Heriot-Watt University

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1 Heriot-Wtt University Heriot-Wtt University Reserch Gtewy Experimentl chrcteristion of sher nd compction nd mechnisms in porous sndstone y comintion of AE nd 3D-DIC Chrlmpidou, Elli-Mri Christodoulos; Hll, Stephen Alexnder; Stnchits, Sergei; Viggini, Giochino; Lewis, Mrgret Helen Pulished in: EPJ We of Conferences DOI: /epjconf/ Puliction dte: 2010 Document Version Peer reviewed version Link to puliction in Heriot-Wtt University Reserch Portl Cittion for pulished version (APA): Chrlmpidou, E. M., Hll, S. A., Stnchits, S., Viggini, G., & Lewis, M. H. (2010). Experimentl chrcteristion of sher nd compction nd mechnisms in porous sndstone y comintion of AE nd 3D-DIC. EPJ We of Conferences, 6, [22009]. DOI: /epjconf/ Generl rights Copyright nd morl rights for the pulictions mde ccessile in the pulic portl re retined y the uthors nd/or other copyright owners nd it is condition of ccessing pulictions tht users recognise nd ide y the legl requirements ssocited with these rights. If you elieve tht this document reches copyright plese contct us providing detils, nd we will remove ccess to the work immeditely nd investigte your clim.

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3 Experimentl chrcteristion of sher nd compction nd mechnisms in porous sndstone y comintion of AE nd 3D-DIC E.-M. Chrlmpidou 1, 2,, S.A. Hll 2, S. Stnchits 3, G. Viggini 2, nd H. Lewis 1 1 Heriot-Wtt University, Institute of Petroleum Engineering, Edinurgh EH14 4AS, Scotlnd 2 Lortoire 3S-R, CNRS/Grenole Universities, Domine Universitire, BP53, Grenole, Frnce 3 GFZ Germn Reserch Centre for Geosciences, Telegrfenerg D423, Potsdm, Germny Astrct. Sher nd compction nds represent two fundmentl filure modes in sndstones. The identifiction of these fetures in oth the nturl outcrops nd the lortory scle yields useful insights into the mechnisms of loclised deformtion. Severl conventionl nd dvnced experimentl full-field methods hve een developed in order to shed light into the processes involved, s well s qulify nd quntify prmeters such s the orienttion of these loclised nds. In this pper, we present results of two trixil tests, investigting sher nd compction nds in porous sndstone, using nlysis of Acoustic Emission recorded during deformtion nd 3Dvolumetric Digitl Imge Correltion sed on pre- nd post deformtion x-ry tomogrphy imges. 1 Introduction Sher nd compction nds hve een oth recognised in nture [e.g., 1-3] nd reproduced in lortory [e.g., 4-9] over numer of yers. The optiml gol of ll such studies is the etter understnding of the mechnisms eing involved in these filure modes. In fct, compction loclistion in nturlly deformed systems cn led to significnt porosity nd permeility chnges. The evolution of fetures such s sher or compction nds, cn, therefore, hve importnt impliction for quifer mngement, fluid injection or withdrwl, oil nd gs production, storge of CO 2 or wste disposl. 2 Experimentl techniques At the lortory scle, the development nd propgtion of loclised deformtion in rocks cnnot e fully investigted vi conventionl techniques, i.e., nlysis of force nd displcement mesurements tken t the oundries of the specimen [10]. Consequently, the use of full-field mesurements, (e.g., Ultrsonic Tomogrphy (UT), Acoustic Emission (AE), x-ry Computed Tomogrphy (x-ry CT) or e-mil : elm.chrlmpidou@pet.hw.c.uk

