SEISMOELECTRIC LABORATORY MEASUREMENTS IN A BOREHOLE. Zhenya Zhu and M. Nafi Toksoz

Transcription

1 SEISMOELECTRIC LABORATORY MEASUREMENTS IN A BOREHOLE Zhenya Zhu and M. Nafi Tksz Earth Resurces Labratry Department f Earth, Atmspheric, and Planetary Sciences Massachusetts Institute f Technlgy Cambridge, MA 2139 ABSTRACT The seismelectric lgging methd is based n measuring the electric field generated by seismic waves in a fluid-filled brehle. Tw kinds f electrmagnetic (EM) fields can be generated within the frmatin and at the interface f frmatins. One is a statinary r lcal EM wave and the ther is a radiating EM wave. In this paper, we make varius fractured brehle mdels with artificial materials r natural rcks and measure the electric field generated by a seismic surce in a water-filled brehle. The experimental results shw that the Stneley wave generates bth a statinary EM wave at the brehle wall and a radiating EM wave n the fracture, which prpagates with light speed in the brehle. When the aperture f the fracture increass, the amplitude f the seismelectric wave decreases due t the lw in cncentratin in the fracture. In a layered brehle mdel, a thin, permeable glued-sand zne is sandwiched between tw nnpermeable r lw-permeable layers, and the Stneley wave generates tw kinds f seismelectric signals at the permeable zne. Cmpared with the acustic wavefrms in the same brehle, the seismelectric wavefrms are mre effective in determining and characterizing a fracture r a fractured zne filled with a permeable layer. INTRODUCTION At the interface between fluid and slid, adsrptin f electric charge t the surface f the slid creates an excess f mbile ins f ppsite charge t that f the fluid (Bckris and Reddy, 197). Thus, a duble layer is frmed n the slid surface. When a seismic wave prpagates in a tw-phase medium fslid and fluid, the mechanic waves generates a mvement f ins in the fluid. The mvement f the charges induces an electrmagnetic (EM) field. This phenmenn, that a seismic wave induces an electrmagnetic 9-1

2 Zhu and Tksz wave in a tw-phase medium, is referred t as seismelectric cnversin. On the ther hand, when an utside electric field induces vibratin f the charges in the fluid, the interactin between fluid and slid generates a seismic wave. This prcess is referred t as electrseismic cnversin. Theretical studies (Pride and Haartsen, 1996; Haartsen, 1995) cnfirm the mechanism f these cnversins. Inside the hmgeneus, prus medium, the seismic wave induces a statinary seismelectric field which exists nly in the area disturbed by the seismic wave. At the interface f frmatins with different prperties, such as prsity, permeability, r lithlgy, the seismic wave induces a radiating seismelectric wave which prpagates with light speed and can be received anywhere. Early labratry experiments fcused n the measurements f the streaming ptentials generated by lw frequency (under 1 Hz) vibratin r steady fluid flw in a fluid-saturated prus medium (Cerda and Kiry, 1989). Recent labratry experiments bserved the seismelectric cnversin at high frequency (khz) range (Zhu and Tks6z, 1996). Recent surface experiments in the field (Thmpsn and Gist, 1993; Butler et al., 1996; Mikhailv et al., 1997a) cnfirm that seismelectric signals frm varius interfaces in the subfrmatin can be detected. Due t the expnential decay f the electric field in a cnductive medium, this methd has a limited penetratin depth in field measurements. It is difficult t apply this methd t the explratin f petrleum gelgy. Field experiments (Mikhailv et ai., 1997b) have been cnducted in a brehle, but als shw depth limitatin because the surce was at the surface. T vercme the abve limitatin and t apply this methd t petrleum gephysics, we have fcused ur studies n brehle measurements f seismelectric cnversin and develped a new methd called "seismelectric lgging in a brehle." The experimental results f ur studies shw that the seismelectric signals can be generated by the acustic surce in a brehle and measured by an electrde in the same brehle (Zhu and Tks6z, 1997). These measurements can be applied t bth shallw and deep brehles and btain detailed infrmatin abut fluid flw in the frmatin. In this paper, we cnduct seismelectric measurements in fractured brehle mdels. Bth the acustic surce and the electrde are in the same brehle. When the Stneley wave prpagates past a fracture, it generates a flw f in-carrying fluid in the fracture, thus creating a radiating electrmagnetic wave that prpagates with light speed in the brehle and surrunding frmatins. By recrding and cmparing the electric signals and acustic waves generated by the same acustic surce, we investigate which measurements mre easily identify a fracture r fractured zne. BOREHOLE MODELS AND MEASUREMENTS T simulate a fracture intersecting a brehle, we make brehle mdels f tw separate cylinder blcks with brehles in their centers (Figure 1). The materials f the blcks are artificial material (Lucite) r natural rck (slate). The diameter f the brehle is 9-2

