J. Mt. Sci. (2016) 13(5): 930-938 e-mail: jms@imde.ac.cn http://jms.imde.ac.cn DOI: 10.1007/s11629-014-3018-0

930

Abstract: How to find more effective way to stabilize

the borehole wall in the fault gouge section is the key

technical challenge to control the stability of the

borehole wall in the Wenchuan fault gouge section

during the process of core drilling. Here we try to

describe the characters of deep fault gouge in fracture

zones from the undisturbed fault gouge samples

which are obtained during the core drilling. The X-

Ray Diffraction (XRD), X-Ray Fluorescence (XRF)

and Scanning Electron Microscope (SEM) provided

the detailed information of the fault gouge’s

microscopic characteristics on the density, moisture

content, expansibility, dispersity, permeability, tensile

strength and other main physical-mechanical

properties. Based on these systematic experimental

studies above and analysis of the fault gouge

instability mechanism, a new technical procedure to

stabilize the borehole wall is proposed — a low water

and a low loss low permeability drilling fluid system

that consists of 4% clay + 0.5% CMC-HV + 2% S-1 + 3%

sulfonated asphalt + 1% SMC + 0.5% X-1 + 0.5% T

type lubricant + barite for core drilling in fault gouge

sections.

Keywords: Fault gouge; Microscopic characteristics;

Borehole wall stability; Drilling fluid

Introduction

Fault gouge is unconsolidated or weakly

consolidated, crushed and decayed rock, that

develops in fault zones (Burchfiel et al. 2008; Han

et al. 2010), usually produced by fault movements

during successive slip events. Fault gouge records

information of the fault formed by relative

movement between the wall rocks. The physical-

mechanical properties of fault gouge may influence

earthquake mechanisms, characteristics of ground

motion near a fault, and resulting failure

characteristics of engineered structures (Faulkner

and Rutter 2000; Geng et al. 1985; Sykes et al.

Influence of the characteristics of fault gouge on the stability

of a borehole wall

WANG Sheng1 http://orcid.org/0000-0003-4310-1891; e-mail: yongyuandewangsheng@sina.com

CHEN Li-yi1 http:// orcid.org/0000-0002-7920-0325; e-mail: cly@cdut.edu.cn

HUANG Run-qiu1 http://orcid.org/0000-0003-2560-4962; e-mail: hrq@cdut.edu.cn

LI Zhi-jun1 http://orcid.org/0000-0002-3580-6421; e-mail: lizhijun0613@126.com

WU Jin-sheng2 http://orcid.org/0000-0003-4451-1649; e-mail: wjs@cgiet.com

YUAN Chao-peng1 http://orcid.org/0000-0003-4775-1138; e-mail: 1065166678@qq.com

1 State Key Laboratory of Geohazard Prevention & Geoenvironment Protection, Chengdu University of Technology, Chengdu Sichuan 610059, China

2 The Institute of Exploration Technology of CAGS, Chengdu 611734, China

Citation: Wang S, Chen LY, Huang RQ, et al. (2016) Influence of the characteristics of fault gouge on the stability of a borehole wall. Journal of Mountain Science 13(5). DOI: 10.1007/s11629-014-3018-0

© Science Press and Institute of Mountain Hazards and Environment, CAS and Springer-Verlag Berlin Heidelberg 2016

Received: 13 February 2014 Revised: 8 June 2015 Accepted: 14 September 2015

J. Mt. Sci. (2016) 13(5): 930-938

931

1999). Many researchers have studied stability of the

borehole wall in the fault gouge and made significant advances. Westergard (1940) published the first article about the stability of a borehole wall and analyzed the stress distribution in a vertical borehole wall.

In the first 20 years of research on the stability of borehole walls, researchers defined borehole wall collapse by saying that “the original strength of the rocks could not resist the failure stress”. In the mid-1950s, researchers realized that the true reason for most collapses was hydration of the wall rock, when it comes in contact with water-based drilling fluid (Hale and Mody 1992; Yu et al. 2001; Wang et al. 2012). Gray and Chenever (1988) first pointed out that the reason for borehole wall collapse is not only a matter of stress but also of the hydration of the shale in the wall rock. Chen and Jin (2005) had similar finding using the same method. Xu (1997) established a laboratory borehole wall evaluation and testing method to determine the borehole wall stability mechanism, and designed a technical drilling method that can stabilize borehole walls.