4 Digitl Imge Correltion (DIC)), hs ecome incresingly common [11]. Such non-conventionl lortory techniques llow mesurements of field of properties or processes: e.g., ultrsonic velocity (UT), density (x-rys CT), nd displcement/strins (DIC). It is worth noting tht ech pproch hs certin resolution nd sensitivity to different physicl properties; thus, much cn e gined y using them in conjunction. In this work, we comine non-destructive methods such s UT, AE, x-ry CT, 3D-DIC with thin section investigtions (destructive method), to study loclised deformtion in sndstone. In this pper, however, we limit the scope to results otined y AE nlysis nd 3D-volumetric DIC on low resolution (~90 μm voxel size) pre- nd post-deformtion x- ry tomogrphy imges. Results on high-resolution (~30 μm voxel size) pre- nd post-deformtion x-ry tomogrphy imges re presented elsewhere. 3 Mteril nd experimentl progrm The mteril studied here is qurtz rich sndstone of 22% verge porosity, coming from the Woustviller qurry in the Vosges Mountins, Frnce [12]. This Vosges Sndstone hs men grin size dimeter of out 300 µm nd yields 93% qurtz, 5% feldspr, 1% cly nd 1% mics [13]. It ws shown tht the ehviour nd filure mode of sndstones re pressure dependent [e.g., 13, 5, 14]. Bésuelle [12] investigted the ehviour of cylindricl specimens of this prticulr Vosges Sndstone under low (10-30 MP) nd medium (40-60 MP) confining pressures; t these two pressure rnges, dilting nd compcting sher nds developed, respectively. In this work, we vlidte tht compcting sher nds develop in specimens with slightly different geometric configurtion loded under trixil compression t 50 MP confining pressure, while, under much higher confining pressures ( MP) essentilly horizontl deformtion nds develop. Here, we present two trixil compression tests, conducted t 50 MP nd 160 MP confining pressure. Both experiments come from much lrger experimentl cmpign [15]. Cylindricl specimens (40mm in dimeter nd 80 mm in height) were cored perpendiculr to the sedimentry edding. In ddition, the specimens hd two opposite flttened surfces throughout their height. This prticulr geometry is necessry for the UT (not presented in this pper). Bésuelle [12] used cylindricl specimens of the sme size nd showed tht under 50 MP confining pressure, conjugte compcting sher nds developed in the top prt of the smples. To void the onset nd propgtion of conjugte nds, under this pressure rnge, nd to enforce the occurrence of distinct sher nd in the middle of the smple, we mchined two notches (2.5 mm width nd 2.0 mm high) on the flttened surfces in different heights (Figure 1). The specimen loded under 160 MP did not hve ny notch (Figure 1). c Fig. 1. Stress devitor versus xil strin nd schemtic picture of the specimens:. sher nd,. compction nd, c. Photogrph of the AE sensors on the specimen surfce. Both trixil compression tests (dry) were crried out t GFZ, Potsdm. AE nd ultrsonic trnsmission signls were recorded during the tests using 14 P- nd 4S- piezoelectric sensors glued on the surfce of the specimens (Figure 1) nd 2 P-sensors incorported in the top nd ottom cps. Two verticl nd two horizontl strin guges were instlled directly on the rock nd n internl lod

5 trnsducer ws used to monitor pplied xil stress. The two specimens were sujected, initilly, to hydrosttic loding followed y devitoric loding. At 50 MP confining pressure, the xil loding stopped ner the pek stress, while, t 160 MP confining pressure, the xil loding stopped t higher level of xil strin (compred to tht corresponding to the pek stress). The two experiments (Ve1 nd Ve2) presented here, were nlysed using AEs recorded during deformtion, s well pre- nd post- deformtion low resolution (~90 µm voxel size) x-ry scns (t Lortoire 3S-R, Grenole). Along with the AE nlysis (3D), 3D volumetric DIC of the pre- nd post- mortem x-ry tomogrphy imges will e presented for oth specimens. 4 Results nd discussion Figure 1 presents the stress devitor q = (σ σ r ), where σ nd σ r re the xil stress nd the confining pressure respectively (the ltter eing constnt during the devitoric loding), s function of the xil strin. For Ve2, the loding ws stopped t out 1.4 % xil strin nd, fter complete unloding, 0.54 % xil strin ws recovered. In this specimen, sher nd developed. For Ve1, the loding ws stopped t out 1.65 % xil strin nd, fter complete unloding, 1.08 % xil strin ws recovered. This specimen filed y compction nds. In the following, results from the AE, 3D volumetric DIC nlyses re presented seprtely for the cse of the sher nd compction nd. 4.1 Sher Bnd Figure 2 depicts the time history (in seconds) of the stress devitor nd the cumultive AE numer, seprted into incresing time intervls moving from left to right. Three 2D projections of 3D distriutions of AE event loctions (one perpendiculr nd two prllel to the smple xis) re shown in Figures 2, 2c, 2d. The notches ct s stress concentrtors; therefore, they encourge loclistion to initite in the regions round them. Indeed, AE events nucleted from the notches in oth upper nd lower directions, however, they, finlly, loclised in the region in etween the notches (i.e., middle prt of the smple). c d Fig. 2. Stress devitor s function of the numer of AE events in different time intervls; colour chnges to highlight most recent events; -d. Mps of AE event loctions for the sme time intervls viewed perpendiculr to the specimen s height, prllel to the specimen s height in projection perpendiculr to the flttened surfces projections nd prllel to the specimen s height in projection prllel to the flttened surfces projections.

