DEVICE FOR GENERATING AN AXIAL LOAD IN A DRILL STRING ASSEMBLY

Abstract

The invention relates to drilling equipment. A device for generating an axial load in a drill string assembly with a bottomhole motor powered by drilling fluid comprises a hollow cylinder barrel and a spring-loaded flow-through plunger rod with a key, the rod and key forming a subassembly. The hollow cylinder barrel is provided with a sealing collar and with bilateral longitudinally oriented grooves that are arranged along the internal generatrix of the cylinder barrel and receive with longitudinal and transverse clearance the key, having a profiled surface, such that the key is capable of moving along the grooves together with the flow-through plunger rod. The length of the bilateral longitudinally oriented grooves is dependent on the maximum length of the working stroke of the flow-through plunger rod. The result is an improvement in the operating efficiency of the drill string together with expanded functional capabilities.

Claims

1. An axial load device is fitted in the drill string assembly with a downhole motor drilling operating with mud, comprising a hollow cylinder body with a sealing bush, a spring-loaded flow-type piston rod with a key, wherein a hollow cylinder body having two-way longitudinal key slots along the inner generator line of the cylinder body, in the said slots placing a key with longitudinal and transverse gaps and profiled surface adapted to move along the body slots together with the flow-type piston rod, adapting gaps so that the profiled key has at least two degrees of freedom in the body slots and in the flow-type piston rod slots for axial movements relative to the cylinder body, and installing the flow-type piston rod and the profiled key in its slots as one assembly unit, and including the profiled key with its main natural vibration frequency is directly proportional to its length and the velocity of propagation of the generated flexural wave over its body and inversely proportional to the square of its thickness, wherein the length of the two-way longitudinal slots is determined based on the maximum stroke of the flow-type piston rod operating in the Eulerian area of stable equilibrium, considering its moment of inertia of a cross section and the permissible critical stress generated by the optimum axial load on the drill bit, which is determined by the operating characteristics of the mud motor: according to the maximum torque and the performance factor.

2. An apparatus according to claim 1, wherein it has two-way longitudinal slots along the inner generator line of the cylinder body parallel to its axis, or as a helical evolute spiral with the left of right rise, and the rise angle of the spiral of the two-way longitudinal slots of the hollow cylinder body is the same as the angle of possible torsion of the drill string under the reactive torque of the mud motor, but with the opposite sign,

3. An apparatus according to claim 1, wherein it has spring-loaded flow-type piston rod and profiled key.

4. An apparatus according to claim 1, wherein it has a streamlined profiled key made as an elastic plate with tapered or rounded ends, or wing-shaped flat and skewed, or with a spherical front surface, symmetrical or asymmetrical drop-shaped, with displaced gravity center, placed in the piston rod and body slots with the side closest to the gravity center toward the flow of mud or turned with the farther side from the gravity center.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The essence of the claimed technical solution is explained with examples of its embodiment, shown in the attached figures, where:

[0039] FIG. 1 shows a longitudinal section of the axial load device, with a spring-loaded assembly unit: including a spring-loaded (4) flow-type piston rod (2) with slots (9), in which a key (3) is engaged; the assembly unit is installed inside a body (1) with the possibility of axial motion along the oriented body slots (5), and the spring interacts with the rod through a load washer (8) and with the body through a sealing bush (6) with seals (7); letters A and B indicate threads for connection with the drill string (not shown);

[0040] FIG. 2 shows the device without a spring, indications are the same as in FIG. 1;

[0041] FIG. 3 shows a fragment of the device body part section with a spiral key slot (5) in retracted (transport) piston rod (2) position and an angle (a) showing the rise direction (turn of the spiral body slots);

[0042] FIG. 4 shows a device with spiral key slots (5) in extended (operating) piston position (2), with the initial position of the key (I), the body, indicated with a dashed line, and the end position of the key (II); FIG. 4 also has the following letter indications for sizes: (L.sub.n) is the length of the key slots of the body, (l) is the key length and (L.sub.cr) is the length of the extended part of the piston; also L.sub.cr=L.sub.n−1; Sections C-C and D-D are shown below, correspondingly in FIGS. 10 and 11;

