If cuttings carrying performance is poor and cuttings removal is not in time during the drilling of horizontal wells, drilling cuttings will accumulate in the lower sections, leading to backing pressure, BHA binding and even drill pipe sticking. In this paper, a new type of Helmholtz pulse grinding bits suitable for horizontal wells was designed based on the theory of Helmholtz oscillation chamber to generate pulse, jet pump and high pressure jet after the formation of cuttings beds was analyzed. In this type of bit, a high-speed pulse jet is used to assist rock breaking, a reverse jet is used to remove the cuttings at the bottom of the bit under negative pressure, and its inner grinding structure is used to reduce the particle size of cuttings.
Wei Zheng*, Gao Deli & Liu Yongsheng
MOE Key Laboratory of Petroleum Engineering, China University of Petroleum, Beijing 102249, China
Received 16 June 2017; accepted 25 August 2017
By using this bit, efficient cuttings removal and rock breaking will be both realized, the chip hold-down effect will be reduced and the cuttings beds in a horizontal well will be also removed. Then, the hydraulic models were established for a pulse generation device, an efficient rock breaking device and a reverse swabbing device, respectively. It is shown from the simulation results that the optimal resonance flowrate increases with the increase of the diameters of an inlet chamber and a feedback chamber and with the decrease of the diameter of a resonance chamber, and it is approximately in linear relationship with each factor.
The optimal flowrate ratio of the reverse swabbing device increases first and then decreases with the increase of dimensionless flowrate ratio, and decreases with the increase of dimensionless area ratio. It is indicated from example analysis that the inherent frequency of Helmholtz oscillation chamber is 24.00 Hz, the optimal oscillation flowrate is 23.92 L/s and the optimal flowrate ratio is 0.59. Based on case studies, the accuracy of hydraulic models is verified. It is concluded that this new type of bits provides a new solution to the accumulation of cuttings beds.
With the rapid development of global unconventional oil and gas exploration and development, highly-deviated wells and horizontal wells are increasingly applied widely in order to further improve the efficiency of oil and gas exploration and development. However, if the cuttings carrying performance is poor and the cuttings removal is not in time during the drilling, drilling cuttings usually accumulate in the low side in the migration process and form cuttings bed, which restricts the drilling rate and drilling efficiency and even causes such drilling complications as pump suffocation and drill pipe sticking. Conventional solutions to the above mentioned problems include frequent “short tripping”, drilling speed improvement, and displacement increase. Although these solutions have some effects, they cannot fundamentally eliminate the cuttings beds that may appear at any time during the drilling .
Due to gravity, the cuttings settle on the low side of wellbore in the process of the wellbore flow and form cuttings bed at the section with a large well deviation . Affected by the drag, lift and buoyancy of fluid, cuttings roll on the surface of the cuttings bed or are lifted to the upper fluid in the migration process [3–5]. Based on the theories about cuttings migration, some scholars have established the basic cuttings migration models for horizontal wells, taking into consideration of the thickness of cuttings bed and particle size of cuttings  and the time–lapse model for cuttings migration. The corresponding basic model for cuttings migration has also been gradually improved and developed. For example, the average cutting migration velocity has been taken into account in the analysis of the equivalent slip velocity , and the empirical formula for cuttings migration in the time–lapse model has been corrected .
The experimental study of cuttings migration shows that wellbore curvature, drilling fluid velocity and rheological properties of drilling fluid have a great influence on cuttings rolling and suspension mechanism , and turbulent fluid is an effective cuttings migration medium . There are many ways to remove the cuttings beds , including by enhancing hydraulic parameters , the drill string rotation [13,14] and the application of the improved jet mill to the gas drilling of horizontal wells. Based on the cuttings comminution theory, the jet grinding bit can effectively remove the cuttings beds in the horizontal well [15,16].
However, the relevant theoretical and experimental studies are based on steady-state fluids, and there is little research on pulse fluids. Compared with the conventional steady-state fluid, the pulse jet can greatly improve rock breaking and cuttings removal efficiency with its asymmetric and non-uniform cutting of rocks . Relevant numerical simulation and experimental researches also prove that the pulsed jet drilling technology can improve the development effect of oilfield, while improving the drilling efficiency . In addition, the design of jet drilling bit is only restricted to gas drilling, and there has been no relevant research on conventional drilling fluid circulation drilling.
