exploringdeep-rockmechanicsthroughmechanicalanalysisof...

Research ArticleExploring Deep-Rock Mechanics through Mechanical Analysis ofHard-Rock In Situ Coring System

Jianan Li1 Heping Xie12 Ling Chen 1 Cong Li12 and Zhiqiang He12

1School of Mechanical Engineering State Key Laboratory of Hydraulics and Mountain River EngineeringCollege of Water Resource and Hydropower Sichuan University Chengdu 610065 China2Guangdong Provincial Key Laboratory of Deep Earth Sciences and Geothermal Energy Exploitation and UtilizationInstitute of Deep Earth Sciences and Green Energy College of Civil and Transportation Engineering Shenzhen UniversityShenzhen 518061 China

Correspondence should be addressed to Ling Chen chenlingscuscueducn

Received 2 July 2020 Revised 20 August 2020 Accepted 20 August 2020 Published 7 September 2020

Academic Editor Fengqiang Gong

Copyright copy 2020 Jianan Li et al $is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Exploration of deep-rock mechanics has a significant influence on the techniques of mining and rock mechanics Rock coringtechnique is the basic method for all rock mechanics study With the increase of the drilling depth and increasing strength of thehard rock how to obtain high-quality rock core through various coring techniques is an eternal work Here an innovative methodis applied to design the new coring system tomaximize the efficiency of operation$e stress conditions or parameters of rock corein the coring are analyzed and the mechanism of the core with in situ stress is shown in this paper $e conflict of the core andcoring tool chamber is proposed for the innovative design $e innovative design method is fulfilled by the theory of inventiveproblem solving (TRIZ) An improved coring system for the full-length core with in situ stress was obtained with the solutions ofimproved coring mechanism cutting mechanism and spiral drill pipe

1 Introduction

In the past human activities are mainly on the subsurfacerange of the earthrsquos surface whose depth is less than 100mHowever with the development of the human civilizationpeople living on the earthrsquos surface encountered a lot oflimitations $e growth of human knowledge about earthrsquoscrust and the advancement in engineering helped mankindto study and analyze deeper parts of earthrsquos crust $issteered the development of various industrial activities suchas coal mining and oil drilling $e activities in deeper partsof earthrsquos crust have not only helped in better understandingof geoscience but also finding a lot of new resources forpeople such as gas hydrate and geothermal energy [1ndash3] Atpresent the mineral resources in the shallow part of theearth have been gradually exhausted and the developmentof resources has been moving towards the deep part of theearth However the basic research on the deep developmentis not enough and the basic law is still unclear [4] Further

coring methods provided a better understanding of thestructure and stress distribution of rocks underground Rockcoring technique is the basic method for all rock mechanicsbecause of its ability to provide the mechanical behaviorchemical characterization and structure of rock and thisinformation gives a better understanding of the structureand stress distribution of rocks underground [5] All theinformation mentioned above has significant impact on rockengineering [6] $e rock core at the deep is the mostimportant part of the earthrsquos crust as it has or bears allgeological information [7] Based on the different conditionsof underground deep-rock coring techniques can be clas-sified into three categories that is oil core drilling tech-niques geological core drilling techniques and scientificcore drilling techniques [8] According to the coringmethod each method can be divided into wireline coringand coring by lifting drilling pipe [9] As wireline coring hasbetter efficiency and continuous coring ability this methodhas wide application in hard-rock coring by vertical drilling

HindawiAdvances in Civil EngineeringVolume 2020 Article ID 8899156 11 pageshttpsdoiorg10115520208899156

Based on the underground condition different core barrelstructures to obtain high-quality core are developedHowever the core always gets cracked or broken during thecoring process due to stress developed when drilling diskingphenomenon crack and so on Moreover the core recoverymethod is still the main method for obtaining the maininformation of the underground condition especially for thestress distribution in the core [10 11]When drilling depth isincreased we can find that the strength of hard rock is alsoincreased As a result core blocking occurs which poses agreat challenge to drilling $erefore how to obtain high-quality rock core is always an eternal work for a coringtechnique $e quality of core is not only limited by thecoring technology but also by the strong crustal stress on thedeep rock [12] High ground stress is the main factor af-fecting the special mechanical behavior of deep rock [13 14]It is well known that stresses are commonly assumed to becaused by primary and secondary sources Assuming there isno man-made stress on the rock core the core only hasprimary stress on it caused by the cumulative effect of eventsthat happened in the geological history of the rock forexample gravitational tectonic residual and terrestrialstresses [15 16] For the hard rock there is a littleachievement in obtaining this primary stress or in situ stressin coring technique In China a key research project isproposed by the government to obtain the hard-rock corewith in situ conditions $e ultrasonic drilling method isapplied to reduce the stress relief in cutting $e explorationof the deep-rock mechanism was used to understand thechallenges to be faced and the innovative method is appliedto assist designers in finding the relevant knowledge in allresearch fields for designing the structures $is paper an-alyzed the stress conditions of rock core during the coringand showed the mechanism of the core with in situ stress$e structure is analyzed to fit this in situ stress conditionand the conflict between core and coring tool chamber isproposed for the innovative design $e innovative designmethod is based on the theory of inventive problem solving(TRIZ) and some new methods are proposed by this theoryAfter comparison of these new structures in rock coremechanic analysis the simulation work of rock core with insitu stress was studied to explore the contribution to deep-rock mechanics An improved coring system for full-lengthcore with in situ stress was obtained with the solutions ofimproved coring mechanism cutting mechanism and spiraldrill pipe