[0043] FIG. 5 shows longitudinal gaps of the key positions (3) in the middle of the piston slots (9): Δx/2, allowing placing the key in the body slots and the piston rod to ensure at least one degree of freedom;

[0044] FIG. 6 shows key profile sections (view from above): (a) drop-shaped key with displaced gravity centre; (b) is a symmetrical key with spherical streamlined surfaces; (c) is a plate symmetrical key;

[0045] FIG. 7 shows overall dimensions: (h) is the width and (l) is the length of a plate-type symmetrical key;

[0046] FIG. 8 shows the process of vortex formation (designated by C arrows) on the upper surface of a plate-type key positioned with gaps in slots, when the mud flows around it with velocity V.sub.fl, and the transverse gap of the key in the piston and body slots: Δy/2 in the initial moment of time;

[0047] FIG. 9 shows the process of the vortex formation (C arrows) on the upper surface of the drop-shaped key with displaced centre of gravity, when the mud flows around the key with a given velocity (V.sub.fl);

[0048] FIG. 10 shows the cross-section C-C of the axial load device indicated in FIG. 4, in the initial position of the key (I).

[0049] FIG. 11 shows the cross-section D-D of the axial load device indicated in FIG. 4, in the end position of the key, in a spiral slot in the end of the piston stroke (II).

[0050] FIG. 12 shows a phenomenological BHA model with an axial load device, a spring K1 and a damper C (hydraulic resistance forces in the device and the assembly below the device, including the MM and the drill bit).

[0051] FIG. 13 shows drawing 2—power schematics

[0052] FIG. 14 shows a graph of the dependency of the drill bit movement with the DA during the drilling

[0053] FIG. 15 shows a graph for smoothed characteristic of the dynamically disturbed DA.

[0054] Following items are indicated in the figures:

[0055] 1—hollow cylinder body (body); 2—flow-type piston rod (piston); 3—key with profiled surface (profiled key, key); 4—spring; 5—oriented key slots of the body (body slots); 6—sealing bush; 7—seals; 8—load washer; 9—slots of the flow-type piston rod (piston slots).

[0056] Following items are indicated in the figures with letters: A and B are the threads for connection with the drill string; L.sub.n is the length of the body key slots; l is the length of key; L.sub.cr is the length of the extended part of the piston rod (required stroke), and

[0057] L.sub.cr=L.sub.n−1; D is the outer diameter of the flow-type piston rod, d is the inner diameter of the flow-type piston rod;

[0058] Δx is the longitudinal gap of the key in the piston rod slots; Δy is the transverse gap of the key in the piston rod and body slots;

[0059] α is the rise (turn) angle of the spiral body slots;

[0060] h is the key width;

[0061] C are the vortex-type flows of fluid;

[0062] V.sub.fl are the mud velocity values;

[0063] ϕ is the angle of the key turn in the slots (“angle of attack” [5])

[0064] C-C and D-D are cross-sectional views of the device, correspondingly in the initial position of the key—3 (I) and the end position of the key 3 (II), in a spiral slot, in the end of the piston rod stroke;

[0065] a); b) and c) are indications of the versions of the key surface shape;

[0066] Equation: L=(ν.sub.k.Math.t).Math.sin(ω.sub.d.Math.t) of the movement (L) of the piston rod with a key in the body of the device, in sync with the dynamic processes happening in the bottom hole when the rock is crushed with a drill bit (not shown), e.g. in a quasiharmonic dependency, where ν.sub.k is the mechanical drilling speed of the channel or the speed of the key movement with the piston rod within the drilling assembly, t is the time of mechanical drilling, ω.sub.d is the frequency of the ground-induced drill vibrations [13].