6 Volumetric nd sher mximum sher strins were otined y 3D-volumetric DIC using the code TOMOWARP, sed on the work of Hll [16], [see lso 17]. The mximum sher strin is ε s = 2/3 [(ε 1 ε 2 ) 2 + (ε 2 ε 3 ) 2 + (ε 3 ε 1 ) 2 ] 1/2, with ε 1, ε 2 nd ε 3 the mjor, intermedite nd minor principl strins, respectively, while the volumetric strin is ε v = [ε 1 + ε 2 + ε 3 ]. The strin fields, derived from the low resolution x-ry tomogrphy imges, indicte tht two compcting sher nds initited from the notches nd propgted to the centre of the specimen. Figure 3 presents oth sher nd volumetric strins in the region where the two nds meet. Strins were clculted over step equivlent to 3 times the grin size nd the clcultions were performed in correltion window equl to 3 times the grin size (Figure 3). However, it ppers tht when clcultions were performed in smller correltion window (equivlent to 1.5 times the grin size) the sher strin field is etter resolved, while the volumetric strin field is quite noisy (Figure 3). Fig. 3. Volumetric nd sher strins in region where the two compcting sher nds meet. Strins re clculted over step of 3 grin size nd correltion window equivlent to 3 grin size () nd 1.5 grin size (). The thresholded 3D geometry of the sher nd, derived from the sher strin field, is shown in Figure 4. The verticl column in the centrl prt of the smple is due to ring rtefcts. The nd ppers to e rther plnr. Figures 4, c show the medin vlues of ll verticl projections of volumetric nd sher strins, respectively. c Fig D thresholded views of the sher strin field;. medin vlues of the volumetric strin projections; c. medin vlues of the sher strin projections. All imges were clculted for correltion window equivlent to 3 times the grin size nd over step equivlent to 3 grins 4.2 Compction Bnd Similrly to Figure 2, Figure 5 presents the time history (in seconds) of the stress devitor nd the cumultive AE numer; the three 2D projections of 3D distriutions of AE event loctions (one perpendiculr nd two prllel to the smple xis) re shown in Figures 5, 5c, 5d. This specimen hd no notches. An intensive AE event ctivity is recorded ner the top edge of the smple, while AE events lso loclised, ner the mid-height of the top prt of the smple, in horizontl nds. Once the nds propgted thought the width of the smple, new nds developed. The numer of the nds incresed with incresing xil strin. Before the unloding, one more nd, this time on the