2 Mechanical Analysis in Rock Coring System

$e rock core with in situ conditions under a borehole istaken for themechanical analysis As the rock core with in situcoring needs to retain stress distribution as in the under-ground then the contact between core barrel and rock corewill be very hard At the same time contact force needs to bein equilibrium with rock core in the in situ state$erefore toobtain a hard-rock sampler a simple method is used in whicha steel cylinder inbuilt with drill bits bore a rock mass up tothe required depth based on the conditions of the rockstructure [6] During coring lubricating oils will be used for

cooling purposes and when coring operation is over thebored holes are assumed to be filled with lubricants or fluidsin this coring or drilling method $erefore the core forceanalysis is shown in Figure 1

As mentioned in the Introduction section the generalidea of coring with in situ conditions should keep the forcebetween core barrel and rock core in equilibrium Because ofthe in situ stress there is always a plastic-elastic effect zoneas shown in Figure 1 $e forces developed during opera-tions are mainly caused by gravity G friction force Ff corehydrostatic pressure P0 and the in situ stress of deep rock$e interaction and influence of forces determine the effi-ciency and quality of core $e mechanical analysis ispresented to find the mechanism for low quality of core Inthe deep-rock environment the primary stress of the rockcore is the stress in the vertical direction which is equal tothe pressure generated by the weight of the rock mass in thevertical direction and is expressed as

σv c middot h (1)

where c represents the volume weight (Nm3) and h is thedepth of the core location (m)

In the system with existing coring tool and the boreholecoring technology the water pressure in the vertical di-rection of the core is equal to the stress in its vertical di-rection which can be expressed as

σv ρwgh + p0 (2)

In the above equation ρw denotes the density of water g

denotes the acceleration of gravity and p0 indicates thepressure value of water coming out of the horizontal surfaceunder hydrostatic pressure

$e strength of the core barrel and its related structureshould be greater than the damage intensity force at the corelevel according to the pressure characteristic of the rock core

While the core is subjected to the original horizontalstress in the vertical direction the force due to friction andgravity will cause the core to become shorter and thicker Inaddition because the core is located in the core barrel thehorizontal deformation is restrained by the core barrel thusincreasing the horizontal stress $is change causes not onlyincreases in the frictional force when the core slides into thecore barrel but also increases in the original stress on thecore barrel

In this paper the influence of the increase of the horizontalstress of the core is analyzed According to distribution law ofhorizontal principal stress with depth [17] the horizontal stressσh of the core is mainly from the stress in the vertical directionAt the same time the magnitude of the horizontal stress willincrease with the increase of core depth and gradually turn intoa major stress which can be expressed as

σh 00238h + 7648 (3)

In addition because the rock core is squeezed into thecore barrel the force analysis of the core will need to bereanalyzed and the analysis results are shown in Figure 2

In the vertical direction the core is subjected to thefrictional force generated by the friction force Ff and by its

2 Advances in Civil Engineering

own gravity As the core length l increases the verticalsupport force FN is also increasing and its concrete forcecan be expressed as

FN π dl middot σh (4)

In addition because the rock core is surrounded by thewater the gravity of the core in the water can be expressed as

G πd

2gh ρR minus ρw( 1113857

4 (5)

G

P0

P0

σh

Ff Ff

N + σv

Hard rockcore

Corebarrel

Piston

(a)

P0

R0

R0R

σh σhσ0

Samplingarea

Elasticregion

Plasticregion

Core

(b)

Figure 1 Mechanical analysis of coring system (a) $e schematic diagram of the core axial force under the core bit structure and (b) theradial force of the core

G

P0

P0

σh

FfFf

N

Core

Figure 2 Mechanical schematic diagram of rock core

Advances in Civil Engineering 3

$e water environment of the core is in the range of corelength the pressure change is very small and the waterpressure on the core will be the same When the core entersthe core barrel as the depth of sampling increases thecorresponding friction force will increase the friction co-efficient μ is about 035 and the friction force can beexpressed as

Ff μ middot FN πμ dl middot σh (6)

At this point the total support force N of the rock corecan be expressed as

N G + Ff πd


4+ μπ dl middot σh (7)

$ough the rock core is in the core barrel the conditionof rock core is related to deep rocks in the earth

$erefore the bottom of the core is the most vulnerabledeformation position and the stress at the bottom of thecore can be expressed as

σN 4N

πd2 gh ρR minus ρw( 1113857 +

4l

dmiddot μσh (8)

If the strength of the bottom volume of the rock corereaches the yield strength of the core the rock core will havea severe plastic deformation when it enters the core barrelAs the extrusion intensifies the core will be damaged$erefore the yield strength at the bottom of the core is themaximum or limit stress at the entry point of the rock intothe fidelity cylinder According to the limit conditions therock core can enter the length of the core cavity when theyield strength occurs and the limit length can be expressed as

σN 4N


4l

dmiddot μσh lt σY (9)

