Determining key features of the operation of percussion downhole drilling machines

Dnipro University of Technology is a leading institution where an authoritative scientific school was formed to solve the fundamental issues of percussive-rotary drilling with the help of hydraulic hammers. This method makes it possible to intensify significantly the mining processes of rock mass breaking. In order to perform the specified function as fully as possible, hydraulic hammers are to work in a certain technological mode and have appropriate technical characteristics; the paper deals immediately with the analysis of those characteristics. The original designs of hydraulic hammers proposed by the authors are distinguished by a high degree of reliability of the interaction of individual parts and assemblies. Simulation of the operation of hydraulic hammers under appropriate mode-parametric support revealed a number of their significant advantages, i.e. in terms of creating effective conditions for rock mass disintegration. It has been proven convincingly that some features of the approaches to the implementation of a hydraulic well washing programme correspond to the stability of a high-quality process of the downhole work of percussion machines. As a result of the research, a composition of some rational formulations of flushing fluids, which contribute to a significant acceleration of the development of destructive processes, was clarified.


Introduction
Further sustainable innovative development of mining and other related industries is impossible without wide application of drilling well technologies.And for the latter, in the near future, there are no real technically and technologically justified alternatives [1].
In the generalized sense of this well-established and widely used terminological expression, drilling different-purpose wells helps solve a significant range of production and theoretical issues related to the need to search for promising structures for the available useful components and study the specific geological and technical conditions of stratified mineral deposits with further creation of powerful industrial complexes for their effective extraction and partial local processing [2].
Modern engineering and technologies of well construction face rather complex but extremely necessary tasks of obtaining comprehensive multifaceted information, which source, first of all, is represented by core samples of rock formations [3].In relation to wells of the large number of operational wells, the main requirement is the formation of a communication channel suitable for long-term operation with a mineral deposit in the rock layer.Taking into account the above, the following can be stated confidently: completeness and perfection of the geological and industrial 1254 (2023) 012053 IOP Publishing doi:10.1088/1755-1315/1254/1/012053 2 task of deposit development depends on the quality of operations related to well construction in the rock mass; moreover, the latter should contain comprehensive and concrete details.
The multi-factorial cycle of well construction with its indispensable component in the form of various bottomhole drilling processes (in other words, phenomena of rock mass breaking), will be effective only while observing a certain mode of interaction of a drilling rock-breaking tool with the rock mass.Thus, in terms of well layout, the connection links of a rock-breaking tool with the surface energy and power equipment, there is a significant number of functional elements designed to rationalize maximally each factor of the complex operation of rock bottomhole disintegration.The above should be supplemented by the fact that well technologies allow carrying out effectively a cycle of operations to solve complicated issues, e.g.preparation of territories for future construction of civil and industrial complexes as well as necessary technological support for proper execution of the specified operations [4].
Summing up, it is possible and necessary to highlight significant complexity of the structural design of drilling processes, which features are determined by the ultimate goal of well construction.Due to the fact that well drilling can have different purposes, the specified rock formations have a significant range of options for their dimensional design and spatial position in the rock mass.The well depths determined by the stratigraphic location of a specific mineral deposit or by special instructional requirements for the technology of drilling and testing differ greatly in their absolute values.As for the wellbore diametrical characteristics, their values are specified, in particular, by the requirements for the representative qualities of rock samples as well as the overall dimensions of the research and operational equipment of wells.
In such a formulation of the analysis of problematics of drilling operations, one can trace a clear need to apply the substantiated (from the standpoint of minimized capital costs) approaches to the development of regulations for their construction; that will also contribute to the complete avoidance of complications and accidents in shafts of the considered specific mining operations.