In view of this, a new type of Helmholtz pulse grinding bit was designed based on the Helmholtz oscillation chamber , jet pump and water jet theory. In this type of bit, Helmholtz oscillation chamber is adopted to form a large-scale vortex ring structure and generate pulsed jet, so as to assist in rock breaking and cuttings clearing; a negative pressure is resulted in the swabbing chamber by the high-velocity reverse jet, and then the cuttings at the bottom of the bit are swabbed to reduce the chip hold-down effect; and the cuttings particle size is reduced depending on the high pressure force of internal grinding, to facilitate cuttings carrying. The design can be applied not only to the gas drilling in horizontal wells, but also to the conventional drilling with drilling fluid. Through theoretical calculation, case analysis and parameter study, it can be concluded that the new Helmholtz pulse grinding bit can effectively remove the cuttings bed.
2. A Helmholtz pulse grinding bit
The Helmholtz pulse grinding bit consists of a Helmholtz oscillation chamber, a lower jet channel, a swabbing chamber, a reverse high-speed flow channel, a mixing chamber, an accelerating chamber, an internal grinding chamber, an internal grinding machine and a diffusion chamber. The Helmholtz oscillation chamber is composed of an inlet chamber, a resonance chamber, a feedback chamber and a diverging section.
Fig. 1. Schematic diagram of a Helmholtz pulse grinding bit. Note: 1. Internal grinding chamber; 2. Accelerating chamber; 3. Mixing chamber; 4. Swabbing chamber; 5. Lower jet channel; 6. Helmholtz oscillation chamber; 7. Diffusion chamber; 8. Internal grinding machine; 9. Throat; 10. Reverse high-speed flow channel.
Different from the conventional PDC bit, the Helmholtz pulse grinding bit consists of a pulse generation device, a high-efficiency rock breaking device and a negative-pressure swabbing device. There is no chip space in the structure. Instead, the cuttings are discharged through circulation in swabbing chamber – mixing chamber – accelerating chamber – internal grinding chamber – diffusion chamber. Its structure and flow channel are shown in Fig. 1, and the relevant structural parameters are shown in Table 1.
Table 1. Structural parameters of a Helmholtz pulse grinding bit.
2.2. Working principle
As shown in Fig. 1, the steady-state drilling fluid forms a high-velocity jet flow through the inlet chamber; in the resonance chamber, the unstable shear layer of the jet flow generates a pressure perturbation wave; in the feedback chamber, the jet flow produces a pressure transient and reflects upstream through the contraction section; and the pressure transient upward and the pressure perturbation wave produced by the unstable shear layer interfere with each other in the resonance chamber, forming a large-scale vortex ring structure. Under the action of large-scale vortex ring structure, steady-state flow is transformed into pulse jet. The resulted pulse jet flows through the diverging section to the lower jet channel and the reverse high-speed flow channel, respectively.
The asymmetric and non-uniform jet produced by downward pulse jet assist in the high-efficiency rock breaking, agitation and cuttings removal. The upstream pulse jet flows through the reverse high-speed flow channel, and forms negative pressure in the swabbing chamber by virtue of its high velocity. Under the action of negative pressure, cuttings are pumped into the mixing chamber. In the mixing chamber, cuttings and reverse jets form two-phase high-speed turbulence, and flows through the throat into the accelerating chamber. In the accelerating chamber, due to the effect of the viscous force of the drilling fluid, the cuttings move at a high-speed into the inner grinding chamber, and are crushed under the action of the high pressure force and the water wedge effect between particles as well as between particles and internal grinding machine. Cuttings are discharged through the diffusion chamber after they slow down.
In the Helmholtz pulse grinding bit, a high-speed pulse jet is generated by a pulse generation device to assist in high-efficiency rock breaking, a negative pressure is formed in the swabbing chamber and at the bottom of the bit by a reverse swabbing device to enable the cuttings backflow, and the particle size of cuttings is reduced by means of its inner grinding structure. It thus provides an effective means for reducing the chip hold-down effect and for removing the cuttings beds in horizontal wells. Its advantages include:
- A large-scale vortex ring structure is formed in the Helmholtz oscillation chamber, which converts the steady-state flow of the circulation drilling into pulse jet. The pulse jet impacts the rock asymmetrically and nonuniformly, which improves the hydraulic assistance in rock breaking.
- The high-speed jet produced by the reverse high-speed flow channel forms negative pressure in the swabbing chamber and at the bottom of the bit and swabs the cuttings, which reduces the chip hold-down effect and improves the cuttings removal efficiency of the drilling fluid.
- The inner grinding structure is used to reduce the particle size of cuttings, which is favorable for the cuttings carrying of the drilling fluid and the cuttings bed removal of horizontal wells.
3. Hydraulic model of Helmholtz pulse grinding bits
3.1. A pulse generation device
Based on the theory of Helmholtz oscillation chamber, a pulse generation device was designed so that a large-scale vortex ring structure was formed through pressure wave interference, and then the steady-state flow was converted into pulse jet, as shown in Fig. 2.