Bradley (1979) evaluated the rock shear failure of a borehole wall using the Drucker-Prager failure criterion, based on Fairhurst (2011) and concluded that wall leakage and collapse were caused by stress associated with swelling of wall rock minerals. Lee and Deng (2011) examined undisturbed fault materials, including fault gouge of the Longmenshan fault zone in Shenxigou Section using the usual laboratory geotechnical tests. In the Lee and Deng (2011) study, they found that higher porosity in a fault gouge correlates with the lower peak shear stress and shear failure of fault gouge in plastic or half brittle characteristics. Chen et al. (2011) studied mineral composition of rocks around the co-seismic ground rupture zone, fault breccia, old and new fault gouges, and friction-sliding properties of sedimentary rock and fault gouges in the Longmenshan fault zone.

Five exploratory boreholes were drilled in the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD project), which aims to collect information on characteristics of the Wenchuan fault. The first borehole was drilled starting on Nov. 4, 2008, and the last one (well WFSD-4) was started on Aug. 6, 2012. The drilling conditions are

very poor due to frequent vibrations of the equipment, which can badly damage required samples (Zhang et al. 2012).

The fault gouge core can also easily swell and shrink by hydration during drilling. For example, a drilling pipe and other tools got stuck three times because of the swelling of the borehole. Therefore, the key technical problem is to overcome these issues during core drilling of the fault gouge zone. The difficulties encountered with borehole wall stability during core drilling for the WFSD project made it clear that further study should include:

(1) Systematic research on the microscopic characteristics and the physical and chemical properties of undisturbed fault gouge samples from the deep fault zone;

(2) Analysis of the borehole wall failure mechanism in the fault gouge section, and development of a new borehole wall stabilization method for fault gouge wall stability.

1 Testing of the Fault Gouge

1.1 Collection of fault gouge in the borehole

Samples, of the core drilling for this research, were collected from borehole WFSD-1 at a depth of 585 meters (core samples shown in Figure 1-b). Cores were comprised of grey black, wet, soft and plastic materials. Fault gouge samples were packed in water-tight bags immediately after they were retrieved from the borehole to maintain the hydration state of any expandable mineral phases. Samples were sent to the Mud Laboratory of the State Key Laboratory of Geo-hazard Prevention and Geo-environmental Protection (SKLGP) at Chengdu University of Technology for laboratory tests.

1.2 Mineralogical composition of the fault gouge

The fault gouge samples from WFSD-1 were analyzed using X-Ray Powder Crystal Diffraction with a DMAX-C diffractometer, Cu Ka radiation, and a Ni filter. The chemical composition of the fault gouge was analyzed using X Ray Fluorescence, and was imaged with Scanning Electron Microscope (SEM).

J. Mt. Sci. (2

932

The csamples arAl2O3 (24.3byFe2O3, K

XRD sand the rewhich illusfault gougequartz (36images of t

The fauwhen obse

Table 1 C

ElementContent

Table 2

Sample number 1 2

Notes: Tminerals.

2016) 13(5): 93

chemical comre in Table 38%) being t

K2O, MgO andspectrum anaesult of analstrates that te samples 16%) and chhe fault gougult gouge samerved under

(a) Drilli

Chemical com

t type Si (%) 59

Mineral comp

Montmor--

There are unk

30-938

mpositions 1, with SiO2

the most abud CaO. alysis are sholysis are shothe mineral #, and 2# ahlorite (12.5ge are shownmples are blar an SEM,

ing field of bor

(a) Sample 1#

Figure

mposition of fa

O2 Al2O3 9.08 24.38

position of fau

rillonite Ill 41 47

known uncrys

of fault go2 (59.08%),

undant, follo

own in Figurown in Tablcompositionare illite (445%). The Sn in Figure 3.ack in color ashow cemen

rehole WFSD-

Figure

#

e 2 X-Ray Diff

ult gouge

Fe2O3 5.59

ult gouge

ite Kaolin1 -7 -

stallized mine

ouge and

owed

re 2, le 2

ns of 4%), SEM . and, nted

textirresmacolumaconpseagghascrysminthe

-1#

1 Fault gouge

fraction (XRD

K2O MgO4.97 2.38

Test outcnite Chlor- 10 - 15

erals; The out

tures and bregular and aaller than 1 µumnar, hessively aggr

ncretions. eudohexagongregated in as a loose ststals form nerals are ti structure,

(b) Co

e sample.