DETAILED DESCRIPTION

[0067] The DA operating with drilling mud and consisting of a rock-crushing tool, such as a drill bit, a small mud motor (MM), a string of coiled and rigid drill tubing, is fitted with an axial load device without anchor, for instance, between the coiled and rigid tubing, the said device including a hollow cylinder body with a sealing bush, a spring-loaded flow-type piston rod with a key, which is distinct from the existing options due to two-way longitudinal key slots on a cylinder body, along the inner generator line of the cylinder body parallel to its axis, or as a helical involute spiral with a left or right rise; in the said slots a key with longitudinal and transverse gaps and profiled surface is positioned so that it can move along the body slots together with the flow-type piston rod, and the gaps are adapted for the profiled key to have at least two degrees of freedom in the body slots and in the flow-type piston rod slots for axial movements relative to the cylinder body; meanwhile, the flow-type piston rod and the profiled key are made with the possible formation of an assembly unit, which may be spring-loaded or without a spring, and the shape of the key is selected to be streamlined, such as an elastic plate with tapered or rounded ends, or wing-shaped flat and skewed, or with a spherical front surface, symmetrical or asymmetrical drop-shaped, with displaced gravity centre, placed in the piston rod and body slots with the side closest to the gravity centre toward the flow of mud; or turned with the farther side from the gravity centre, and the dimensions of the key are adapted in a way to make its main natural vibration frequency directly proportional to its length and the velocity of propagation of the generated flexural wave over its body and inversely proportional to the square of its thickness; meanwhile the length of the body slots and, correspondingly, the required stroke of the flow-type piston rod, are selected depending on the performance characteristics of the MM and the operating conditions of the flow-type piston rod in the Eulerian area of stable equilibrium, considering its moment of inertia of a cross section, based on the optimum axial load on MM and the drill bit at permissible values of the critical stress occurring in the piston rod.

[0068] The invention describes several possible embodiments of the device that differ in the structural characteristics of the key placement in the oriented slots of the hollow piston rod and in the corresponding slots of the cylinder body, the said slots having various degrees of freedom; in one embodiment it is made with a spring-loaded key within an assembly unit together with the piston rod and with one degree of freedom, and in another embodiment the key is positioned in the guide slots with longitudinal and transverse gaps providing at least two degrees of freedom and the possibility of self-excited vibrations, while in the third embodiment it has the key with different profiles of outer surfaces and positioned in the oriented slots, with a symmetrical or a displaced centre of gravity (mass), with additional slot lines orientation options with rated incline (rise) angles.

[0069] The angle of rise of the slot spiral is the same as the torsion angle of the tubing assembly (for example, coiled tubing assembly) placed under the device above the MM, caused by its reactive moment, but with the opposite sign, i.e., with the opposite direction.

[0070] The device in FIG. 1 forms a technical system including a hollow cylinder body (1), with two-way longitudinal slots (5); a sealing bush (6) with sealings (7), a flow-type piston rod (2) with slots (9), where a profiled key (3) is placed with longitudinal and transverse gaps: Δx and Δy (gaps are shown in FIGS. 5 and 8) in the flow-type piston—the rod and the body, so that the key is positioned with at least two degrees of freedom.

[0071] When necessary, longitudinal slots (5) may, aside from the longitudinal position in the body (parallel to the axis of the device), be positioned along the generator line as a helical, for example, involute, spiral (see FIGS. 3 and 4), with a left or right rise, at the rated angle α.

[0072] To alter the viscoelastic properties of the mechanical system, the profiled key as an assembly unit together with the piston rod may be spring-loaded (4) with a particular rigidity, which allows for varying force “transmission coefficient” [6]:

[00001]$k=\frac{P_d}{F_p}$

and the frequency ratio

[00002]$\frac{{\omega }_0}{\omega },$

where ω.sub.0 the natural frequency of the technical BHA system, ω is the frequency of the disturbing load generated as the rock crushing tool is working; P.sub.d is the dynamic disturbing load on the bottom hole, F.sub.p is the force transferred through the device to the drilling assembly located above. Natural frequencies ω.sub.0 of the BHA technical system can be determined rather precisely for calculations as follows [9]:

[00003]${\omega }_0=\frac{5}{\sqrt{x_t}}$

where x.sub.t is the linear deformation of the assembly in centimetres, for example, for coiled tubing of the length 12.7 m and diameter 30 mm, wall thickness 2.5 mm, made of steel grade 12Kh18N9T (Young modulus E=(1.3 . . . 1.9)105 MPa); under force P.sub.d=0.2 . . . 0.6 kN; x.sub.t=0.90 . . . 2.71 cm, ω.sub.0, natural frequency of the coiled tubing assembly will be 5 . . . 3 Hz.