Fig. 2. Schematic diagram of a pulse generation device.
When the frequency of the pressure disturbance in the Helmholtz oscillation chamber is m integral multiple of or equal to the inherent frequency of the Helmholtz oscillation chamber, it can generate resonance and generate pulse jet. According to the analysis of the inherent frequency of the Helmholtz oscillation chamber, fHEL[20,21], and combined with the expression of Strouhal number , the optimal resonance displacement of the Helmholtz oscillation chamber is obtained, which is expressed as below:
where, m represents the multiple of the pressure disturbance frequency and the inherent frequency of the Helmholtz oscillation chamber; similarly n: the characteristic coefficient of fluid; fHEL: the inherent frequency of the Helmholtz oscillation chamber; D: the resonance chamber diameter; u: the velocity of fluid flowing through the resonance chamber.
The inherent frequency of the Helmholtz oscillation chamber is expressed as:
R1 and R2 respectively represent the equivalent flow resistance of the inlet chamber and feedback chamber, 1/(m·s); similarly L1 and L2:respectively the equivalent flow inertia, 1/m; C: the equivalent flow capacity of the resonance chamber, m/s2.
3.2. A high-speed rock breaking device
The high-speed rock breaking device makes rock breaking through the asymmetric and non-uniform impact of pulse jet, as shown in Fig. 3. For the convenience of research and analysis, a cross section at the end of the feedback chamber is taken as Section A, similarly that at the end of the lower jet channel: Section B, that at the entrance of the reverse high-speed flow channel: Section C, that at the end of the swabbing chamber: Section D, and that at the exit end of the reverse high-speed flow channel: Section E.
Fig. 3. Schematic diagram of a high-efficiency rock breaking device
3.2.1. Rate and pressure of flow through Sections B and C
The drilling fluid flows into the diverging section through Section A, diverges in the diverging section, and then flows to the high-efficiency rock breaking device and the negative-pressure swabbing device respectively. The drilling fluid diverges in the diverging section, and converges in the mixing chamber. The flow channels of Sections B–E and Sections C–D are in parallel, and the pressure head of the two flow channels is equal, which is expressed as below:
where, kB and kC respectively represent the discharge modulus of two flow channels; similarly lB: the length from Section B to Section E, m; lC: the length from Section C to Section D, m. According to the continuity equation, the relationship between the drilling fluid flowing through Sections A, B and C can be expressed as:
According to the simultaneous equation of Equations (3) and (4), the drilling fluid flowing through Sections B and C can be expressed as below:
The drilling fluid flowing through Section A is half that pumped by the drilling pump.
At the same time, taking the ground as the datum plane, the Bernoulli equation of Sections A and B, and Sections A and C is as below:
According to the simultaneous equation of Equations (5) and (6), the pressure of Sections B and C can be expressed as below:
The pressure of Section A is:
where, ps represents the pressure of the drilling pump, MPa; similarly Δpg: that of the ground manifold, MPa; Δpst: that of the drilling tool, MPa; Δp1: that of the screw, MPa.
3.2.2. Rate and pressure of flow through Sections D and E
The drilling fluid flows through Section C of the high-efficiency rock breaking device into Section D of the negative-pressure swabbing device. The uneven distribution of drilling fluid flow is often caused by bottom turbulence. Therefore, f is defined as the flow correction factor, and the drilling fluid flowing through Section D is expressed as below:
In this process, the drilling fluid will also have pressure loss, which is manifested as the bit pressure drop ΔpB. Then the pressure of Section D is expressed as below:
Considering the drilling fluid will have pressure loss in the lower jet channel and swabbing chamber, Kpj is defined as the pressure correction factor of drilling fluid in the lower jet channel and Kps is defined as the pressure correction factor of drilling fluid in the swabbing chamber, and thus the pressure of Section D after correction is expressed as below:
Meanwhile, the drilling fluid has no flow loss in the reverse high-speed flow channel, that is, the flow through Section E is exactly the flow through Section B.
In this process, considering the pressure loss of the drilling fluid in the reverse high-speed flow channel, Kpa is defined as the pressure correction factor of the drilling fluid in the reverse high-speed flow channel, and then the pressure of Section E is expressed as below:
3.3. A negative-pressure swabbing device
The negative-pressure swabbing device is designed based on jet pump theory, which swabs the cuttings by virtue of the high velocity of reverse jet. Its schematic diagram is shown in Fig. 4.
Fig. 4. Schematic diagram of a reverse swabbing device.