3 Seismelectric Measurements in a Brehle 1.27 cm. Three mdels are cmpsed f tw Lucites, tw slates, and ne Lucite and ne slate, respectively. Between the tw blcks there is a gap, saturated with water (Figure la). Figure Ib shws a layered brehle mdel where a thin epxy-glued sand layer is sandwiched between tw slate r Lucite blcks. During the measurements, a PZT-tube transducer,.9 cm in diameter, is placed at the lwer sectin f the brehle and excited by an electric square pulse f 1 JLs in width and 75 V in amplitude. The electrde is a pint receiver,.5 mm in diameter and 1. mm in length, made f a shielding cable wire. After ging thrugh a pre-amplifier with 6 db gain and a filter, the received electric signals are displayed and recrded n a digital scillscpe (Zhu and Tksiiz, 1996). The whle mdel with the surce transducer and the electrde are saked in water whse cnductivity is abut.18 ms. We fix the surce transducer at the lwer sectin f the brehle, mve the electrde alng the brehle and acrss the fracture, and recrd the received electric signals at each step. All recrded electric wavefrms are time delayed t avid the huge electric influence by the high-vltage surce pulse. T cmpare the electric signals with the acustic field in a brehle, a hydrphne (B&K 813) replaces the electrde and measures the acustic wavefrms under the same cnditins. The amplitude f the acustic wavefrms are nrmalized by the maxim1.!m in each plt. RESULTS IN FRACTURED BOREHOLE MODELS We perfrm experiments in three brehle mdels cmpsed f tw blcks with a waterfilled gap (Figure la). The materials f the tw blcks are Lucite-Lucite, slate-slate, and slate-lucite, respectively. The aperture f the gap is.2 mm. Figure 2 shws the electric signals (Figure 2a) and the acustic waves (Figure 2b) in the Lucite-Lucite brehle mdel. Frm the acustic wavefrms (Figure 2b), we knw that there is a fracture arund trace 7. At the fracture, the Stneley wave generates a P-wave prpagating acrss the fracture and int the upper sectin. The Stneley wave splits int tw waves prpagating with P-wave and Stneley wave velcities, respectively. In Figure 2a the amplitudes f the electric wavefrms are nrmalized by the clip vltage f 2JLV. We see sme electric cmpnents prpagating with Stnely wave velcity. They are statinary electrmagnetic waves generated by the Stneley wave. There is anther electric cmpnent which starts frm the fracture (trace 7) and prpagates with high speed. It arrives at the same time n each trace. Because the Stneley wave frces the in-carrying water in the fracture t vibrate hrizntally, the flw f free charge in the fluid induces a radiating electrmagnetic wave which can be received at any place in the brehle. In a nnpermeable Lucite mdel, a duble layer frms n the brehle wall and n the surfaces f the fracture. When a surface wave (Stneley wave) prpagates alng a brehle, the wave excites the brehle wall and the ins attached t the wall t vibrate. Because f the cnsistency f the brehle alng its axis and the directin f 9-3

4 Zhu and Tksz wave prpagatin, the Stneley wave generates a statinary electrmagnetic wave at the wall. Only at the fracturedes the Stneley wave generate the radiating electrmagnetic wave. T investigate the effect f a fracture aperture n the seismelectric cnversin, we change the aperture between the tw Lucite blcks and measure the electric signals. Figure 3 shws the relatinship between the aperture f a fracture and the amplitude f the seismelectric signal generated by the Stneley wave in the brehle mdel. The larger the aperture, the smaller the amplitude f the electric signal. The number f free ins in the fluid depends n the surface area. When the aperture increases, the surface area and the number f free ins d nt increase. Because the cncentratin f ins in the fracture decreases, the strength f the streaming electric current generated by the same Stneley wave decreases. These results cnfirm that the seismelectric signals at a fluid-saturated fracture are related t its aperture and the electrchemical prperties f the fractured frmatin. The same measurements are cnducted with slate-lucite and slate-slate brehle mdels. Figure 4 shws the seismelectric signals (Figure 4a) and the acustic waves (Figure 4b) in the slate-lucite mdel. The acustic surce is fixed in the slate sectin. Slate is a very hard rck, and its P-wave velcity is 6,95 m/s. Therefre, the acustic surce generates a strnger Stneley wave in the slate sectin than in the previus Lucite-Lucite mdel. At trace 7 in Figure 4b, the Stneley wave splits int bth a P-wave and a Stneley wave in the Lucite sectin. This cnfirms that there are tw materials with a fracture between them. Frm the slpes f the seismelectric signals in Figure 4a, we knw that the Stneley wave generates a statinary EM signal at bth the slate and Lucite sectins, and it generates a radiating EM signal at the fracture. The signal is received in the Lucite sectin. Cmpared with the radiating signal in the Lucite-Lucite mdel (Figure 2a), the radiating EM signal is strnger in the slate Lucite mdel (Figure 4a). Therefre, the seismelectric cnversin is relat,,-d t the electrchemical prperties f the fracture frmatin. Figure 5 shws the electric signals (Figure 5a) and the acustic waves (Figure 5b) recrded in the slate-slate brehle mdel. The Stneley wave velcity is the same in bth slate sectins. We can see that the amplitude f the Stneley wave becmes smaller at the fracture (traces 6 and 7 in Figure 5b). The P-wave is very weak in this hard rck mdel. In Figure 5a, we bserve the seismelectric signals generated by the Stneley wave. We als recrd the radiating EM signals, which are generated at the fracture. These signals have the same arrival time n each trace with the Stneley wave arriving at the measurement pints later. In this mdel, the amplitude f the signals is smaller than in previus brehle mdels because it depends nt nly n the fracture aperture, but als n the electrchemical prperties f the surrunding rck. Our experiment results shw that a radiating EM wave is an indicatr fr a fracture r fractured zne. When the brehle is surrunded by the same frmatin, the Stneley wave generates a seismelectric signal that prpagates with the same velcity. If the Stneley wave generates an electric signal that prpagates with electrmagnetic speed, 9-4