) analysis of fa

O CaO 8 1.05

ome (%) rite Qu

4329

tcome is just

recciated struangular lameµm. The quarexagonal bregated and

Chlorite nal flakes or ta scaly or rotructure, angrid structuny tufted an

or are ar

ore sample of

(b) Sample 2

ault gouge.

TiO2 SO3 0.98 0.73

uartz Ano3 3 9 3

the relative

uctures. Theelli and its rtz crystals a

bipyramid d in hard-g

crystalstabular shaposette form.

nd many ofures. The fnd form conrranged in

fault gouge

2#

Na2O o 0.52 0

orthose Do3 5

amount of c

illite forms crystals are

are equiaxial,in shape, grains and s show pes, and are Sample 1#

f its quartz fragmentary ncretions in

a discrete

others 0.32

olomite

crystallized

,

condition. small contaminerals castructure iare apparentraces after

1.3 Physicfault g

The m(such as permeabilitwere separusing the content wit

The crystal act areas. Flan be seen is discontinuntly spatiallyr experiencin

cal-mechangouge

main physdensity, moty and tensi

rately tested.cutting-ringth the oven d

faces are smlaky and tabin sample 2

uous, in whiy arranged, ang extrusion.

nical prope

ical-mechanoisture contile strength). The density method an

drying metho

(a)Sample 1

(c) Sample 2

Figure 3 Sca

mooth and hular fault go#, but the flch the mine

and show fold

rties of the

nical propertent, dispers) of fault goy was measu

nd the moisod. Six group

#

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anning Electro

have ouge flaky erals ding

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Expansion loaded dilatoved with a 2ferent solutioride solutyacrylonitrilution, 20% s

ud solution. nfigured for rformance, wntonite in cleer aging forferent solutio

The dispers

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J. Mt.

e tested in thhe test errors was mea

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was preparedear water anr 24 hours. ons was obsesity was estim

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(d) Sample 2#

e of fault goug

Sci. (2016) 13

he experimens. asured withples were air

eve, after soawater, 20%

ding, 20% m salt solutiosphalt solutiomud solutioed drilling fld with 4% h

nd was ready The expanserved and recmated by cal

#

#

ge.

3(5): 930-938

933

nt to reduce

h a WZ-2 r-dried, and aking in six potassium

hydrolysis on, 20% S-1 on and base n, specially luid sample high quality y for testing sion in the corded. culating the

J. Mt. Sci. (2

934

recycle rateat 77°C. mashed aSpecimens size of specthe sample3, 4, and thto roll 16 htaken out tthen all of onto 40 mThe fault goin an oven then the re

The pdeterminedinstrumenthigh pressuThe pressuon both enthe inlet prand calculaDarcy’s Lawstrain-contcomposed shear transand displVertical preduring theforce was puntil the failure theincluding: cohesion, wLaw.

1.3.1 Mois

The mmeasured d3. The memean wet ddry density

Table 3 M

Moisture cMoisture cDry densityPorosity (%

2016) 13(5): 93

e after the saAir-dried fnd passed were weighcimen rangeses were placehe pots were

hours at 77°Cto cool down the liquid a

mesh sieves aouge materia at 105°C uncycle rate co

permeability d with a t, and the auure nitrogen

urized gas mnds of the fauressure, outleated the permw. The tensiltrolled directof a shear b

smission equlacement messure was e

e experimentput under thsamples we

e shear streangle of

were determ

sture conte

moisture coduring six teean moisturedensity was

y was 2.18 g/

Moisture conte

content (%) content (%) y (g/cm3)

%)

30-938

ample was rofault gouge

through a hed to obtains from 6 to 1

ed in pint pote put into fiveC. After rollinn to room teand fault gouand sifted inal was put inntil the weighuld be calcul of the fa rock gasuxiliary equin gas bottle method estabult gouge saet pressure, meability of fle strength wt shear appabox, vertical

uipment, dynmeasurementexerted on thts. Next, a

he shear boxere cut failuength indexinternal fri

mined with

ent and den

ontents and ests are showe content w2.34 g/cm3, cm3.

ent and density

Sample 7.32 2.31 2.15 20.69

lled for 16 ho samples w sample sin 30 grams 10 meshes), tts numberede roller furna

ng, the pots wemperature, uge were poun running wa

to the sieve ht was constlated.

