© V. Moisyshyn and K. Levchuk, published by IFP Energies nouvelles, 2017

Introduction

The scientific and technical literature broadly covers the methods of elimination Drill String (DS) sticking using firing gear [1]. In general, mathematical modeling process is quite difficult, therefore to solve specific applied the impact theory we had to use some simplifications and assumptions that satisfy receive convenient engineering solution. Very important also is the right choice of mathematical description of the construction work which is related to the shock [2]. Methods of approximate calculation of impact include:

• classic Newton method, which provides body and solid contact area;

• Hertz method, which provides solid body and contact areas is elastic;

• elasticity method, which provides elastic body, but the distribution of stress on bodies is immediate;

• plane wave Saint-Venant method;

• combined method that combines lists.

The classical Newton’s method gives static elasticity solutions for contact zone and plane wave method to other bodies. Sufficiently accurate results are received if the percussion length of 3–5 times the periods of natural vibrations of the DS. But according to the studies [3] and experimental data [4] blow hammer and anvil last less than 0.1 s, and the greatest period of free oscillations of the drill pipe is 0.2–0.3 s. It should also be noted that the ratio of the classical theory of impact is impossible to determine the duration and force of percussion, tensions in DS, and accelerate their movement.

Despite a sufficient number of publications and developed advanced drilling technologies [5], potential risk of stuck DS is high. Thus, the task of developing new technological solutions and sophisticated technical means to release the drill string sticking remains relevant [6].

Modern methods of elimination stuck are based on the use of mechanical action stuck DS in the area. These methods are based on creating disturbances vibration, percussion or explosive loads applied in the area of the stuck DS. One of the promising areas of DS is releasing drill jars [7], which can eliminate the accident in the most difficult cases.

1 Mathematical model of elastic waves emitted by a drill pipe with a jar

Moving parts of the travelling system are presented by multiple mass m₀ = 9855 kg and rigidity of travelling ropes c₀ = 53 MN/m. Mechanical scheme of DS with a firing gear can be represented by discrete-continuous system comprising four sections (Fig. 1). For research of the selected DS layout we took the parameters that are presented in Table 1. The DS sections are composed of thin cylindrical steel pipes with outside diameter D, inside diameter d and total length l. Stuck part of the drill string will be divided into two sections: the third – a loose part, located above sticking zone and the fourth – stuck DS part. The density of the drilling fluid is q = 1200 kg/m³. Young’s modulus (modulus of elasticity) for all three sections is E = 210 GPa. Coefficient of dry friction was taken f₀ = 0.3 to the fourth section.

The mathematical model for the proposed scheme: travelling block + loose drill string + mechanical jar + sticking drill string (Fig. 1) describes dynamic processes in DS. According to the theory of elasticity [8], wave differential equations [9] for the cross-cuts of each of the four sections can be written as: (1) where ; – multiple coefficient of viscous resistance (α – coefficient drilling fluid interaction with DS section, m = ρFl – mass of section); F – cross sectional area of drill pipes; – speed of elastic waves propagation (E – Young’s modulus); ; g – gravitation acceleration; f₀ – coefficient of DS dry rod friction; x – current coordinate of DS section cross-cut; t – current time.

Next, we have to attach boundary conditions at the ends and docking sections of DS to the dynamic equations of the drill string motion: (2) (3) (4) (5)

From static equilibrium equations of the mechanical system we got initial motion conditions u_i (x_i, 0): (6) (7) (8) (9) where P_st – stuck force of the drill string. Indexes in equations (2)–(9) indicate the number of section.

At the beginning of the motion in the position of static equilibrium speeds in the current DS cross-cuts are .

Figure 1 Drill string with built-in mechanical jar.

2 Methods determining the boundaries of the sticking drill string

Before a decision on the choice of the liquidation of the accident, we should determine the type and location of stuck. The simplest way to determine the upper boundary sticking is an approximate calculation [10]. At the same time, it is believed that every 1000 m loose part of the stuck DS with force an effort that exceeds its own weight the drill string 200 kN is stretched in accordance with Table 2.

The upper boundary of stuck places is usually determined by the formula: (10) where 1.05 – coefficient takes into account hard locks; P₁, P₂ – tensile forces; Δl – elongation the drill string with the subtraction load P₁ − P₂. For the selected layout of the drill string: (11)

An oilman determines a place of the sticking DS more precisely using the identification stuck. This device is the electromagnet, the effect of which is based on the conversion of the sensor indications of magnetic waves in a stick impulses induction of electromotive force [11]. These indications are transmitted by cable to the drilling rig and recorded. Slight areas of pipes are magnetized using the direct current.

