Borehole breakout and drilling-induced fracture analysis from image logs

Transcription

1 Borehole breakout ad drllg-duced fracture aalyss from mage logs M. Tgay, J. Reecker, ad B. Müller Itroducto Borehole breakouts ad drllg-duced fractures (DIFs) are mportat dcators of horzotal stress oretato, partcularly asesmc regos ad at termedate depths (<5 km). Approxmately 9 % of the stress oretato dcators the World Stress Map (WSM) database have bee determed from borehole breakouts ad DIFs. Furthermore, borehole breakouts ad DIFs provde the majorty of stress oretato dcators petroleum ad geothermal systems. Here we preset a broad overvew of the procedures for terpretg borehole breakouts ad DIFs from mage log data ad for WSM qualty rakg of stress oretatos derved from these features. Borehole Breakouts Borehole breakouts are stress-duced elargemets of the wellbore cross-secto (Bell ad Gough, 979). Whe a wellbore s drlled, the materal removed from the subsurface s o loger supportg the surroudg rock. As a result, the stresses become cocetrated the surroudg rock (.e. the wellbore wall). Borehole breakout occurs whe the stresses aroud the borehole exceed that requred to cause compressve falure of the borehole wall (Zoback et al., 985; Bell, 990). The elargemet of the wellbore s caused by the developmet of tersectg cojugate shear plaes that cause peces of the borehole wall to spall off (Fgure ). The stress cocetrato aroud a vertcal borehole s greatest the drecto of the mmum horzotal stress (S h ). Hece, the log axes of borehole breakouts are oreted approxmately perpedcular to the maxmum horzotal compressve stress oretato (S H ; Plumb ad Hckma, 985). Drllg-Iduced Fractures DIFs are created whe the stresses cocetrated aroud a borehole exceed that requred to cause tesle falure of the wellbore wall (Aadoy, 990). DIFs typcally develop as arrow sharply defed features that are sub-parallel or slghtly cled to the borehole axs vertcal wells ad are geerally ot assocated wth sgfcat borehole elargemet the fracture drecto (ote that DIFs ad breakouts ca form at the same depth orthogoal drectos). The stress cocetrato aroud a vertcal borehole s at a mmum the S H drecto. Hece, DIFs develop approxmately parallel to the S H oretato (Fgure ; Aadoy ad Bell, 998). Itroducto to Borehole Imagg Tools Borehole magg tools provde a mage of the borehole wall that s typcally based o physcal property cotrasts. There are curretly a wde varety of magg tools avalable, though these predomately fall to two categores: resstvty ad acoustc magg tools. Resstvty magg tools provde a mage of the wellbore wall based o resstvty cotrasts (Ekstrom et al., 987). Resstvty magg tools have evolved from dpmeter tools ad cosst of four- or sxcalper arms wth each arm edg wth oe or two pads cotag a umber of resstvty buttos. Resstvty mage tools provde the same formato o borehole dameter ad geometry as the older

2 dpmeter tools, however the resstvty buttos also allow hgh-resoluto resstvty mages of the borehole wall to be developed (see WSM four-arm calper log gudeles for a detaled descrpto of dpmeter tools ad assocated log data). There are a wde varety of wrele resstvty magg tools avalable, some of the more commo tools are the Formato Mcro Scaer (FMS; from Schlumberger), Formato Mcro Imager (FMI; from Schlumberger), Ol-Based Mcro Imager (OBMI; from Schlumberger), Smultaeous Acoustc ad Resstvty tool (STAR; from Baker Atlas), Electrcal Mcro Scaer (EMS; from Hallburto) ad Electrcal Mcro Imager (EMI; from Hallburto). Furthermore, recet years have see the developmet of a rage of loggg whle drllg (LWD) or measuremet whle drllg (MWD) resstvty mage loggg tools, such as the Resstvty At Bt (RAB; from Schlumberger) ad STARtrak (from Baker Iteq). For more detals o resstvty mage loggg tools see Ekstrom et al (987) or Asquth ad Krygowsk (2004). Fgure : Schematc cross-sectos of borehole breakout ad drllg-duced fracture (DIF; fgure adapted from Hlls ad Reyolds, 2000). Borehole breakouts form whe the crcumferetal stress aroud the wellbore exceeds the compressve rock stregth ad thus are oreted parallel to the mmum horzotal stress (σ h ). DIFs form whe the crcumferetal stress exceeds the tesle stregth of the wellbore wall ad are thus oreted parallel to the maxmum horzotal stress (σ H ). Acoustc magg tools utlse a rapdly rotatg pezoelectrc trasducer to emt a focused hghfrequecy soc pulse to the borehole wall (Asquth ad Krygowsk, 2004). The acoustc magg tool the records the ampltude of the retur echo as well as the total travel tme of the soc pulse. The acoustc wave travel tme ad reflected ampltude s measured at umerous azmuths sde the wellbore for ay gve depth. Ths data s the processed to mages of the borehole wall reflectace (based o retur echo ampltude) ad borehole radus (based o pulse travel tme). There are a wde varety of acoustc magg tools avalable, some of the more commo tools are the Borehole Televwer (BHTV, from Schlumberger), Ultrasoc Borehole Imager (UBI; from Schlumberger), Crcumferetal Borehole Imagg Log (CBIL; from Baker Atlas), Smultaeous Acoustc ad Resstvty tool (STAR; from Baker Atlas), Crcumferetal Acoustc Scag Tool-Vsualzato 2

