Variable frequen cy of pu lse hydraulic fracturin g for im proving
permeability in coal seam
L i Qu angu i,L in Ba iqu an ,Zh ai Ch eng,Ni Gua nh ua ,Peng Sh en,Sun Chen ,Che ng Y a nying
Scho ol o f Safe ty En gin eer i n g,Ch i n a U n iver si t y o f Minin g &Tech no logy ,X uzh ou 221116,Ch ina
a r t i c l e i n f o
A r ticle h ist or y:
Receiv ed 25Ma rc h 2013
Receiv ed in r e vised f or m 22A p r il 2013A ccep t ed 11Ma y 2013
A va ilab le o n lin e 9No v em b er 2013K eywo rd s:
V ar iab le fr eq u en cy
Pu lse h yd r au lic fr a ctu r in g Fr act ur e A E
Fat igu e
a b s t r a c t
Variable frequen cy,a n ew pat t ern of pu lse h ydraul ic fract u ring,is presen ted for im provin g perm eabili t y in coal seam.A variable frequ ency p uls e h ydrauli c fractu ring t est ing syst em w as b uil t,th e m ou ld wit h t riaxial l oadin g was d evelo ped.B as ed on th e mo nit or m et hod s of pressu re sens or and acou st ic em is sion ,t he tri al s o f t w o pat t erns of pu lse hyd rau lic fract urin g o f s ingl e freq uency an d vari ab le freq uen cy were carri ed ou t ,an d at last fract urin g m ech anis m was an alyzed.Th e resul ts sh ow th at t he effect o f variabl e freq uency o n fract u re ext en sio n i s b ett er t h an t h at of sin gle frequen cy based on t he an alysis of m acro -scopi c gu res and AE.An d t h e sh ortage of sin gle freq uency i s s omew hat rem edied wh en t he frequ ency is vari able.Und er variable freq uency,t he press ure pro cess can be divid ed in t o t hree st ages :l ow fre-qu ency band ,pres sure st abil it y band an d hi gh frequ ency band ,and rup tu re p ressu re o f t he sam ple is sm all er t han t hat of t he con dit ion o f sin gle frequen cy.B ased on t he Min er fat igu e t heory,t he effect of di fferen t l oadin g sequen ces o n sam ple ru pt ure is discus sed and th e resul ts show th at i t i s bet ter t o select t he sequ ence o f low frequen cy at rs t and th en hi gh freq uency.Our achievem ent s can give a b as is for th e im p rovem en t and opt im izat ion of t he p uls e hyd raulic fract uri ng techn ology.
2013Pu blis hed by Elsevier B.V.on behalf of Chin a Un iversit y of Minin g &Tech nol ogy.
1.In t ro d u ction
W ith th e in cr easin g d ep th o f co al min in g ,th e p re ven tion an d co n tro l o f ga s d isaster b ecom es m o re d if cu lt,an d it is d if cu lt fo r or ig in al te ch no logies t o me et t he d em an d s o f safe min in g [1–4].Sev era l e ld tests of p u lse h y d rau lic fractu r in g (PHF)h ave be en con d u cted as a m ea su r e of r egion a l ga s co n tro l.Th e r esu lts sh o w th at th e effe ct o f imp r ov in g p erm eab ility an d g as d rain age is in cr eased m ark ed ly afte r th e im p lem en tat io n of PHF.T h e PHF tech n olog y p r ov id es a n e w men ta lity fo r imp ro vin g p er me ab ility an d co n tro llin g ga s in co al seam.
As on e o f ma jo r stimu lation m ea su r es,h yd ra u lic fractu rin g h as be en w ide ly u sed in oil in d u st ry,an d cor relation al r ese arch is relativ ely co m pr eh en siv e.Re sea rch r esu lts h av e g rea t r eferen tial sign i can ce o n coal seam h yd ra u lic fr actu rin g su ch as b asa l p r in ci-ple o f fr actu re d ev elo p m en t an d p ro pa gatio n a n d ru le o f fractu re orien ted ex ten sio n [5–7].In th e last few ye ars,th e stu dies o n co al sea m h y d rau lic fractu rin g h av e m ad e som e ach ieve me nt s,th e q u a-si th re e-ax is h y d rau lic fra ct u rin g tests w ere carr ie d ou t to stu d y th e coa l h y d rau lic fra ct u rin g me ch an ism th oro u gh ly,a n d crack pr op ag ation la w o f h yd rau lic fractu r e in coa l-r ock mass ha d b ee n stu d ied [8,9].How e ver ,as an exte n sion tech n olog y of co al se am hy d rau lic fra ctu r in g ,PHF sh o u ld b e stu died in co n n ection w ith
its self-p r op er ties b eside s th e r esults ab o ve.PHF is a tech n o lo gy th at w ate r w ith a certa in fre qu e n cy is p um p ed in to bo re h oles an d it cau ses th e fractu re s to d eve lo p an d exte n d an d a t last fr actu re n etw o rk is fo rm ed in sid e th e co al seam.T h e r ela tio n sh ip be tw een p u lsatin g p ara met ers an d fr actu re e xten sion in co al is on e re sea rch emp h a sis o n PHF.A s on e o f p u lsatin g p ara mete rs,fr eq u en cy is in d ir ect relation w ith th e effect an d ef cien cy o f PHF.Tw o asp ects sh o u ld be ta ke n in to co nside ration w h en fre-qu e ncy is sele ct ed :(i)re so n an t freq u en cy ,th e pr ob ab ility o f co al reson a n ce d estro y is in cr eased w h en th e fre qu e n cy is clo se to in h ere n t fre qu e n cy of co al;(ii)freq u en cy p ro vid ed by e qu ip me n t.A resea rch sho w ed th a t th e in h ere nt fr eq u en cy o f co al v aried in a lar ge r an ge fr om 70to 400Hz,an d fu rth erm o re it w as m o re co m-plica ted w h en th e coa l w as d rilled or in jected w ith w a ter,so in h er-en t fr eq ue n cy of coa l is ju st a g u id elin e fo r fr eq u en cy sele ctio n [10].At p r ese n t,th e p lu n g er pu lse in je ctio n p u mp is co m mo n ly used for PHF,th e fre qu e ncy p rov id ed b y w h ich is fro m 0to 25Hz an d a sin g le freq u en cy ,su ch as 20an d 25Hz,is u se d th rou g h ou t th e p ro cess fo r th e lim itation o f th e eq u ip m en t.W ill th e e ffect o f PHF b e e nh a n ced if d ifferen t fre qu e n cies are used in th e sa me p ro ced u re an d w ill it mak e u p for th e lim ita tio n of sin gle freq u en cy u sin g th e ad ju stm en t an d com b in ation o f fr eq u en cy?V a ria ble fre qu e n cy o f PH F is p r ese n ted in th is p ap er ,an d a triax ial PHF test sy st em is bu ilt a nd PHF te sts at d iffer en t fre-qu e ncies are con d u cted.Th e effect o f v ariab le fr eq u en cy o n frac-tu re d eve lo pm en t an d exte n sion in samp les is inv estig ated.T h e
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In t er n at ion a l Jo u rn a l of Min in g Scien ce an d Tech n o lo gy 23(2013)847–85
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