According to formula (9) the limit length of the corerock that can enter barrel is given as follows

lltd

4σY minus gh ρR minus ρw( 1113857

μσh

1113888 1113889 (10)

In the traditional penetration coring device limestone isassumed for the coring rock in the deep underground $eother parameters of the rock core conditions are shown inTable 1

Substitute the above parameters into equation (10) andobtain the limit length llt 102mm In the actual coringprocess a part of the core is usually lost so it is impossible tomake a standard core with length of 100mm It is necessaryto study how to obtain longer cores

In order to obtain the in situ conditions of rock core thecontact point of hard rock and core barrel should be strongenough to keep the stress in horizontal direction Taking thedrilling system as a base for the sampling mechanismcutting area debris removal and contact form of core barrelshould be optimized to improve the core recovery rate andobtain better core morphology as shown in Figure 3 andTable 2 However there is a conflict relation between hardcontact and high friction at the entry point of hard-rock core

into core barrel For the better design of coring system theinnovative design method is applied Moreover the solutionof the new design is also applied to the method of knowledgeengineering to obtain the exact and required knowledge forthis new design

3 Design Solution Strategy of Hard-Rock Sampler

As core drilling process involves various complex functionssuch as drilling sampling storage and fidelity and in theprocess of the designing and implementation of the hard-rock sampler a series of design conflicts that are moredifficult to break through and solve usually exist If we wantto maintain the integrity of the sampled drill core strengthbetter during design other parameters related to themodified structure may be affected by the change of drillingand mining methods $e result may cause the device tohave a negative impact on other performances and create aconflict Relying on the design criteria that are summed upby the predecessors often is not a good way to start with$eabove problems can be solved easily using the conflictresolution theory theory of inventive problem solving(TRIZ) [18]

$e innovation of various equipment structures isfundamentally to solve or improve the design problem andcreate a new competitive solution Conflicts generally existin design particularly technical conflicts and physicalconflicts are the main forms of existence It is universallybelieved that in the process of improving the conflictproblem it is not possible to solve the problem completelybut can reduce the degree of conflict with a compromiseformula [19] In the actual problem analysis process in orderto facilitate the definition of a technical conflict in a systemTRIZ theory transforms a specific problem into a standardTRIZ problem with the help of 39 general technical pa-rameters $en the technical conflict is solved by the in-vention principle and the physical conflict is solved by fourseparate principles [20]

Here when mechanical properties mainly friction forceof deep hard are analyzed there is a need to reduce andeliminate the effect of drill core pile and the degree ofdistortion by designing a new type of mechanical device Inorder to solve the design problem and conflict of the hard-rock sampler a set of innovation oriented design solutionstrategies should be set up Based on TRIZ conflict reso-lution theory substance-field model the design strategyprocess of a hard-rock sampler as shown in Figure 4 can beestablished and it includes the following parts [21ndash23] (1)the stage of analyzing drilling module problem based on themechanical properties and the fidelity coring requirementsthe opportunity recognition and problem discovery arerealized and the engineering definition of the problem iscompleted (2) the stage of problem transition and conflictdefinition problem-oriented conflicts are resolved thenexpressing the defined engineering problems in the way ofdemand and clear solution direction (3) the stage of in-novation design we usually establish substance-field model


and conflict solutions for the design of hard-rock samplerand the separation principle the invention principle or thestandard solution can be used to reduce the solution rangeaccording to the specific conflict characteristics (4) the stageof project evaluation and verification we turn the design

plan into an application plan and judge its effectiveness incombination with manufacturing and existing environmentIf a conflict is generated return to the design phase and solvethe conflict

As an important tool for description and analysis ofTRIZ theory substance-field model decomposes the func-tion of the system into two substances (S1 S2) and a field (F)[24] A function consists of the three components that istwo substances and a field A matter achieves a specificfunction by field interaction $e conflict between the threeelements of the substance-fieldmodel and the expected effectcan be solved by using 6 general solutions and 76 standardsolutions It usually consists of four types which are usefuland full-interaction models useful but insufficient inter-action models useful but excessive interaction models andharmful interaction models $e functional components ofthe core module of the hard-rock sampler can be describedin the form of the substance-field model as follows

langF S1 S2rang (11)

here F represents the way of drilling and collecting the coremodule (embodied in the way of the field) S1 representscore module of hard-rock sampler and S2 represents drillingmaterial like hard rock $e substance-field model of hard-rock sample belongs to the second type of useful but in-adequate interaction models Materials S1 and S2 can bedefined as a useful but inadequate trigger for design elementsand performance design elements Field F can be defined as auseful but inadequate structural design performance

$e core structure of the hard-rock sampler is designedto maximize the horizontal pressure characteristics of the insitu core and to improve the strength and integrity reducethe pressure stress caused by friction in vertical directionreduce the constraints imposed on the core in horizontaldirection of the fidelity coring tube and weaken the changeof the horizontal stress component in the core Generallythe solving process of this method is as follows choosesolution tools choose a solution synthetically and judgewhether the solution is complete and effective