3 (CAST-V; from Hallburto) ad the LWD/MWD AcoustCalper tool (ACAL; from Hallburto). For more detals o acoustc mage loggg tools see Asquth ad Krygowsk (2004). I addto to resstvty ad acoustc mage loggg tools, there are a rage of other, curretly less commo, tools that also provde mages of the borehole wall ad whch may be used for borehole breakout ad DIF aalyss. Optcal mage loggg tools, such as the Optcal Televewer (from Schlumberger) ad Dowhole Vdeo tool (from Dowhole Vdeo), are wrele tools that utlse cameras to drectly mage the wellbore wall. Fally, borehole breakouts ad DIFs ca also be terpreted from MWD/LWD desty magg tools, whch provde formato o bulk desty ad photoelectrc factor (Pe) at a varety of azmuths aroud the wellbore ad ca be used to develop formato desty ad Pe mages. LWD/MWD desty mage loggg tools clude the Azmuthal Desty Neutro Vso (advso; from Schlumberger), Lthotrak (from Baker Iteq) ad Azmuthal Lthodesty tool (ALD; from Hallburto). Iterpretg Breakouts ad DIFs from Resstvty Image Data Resstvty mage loggg tools provde the same formato of borehole dameter ad geometry as the older dpmeter logs ad, thus, ths data ca be used to terpret breakouts the same way as for four- or sx-arm calper logs (see WSM gudeles o four-arm calper aalyss). However, resstvty magg logs also provde a hgh-resoluto pcture of the wellbore wall based o resstvty cotrasts that allows for the drect observato of borehole breakout. Borehole breakout typcally appears o resstvty mage logs as broad, parallel, poorly resolved coductve zoes separated by 80º (.e. observed o opposte sdes of the borehole) ad ofte exhbtg calper elargemet the drecto of the coductve zoes (Fgure 2; Bell, 996). Breakouts are typcally coductve ad poorly resolved because the wellbore fracturg ad spallg assocated wth the breakout results poor cotact betwee the tool pads ad the wellbore wall, whch tur causes the tool to partally or fully measure the resstvty of the electrcally coductve drllg mud rather tha the formato. However, t s mportat to ote that breakouts wll appear as resstve, rather tha coductve, zoes resstvty mages ru ol-based mud (such as usg the OBMI tool). Drllg-duced fractures ca oly be observed o mage logs. DIFs typcally become fltrated by drllg mud ad, thus, appear o resstvty mage logs as pars of arrow, well defed coductve features (resstve ol-based mud mages) separated by 80º (Fgure 3; Aadoy ad Bell, 998). Furthermore, ulke atural fractures that ted to cross-cut the wellbore, DIFs are usually alged subparallel or slghtly cled to the borehole axs vertcal wells (Fgure 3). Iterpretg Breakouts ad DIFs from Acoustc Image Data Borehole breakouts are typcally terpreted from acoustc mage log data usg the borehole radus (or travel tme) mage combato wth mages of the reflected ampltude. Borehole breakouts appear as broad zoes of creased borehole radus (or travel tme) observed o opposte sdes of the borehole (Fgure 4a). However, breakouts typcally have rough ad varable surfaces ad thus ca also ofte be observed o reflected ampltude mages as broad zoes of low ampltude (Fgure 4a). Drllg-duced fractures are prmarly observed o the reflected ampltude mage. Both atural ad drllg-duced fractures are poor reflectors of acoustc eergy. Hece, DIFs appear as arrow zoes of low reflectvty separated by 80º ad typcally sub-parallel or slghtly cled to the borehole axs (Fgure 4b). DIFs are ot commoly assocated wth ay borehole elargemet ad thus are ofte ot well exhbted o borehole radus mages. However, both atural ad drllg-duced fractures may appear o borehole radus mages as arrow zoes of creased borehole radus (Fgure 4b). 3

