W_s − H₀ (W_s is fluid column pressure).

T(t) is the torque of bit, which consists of two parts: (1) T_c is used to overcome the shear strength of rock. (2) T_f is used to overcome the frictional force during drilling.

$$ T\left( t \right) = T_{\text{c}} + T_{\text{f}} $$

(3)

When the stick-slip effect is produced, the boundary conditions of the model have discontinuous properties. For simplicity, we suppose that the bit does not rotate in reverse. The bit stops turning when t = t_k.

$$ \frac{{{\text{d}}\Phi }}{{{\text{d}}t}} = 0 $$

(4)

Equation (1) is deformed into

$$ C\left( {\Phi _{k} -\Omega _{0} t} \right) = T_{0} - T\left( t \right) $$

(5)

In the stage of stick, drill string continuous rotation. The torque applied to the bottom hole assembly by the drill string is

$$ T\left( t \right) = T_{0} - C\left( {\Phi _{k} -\Omega _{0} t} \right) $$

(6)

As time increases, T(t) begins to accumulate until its size reaches the torque needed to overcome the cutting rock:

$$ T_{\text{c}} + T_{\text{f}} = T_{0} - C\left( {\Phi _{k} -\Omega _{0} t_{p} } \right) $$

(7)

3 Research and Development of Torsional Impact Tools

At present, the main methods to reduce and solve the stick-slip vibration are to optimize the parameters of drilling parameters, reduce the weight on bit (WOB), or improve the performance of drill fluid [1]. This paper mainly adopts the way of using the torsional impact tool and the optimization of the bit structure to reduce the vibration.

3.1 The Structure of Torsional Impact Tool

The main structure of the tool is shown in Fig. 2, where the current divider, the starter, and the hydraulic hammer are the core components of the tool,

Fig. 2.

Structure profile of torsional impact tool

where 1 is short shell, 2 is hydraulic hammer shell, 3 is hydraulic hammer, 4 is transmission joint, 5 is diversion device, 6 is starter, and 7 is current divider.

The torsional impact tool can convert the fluid energy of the drilling fluid into axial, circumferential, high-frequency, and stable mechanical shock energy, and transfer the mechanical energy to the bits, which can well restrain the stick-slip vibration and the axial bit bounce with the PDC bits.

The diversion device distributes the drilling fluid transmitted from the drill collar, where fluid containing large particles is discharged through the current divider. The drilling fluid that does not contain large particles is transported to the starter and the hydraulic hammer to avoid that tool was jammed shut during the working process. The fluid from the diversion device opens the starter, which let hydraulic hammer to realize periodic and repeated torsional shocks. The torsional impact force formed by the hydraulic hammer is transmitted to the bits through the transmission joint, thus overcoming the viscous and slippery vibration of the PDC bit.

3.2 The Transfer Efficiency of Torsional Impact Tool

Impact energy transmitted by high-frequency torsional impactor to PDC bits will affect the rock breaking effect and eliminate the stick-slip effect of the PDC bits. Without considering loss of energy about heat, sound, and light during the impact process, energy transfer efficiency of impact system is defined as the ratio of the work done by the impact system to break the rock and initial kinetic energy of an impact system, which we shall denote by η.

$$ \eta = \frac{{W_{ 1} + W_{ 2} }}{W} $$

(8)

where W₁ is the part of the impact energy that is eventually used for rock breakage, J, W₂ is the energy lost in the impact process of the impact system, J, which is equivalent to the deformation energy of the impact system, and W is the initial energy of the impact system.

Considering the impact of shock wave resistance, the equation of energy transfer efficiency of the impact system is derived:

$$ \eta = \frac{{4\pi Er^{2} }}{{4\pi Er^{2} + lk}} \cdot Z $$

(9)

where E is the modulus of elasticity of a tool material, GPa; k is the compressive strength of rock, MPa; l is the distance from the impact to the working face of the rock, m; and Z is the wave resistance coefficient.

According to the characteristics of rock mechanics and tools in drilling strata, the relationship between the diameter and length of PDC bits and the energy transfer efficiency of impact system of high-frequency torsional impactor is analyzed, as shown in Figs. 3 and 4 .

From Fig. 3, we can see that the cross-sectional area of PDC bits has a logarithmic relationship with transfer efficiency. With the increase of PDC bits area, the energy transfer efficiency of the system is higher, and the performance of high-frequency twist impactor is more stable than that of PDC bits.

Fig. 3.

Relation diagram of energy transfer efficiency η and bit area S

As we can see from Fig. 4, the energy transfer efficiency is linearly decreasing with the increase of PDC bits length. That is, the longer the length of PDC bit is, the more energy loss will be. Therefore, when selecting the combination of PDC bit and high-frequency torsional impactor, we should choose ultrashort single drill bits as much as possible, which is helpful to improve the application efficiency of torsional impactor.

Fig. 4.

Relationship between energy transfer efficiency η and bit length L

4 Laboratory Experiment

4.1 The Experimental Condition

The drilling company of Bohai drilling Tarim Oilfield has used the torsional impact tool since 2010. The tool is mounted on the PDC bit, and its performance parameters are shown in Table 1.

Table 1. The parameters of tool performance

The torsional burst experiment of PDC bit was carried out in the laboratory using the simulation test system of rock breaking with torsional impact tool. The diameter of the drill bits was 50.0 mm, the cutting tooth diameter was 13.2 mm, and rotate speed was designed to be 200 r/min. Tap water is used as drilling fluid for carrying cuttings and cooling bits. In this experiment, the red sandstone is selected as the experimental rock sample. The PDC bit is double teeth, and the drilling pressure is 5 kN.