4 Design Analysis of the Hard-RockCoring System

41 Innovative DesignMethods To maintain the integrity ofthe in situ coring pattern of the hard-rock sampler in deep-rock mass environments we use the abovementioned designflow to improve the design of hard-rock sampler by drillingrecovery collection module According to processes (1) and(2) the design problems are defined and the design re-quirements are analyzed In the traditional drilling the coresample is subjected to the existing horizontal stress and the

Table 1 $e core parameters for calculation

Core diameter(m)

Compressive strength(Pa)

Acceleration due to gravity(Nkg)

Depth of core(m)

Water density(kgm3)

Rock density(kgm3)

Friction angle(deg)

005 9E+ 07 98 1000 1000 2000 35

d1

d2

D

L

Figure 3 Structural parameters of coring system

Table 2 Structural parameters and optimization objectives oftraditional coring

Key parameter Optimization objectiveD Reduced1 Reduced2 ConstantL IncreaseFf ReduceCoring mechanism Rotary drilling


compressive stress caused by the friction and gravity force inthe vertical direction which causes the core to becomeshorter and thicker $e core is located in the fidelity corebarrel and the horizontal deformation is restrained by thecore barrel causing the horizontal stress component in thecore to change $e size of the change not only causes thefrictional force of the core sliding into the core barrel toincrease but also changes the original stress state of the corebarrel $e above analysis shows that the core is subjected tothe friction generated by the extrusion force FN in thevertical direction As the coring length increases thecomponent N in the vertical direction also increases


When improving the design frictional forces are mainlytaken into account Friction is the main force to keep the corefixed and is due to the basic physical contact between two bodiesas shown in Figure 5 However the friction force also changesthe force distribution of the core resulting in the pile effect of thecore and the increase of in situ distortion of the core It is hopedthat the process of the hard-rock sampler drilling and collectingmodule can improve the other mechanical properties by re-ducing the extrusion FN the friction force Ff and the stress σh

while ensuring the core-taking lengthFor the existing mechanical characteristics the method of

reducing friction according to the core characteristics can bedivided into the following aspects structure contact mediumandmovement control method According to processes (1) and(2) the substance-field model of the hard-rock sampler isestablished As shown in Figure 6 as the existing structure canonly perform coring operations and its effect on maintainingthe original mechanical properties is not obvious the type ofthe substance-field analysis model should be a useful but notsufficient interaction model $is type can adopt the standardsolutions of the second and the third class of the substance-fieldmodel including 23 standard solutions of ldquoenhanced objectfield modelrdquo and 6 standard solutions of ldquotransformation tosupersystem or microlevelrdquo further reducing the standardsolution space and adopting comprehensively conflict

resolution theory and substance-field model to improve theeffectiveness and efficiency of the solution

Classical conflict matrix can be based on the design of thesystem to produce two conflicting technical parameters sothat the innovative design can be found out from the in-vention principle of the conflict directly from the matrixtable and using the principle to solve the problem [25 26]

According to the analysis of the problems the determi-nants of hard-rock sampler conflict parameter to be improvedare determined to reduce the extrusion force FN the frictionforce Ff and the stress σh but the depth of the core conflictswith it (the contact area between the core and the core barrelduring the movement) so refer to 39 common technicalparameter definitions hope to improve the parameters (10)force (11) stress and pressure deteriorating parameter and(5) the area of the moving objects Corresponding to thetheory of TRIZ in Table 3 the collision matrix is queried

Application solution is designed by comprehensivelyanalyzing the solution of the invention principle and thestandard solution of the substance-field analysis

σN 1n

middot4N

πd2

1n

middot gh ρR minus ρw( 1113857 +4l

dmiddot μσh1113890 1113891lt σY (13)

42 Innovative Design of Coring Mechanism As the wholesystem is divided into n parts each part cannot yield the rockcore and the new structure has the ability to obtain the no-damage core $e principle of invention (19) periodic effectsuggests replacing continuous action with periodic action orimpulse action and the principles of invention (15) dynamiccharacteristics prompts to separate objects so that its variousparts can change the relative position $e second standardsolution prompted S2 (core barrel) separation and get the design(1) $e design of penetration coring is improved to self-ad-vancing rotary drilling and coring at the same time the drill pipebody is separated from the core barrel and the core barrel madeof a new material with smaller friction coefficient is separatedfrom the rock core so as to minimize the drilling disturbance to

02 01 Programevaluation

Programgeneration

Choose theoptimal soluation

Conflictresoluation

theory

Solution 2

Solu

tion

1

Technicalconflict

Separationprinciple

Physicalconflict

Inventionprinciple

Subatance-field analysis

Incomplete

Harmful

Insufficient

Determinethe type

of problem

Solve theproblem

Standardsolution (11)

Separationprinciple (12)


Build thesubstance-field

model of thestructure

Analyze thedesign

requirements

Define thedesign issuesof module or

structureSTEP A STEP B STEP C

Pretreatment andanalysis

Star

03

04

Invalid

EffectiveEnd

Figure 4 Design process of core system based on TRIZ conflict resolution theorymdashsubstance-field model


the core and reduce the friction as shown in Figure 7 (2)Principle of multistage and equal diameter cutting is designed tocomplete the rock cutting step by step $e outer diameter ofnew reaming tool is almost the same as that of drill pipe bodywhile the outer diameter of common core bit is about 5mmlarger than that of drill pipe Taking the design in Table 4 as

an example the cutting area of new reaming design is about30 lower than that of traditional blade design [27] (3)$enew design reduces the annulus area of cuttings whenreturning which causes cuttings-sticking or friction in-creasing as cuttings-entering the core barrel According tothe principle of innovation (28) the spiral core drill pipe is