Acoustic cement bond log sonde is the most advanced method to determine the place of the sticking and allocation of plots with different degrees of pressing [12]. Amplitude and duration distribution refracted waves are measured using acoustic logging. Emitter generates sound pulses at a frequency of 10–30 Hz, which picks up the receiver. Elastic vibrations transmitted through the drilling fluid in DS, in which waves propagate at the speed of sound. Locations are stuck by a loss of energy that is not returned to the receiver. The intensity of the forces are measured at magnitude stuck energy that is absorbed by the rock. As a result of geophysical studies wells can get the curve that allows you to allocate areas of the sticking DS intensity and pressure forces distributed rocks in the pipe.

3 The sticking drill string

The drill pipe and the rock hole that surrounds it should be considered as a single structure in the case of simulation of wave characteristics of fourth section of the sticking DS. Given the density ρ_r = 3000 kg/m³ and Young’s modulus E_r = 129 GPa of rock we calculated multiple density ρ₄, multiple cross sectional area F₄ and multiple Young’s modulus E₄. Thus, the sticking drill pipe consists of two bodies: steel pipe and rock stuck, unlimited nested in the hills. If we consider the section of described construction, the conditional parameters of rock stuck are completely determined by the stuck power pressure rock P_st, which is the sticking drill pipe (Fig. 2): (12) (13) where f(x₄, φ) – distributed power intensity rock stuck; ρ_r, F_r – rock density and rock strength equivalent cross-cut of the rock sticking area.

Figure 2 Cross-cut sticking drill pipe.

4 Wave model of a mechanical jar

Let us review a percussion of two parts of the drill string. We consider the impact as being elastic: the possibility of residual deformation and dissipation energy is excluded. Therefore, during the formation of waves of an absolutely elastic percussion, we can observe the division of the energy into kinetic and potential. The process of partial transformation of kinetic energy into potential energy gradually spreads on adjacent layers of the drill pipe, whereas deformation spreads from one section to another section. When a percussion wave moves on rod of the drill string, starting from impact zone, dynamic characteristics change is explained by material properties and cross-cuts of the drill pipes, as well as explained by the external environment. According to this, we will consider that the parameters of the wave, moving on rod of the drill string, will not change till it reaches the boundary surface. The boundary surfaces in DS are the following: end sections, adjusted to each other, and stuck surface of the drill string.

The demonstrated mechanic system (Fig. 1) has four boundary surfaces. As a result, a multiple reflection, refraction and imposition of shock waves are possible. It can cause a complication of general picture of wave process. This is why it is advisable to draw a wave diagram (Fig. 3), so as to demonstrate the drill pipes’ wave process. From Figure 3, we can see – the duration of the impact motion wave between the sections of the drill string; , , – efforts and speed on the section borders of the drill string before the percussion; – impact forces; – the speed of the waves between sections of the drill string.

Since all pipes are forged from the same material, and the distance between the impact zones and the drill string sticking l₃ < l₂ < l₁, thereafter t₃ < t₂ < t₁. Thus, the bouncing from the top of the drill string will take place times less than from the first section of the drill string and, consequently, times less than from the stuck boundary surface of the drill pipe. The percussion will be lasting as long as the impact forces will be gaining positive values and both components of the drill string will spring back.

The wave diagram is an important element in order to graph wave equation. At the beginning of a percussion, burdened drill pipes with end speed and ultimate tensile strength met with anvil – sticking component of the drill string, while its upper butt had a speed and a strike force : (14)

From the emplacement, percussion wave with force and speed spreads on the third section of the DS and then lapse of t₃ reaches the stuck place of a static forth section of the drill string, under the force of : (15)

Similarly, in the following intervals 2t₃, impact forces on the drill pipes’ boundary sections DS (Fig. 3) can be calculated from the following equation system: (16) where i = 2, …, 2n₂. The lower part of the first section of the drill string, which had, before the percussion, speed and tensile strength will meet through the time interval t₂ with the wave, which had spread with speed and force : (17)

Here , i = 1–4 – the impedance of the type of barrel drill hole. Impact forces and speeds are calculated step-by-step from these equation systems (14)–(17). Similarly for the upper part of the drilling string, which had, before the strike, speed and tensile strength will meet with a wave after a certain time t₁, that derived from the second section with the speed and force . Speeds , and forces , are calculated from the following equation systems: (18) where – air impedance. It is important to mention that impact force appears later in the sticking zone than in the zone of an actual percussion. This delay depends on the general distance between anvil, the place of the sticking of the DS and the type of borehole, so it means that it can be calculated due to duration of the wave’s passage till the place sticking t₃. The strike will be lasting till the moment when the force becomes negative and both parts of the drilling stick will spring back. Usually the wave never reaches the top of the drill stick, as till that moment t₁ + t₂ percussion ends up.