ault gouge s permeabipment inclu and gas meblished pressmple, measuand export flfault gouge w

was tested wiratus which l pressure,

namometer, t system. he samples horizontal

x bit by bit ure. Upon es of soil ction and Coulomb’s

nsity

densities wn in Table was 7.35%, and mean

y of samples

1 Sample7.33 2.33 2.17 20.71

ours were ieve. (the then

d 1, 2, aces were and ured ater. and tant,

was bility uded eter. sure ured flow, with ith a was

1.3

unlcleasolumualon

varsolusulfHP1 sothe7.55Figmosteesubsulfslowsolusolu

Figure 4 soaking sol

e 2 Sampl7.33 2.34 2.17 20.73

.2 Expansib

Expansibililoaded dilatar water, KCution, sulfon

ud solution. ng with time

Figure 4 sies significautions, genefonated asphAN-NH4 soluolution. The se different s5%, 6.15% anure 4 also sst rapidly weply to abou

btle change. Tfonated asphwly when soaution, in waution to valu

Expansionbilutions.

le 3 Samp7.35 2.34 2.18

20.73

bility

ity was meaometer afte

Cl solution, Hnated aspha The result

e are shown inshows that antly when

erally the exhalt solutionution > in ba expansibilitsolutions arend 5.25%, rehows that thwhile soaket 40% in 10 The expansiohalt solutionaking in KCl ater-base mues lower than

lity curves of

ple 4 Sam7.37

2.362.20

3 20.7

asured usinger soaking HPAN-NH4 salt solution,ting expansn Figure 4. fault gouge

n soaked inxpansion in n > in KCl soase mud soluty rates aftere 40.25%, 13espectively, ahe fault gou

ed in water, minutes, foon curve risen, but the solution, in ud solution n 10% in 1 ho

f fault gouge

mple 5 Sa7 7.36 2.30 2.275 20

g the WZ-2 samples in

solution, S-1 , and base sion curves

e expansion n different water > in

olution > in ution > in S-r soaking in .15%, 8.15%,

after 1 hour. uge expands , and rises

ollowed by a es steeper in curves rise

HPAN-NH4 and in S-1

our.

in different

ample 6 37 38 21

0.77

,

1.3.3 Disp

Table samples rol

The diof the fauwhich charfault gougdispersion recycle rate(accountedgouge can b

1.3.4 Perm

The expesample 1, 9.8312×10-

11.9087×10The

9.7608×10that pore fault zoneadditional s

1.3.5 Tens

Figuregouge at di

The pwith the Y-the fault ggives the inThe strengmoisture cregression

Table 4 D

Pint pot nu1 2 3 4

Notes: BRweight = Wrecycle rate Table 5 Inin different

MC- parameter IFA

CS

Notes: IFcohesive str

persity

4 shows tlled for 16 hoispersity of t

ult gouge disracterizes the particles from water.e of the faul

d for 10.3%be dispersed

meability

erimental per2, 3, 4, 5

16, 13.480-16 and 7.689average pe-16m2. The pfluids canno

e which crestresses.

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e 5 shows theifferent moisoint of inte

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content can b method from

Dispersity of th

mber BR-w30 30 30 30

R-weight = WWeight after e (%).

nternal frictiont moisture con

Original MC

1

36.1 3

36.9 2

FA = Internarength (kPa); M

the recycle ours at 77°C.the fault gouspersed intohe connectinand the ab Table 4 shot gouge is ve), which m

d in water in l

rmeability of t5 and 6 ar876×10-16, 98×10-16 m2. ermeability permeabilityot release efeates condi

th

e failure curture content

erception of he cohesion se the inclinaion angle of

ons at diffebe determin

m the failure

he fault gouge

weight AR-w2.8 2.9 3.4 3.2

Weight before rolling (g);

n angle and content values

10% MC 15%

34.4 17.2

25.6 24.5

al friction anMC = moistur

rate after

uge is the dego tiny particng force amility to prev

ows that the ery low in w

means that flarge quantit

the fault gougre 8.9125×1

6.7354×1

is very ly has the efffectively in itions for h

rves for the fs. a failure cu

strength valuation of the the fault gouerent valuesed by the lin strength cur

weight RR-rat9.3 9.7 11.3 10.7

rolling (g); ARR-rate = R

ohesive streng

% MC 20% M

2 2.9

5 10.2

ngle (°); CS re content.

the

gree cles,

mong vent roll

water fault ties.

ge of 10-16, 10-16,

low: ffect the high

fault

urve ue of

line uge.

s of near rves.