G

Useful and fully effectiveUseful but not sufficient

Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv

Figure 6 Substance-field model of the hard-rock sampler drilling

G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure

Figure 5 $e distribution of elements for innovative design by mechanical analysis

Table 3 Hard-rock sampler drilling conflict matrix

Types Improved parameters Deteriorating parameters Invention principle

Technical conflict (10) Force (5) $e area of the moving objects

(19) Periodic effect(10) Prerole

(15) Dynamic characteristics(10) Prerole

Technical conflict (11) Stress and pressure (5) $e area of the moving objects(15) Dynamic characteristics

(36) Phase change(28) Replace with mechanical system


Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)

Figure 7 Innovative design of coring mechanism (a) Self-advancing rotary drilling and coring (b) Assembly relationship of drillingcomponents

Table 4 Structural parameters of coring bit assembly

Design method d1 (mm) D1 (mm) D2 (mm) Reaming area (mm2) Area reduction ()

Traditional design 80 89 94 1913 30

Innovative design 80 89 90 1335

Coring bit

Reaming bit

Spiral groove

D1d1 D2

Table 5 Drilling parameters

Speed Weight on bit (WOB) Circulation rate300 rmin 180 kg 40 Lmin


Figure 8 Experimental equipment and coring bit assembly

Bit torque

T (N

middotm)

Rotational speedDrilled length (cm)

0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))

Figure 9 Drilling parameters of marble

05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed

Drilled length (cm)0 40

3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350

Figure 10 Drilling parameters of dolomite


designed innovatively to increase the return channel andthe spiral mechanism is used to increase the return force ofcuttings and the drilling force of core bit

5 Experimental Study on Rotary Drilling andCoring System

In order to experimentally analyze the dynamical behaviorof new coring system a drilling test on coring bit assembly indesign was conducted which was the foundation of in situcoring system designing

51 Experimental Equipment and Parameters As shown inFigure 8 the experimental equipment mainly includesthe following (1) rotating unit because of the limitationof indoor space electric motor is used instead of hy-draulic motor (2) hoist unit it provides downwarddrilling power and hoist coring bit assembly to obtainrock core (3) circulation equipment it is used to assistthe upper return of rock cuttings and maintain the liquidlubrication between the core and the core barrel at acertain pressure $e main drilling parameters are shownin Table 5

52 Result Analysis Two coring tests were undertaken inmarble and dolomite $e calculated torque (T) and rota-tional speed (n) for dolomite and marble are presented inFigures 9 and 10 as a function of time One core in marbleand one in dolomite are shown in Figure 11

As a uniform material marble shows a relatively stablecurve with little fluctuation of drilling torque However do-lomite consists of nonuniform layers and the results of drillingtorque fluctuate violently In the process of experiment two40 cm long cores of marble and dolomite were obtained whichproved the effectiveness of innovative design

6 Conclusions

Due to the serious ldquostake effectrdquo of hard-rock core in coringprocess the in situ coring of the hard rock deeply underground(gt1000m) is a great challenge $is paper has done the me-chanical analysis of the coring system of the hard rock anddesigned a new coring system for hard-rock core with in situstress In mechanical analysis the core barrel has a seriousfriction problemwith the hard-rock core It not only causes thesurface damage due to the friction force but also limits the corelength for the condition of forming ldquostake effectrdquo Based on this

conflict by friction four innovative structures are proposed toreduce the damage by friction and enhance the good effect tokeep the in situ stress$e innovative designmethod of TRIZ isapplied to improve the coring structure contact medium ofbarrel and core and motion control method In this me-chanical analysis it is proved that all of these innovative designscan obtain the better quality of the hard-rock core than thetraditional coring method In the future this new designmethod and coring system could be employed for the futureexploration of the deep-rock mechanics

Data Availability

$e data used to support the findings of this study cannot beshared

Conflicts of Interest

$e authors declare that they have no conflicts of interest

Acknowledgments

$is work was supported by the National Natural ScienceFoundation of China (Grant nos 51827901 and 51805340)$e financial aids are gratefully acknowledged

References

[1] K Abid G Spagnoli C Teodoriu and G Falcone ldquoReview ofpressure coring systems for offshore gas hydrates researchrdquoUnderwater Technology vol 33 no 1 pp 19ndash30 2015

[2] I Tomac and M Sauter ldquoA review on challenges in the as-sessment of geomechanical rock performance for deep geo-thermal reservoir developmentrdquo Renewable and SustainableEnergy Reviews vol 82 no 3 pp 3972ndash3980 2018

[3] H Xie M Gao R Zhang G Peng W Wang and A LildquoStudy on the mechanical properties andmechanical responseof coal mining at 1000m or deeperrdquo RockMechanics and RockEngineering vol 52 no 5 pp 1475ndash1490 2019

[4] M Z Gao R Zhang J Xie G Y Peng B Yu andP G Ranjith ldquoField experiments on fracture evolution andcorrelations between connectivity and abutment pressureunder top coal caving conditionsrdquo International Journal ofRock Mechanics and Mining Sciences vol 111 pp 84ndash932018