4.2 The Experimental Result

Rudat (2011) theoretically analyzes [12] that in conventional drilling process, because there is the stick-slip vibration impact, drill need to accumulate a certain torque energy to the broken rock. This torque curve of bit will produce a cyclical fluctuation, and this stick-slip vibration will affect the bit service life.

Figure 5 shows the curve of the relationship between the torque and the drilling time of the bit. During conventional drilling, the drill bit produces a distinct periodic viscous stick-slip vibration, which leads to a large periodic fluctuation about bit torque. When the bit torque is not sufficient, the sticky occurs, and the rotate speed is reduced. So, the energy is accumulated. When the accumulated energy reaches the energy of the broken rock, the bit is suddenly released, and slipping occurs. The more the torque is accumulated, the greater the fluctuation after the release. However, the technology of torsional rock breaking changed the rock breaking way of PDC bits, from continuous torque input to continuous torque and periodic high-energy impact superposition. By applying a certain frequency of torque, the torque fluctuation of the bit can be reduced obviously, so it can cut the rock steadily and avoid the damage to the drill string.

Fig. 5.

The change curve of drill bit vibration with time

It is proved by laboratory experiments that the effect of torsional impact can obviously inhibit the stick-slip effect. In the process of drilling, when the high-energy drilling fluid flows through the tool, the torsional impact force (about 2kN) and the impact frequency (about 1500r/min) produced by the tool are transmitted directly to the PDC bit, which makes the bit does not need to wait for storing enough torque energy to cutting formation. It can effectively avoid the stick-slip phenomenon of PDC bit, so as to improve the speed of mechanical drilling.

5 The Field Test

5.1 Application and Effect of Tools

The drilling tool combination used in the torsional impact tool test site of QG5 well is Φ215.9 mm drill bit + Φ180 mm torsional impact tool + Φ165 mm drill collar + Φ214 mm centralizer + 215.9 mm drilling string. The bit has five blades with five nozzles whose diameter is 15.8 mm and the PDC cutter whose diameter is 13 mm. The field drilling parameters are as follows: the drilling pressure is 80–140 kN, rotate speed is 50–80 r/min, and the delivery volume is 30–32 L/s. The lithology of the drilling strata is dominated by sandstone mudstone. The rate of penetration in this well is 4.29 m/h. The rate of penetration of the conventional drilling assembly in the adjacent well is 1.75–2.12 m/h. By comparison, the QG5 well uses the torsional impact tool, and the rate of penetration is greatly improved. The tool worked in well after it is tested at wellhead, when the tool worked for 128 h, pulling it out the well, it can be seen that the appearance of the torsion impact tool is good (Fig. 6) and the drilling bit wears slightly (Fig. 7).

Fig. 6.

Resonator arrive wellhead

Fig. 7.

Abrasion status of PDC bit

5.2 Influence of Drilling Parameters on the Speed Effect of the Tool

The lithology of the 4020–4185 m section is the same, all of which are gray sandstone. According to different drilling pressures and rotate speeds, the average value of drilling time is obtained and drawn in Fig. 8.

Fig. 8.

Relationship between drilling time and well depth

1, 2, 3, and 4 represent four different drilling parameters of drilling processes, respectively. In the first stage, drilling pressure is 80 kN, rotate speed is 50 r/min, the average drilling time is about 11 min/m, and it is drilling into 4110 m. In the second stage, drilling pressure is 80 kN, rotate speed is increased to 80 r/min, the average drilling time is about 15 min/m, and it is drilling into 4132 m. In the third stage, drilling pressure is decreased to 50 kN, rotate speed is 80 r/min, and the drilling time continues to increase with an average of about 17 min/m. In the third stage, it is drilling into 4156 m, where drilling pressure is increased to 80 kN and rotate speed is decreased to 50 r/min. At this time, the drilling time decreased significantly with an average of about 14 min/m. Through real drilling data, we can see that the best drilling parameters exist in the drilling process, that is to use high drilling pressure and low speed to maximize drilling speed.

6 Conclusion

• 1. A torsional impact tool, which produces 1500 Hz/min impact frequency, has been independently developed. The main structure of the tool is introduced, and the working state and speed increasing mechanism of the tool are analyzed.

• 2. Through the laboratory experiments, it is proved that the periodic torsional impact load can effectively restrain the stick-slip effect of the PDC bit.

• 3. The transfer efficiency model of torsional impact energy is given. The model analysis showed that the PDC bit length, energy decay faster, so torsional impact energy transfer efficiency is low. When the PDC bit section is large and the length is small, energy transfer efficiency of high-frequency torsional impacting tool is high.

• 4. The field application shows that when it adopts drilling parameters with high drilling pressure and low speed, torsional impactor and the optimized PDC bit can effectively eliminate the stick-slip vibration effect of PDC bit, and reduce the torsional vibration of drill string, then effectively protect the matching parts consisting of drill bits and drilling tools. Moreover, it can effectively reduce drilling costs.

• 5. The QG3 well experiment in Tarim Basin of Xinjiang shows that the drilling technology with the torsion impact tool and PDC bit increases the penetration rate more than 1 times compared with the conventional drilling tool and PDC bit