Related work
According to the conducted analysis of literature sources and information reports of production organizations engaged in drilling and related operations, it is possible to talk about current and prevailing trend -improvement of rock-destroying tools to increase their operating resource on the well bottom [5].The following remark will be relevant here: a new rock-breaking tool is intended mostly for its use in terms of classic rotary drilling.The given data can be also explained by the fact that almost all technical means of drilling operations are designed and adapted to carry out corresponding operations using rotation (rotary, in case of oil-and-gas well construction) method of rock mass breaking.The latter is the most widely used one; though, it has significant defects, which essence is exclusively in those factors being the basis of functional principles of the method under consideration [1].
Generally, mechanical methods of rock mass breaking are distinguished by a too low coefficient of efficiency due to the fact that they are characterized by unjustified energy dissipation in most technological schemes of their implementation.That refers to large power losses due to drill string friction against the well walls during its necessary rotation as well as bottomhole interaction of a rock-breaking tool with the rock mass with further heat release.The described phenomena are objective and the ones that cannot be eliminated without fundamental transformations of a technological scheme of the rotary drilling method [4].In fact, such a state of affairs has resulted in the development of a number of effective and promising non-mechanical methods that apply useful phenomena of physical or chemical origin to form breaking forces within the drilling tool-rock contact zone of a special design.
Modelling, extensive testing, and, in some cases, even industrial implementation of multifaceted forms of non-mechanical methods have proven their important potential advantages and wide opportunities.However, their relatively large-scale application in the practice of well construction is hindered by the need to make changes, sometimes quite significant, in the construction schemes of the corresponding technological cycles.It will be appropriate here to provide examples of partial application of some principles of non-mechanical methods of forming wells in rock mass.In particular, that refers to high-pressure jets of washing fluid, which hydraulic organs are woven organically into the drill bit structure [6].In the context of such a technical solution, it is possible to intensify significantly a course of destructive processes and their further development in terms of effective drilling mud removal from the well bottom [7].Additional introduction of interaction of high-pressure jets with a rock mass of various-origin abrasive materials into a circulation chain can bring significant positive changes at almost all stages of the formation of a rock mass breaking zone [8].
While applying this method, it is possible to observe an extremely intensive development of complex deformation and subsequent breaking phenomena in the rock mass; ultimately, that allows obtaining high indicators of the wellbore deepening.It can be explained by the significance of dynamic level of application of breaking forces to the rock, which cannot be achieved by other methods and techniques.The conducted studies proved the expediency of such an interpretation of effective rock-breaking methods, when the variants of conditional periodicity of applying the required values of axial load occurs due to the presence of not only abrasive-jet percussion effects but also the generation of directed shock pulses by special impact machines.A convincing advantage of the described modifications of the well formation methods is the possibility of controlled adjustment of the frequency characteristics to a higher degree of a dynamic process of the breaking element-rock interaction.It is implemented by complementing the design with special downhole machines and a corresponding complex tool of certain design of components, developed to act as a so-called downhole regulator as comprehensively as possible.They should function to coordinate the requirements of a well construction technology regarding the proposed combined drilling methods and capabilities of the drilling mechanisms.Percussion downhole machines are to form appropriate pulses of a clearly defined amplitude with the possibility of direct and operational adjustment from the surface; according to the general definition of this concept, a drilling tool should be able to relay and transform shock pulses to the rock bottomhole of the well as fully as possible [9].
The design and implementation of special measures of a hydraulic well flushing programme with the mandatory use of appropriately activated technological indicators of circulating drilling fluids is the additional and very influential factor for a proper downhole breaking process [10].
The represented reasoned data indicates indisputably the available urgent needs for further comprehensive development and industrial implementation of sufficiently effective technical means and technologies for well construction with the adequate multi-category indicators.