[5] Z Q He L Chen and T Lu ldquo$e optimization of pressurecontroller for deep earth drillingrdquo ermal Science vol 23p 123 2019

[6] G Angeli and B Alberini ldquoA comparison of radiated energyfrom diamond-impregnated coring and reverse-circulation

Figure 11 Recovered core (bottom marble Top dolomite)


percussion drilling methods in hard-rock environmentsrdquoGeophysics vol 80 no 4 pp 13ndash23 2015

[7] M Gao Z Zhang Y Xiangang C Xu Q Liu and H Chenldquo$e location optimum and permeability-enhancing effect ofa low-level shield rock roadwayrdquo Rock Mechanics and RockEngineering vol 51 no 9 pp 2935ndash2948 2018

[8] D Xiangang P G Ranjith and M S A Perera ldquo$e brit-tleness indices used in rockmechanics and their application inshale hydraulic fracturing a reviewrdquo Journal of PetroleumScience and Engineering vol 143 pp 158ndash170 2016

[9] J Tian J Li W Cheng et al ldquoWorking mechanism and rock-breaking characteristics of coring drill bitrdquo Journal of Pe-troleum Science and Engineering vol 162 pp 348ndash357 2018

[10] R Zhang G-S Li and S-C Tian ldquoStress distribution and itsinfluencing factors of bottom-hole rock in underbalanceddrillingrdquo Journal of Central South University vol 25 no 7pp 1766ndash1773 2018

[11] C Ljunggren Y Chang T Janson and R Christiansson ldquoAnoverview of rock stress measurement methodsrdquo InternationalJournal of Rock Mechanics and Mining Sciences vol 40 no 7-8 pp 975ndash989 2003

[12] P Yan Q He W Lu Y He W Zhou and M Chen ldquoCoringdamage extent of rock cores retrieved from high in-situ stresscondition a case studyrdquo KSCE Journal of Civil Engineeringvol 21 no 7 pp 2946ndash2957 2017

[13] F-Q Gong X-F Si X-B Li and S-Y Wang ldquoExperimentalinvestigation of strain rockburst in circular caverns underdeep three-dimensional high-stress conditionsrdquo Rock Me-chanics and Rock Engineering vol 52 no 5 pp 1459ndash14742019

[14] F Q Gong W X Wu T B Li and X F Si ldquoExperimentalsimulation and investigation of spalling failure of rectangulartunnel under different three-dimensional stress statesrdquo In-ternational Journal of Rock Mechanics and Mining Sciencevol 122 Article ID 104081 2019

[15] W Wang J-G Deng B-H Yu X-J Zheng C-L Yan andY Deng ldquoCoupled effects of stress damage and drilling fluidon strength of hard brittle shalerdquo Journal of Central SouthUniversity vol 23 no 12 pp 3256ndash3261 2016

[16] M Z Gao S Zhang J Li and H Y Wang ldquoDynamic failuremechanism of coal and gas outbursts and response mecha-nism of support structurerdquo ermal Science vol 23 p 1222019

[17] X P Li B Wang and G L Zhou ldquoResearch on distributionrule of geostress in deep stratum in Chinese mainlandrdquoChinese Journal of Rock Mechanics and Engineering vol 31no S1 pp 2875ndash2880 2012 in Chinese

[18] Y Li and W Q Li Innovative Design Methods Science PressBeijing China 2012 in Chinese

[19] I M Ilevbare D Probert and R Phaal ldquoA review of TRIZand its benefits and challenges in practicerdquo Technovationvol 33 no 2-3 pp 30ndash37 2013

[20] C Marcel K Robert and H Slavomır ldquoUse the method ofTRIZ in optimizing automated machine for ultrasonicweldingrdquo Procedia Engineering vol 192 pp 80ndash85 2017

[21] A Czinki and C Hentschel ldquoSolving complex problems andTRIZrdquo Procedia CIRP vol 39 pp 27ndash32 2016

[22] W Xia K Wang Y Li and Y Xiong ldquoInnovative design foradaptive detection module of in-pipe robot based on TRIZrdquoJournal of Mechanical Engineering vol 52 no 5 pp 58ndash672016 in Chinese

[23] X Guo J Wang W Zhao K Zhang and C Wang ldquoStudy ofmedical device innovation design strategy based on demand

analysis and process case baserdquo Multimedia Tools and Ap-plications vol 75 no 22 pp 14351ndash14365 2016

[24] S D Savransky Engineering of Creativity Introduction toTRIZ Methodology of Inventive Problem Solving CRC PressBoca Raton FL USA 2000

[25] Z H Bai S Zhang and M Ding ldquoResearch on productinnovation design of modularization based on theory of TRIZand axiomatic designrdquo Advances in Mechanical Engineeringvol 10 no 12 pp 1ndash15 2018

[26] D Chybowska and L Chybowski ldquoA review of triz tools forforecasting the evolution of technical systemsrdquo ManagementSystems in Production Engineering vol 27 no 3 pp 174ndash1822019