Angstrecalcfriccon

the10.2fricwithfriccom

2

2.1

is instmetcomResorgcheleadof tmec

te

AR-Roll

gth

MC

=

Figmo

gles of intength at difculated (Tabction angles antent is show

The cohesi original in s2 kPa at 20%

ction angle dh increasing

ction angle’smpared with

Results an

1 Instabilityin fault go

The stabilita worldwi

tability mechthods are d

mplexity, ansearch on boganic combinemistry andding to a goothe deeper schanism. On

gure 5 Fauisture content

J. Mt.

ernal frictiofferent moisble 5). Theand cohesive

wn in Figures ve strength

situ undisturb% moisture codrops from 3g moisture s value de the tensile st

nd Discuss

y mechanisouge sectio

ty of a borehde technicahanisms andifficult to red because oorehole wallnation and id mechanicod appreciatistrata and only then can a

lt gouge faits.

Sci. (2016) 13

on and thsture conten

e variation e strength wi 6 and 7. drops from

rbed moisturcontent, and 36.1° to 2.9°. content, thecreases motrength.

sion

sm of borehon

hole wall dural problemd technical sesolve becauof research l stability shintegration ocs perspection of the chaof the borehoa proposal b

ilure curves

3(5): 930-938

935

he cohesive nts can be of internal

ith moisture

36.9 kPa at e content to the internal In general, he internal

ore sharply

hole wall

ring drilling . Borehole stabilization use of their limitations.

hould be an of both the tives, thus aracteristics ole collapse e developed

at different

J. Mt. Sci. (2

936

for a technthe drilling

The reshow that permeabilitcontact witand tensilecontact witthat the ppermeate eblocked by stress decreincrease o

Figure 6 content.

Figure 7 V

2016) 13(5): 93

nical plan tog fluid. esults of thefault gouge ty, and a st

th water, as we strength thath water. Thpore fluids effectively b fault gouge ease by drain

of additiona

Variation of

Variation of coh

30-938

o stabilize th

e physical-m has a low ptrong expanswell as a stroat quickly dehe low permin the fau

because the and consequning is muchal stress. Ad

internal fricti

hesive strengt

he borehole

mechanical tporosity, a wsibility when

ong dispersibecreases whemeability meult zone can

fault zone uently the rath slower thandditional st

ion angles wi

th with moistu

and

tests weak n in

bility en in eans nnot was

te of n the tress

theconcauchethefaurocstresurconof intebor

ith moisture

ure content.

together abnormalfault zonebalance instress ardistributeconcentraaddition, can easilylow strengmechanicfault goug

Thusdeformatiwall undstress. It clearly hysmectite with watecore drilltube areexpansionthicker thcausing th

2.2 Meth wall i

The project is which thebit and bdrilling tostuck. Thcollapse osection is fluid, to in

borehole, ncentration used by groemical intera fault gougelt gouge is ks restrict thess. This rounding ro

ntact with wathe fault

eractions earehole.

with the tecly high stres

e. During drin the materiround the ed leading tation near ta fault gouge

y collapse begth and the al and chem

ge mechanica, the faion causes cer the cond has shownydrates and sharply exp

er. When haing is used, pushed a

n of the corehan the drilhe inside pipe

hods to stabin the fault

adopted dril diamond wi

e clearance bborehole walools may ea

he main soluof the boreh to adjust thncrease the f and to phenomena

ound stressctions betwe

e because thswellable c

he swelling stress in

ock stress aater causes t

gouge. Thasily cause

ectonic stresss concentrailling, the exiial is disruptborehole isto a very the boreholed section of

ecause fault gdrilling fluid

mical properal propertiesault gougeclosure of thdition of hi

n that the f the interlay

pands when alf-closed pip, the two-haapart becaue, which makll pipe’s ins

pe to get stuck

bilize the bt gouge sec

lling methodwire-line corebetween the cll is small, asily expandutions to dehole in the he density of fluid column

decrease around th. There ar

een the drillinhe major minclay, but boand produc

turn incrand at the the decrease hese physic the collap

s forms an ation in the isting stress ted and the s then re-high stress le wall. In f a borehole gouge has a d affects the rties of the .