Results
A complex cycle of well drilling consists of several sequentially performed operations; rock breaking is the most responsible one from the viewpoint of reaching high production and economic indices [11].A rock breaking method determines almost all technical and technological features of well drilling regulations.It is the method that influences directly the course of downhole breaking processes, which, under conditions of well construction in the rocks prone to manifestation of various complications, must be planned in a logical sequence to prevent violation of the execution completeness of all necessary well technological operations [12].
Overall, a drill well is a complex geotechnological structure that requires certain (and sometimes quite cumbersome) effective measures to maintain them in a certain working condition.Here, great attention should be paid to the needs of high rates of wellbore deepening [13].In order to fulfil the specified criterion, it is necessary to create rational conditions for a stable flow of a rock breaking process with restrictions concerning the resource of the effective tool operation.
In the context under consideration, several approaches to optimization of percussive rock breaking can be mentioned, i.e.: use of a drilling tool with increased wear resistance (this includes a tool equipped with ultra-hard materials); use of the principles of toolless rock mass breaking (physical and chemical methods of rock disintegration are a characteristic feature of the case under consideration); involvement of highly mobile elements in the interaction with a wellbore with the possibility of their operational circulation replacement (principles of combined ball jet drilling are traced here); creation of certain conditions to intensify downhole breaking processes due to dynamic impact of a special tool (a drilling tool layout includes the machines generating shock pulses of different amplitudes and frequencies) [5].
A drilling tool made with the use of relatively wear-resistant cutting structures, is distinguished, for the most part, by a rather high cost, determined not only by a significant indicator of the consumable costs but also by the complexity of the technological manufacturing of tool structures.That is also complicated by special requirements for bottomhole operation of the latter.It narrows somewhat the area of possible rational use of such a drilling tool and requires preliminary analytical and laboratory studies regarding the expediency of its use in specific lithological variants of rocks [1].
In terms of limited funding for drilling operations and the need to construct wells in complicated geological conditions, other considered approaches to the optimization of bottomhole rock breaking, separately or in a complex combination, are seen as the most acceptable ones from many leading positions.
Further developments were based on a rational technological approach, meaning implementation of significant dynamic loads on the wellbore using a hydraulic energy source.That makes it possible to obtain the mode of well deepening characterized by the intensive formation of zones of rock mass breaking and re-breaking of a very effective volumetric nature [14].It is possible to create such conditions due to the involvement of metal balls that move with the flow of washing fluid at high speeds and impact the rock downhole.According to the features of the methodology of a briefly described modernized principle of ball jet drilling, metal balls also perform the work of processing the peripheral wellbore areas together with the percussion machines under certain technical and technological regulations [5].
The considered combined drilling method was studied rather comprehensively by means of analytical and bench modelling.The following can be noted among its main results: the method is characterized by active development of breaking deformations in the form of cracks and holes; the method is characterized by a clear dependence of the scale of breaking processes on the application of axial load and its subsequent physical results (compared to cases of rock breaking in the absence of dynamism of axial load application).This is illustrated both qualitatively and quantitatively on the corresponding graphs in figure 1 and figure 2.
The developed ball jet or hydromechanical method is distinguished by its exceptional featureit is the possibility of obtaining a wide range of load application speed v, which is known to determine usefully the effectiveness of breaking processes.Along with a gradual increase in v values, there is a natural increase in the scale of breaking process development, followed by the formation of corresponding breaking and re-breaking zones.
The data shown in figure 1 demonstrate clearly that the penetration depth of breaking disturbances h into the rock mass, in comparison with the case of static application of axial load, increases by approximately 1.3-1.4times.Certain dependence of the results of breaking processes on the petrophysical characteristics of rocks was also revealed.In this case, influence of a mineral grain size can be observed (constant fluctuations between the absolute values of the depth of breaking hole formation h within the range of 6-14% is noted here), which is necessary to take into account while developing regulations for the operation of designed drilling devices for hydromechanical well drilling.
Pre-qualification studies of the devices under consideration have proved that they exclude  completely the most influential shortcoming of the ball jet drilling method -lack of proper treatment of the peripheral well zone.This is solved by complementing the device layout with a special design of rock-breaking organs, which, together with balls, form a well drilling contour.The effectiveness of this approach is the availability of an additional component in the complex downhole layout of a concrete hydromechanical drilling device.It allows forming the combined impact pulses on the rock mass deformed by active impacts of balls.
Under the proposed technical-technological scheme of drilling, there is a potential possibility of total exclusion of the suspension of well borehole deepening due to its complex curvilinear narrowing in the bottomhole part.Proper conditions for wellbore deepening are formed by the device rotation together with a rock-breaking organ and by creating the necessary static axial load with an active dynamic component (figure 2).