[27] J J Wu S H Zhang and L Shi ldquoDesign and application ofsmall diameter of drill tools with dual hydraulic circulation indouble tuberdquo Journal of Central South University Science andTechnology vol 45 no 1 pp 186ndash192 2014 in Chinese


Based on the underground condition different core barrelstructures to obtain high-quality core are developedHowever the core always gets cracked or broken during thecoring process due to stress developed when drilling diskingphenomenon crack and so on Moreover the core recoverymethod is still the main method for obtaining the maininformation of the underground condition especially for thestress distribution in the core [10 11]When drilling depth isincreased we can find that the strength of hard rock is alsoincreased As a result core blocking occurs which poses agreat challenge to drilling $erefore how to obtain high-quality rock core is always an eternal work for a coringtechnique $e quality of core is not only limited by thecoring technology but also by the strong crustal stress on thedeep rock [12] High ground stress is the main factor af-fecting the special mechanical behavior of deep rock [13 14]It is well known that stresses are commonly assumed to becaused by primary and secondary sources Assuming there isno man-made stress on the rock core the core only hasprimary stress on it caused by the cumulative effect of eventsthat happened in the geological history of the rock forexample gravitational tectonic residual and terrestrialstresses [15 16] For the hard rock there is a littleachievement in obtaining this primary stress or in situ stressin coring technique In China a key research project isproposed by the government to obtain the hard-rock corewith in situ conditions $e ultrasonic drilling method isapplied to reduce the stress relief in cutting $e explorationof the deep-rock mechanism was used to understand thechallenges to be faced and the innovative method is appliedto assist designers in finding the relevant knowledge in allresearch fields for designing the structures $is paper an-alyzed the stress conditions of rock core during the coringand showed the mechanism of the core with in situ stress$e structure is analyzed to fit this in situ stress conditionand the conflict between core and coring tool chamber isproposed for the innovative design $e innovative designmethod is based on the theory of inventive problem solving(TRIZ) and some new methods are proposed by this theoryAfter comparison of these new structures in rock coremechanic analysis the simulation work of rock core with insitu stress was studied to explore the contribution to deep-rock mechanics An improved coring system for full-lengthcore with in situ stress was obtained with the solutions ofimproved coring mechanism cutting mechanism and spiraldrill pipe

2 Mechanical Analysis in Rock Coring System

$e rock core with in situ conditions under a borehole istaken for themechanical analysis As the rock core with in situcoring needs to retain stress distribution as in the under-ground then the contact between core barrel and rock corewill be very hard At the same time contact force needs to bein equilibrium with rock core in the in situ state$erefore toobtain a hard-rock sampler a simple method is used in whicha steel cylinder inbuilt with drill bits bore a rock mass up tothe required depth based on the conditions of the rockstructure [6] During coring lubricating oils will be used for

cooling purposes and when coring operation is over thebored holes are assumed to be filled with lubricants or fluidsin this coring or drilling method $erefore the core forceanalysis is shown in Figure 1

As mentioned in the Introduction section the generalidea of coring with in situ conditions should keep the forcebetween core barrel and rock core in equilibrium Because ofthe in situ stress there is always a plastic-elastic effect zoneas shown in Figure 1 $e forces developed during opera-tions are mainly caused by gravity G friction force Ff corehydrostatic pressure P0 and the in situ stress of deep rock$e interaction and influence of forces determine the effi-ciency and quality of core $e mechanical analysis ispresented to find the mechanism for low quality of core Inthe deep-rock environment the primary stress of the rockcore is the stress in the vertical direction which is equal tothe pressure generated by the weight of the rock mass in thevertical direction and is expressed as

σv c middot h (1)

where c represents the volume weight (Nm3) and h is thedepth of the core location (m)

In the system with existing coring tool and the boreholecoring technology the water pressure in the vertical di-rection of the core is equal to the stress in its vertical di-rection which can be expressed as

σv ρwgh + p0 (2)

In the above equation ρw denotes the density of water g

denotes the acceleration of gravity and p0 indicates thepressure value of water coming out of the horizontal surfaceunder hydrostatic pressure

$e strength of the core barrel and its related structureshould be greater than the damage intensity force at the corelevel according to the pressure characteristic of the rock core

While the core is subjected to the original horizontalstress in the vertical direction the force due to friction andgravity will cause the core to become shorter and thicker Inaddition because the core is located in the core barrel thehorizontal deformation is restrained by the core barrel thusincreasing the horizontal stress $is change causes not onlyincreases in the frictional force when the core slides into thecore barrel but also increases in the original stress on thecore barrel

In this paper the influence of the increase of the horizontalstress of the core is analyzed According to distribution law ofhorizontal principal stress with depth [17] the horizontal stressσh of the core is mainly from the stress in the vertical directionAt the same time the magnitude of the horizontal stress willincrease with the increase of core depth and gradually turn intoa major stress which can be expressed as

σh 00238h + 7648 (3)

In addition because the rock core is squeezed into thecore barrel the force analysis of the core will need to bereanalyzed and the analysis results are shown in Figure 2

In the vertical direction the core is subjected to thefrictional force generated by the friction force Ff and by its





G πd


4 (5)

G

P0

P0

σh

Ff Ff

N + σv

Hard rockcore

Corebarrel

Piston

(a)