Figure 3 Wave diagram of the drill string.

5 Findings

The research was conducted for the chosen component of the drill string with the modeling of different length sticking force with equally distributed load. The given wave dependencies are demonstrated in Figure 4.

The duration of the percussion was defined by the period of time from the moment of meeting t_u till the moment of separation of the hammer and the anvil. According to the wave diagram (Fig. 3) release of the sticking zone was considered as step-by-step summary displacement.

In case the integral impact forces occur by P_st and they exceed 100 kN, wave diagrams will have impulsive shape. And the collision momentum itself responds to the Hertz theory: at early convergence of hammer and anvil it quickly increases to the maximum value than it decreases till the complete stop of the drill string. The maximum value of impact force can reach tens of MN. Such stuck is characterized by length of the sticking zones more than 30 m or in places of strong monolithic formations. It is important to add that in practice the drillers meet with several sticking areas, and the sticking length does not exceed more than 20 m (Figs. 5 and 6).

From the researches we can see that with a decrease of pressure force of the rock on the drill pipe, the number of peaks of impact forces on wave diagrams and the speed of liberation of the drill string increase, and collision momentum takes the form of smooth curves.

Since every cross-cut of the drill string's sticking gets negative speed, explained by elastic deformation, and some of the sections – characteristics of the material, therefore the kinetic energy of the drill string before percussion will have integral form: (19) where is the speed of cross-cut of the section DS with coordinate x at the meeting moment of hammer and anvil t_u. The common energy that sticking area gets after the percussion can be calculated as such: (20)

With the help of elaborated computer program for a chosen composition of the drill string, we calculated time of meeting of hammer and anvil t_u = 28 ms, if the way of shock mechanism s = 3 m, rock pressure forces P_st, percussion duration τ, energy A, that was transmitted to the drill string sticking and the number of percussion Nud, needed to liberate the drill string. The results of calculations are summarized within Table 3.

Figure 4 Laws changing impact strength and speed of the stuck area of the drill string and for the pressure rocks Pst = 120 kN.

Figure 5 Laws changing impact strength and speed of the stuck area of the drill string and for the pressure rocks Pst = 90 kN.

Figure 6 Laws changing impact strength and speed of the stuck area of the drill string and for the pressure rocks Pst = 50 kN.

Conclusion

In this study we obtained mathematical and computer model of the firing gear, triggered with the help of longitudinal vibrations of the drill string. Those vibrations were generated by multiple harmonics spectrum. The given theory of dynamic modeling of the drill pipe with a mechanical jar gives us an opportunity to calculate the process of accident liquidation. One should take into account that mechanical jar is embedded into drill pipes section.

The researches have shown [1] that the rapidity of stuck liquidation depends on rock elasticity. The fastest method allows us to release the drill string from loam (E_r = 33 GPa), and the longest from crushed stone (E_r = 50 GPa) and sandy loam (E_r = 78 GPa).

The graphics show that after the meeting with a rock, the collision momentum of plane wave of the drill string (with an increase of rock pressure forces on the pipe) has lower duration and impulsive character. Therefore, while calculating we should take into account geographical data, which will enable us to define the number of stuck, its areas and length, as well as distributed force power on the pipe.

Acknowledgments

We express our sincere gratitude to Dr. Ja. Kuntsyak (Joint Stock Company « Scientific Design Bureau for Testing of Drilling Tools », Kyiv, Ukraine) and Prof. V. Vekeryk (Ivano-Frankivsk National Technical University of Oil and Gas, Ukraine) for consultations on technical issues of borehole drilling.

Scientific advice was obtained from The National Academy of Sciences of Ukraine, Academic Society of Michal Baludyansky (Bratislava, Slovak Republic).

We greatly appreciate Prof. O. Ivasyshyn (G.V. Kurdyumov Institute for Metal Physics of the National Academy Sciences of Ukraine) for his valuable improvements in this manuscript.

References

All Tables

All Figures

	Figure 1 Drill string with built-in mechanical jar.
In the text

	Figure 2 Cross-cut sticking drill pipe.
In the text

	Figure 3 Wave diagram of the drill string.
In the text

	Figure 4 Laws changing impact strength and speed of the stuck area of the drill string and for the pressure rocks Pst = 120 kN.
In the text

	Figure 5 Laws changing impact strength and speed of the stuck area of the drill string and for the pressure rocks Pst = 90 kN.
In the text

	Figure 6 Laws changing impact strength and speed of the stuck area of the drill string and for the pressure rocks Pst = 50 kN.
In the text