Thus, analyzing the data of figure 2 and statistics of the parametric values of stand studies, the following can be stated: by systematic increase in the rotation frequency n and axial load C, together with their reinforcement with dynamic pulses, it is possible to increase wellbore deepening rate u by approximately 30%.This is most clearly seen for a deformed bottomhole; in this case, growth of u is more than 40%.
A modernized ball-jet (hydromechanical) method of rock mass breaking (or abrasivemechanical percussion method of rock breaking as we call it according to its technological essence), is the one that allows achieving high wellbore depth values, i.e. the main indicator that is the main criterion for the excellence of drilling operations; it is identified by a mechanical speed of drilling wells.
For its organic implementation in drilling practice, the method we have modernized does not require any significant technical and technological innovations and changes in the currently used layouts of the available drilling machines, mechanisms, tools etc. [15].
The design of a downhole rock-breaking tool adapted to the processing mode of the peripheral well zone is characterized by simple modification and work cycle stability.In addition to its simplicity, a ball jet device needs only some special hydraulic mode of operation; that was taken into account as much as possible while developing an additional percussion machine for it -a modernized hydraulic hammer [16].In addition to the necessary clear synchronization of the interaction with a jet device, the reasons for its improvement included following significant defects peculiar for the majority of currently operated impact machines, i.e.: excessive complexity of the functioning of components and parts in a "valve-piston-hammer" system with the lack of sufficient technologically substantiated range of movement speeds of the latter.This provokes considerable and unpredictable, in terms of absolute values, pressure fluctuations in the device, reducing significantly the energy intensity of each individual act during the shock pulse generation.In most cases, hydraulic shock devices are also characterized by large alternating loads on power springs with their active wear and failure.
Figure 3 demonstrates a general scheme of the proposed hydraulic hammer, which includes: an upper locking adapter 1 with an internal thread 2 for the column nipple and recesses 3 for a drilling tongs as well as a central circulation channel 4.An adapter 1 with a casing bushing 24 connected by a thread 29 form an acceleration chamber 5. Casing bushings 24 and a cup 18, connected by a thread 31, form an impact chamber 8.In the internal part of the interconnected adapter 1, the casing bushing 24, and the acceleration chamber 5, the following operates: hammer 7, which contains a high-pressure pneumatic chamber 19, a through circulation channel 6, a drainage channel 26 with the circulation holes 25, rubber-metal sealing rings 27, and an anvil 11, which contains through holes 9 and a central circulation channel 10, external splines 16, shank adaptor 20, and sealing elements 21, 22, 28.To remove the washing liquid, the casing bushing 24 contains resettable circulation windows 23.A casing cup 18 in its lower part is equipped with internal splines 17 for the connection with the anvil 11, which also contains a metric thread 15, to which the lower adapter 12 with the recesses 31 for drilling tongs, a vertical circulation channel 13, and a tool-joint thread 14 are connected.
When lowering the hydraulic hammer in combination with the jet machine, with mandatory flushing fluid circulation when reaching the wellbore, the fluid moves as follows: the central circulation channel 4 of the lock adapter 1, the acceleration chamber 5, the through circulation channel 6 of the hammer 7, the impact chamber 8, the through holes 9, and the central circulation channel 10 of the anvil 11, the vertical circulation channel 13 of the lower adapter 12.When the assembly connected to the locking thread 14 of the lower adapter 12 reaches the wellbore, the axial movement of the adapter 12 and the anvil 11 connected to it by the metric thread 15 occurs, which unhindered and stable movement is ensured by the contact of the splined pair "outer splines 16 of the anvil 11-inner splines 17 of the casing cup 18".The occurrence of a hydraulic shock -a generator of shock pulses of the corresponding frequency, occurs when the anvil 11 is moved axially by a structural parametric value L, which overcoming leads to an instant increase in the liquid pressure in the acceleration chamber 5, being the result of blocking the liquid flow to the bottomhole with the help of through holes 9 in the anvil 11, which, together with the shank adaptor 20, are immersed in the high-pressure pneumatic chamber 19 of the hammer 7. The consequence of the indicated interaction will be the rapid accumulation of the necessary kinetic energy reserve by the hammer 7, which will cause it to impact the anvil 11 with a given working movement by distance D.
The accuracy of impacting the anvil 11 is ensured by the anisobaric nature of the processes in the high-pressure pneumatic chamber 19, filled with compressed gas with a critical pressure of P 0.4 MPa and its periodic compression (during the active washing liquid movement) or expansion (when the active movement stops), which corresponds to a specific stage of the circulation process in the acceleration chamber 5. Hermeticity of the high-pressure chamber 19 and insulation of the through holes 9 in the anvil 11 is ensured by the rubber-metal sealing rings 21 and 22. Constancy of the processes of forward and reverse movements of the hammer 7 is possible while fulfilling the conditions for flushing liquid removal from the impact chamber 8 by synchronizing the location of the resettable circulation windows 23 of the casing bushing 24 and the circulation holes 25 made in the drainage channel 26 of the hammer 7. The rubber-metal sealing rings 27 exclude unpredictable connection of the circulation holes 25 of the drainage channel 26 with the circulation windows 23 of the casing bushing 24.Smoothness and isolation of the movement between the pair of "external splines 16 of the anvil 11 -internal splines 17 of the casing cup 18" and the casing cup 18 itself is ensured by the sealing element 28.The technological manufacturing of the hydraulic hammer structure and the possibility of its maintenance are also facilitated by the available threaded connections 29 and 30 between the upper lock adapter 1, casing bushing 24, and casing cup 18, respectively.The installation operations with the lower adapter 12 are convenient as there is the recess 31 for the drilling sickleshaped tongs on its surface.The principle of hydraulic hammer operation in the automatic mode and possible adjustment of the latter are based on the creation of a regular periodic flushing fluid circulation for the hydraulic hammer, which can be achieved by operational transformation of the drilling pump design -a combination of the technological scheme of operation and interaction of plungers, disconnection of a compensator, and modernization of the piston operation cycle.