P0

R0

R0R

σh σhσ0

Samplingarea

Elasticregion

Plasticregion

Core

(b)


G

P0

P0

σh

FfFf

N

Core






N G + Ff πd





σN 4N


4l

dmiddot μσh (8)


σN 4N


4l



lltd


μσh

1113888 1113889 (10)



















Core diameter(m)



Depth of core(m)

Water density(kgm3)

Rock density(kgm3)

Friction angle(deg)

005 9E+ 07 98 1000 1000 2000 35

d1

d2

D

L














σN 1n

middot4N

πd2

1n


dmiddot μσh1113890 1113891lt σY (13)



Programgeneration


Conflictresoluation

theory

Solution 2

Solu

tion

1

Technicalconflict

Separationprinciple

Physicalconflict

Inventionprinciple


Incomplete

Harmful

Insufficient

Determinethe type

of problem

Solve theproblem






Analyze thedesign

requirements




Star

03

04

Invalid

EffectiveEnd





G


Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv


G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure










Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References




































G πd


4 (5)

G

P0

P0

σh

Ff Ff

N + σv

Hard rockcore

Corebarrel

Piston

(a)

P0

R0

R0R

σh σhσ0

Samplingarea

Elasticregion

Plasticregion

Core

(b)


G

P0

P0

σh

FfFf

N

Core






N G + Ff πd





σN 4N


4l

dmiddot μσh (8)


σN 4N


4l



lltd


μσh

1113888 1113889 (10)



















Core diameter(m)



Depth of core(m)

Water density(kgm3)

Rock density(kgm3)

Friction angle(deg)

005 9E+ 07 98 1000 1000 2000 35

d1

d2

D

L














σN 1n

middot4N

πd2

1n


dmiddot μσh1113890 1113891lt σY (13)



Programgeneration


Conflictresoluation

theory

Solution 2

Solu

tion

1

Technicalconflict

Separationprinciple

Physicalconflict

Inventionprinciple


Incomplete

Harmful

Insufficient

Determinethe type

of problem

Solve theproblem






Analyze thedesign

requirements




Star

03

04

Invalid

EffectiveEnd





G


Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv


G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure










Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References




































N G + Ff πd





σN 4N


4l

dmiddot μσh (8)


σN 4N


4l



lltd


μσh

1113888 1113889 (10)



















Core diameter(m)



Depth of core(m)

Water density(kgm3)

Rock density(kgm3)

Friction angle(deg)

005 9E+ 07 98 1000 1000 2000 35

d1

d2

D

L














σN 1n

middot4N

πd2

1n


dmiddot μσh1113890 1113891lt σY (13)



Programgeneration


Conflictresoluation

theory

Solution 2

Solu

tion

1

Technicalconflict

Separationprinciple

Physicalconflict

Inventionprinciple


Incomplete

Harmful

Insufficient

Determinethe type

of problem

Solve theproblem






Analyze thedesign

requirements




Star

03

04

Invalid

EffectiveEnd





G


Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv


G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure










Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References










































Core diameter(m)



Depth of core(m)

Water density(kgm3)

Rock density(kgm3)

Friction angle(deg)

005 9E+ 07 98 1000 1000 2000 35

d1

d2

D

L














σN 1n

middot4N

πd2

1n


dmiddot μσh1113890 1113891lt σY (13)



Programgeneration


Conflictresoluation

theory

Solution 2

Solu

tion

1

Technicalconflict

Separationprinciple

Physicalconflict

Inventionprinciple


Incomplete

Harmful

Insufficient

Determinethe type

of problem

Solve theproblem






Analyze thedesign

requirements




Star

03

04

Invalid

EffectiveEnd





G


Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv


G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure










Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References










































σN 1n

middot4N

πd2

1n


dmiddot μσh1113890 1113891lt σY (13)



Programgeneration


Conflictresoluation

theory

Solution 2

Solu

tion

1

Technicalconflict

Separationprinciple

Physicalconflict

Inventionprinciple


Incomplete

Harmful

Insufficient

Determinethe type

of problem

Solve theproblem






Analyze thedesign

requirements




Star

03

04

Invalid

EffectiveEnd





G


Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv


G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure










Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References



































G


Electricamp force

field

Motor ampsensor

Corecutter

Rockcore

Corebarrel

Frictionpressure

Mechanicalenergy

P0

P0

Ff

σh

Ff

N + σv


G

P0

P0

σh

FfFf

N

Movingdirection

Contact surface

Structure










Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References

































Fluid

Hydraulicmotor

Pistonmechanism

Spiral coring pipe

(a)

Rock core

Core barrel

Spiral drillpipe

(b)






Coring bit

Reaming bit

Spiral groove

D1d1 D2





Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References


































Bit torque

T (N

middotm)


0 40

0

20

40

60

80

100

120

1 2 3 4 5 6 70 80

50

100

150

200

250

300

350

n (r

(min

))


05 1 15 2 25

Bit torque

T (N

middotm)

n (r

(min

))

Rotational speed


3 35 400

20

40

60

80

100

120

140

0

50

100

150

200

250

300

350









6 Conclusions



Data Availability




Acknowledgments


References







































6 Conclusions



Data Availability




Acknowledgments


References

































exploringdeep-rockmechanicsthroughmechanicalanalysisof...

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