4 Fgure 2: Example of borehole breakout terpreted o a Formato Mcro Imager (FMI) log. Breakout s observed both va elargemet the calper 2 drecto (C2 red) ad drectly o the FMI mage as broad poorly resolved coductve zoes oreted towards 00ºN ad 290ºN. These breakouts dcate a approxmately N-S maxmum horzotal stress oretato. Fgure 3: Example of drllg-duced fractures (DIFs) terpreted o Formato Mcro Imager (FMI) logs. DIFs are observed as arrow well defed coductve features separated by 80º ad oreted sub-parallel to the borehole axs. (a) DIFs are oreted towards 00ºN ad 90ºN, dcatg a approxmately N-S maxmum horzotal stress oretato. (b) DIFs are oreted towards 040ºN ad 220ºN, dcatg a approxmately NE-SW maxmum horzotal stress oretato. (c) DIFs are oreted towards 045ºN ad 225ºN. Furthermore, breakouts are also observed co-cdet wth the DIFs. Both the breakouts ad DIFs dcate a approxmately NE-SW maxmum horzotal stress oretato. 4

5 Fgure 4: Example of breakouts ad drllg-duced fractures (DIFs) observed o acoustc mage logs. (a) Borehole breakouts observed o Ultrasoc Borehole Imager log. Borehole breakouts are broad zoes of hgh borehole radus ad, to a lesser extet, low reflecto ampltude oreted towards ºN. The borehole breakouts dcate that the preset-day maxmum horzotal stress s oreted approxmately N-S. (b) Borehole Televewer log showg DIFs oreted towards ºN. DIFs are observed as zoes of low ampltude (left mage) ad, to a lesser extet, hgher radus (rght mage). The DIFs dcate that the preset-day maxmum horzotal stress s oreted approxmately SSE-NNW. Iterpretg Breakouts ad DIFs from Other Image Data Borehole breakouts ad DIFs ca also be terpreted from optcal mage logs ad desty mage logs. O optcal mage logs (e.g. optcal televewer ad Dowhole Vdeo), breakouts appear as broad zoes of borehole elargemet o opposg sdes of the well, whle DIFs appear as arrow fractures usually separated by 80º (Fgure 5). Desty magg logs, lke resstvty magg logs, requre the tool to have drect cotact wth the wellbore wall. Hece, the desty magg tool partally or fully samples the drllg mud rather tha the wellbore wall breakout ad DIF zoes. Drllg mud s less dese tha the formato ad hece breakouts appear as broad low desty zoes separated by 80 degrees (Fgure 6), whle DIFs are sometmes vsble as arrow low desty axal-parallel fractures o bulk desty mages. However, may drllg muds also cota barte, whch has a extremely hgh photoelectrc absorpto factor. Hece, breakouts ca also be observed as broad zoes of hgh photoelectrc absorpto o Pe mages (Fgure 6), whle DIFs appear as arrow hgh Pe fracture zoes oreted sub-axal to the wellbore. 5

6 Fgure 5: (a) Example of borehole breakout take by a dowhole camera. (b) Example of a borehole fracture observed o a dowhole camera (Fgure 4(b) from Asquth ad Krygowsk, 2004). Fgure 6: Example of breakouts o desty ad photoelectrc absorpto factor (Pe) mages from a LWD/MWD Azmuthal Lthodesty tool. Breakouts show up as pars of broad poorly resolved zoes separated by 80º that have low desty ad hgh Pe. The breakouts are oreted approxmately ºN ad dcate that the preset-day maxmum horzotal stress s oreted approxmately ENE-WSW. 6