[0073] If the fraction, due to geological setting, is expected to be

[00004]$\frac{{\omega }_0}{\omega }>\sqrt{2},$

then the assembly unit should rather be spring-loaded, for operation in vibration damping mode, and if oscillator mode is required, then the fraction will be selected based on analysis and experiment as

[00005]$\frac{{\omega }_0}{\omega }<\sqrt{2},$

so the spring is removed from the assembly unit [9,10].

[0074] Direction of the slots along which the key moves may be set with an expected (rated) rise angle (α), possibly as a spiral involute line, which will prevent possible deviation of the borehole path due to reactive moment from the mud motor or known anisotropy of the rock occurrence (FIG. 3) and the direction (right or left) of the drilled channel (well) trajectory modification.

[0075] Angle (α) and directions of the body involute slot lines coiling (and their length) are selected depending on the required length of the borehole, mechanical characteristics of the basic elements of BHA, such as the bending and torsional stiffness of the coiled and rigid tubing assembly, power of the mud motor and, correspondingly, its reactive moment, shape of the key and its degrees of freedom when installed in the slots of the device by means of mounting and testing the device with different assembly units and prepared rock blocks [13] at the test bench. A flow-type piston rod transfers controlled axial load on the coiled tubing and further through the MM to the drill bit.

[0076] For example, to compensate for the BHA torsion due to the reactive moment of MM when drilling a channel with L.sub.k=15 m using a small MM type 2 D 43.5/6.42 (reactive moment of MM is equal to torque M.sub.cr=70 . . . 80 Nm, length of the motor is 2.3 m) and a coiled tubing with L.sub.k=12.7 m, diameter 30 mm and wall thickness 2.5 mm, made of steel 12Kh18N9T, polar moment of inertia of the tubing cross-section is: J=4.12 10.sup.−8 M.sup.4; (G is the shear modulus of elasticity, G=77000 MPa), the expected BHA torsion angle is [14]:

[00006]$\alpha =\frac{M_{\mathrm{cr}}}{GJ}{L}_k=31.1\mathrm{degrees}.$

Then the rise angle for the body slots, as recommended, is taken with the opposite sign: (−) α=−31.1 degrees.

[0077] To calculate vibration damping parameters of the device when creating the axial load (“force transfer coefficient”) with the spring-loaded assembly unit, a phenomenological model for oscillation system of the dynamically disturbed bottom drill string assembly with the device shown in FIG. 1 (two-mass model) is analysed;

[0078] where m.sub.1 is the mass of coiled tubing;

[0079] m.sub.2 is the mass of rigid tubing in the dynamic disturbance zone of BHA, with rigidity coefficient K.sub.2;

[0080] K.sub.1 is the rigidity of the coiled tubing in the assembly;

[0081] C is the damping coefficient of the device, depending on hydraulic resistance forces generated at the mud motion with a certain flow in the device, pipes, mud motor, drill bit and annular space in the borehole;

[0082] K.sub.1 and C form an elastic Maxwell body that is a model of the spring-loaded flow-type piston rod.

[0083] Let us assume that dynamic force P.sub.d (t), which is the reaction of the bottom hole to the axial load generated by the device, is applied on mass m.sub.1, so for a deformed bottom hole we take it as [8, 12]:

P.sub.d=P.Math.cos(ω.Math.t),  (1)

Where P is a static component of the axial force generated by the device;

[0084] ω is the frequency of longitudinal vibrations of the drill bit [13].

[0085] X.sub.i, i=1.3 is the deviation of masses m.sub.1 and m.sub.2 from the equilibrium state.

[0086] A phenomenological BHA model with an axial load device, a spring K.sub.1 and a damper C (hydraulic resistance forces in the device and the assembly below the device, including the MM and the drill bit), is shown in FIG. 12.

[0087] A motion equation may be derived based on the Newton's Laws of motion, for which let us remove the connections and replace them with force diagrams showing the character of the mass loading (FIG. 13).