7 ottom prt of the smple nd ner the mid-height, developed. These nds re inferred to e compction nds. It is worth noting tht these loclised fetures developed only on the top edge nd the middle prt of the specimen, while no AE ctivity ws recorded elsewhere. Most likely, tht is ssocited with smple heterogeneity or higher density of edding plnes in those regions. c d Fig. 5. Stress devitor s function of the numer of AE events in different time intervls; colour chnges to highlight most recent events; -d. Mps of AE event loctions for the sme time intervls viewed perpendiculr to the specimen s height, prllel to the specimen s height in projection perpendiculr to the flttened surfces projections nd prllel to the specimen s height in projection prllel to the flttened surfces projections. The volumetric nd sher strin fields re presented in Figure 6. Strins were clculted over step equivlent to 1.5 grin size for correltion window equl to 3 times the grin size. Figure 6 illustrtes the medin volumetric nd sher field of ll verticl projections. A network of compction nds developed ner the top edge nd the mid-height of the specimen. Nevertheless, the 3D geometry of the compction nds ppers to e more complex compred to the sher nd geometry. Figures 6 nd c show two different compction nd networks, inside the volume of the specimen. In oth cses, three compction nds developed ove the mid-height of the smple, however, their orienttion nd propgtion chnge throughout the specimen. Such oservtion is much etter illustrted in Figure 7, which shows the 3D geometry of the compction nds using the thresholded sher nd volumetric strin field. c Fig. 6.. Medin vlues of volumetric nd sher strins over ll verticl projections;, c. 2D slices of volumetric nd sher strin in different positions inside the specimen.

8 Fig. 7. 2D slices of rotted thresholded 3D view of the compction nds derived from the volumetric () nd sher () strin fields. 5 Conclusions Results on 3D-volumetric DIC, derived from low resolution (voxel size of ~ 90 µm) pre- nd postdeformtion x-ry tomogrphy imges, nd AE nlysis re presented in this pper to chrcterise loclised deformtion in sher nd compction nds. The AE nlysis illustrtes the onset nd evolution of these nds throughout the experiments, while the 3D-volumetric DIC gives field picture of sher nd volumetric strins derived from the deformtion process. In oth deformtion nds compction nd sher hve een recorded. The two methods pper to e consistent, since they disply similr fetures in ech experiment. Acknowledgement Georg Dresen is grtefully cknowledged for the finncil support of the experiments crried out in GFZ. E-M.Chrlmpidou is supported y n Ali Dnesh Scholrship from the Institute of Petroleum Engineering t Heriot-Wtt University, Edinurgh. References 1. M.A. Antonellini, A. Aydin, D.D. Pollrd, J. Struct. Geol., 16, (1994) 2. P.N. Mollem, M.A. Antonellini, Tectonophysics, 267, (1996) 3. K.R. Sternlof, J.W. Rudnicki nd D.D. Pollrd, J. Geophys. Res., 110, B11403, doi: /2005jb (2005). 4. B. Menéndez, W. Zhu, T.-W. Wong, Journl of Structurl Geology, Vol.18, No. 1, pp (1996) 5. A. El Bied, J. Sulem, F. Mrtineu, Int. J. Rock Mech. & Min. Sci., 39, (2002) 6. V. Vjdov nd T.-F. Wong, Geophysicl Reserch Letters, VOL. 30, NO. 14, 1775, doi: /2003GL (2003) 7. P. Bud, E. Klein, nd T.-F. Wong, J. Struct. Geol., 26, (2004) 8. J. Fortin, S. Stnchits, G. Dresen, Y. Gueguen, Pure ppl. Geophys., 166, (2009) 9. S. Stnchits, J. Fortin, Y. Guéguen, G. Dresen, Pure ppl. Geophys., 166, (2009) 10. J. Desrues, nd G. Viggini, Int. J. Numer. Anl. Meth. Geomech, 18, , (2004)

9 11. G. Viggini, S. A. Hll, Deformtion Chrcteristics of Geomterils, IOS Press, Atlnt, USA, p (2008) 12. P. Bésuelle, Phys. Chem. Erth(A), Vol. 26, No. 1-2, pp (2001) 13. P. Bésuelle, J. Desrues, S. Rynud, Int. J. Rock Mech. & Min. Sci., 37, (2000) 14. K. Mir, S. Elphick, I. Min, Geophysicl Reserch Letters, 29, NO. 10, /2001GL (2002) 15. E-M. Chrlmpidou, Experimentl study of loclised deformtion in porous sndstone, PhD thesis, Heriot-Wtt University & Grenole University, Edinurgh, Scotlnd nd Grenole, Frnce, (2010, forthcoming) 16. S.A. Hll, Geophysics, 71, O21 O31 (2006) 17. S.A. Hll, N. Lenoir, G. Viggini, J. Desrues, nd P. Bésuelle, Internl Symp. on Computtionl Geomechnics, Cote d'azur, Frnce (2009)