7 Determg the Average Maxmum Horzotal Stress Oretato: Crcular Statstcs BO ad DIF oretatos are bmodal data. Data betwee 80 ad 360 are equvalet to those from the terval 0-80 (S H vares betwee 0 ad 80 ). Accordg to Marda (972) the mea breakout azmuth θ m (.e. S h ) of a populato of pcked breakout log axs drectos θ s derved by frst trasformg the agles to the terval. θ = 2 θ The the drecto cose ad se of θ has to be added ad averaged ether by the umber of measuremets (for umber weghted mea) or weghted by the BO or DIF legth ad the dvded by the total BO or DIF legth L (legth weghted mea). umber weghted: C = S = = = cosθ sθ legth weghted: L = C = L S = L l = = = l cosθ l sθ where l s the legth of BO or DIF wth oretato θ. The mea azmuth results from: θ m = ½ arcta(s/c) (Make sure that the agles are coverted from rad to deg!) The stadard devato s o s derved as s o = 360/2π (-/2 log e R) /2 wth R = (C 2 + S 2 ) /2. World Stress Map Qualty Rakg All data the WSM database are qualty raked to facltate comparso betwee dfferet dcators of stress oretato (e.g. focal mechasm solutos, overcorg). Image logs provde a much more relable terpretato of borehole breakouts tha four-arm calper logs. Therefore, stress oretatos determed from BO ad DIFs terpreted o mage log data are qualty raked separately. The qualty-rakg crtera are preseted Table ad Table 2. Note that BO ad DIFs must be treated separately durg qualty rakg. For example, a well that exhbts both BO ad DIFs wll receve two stress dcators, oe for the stress oretato determed from the BO ad aother for the stress oretato determed from DIFs. Table : WSM qualty rakg crtera for breakouts terpreted from mage logs a sgle well (s.d. = stadard devato). A-Qualty B-Qualty C-Qualty D-Qualty E-Qualty 0 dstct breakout zoes ad combed legth 00 m a sgle well wth s.d. 2 6 dstct breakout zoes ad combed legth 40 m a sgle well wth s.d dstct breakout zoes ad combed legth 20 m a sgle well wth s.d. 25 < 4 dstct breakout zoes or < 20 m combed legth wth s.d. 40 Wells wthout relable breakouts or wth s.d. > 40 7

8 Table 2: World Stress Map qualty rakg crtera for drllg-duced fractures terpreted from mage logs a sgle well (s.d. = stadard devato). A-Qualty B-Qualty C-Qualty D-Qualty E-Qualty 0 dstct DIF zoes ad combed legth 00 m a sgle well wth s.d. 2 6 dstct DIF zoes ad combed legth 40 m a sgle well wth s.d dstct DIF zoes ad combed legth 20 m a sgle well wth s.d. 25 < 4 dstct DIF zoes or < 20 m combed legth wth s.d. 40 Wells wthout relable DIFs or wth s.d. > 40 Refereces Aadoy, B.S. (990): Iverso techque to determe the -stu stress feld from fracturg data. - J. Petrol. Sc. Eg., 4, Aadoy, B.S. ad J.S. Bell (998): Classfcato of drll-duce fractures ad ther relatoshp to stu stress drectos. - Log Aalyst, 39, Asquth, G. ad D. Krygowsk (2004): Basc well log aalyss. - AAPG Methods Explorato 6, AAPG, Tulsa, Oklahoma, 244 p. Bell, J.S. ad D.I. Gough (979): Northeast-southwest compressve stress Alberta: Evdece from ol wells. - Earth Plaet. Sc. Lett., 45, Bell, J.S. (990): The stress regme of the Scota Shelf offshore easter Caada to 6 klometres depth ad mplcatos for rock mechacs ad hydrocarbo mgrato. - I: Maury, V. ad D. Fourmatraux, eds., Rock at Great Depth, Rotterdam, Balkema, Bell, J.S. (996): Petro Geoscece. I stu stresses sedmetary rocks (part ): measuremet techques. - Geoscece Caada, 23, Ekstrom, M.P., C.A. Daha, M.Y. Che, P.M. Lloyd ad D.J. Ross (987): Formato magg wth mcroelectrcal scag arrays. - Log Aalyst, 28, Hlls, R.R ad S.D. Reyolds (2000): The Australa Stress Map. - J. Geol. Soc., Lodo, 57, Marda, K.V. (972): Statstcs of drectoal data: probablty ad mathematcal statstcs pp., Lodo (Academc Press). Plumb, R.A. ad S.H. Hckma (985): Stress-duced borehole elogato: A comparso betwee the Four-Arm Dpmeter ad the Borehole Televewer the Aubur Geothermal Well. - J. Geophys. Res., 90, Zoback, M.D., D. Moos, L.G. Mast ad R.N. Aderso (985): Well bore breakouts ad stu stress. - J. Geophys. Res., 90, World Stress Map Project,