[0088] Summing up the dynamic forces relative to the corresponding masses, let us have the motion equations for the studied assembly:

[00007]$\begin{array}{cc}\{\begin{array}{c}{m}_2{X}_3^{\&\&}+K_2{X}_3+K_1\left(X_3-X_2\right)=0\\ m_1{X}_1^{\&\&}+C\left(X_2^{\&}-X_1^{\&}\right)=P.\cos \omega t\\ K_1\left(X_3-X_2\right)=C\left(X_2^{\&}-X_1^{\&}\right)\end{array},& \left(2\right)\end{array}$

[0089] where X.sub.i is the space coordinates characterizing the dynamic deviations of the corresponding system points from the state of static equilibrium of the studied model;

[0090] X.sup.&.sub.1 and X.sup.&&.sub.1 are speeds and accelerations of the corresponding points of the system

[0091] X.sub.1 is the disturbing motion of the drill bit along the crushed bottom hole;

[0092] X.sub.2 is the implementation of the device displacement;

[0093] X.sub.3 is the behaviour (displacement) of the dynamically disturbed string.

[0094] The stationary system of linear differential equations (2) of the second order is easily solved by using a complex amplitude method in Mathlab selecting the corresponding single-valued conditions [7,12]. As a result of this calculation, we get the dependency graphs for amplitude displacements X.sub.i(t) assuming the allowed stress-strain state of the DA components and all the operating elements of the assembly are in dynamic equilibrium. A graph in FIG. 14 shows the dependency of the spring-loaded plunger rod DA motion from time during the channel drilling.

[0095] The resulting dependency of amplitude modulations, see FIG. 15, shows that with properly selected viscoelastic properties of the Maxwell body the device shall not only ensure the generation of the required axial load on the drill bit, but damper the amplitude oscillations of the drill bit, i.e., increase the mechanical speed of drilling.

[0096] The length of the piston rod L.sub.cr is selected using known dependencies [14] based on the conditions of its operation in the Eulerian area of stable equilibrium and the allowed values of generated critical stress σ.sub.cr, from the optimal axial load on the drill bit P.sub.cr, which is regulated by the operating characteristics of the MM: the torque, M.sub.ft and the power N.sub.ft in the braking mode:

[00008]$\begin{array}{cc}{L}_{cr}=\sqrt{\frac{{\pi }^2.Math.E.Math.I}{P_{cr}.Math.{\mu }^2}},& \left(3\right)\end{array}$

[0097] where E is the modulus of elasticity of the piston rod material, for example, for steel 40 KhN: E=2.1.Math.10.sup.5

[0098] I is the second moment of the area of the piston rod:

[00009]$I=\frac{\pi \left(D^4-d^4\right)}{64},$

D is the external diameter of the piston rod 35 mm, d is the internal piston rod diameter, depending on the required damping value (C): 12 . . . 25 mm;

[0099] μ is the piston rod length coefficient (for threaded piston rod ends, μ=0.5).

[0100] P.sub.cr is the range of the optimum axial load on the drill bit, for example, for a small MM VZD 2 D 43.5/6.42, based on the operating torque (70 . . . 80 Nm) and the maximum efficiency, it shall be 0.2 . . . 0.6 kN (manufacturer data);

[0101] The permissible stress σ.sub.cr under critical axial load on the drill bit is determined with a known equation:

[00010]${\sigma }_{cr}=\frac{{\pi }^2.Math.E}{{\lambda }^2},$

where λ is the piston rod flexibility; it is recommended that λ>100 . . . 150 [14]. Due to low P.sub.cr values, the length of the body slots and the operating stroke of the piston rod (L.sub.cr) is selected based on design concept, for example 1 . . . 2 m.

[0102] The device with the springless assembly unit works as follows:

[0103] The device is installed within the drill string, above the mud motor, for example between the coiled and the rigid tubing. Pressure loss (drop) when the drill fluid flows inside the piston rod, in the coiled tubing, MM, drill bit attachments and the annular space of the channel and the borehole affects the piston rod area and creates hydraulic load that presses the drill bit to the borehole bottom.

[0104] The device not only helps to intensify the rock crushing process in the bottom hole but facilitates the advancement of the drill assembly in a long horizontal bore hole of the channel due to mechanical vibrations generated by the pulsations of the key operating as a resonator oscillator applying the load as short pulses perpendicular to the rock face, and more energy can be transferred to crush the rock and accelerate the drilling.