e’s plastic he borehole igh ground fault gouge yered illite- in contact pe wire-line alves of the use of the kes the tube side radius, k.

orehole tion

d in WFSD e drilling, in core drilling so that the

ded and get eal with the

fault gouge the drilling pressure in the stress

he borehole e physical-ng fluid and neral of the rehole wall e hydration reases the same time

of strength cal-chemical pse of the

J. Mt. Sci. (2016) 13(5): 930-938

937

To keep the borehole wall stable under high ground stress conditions, the pressure of the drilling fluid must provide a mechanical stress balance. So, increasing the specific gravity of the drilling fluid and the pressure of the drilling fluid are the most commonly used methods for combatting borehole shrinking in the fault gouge section. However, it is not certain what approach is best for implementing a drilling stress balance under the condition of high ground stress adapted to the characteristics of diamond drilling.

Based on our analyses of the fault gouge characteristics and in accordance with the content and sensitive level of hydration and expansion level of the clay minerals, we need a drilling fluid system that can effectively control the filter loss, and can keep the drilling fluid activity at a balance. These qualities combine effective plugging and lubrication performance.

2.3 Development and application of low-water-loss and low-permeability drilling fluid

An adaptable drilling fluid was developed with a non-viscous polymer fluid loss agent S-1 that adjusts to minor-caliber diamond drilling and a treating agent X-1. This fluid has the character dilution into membranes and strong adsorption, thus forming a super-low permeability membrane on the borehole wall. The integrated combination of low water loss with low permeability effectively reduces borehole wall attenuation due to hydration and expansion in fault gouge layers.

Laboratory quadrature experiments were used to adopt various quantities of main material with the drilling fluid, and to test drilling fluids properties including density, filter loss and viscosity. The main materials of a core drilling fluid system, such as clay, S-1, sulfonated asphalt, and SMC, were carried out in cross experiments, and used to analyze the collapse mechanism of a borehole through fault gouge. A preferred core drilling fluid system composition was developed with 4% clay + 0.5% CMC-HV + 2% S-1 + 3% sulfonated asphalt + 1% SMC + 0.5% X-1 + 0.5% T type lubricant + barite. Performance characteristics of this fluid are shown in Table 6.

Low water loss and low permeability drilling fluid in fault gouge core drilling of the WFSD

program has yielded results that are promising for stabilizing borehole walls. Borehole shrinking in the fault gouge layers decreases considerably with this type of drilling fluid, and drill pipe sticking has been reduced; therefore, drilling is less problematic.

3 Conclusions

(1) Undisturbed samples of fault gouge in the deeper part of the fault zone of the Wenchuan earthquake were obtained by core drilling. From these samples we examined the microscopic characteristics of the fault gouge produced by complicated tectonic movement with the help of the XRD, XRF and SEM methods. Furthermore, we measured the moisture content, density, expansibility, dispersity, permeability, tensile strength and other important physical and chemical properties.

(2) We thoroughly investigated the collapse mechanism of borehole walls during drilling from the characteristics of the fault gouge in the Wenchuan earthquake fault zone. These investigations resulted in a successful strategy for borehole wall stabilization, with low water loss and low permeability drilling fluid system, for core drilling through the fault gouge section in the Wenchuan earthquake fault zone. The application of this fluid during core drilling has shown that it performs well, thus laying a solid basis for progress of the WFSD program.

Acknowledgements

The research described in this paper was supported by the Land & Resources Ministry of China, the China Geological Survey, and the research institute of prospecting technology in the Chinese Academy of Geological Sciences, sincere

Table 6 Performance of low water loss and low permeability drilling fluid

D WL FCT FV AV PV LF 1.45 4.6 0.8 28 24.5 18 0.2

Notes: D = Density (g.cm-3); WL = Water loss (mL. (30 min)-1); FCT = Filter cake thickness (mm); FV = Funnel viscosity (s); AV = Apparent viscosity (mPa.s); PV = Plastic viscosity (mPa.s); LF = Lubrication factor.

J. Mt. Sci. (2016) 13(5): 930-938

938

thanks here. This paper has been supported by National Natural Science Foundation of China (Grant Nos. 41272331, 51204027) and the State

Key Laboratory of Geohazard Prevention & Geoenvironment Protection (Grant Nos. SKLGP2012Z007, SKLGP2014Z001, SKLGP2015Z010).

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