An abrasive-mechanical impact method is the one that it allows its versatile technological enhancement.Since the basis of the operation of the drilling machines accompanying its technological cycle is transformation of hydraulic energy, which is supplied in the form of a directed flow of flushing fluid, it is important here to preserve maximally the original characteristic hydraulic parameter of the flow -pressure drop (produced by the surface drilling pump) to overcome the resistances in the circulation circuit of the drilling well, i.e. in the percussion machines themselves [17].
Special experimental studies were conducted at the Department of Oil and Gas Engineering and Drilling of Dnipro University of Technology.Their purpose was to clarify the patterns of influence of surface-active substances (surfactants) and polymer additives on hydraulic supports during the movement of liquid through a pipeline [18].These studies were performed on a special stand (figure 4), where pressure drop ∆P was measured in a standard steel pipe with a diameter of 14 mm and a length of L = 4.5 m, through which the NB-11E pump pumped the studied liquids at different speeds (they were controlled with the help of a valve, marked as B in the diagram, figure 4).DT-10 pressure sensors (marked as M 1 and M 2 in the diagram, figure 4), VT-21 equipment complex, and an oscilloscope were used to measure pressure and determine its difference for the solutions and without surfactants and polymer additives at the tube inlet and outlet [19].The next analytical stage of the research involved determining the value of resistance reduction to liquid movement through a pipeline (in%) [20], which we called hydraulic efficiency A calculated according to following formula where ∆P 0 is drop in liquid pressure at the pipe ends without adding active impurities to the circulating medium; ∆P A is pressure drop at the pipe ends due to addition of polymers and surfactants to the circulation medium.Table 1 represents summary data on the determination of a generalized coefficient of hydraulic resistance and so-called hydraulic efficiency A. While analyzing the data represented in table 1, following conclusions can be drawn: polymer additives and surfactants within a certain range of concentrations reduce hydraulic resistance but the degree of this effect depends on the polymer and surfactant types [21].The greatest reduction of hydraulic resistances can be obtained with the use of polyacrylamide and polyoxyethylene additives.Moreover, it was found that plant polymers lose their effectiveness during the circulation through a pipeline as a result of so-called destruction of molecules, which is evidenced by a decrease in the solution viscosity.To maintain the duration of the effect of hydraulic resistance reduction, it is necessary to use polymers in combination with additives of special reagents-stabilizers [22].
In general, the task of planning an effective hydraulic washing programme means development of compositions and technology for preparing such washing liquids, which would have simultaneously the following most important properties: reducing the coefficient of internal friction, thereby reducing hydraulic resistance; decreasing the surface tension; reducing the manifestation of friction effect during the rotation and contact of a drillstring and a rockbreaking tool with the rock due to lubrication; and preventing swelling, wetting, and falling of clayey and other unstable rocks.
A significant and still insufficiently used reserve of intensification of downhole breaking processes [23] in terms of simultaneously decreasing power losses for the technologically necessary rotation of a drillstring in the wellbore (table 2) is in the possibility to use special flushing fluids treated with appropriate active reagents of complex effect.
Basing on the analysis of the experimental data shown in table 2, it can be stated reasonably that, of all the considered substances, the greatest reduction in the friction coefficient can be obtained when tall oil is added into the drilling fluid [24].
Since it is established and proven that a decrease in the surface tension of flushing fluids σ [25] contributes to the increased mechanical speed of drilling, complex studies were conducted to clarify the nature of the influence of active components on the technological properties of flushing fluids and the results of breaking processes in their application.The data on the laboratory-stand studied of these issues are represented in figure 5 and figure 6.The change in σ indicators must be considered in the context of complex effect of surfactants on the properties of washing liquids with the corresponding consequences.According to the complex experimental data shown in figure 5 and figure 6, following conclusions can be made: when processing washing liquids (in this case, technical water) with the studied surfactants, a decrease, sometimes quite significant, in surface tension σ [26]   rock mass breaking results in the increasing maximum possible mechanical speed of drilling by the basic 1.6 times for percussive-rotary drilling and by 1.3 times for the rotary method of well construction (figure 6) [28].
The considered technical solutions have a relatively simplified implementation, which, however, did not cause any violations of the completeness of a shock pulse generation cycle [29].The above also concerns the use of special washing liquids with the activation of some of their general and special functional properties [30].