[0105] When a profile key is installed in the slots of the device with clearance Δx and Δy, it gives it at least two degrees of freedom, so that at a particular velocity (V.sub.fl) of the mud of corresponding density (ρ.sub.fl), when the mud gets onto the front end of the key, it divides into streams and flows around the key on its edges and surfaces, forming vortex-type flows in alternate manner on both sides, and such flows, also in alternate manner, causing pressure changes, which move along the key surface around which the mud flows as elastic waves, affecting both the mud flow and the key, so that a positive reaction is generated in the system “mud flow-key edge”, and this allows for the generation of self-excited vibrations with frequency f.sub.fl, directly proportional to the mud velocity V.sub.fl (considering the drill bit vibrations [13]) and inversely proportional to the distance L (see FIG. 5) between the front edge of the key and the entry to the hollow body of the device:

[00011]$\begin{array}{cc}{f}_{fl}=\frac{k.Math.V_{\mathrm{fl}}}{L}\left(11\right)& \end{array}$

which changes as the drilled channel is deepened (as the piston rod with the key moves in the hollow body of the device), notably in sync with the dynamic rock crushing by the drill bit, for example, according to quasiharmonic function [9]: L=(ν.sub.k.Math.t).Math.sin(ω.sub.d.Math.t), where ν.sub.k is the speed of the key and piston rod movement or the speed of mechanical drilling (considering the longitudinal oscillations of the drill bit and BHA occurring at 2 . . . 5 m/sec [13], depending on the size type of the drill bit), t is the time of mechanical drilling, ω∂ is the drill bit vibration frequency, which, depending in the key size and its bending stiffness (EI), may occur at a particular frequency ω notably in several vibration modes [5, 6].

[0106] Critical velocity of the self-excited vibrations is determined at the test bench depending on the flow quantity Q.sub.fl (drill pump performance). Pressure changes are transferred through the fluid and through the key forming a vibration system with the possibility of self-excited vibrations [10,11]. The self-excited fluid vibrations frequency is directly proportional to the fluid stream velocity V.sub.fl (which means it may be regulated in the range from 10 m/sec to 30 m/sec) and inversely proportional to the structural length values of the key l, its position relative to the edge of the adapter bushing L, so it also changes as the axial load is generated and transferred to the DA.

[0107] The key surrounded by the vortex-type stream of fluid will oscillate at natural frequency with the possibility of formation, among other, standing flexural waves.

[0108] The main natural frequency of the key oscillations is determined as follows [11]:

[00012]$f_k=k.Math.\frac{h}{l^2}.Math.\sqrt{\frac{E}{\rho }}$

[0109] where h is the key thickness;

[0110] l is the key length (key dimensions are indicated in FIG. 7);

[0111] E is the modulus of elasticity for the key material, when it is made of steel 40 KhN2MA; (E=2.1×10.sup.5 MPa);

[0112] ρ is the density of the key material (7850 kg/m.sup.3);

[0113] k is a shape factor of the key (identified by experiment on the test bench).

[00013]$\sqrt{\frac{E}{\rho }}=5172m/\sec$

is the velocity of elastic waves propagation in an oscillating key.

[0114] For example, with h=0.01 . . . 0.015 m; 1=0.05 . . . 0.1 m; k=2 . . . 5; then natural vibrations frequency of the key is f.sub.k=5 . . . 40 kHz.

[0115] With Q.sub.fl=4 . . . 5 l/sec (required and sufficient value for bottom hole cleaning and conveying the cuttings from 58-60 mm channels and the optimum operating mode for a small MM, such as 2 D 43.5/6.42), considering the BHA vibration velocity: V.sub.fl=10 . . . 40 m/sec, then the self-excited vibrations frequency: f.sub.fl, =1 . . . 20 kHz.

[0116] By selecting the V.sub.fl, (changing the drill pump rate), key dimensions and position, it is possible to synchronize frequencies f.sub.fl and f.sub.k so that f.sub.fl≈f.sub.k, which would lead to operation mode close to resonant, i.e. The key in the device will work as a resonator, intensifying relatively low mud flow vibrations at the discharge of the positive-displacement mud pump. Such operation mode close to resonant would eliminate or considerably reduce the risks of sticking of the DA.