Conclusions
The perspective of thorough analytical development and wide industrial implementation of an innovative abrasive-mechanical impact method of obtaining suitable rock products is shown in the context of the need to increase a well construction rate along with simultaneous reduction of financial and material costs for drilling technological and, immediately, rock-breaking tools.Distinctive features of the specified method are that it requires the use of a special type of rockbreaking elements (in the form of special-design drill crowns and metal hard-alloy balls) as well as jet and percussion hydraulic machines.Such a comprehensively formed bottomhole layout allows intensifying and strengthening significantly the breaking processes and phenomena for rock mass.It can be characterized by current significant technologically controlled development of deformation disturbances in the form of cracks and rock chipping.
There are following particular components to regulate well deepening with the help of balls: speed of applying ball load v to the well bottomhole and axial load C on them during their subordinate movement in the peripheral zone of the well bottomhole.The results of breaking processes make it possible to confirm that the method under consideration demonstrates an increase in the mechanical drilling speed u by a weighted average of 20-30% and an increase in the volumes of breaking zones by 2 or more times in comparison with the corresponding values obtained for the case of static application of axial forces.
The paper represents a technical description of the percussion machine, which can be adapted especially to the conditions of implementation of the abrasive-mechanical percussion method.
The conducted studies show the prospects of using surfactants and polymers as hydraulic resistance reducers (up to 50% or more) in the circulation contour of the well, which will contribute to the additional strengthening of the effect of active washing liquid jets on the rock mass.The use of specified chemically active reagents as a component of flushing fluids also helps reduce significantly the power consumption for drill string rotation in the wellbore by minimizing a coefficient of friction (depending on the type of rocks and flushing fluids, it can be reduced by 1.1-4.0times and even more).Treatment of flushing fluids with certain surfactant also contributes to the reduction of their surface tension σ, which has a positive effect on the results of destructive processes (due to this, it is possible to increase the mechanical drilling speed by a minimum of 30%, especially the indicated case relates to the technological reception of shock impact on the bottomhole).

Figure 1 .
Figure 1.Patterns of the development of breaking processes for different mineralogical rock types under variable values of the axial load application rate.

Figure 2 .
Figure 2. Influence of a physical state of rock mass on its breaking rate in terms of a modernized ball-jet method of well construction.

7 Figure 3 .
Figure 3. Layout diagram of a hydraulic hammer device.

Figure 4 .
Figure 4. Schematic diagram of a laboratory stand to study hydraulic resistances when pumping liquid through a pipeline.

Figure 6 .
Figure 6.Dependence of the mechanical speed of drilling on surface tension σ of flushing fluids: 1 -percussive rotary drilling; 2 -rotary drilling.

Table 1 .
Influence of surfactants and water-soluble polymers on the values of a generalized coefficient of hydraulic resistance and hydraulic efficiency.

Table 2 .
Results of studying the influence of various active additives in flushing fluids on a steel-rock friction coefficient (simulation of well rotational movement of a drill string).