5 Seismelectric Measurements in a Brehle it means there is a fracture r fractured zne in the brehle. RESULTS IN SANDWICHED BOREHOLE MODELS In rder t simulate a brehle that is intersected by a fracture filled with a thin permeable layer, we perfrm experiments with a brehle mdel where a thin, epxyglued sand layer is sandwiched between the slate r Lucite blcks (Figure Ib). The same prcedures are cnducted t recrd the seismelectric signals and the acustic waves in these mdels as were cnducted in the experiments abve. Figure 6 shws the electric signals (Figure 6b) and the acustic waves (Figure 6a) in the slate-sand-slate sandwiched brehle mdel. The thickness f the epxy-glued sand layer is 1. em. The amplitudes f the electric wavefrms in Figure 6b are nrmalized by the clip vltage f 12JLV. Figure 6c shws the amplitude fthe electric signals nrmalized by the amplitude f the Stneley waves at each trace. Frm Figure 6a, we see that the amplitude f the Stneley wave is large befre the electrde enters the fracture (traces 1-5). When the acustic wave enters the sand layer, the amplitude decreases due t its high attenuatin, but the Stneley wave velcity hardly changes. We can als see that the Stneley wave generates a statinary seismelectric wave that prpagates with the same velcity (Figure 6b). The amplitude f the electric signals is larger at the sand layer due t its high prsity and permeability. Figure 6c shws the amplitude f this electric field nrmalized by the amplitude f the Stneley wave at each trace. The amplitude peak at trace 7 indicates the sand frmatin with high prsity and high permeability. In Figure 6b we als see the radiating electrmagnetic wave generated at the sandwiched sand layer due t the cntact f the frmatins and pssible gaps between the layers. This radiating electrmagnetic wave prpagates in the brehle with very high velcity. In this case, the center frequency f the electric signals is abut 3 khz, and it is higher at the slate sectin and lwer at the glued-sand sectin. Thus, the frequency respnse is related t the lithlgy f the frmatin. The curve in Figure 6c is nt clear enugh t determine the thickness f the sand layer because the center frequency varies and the statinary and radiating signals interact. We perfrm similar experiments with a mdel where a thin, glued-sand layer, 1. em in thickness, is sandwiched between tw Lucite blcks. Figures 7a and 7b shw the acustic waves and the seismelectric signals recrded in the Lucite-sand-Lucite brehle mdel. Figure 7c shws the amplitude f the electric signals nrmalized by the amplitude f the Stneley wave at each trace. The nrmalized amplitude f the seismelectric signals at the glued-sand layer is much larger than that in the Lucite sectins due t its high prsity and permeability. Frm the wavefrm variatin f the electric signals, we see there still is a radiating cmpnent in Figure 7c, but it is difficult t separate it frm the main signals because the frequency f the signal is t lw and the glued-sand layer is t thin. The experimental results cnfirm that the nrmalized amplitude f the electric sig- 9-5

6 Zhu and Tksz nals is a gd indicatr fr a fracture filled with a prus medium. CONCLUSIONS In this paper, we measure seismelectric signals generated by Stneley waves in brehles with water r a prus medium filled fractures. The experimental results shw that a statinary electrmagnetic wave is induced at the brehle walls and a radiating electrmagnetic wave is induced at fractures. In the water-filled fracture, the amplitude f the seismelectric signals is related t its fracture aperture. The larger the aperture, the smaller the amplitude f the seismelectric signals. In prus medium filled fractures, the Stneley wave generates a statinary electric wave which is larger than a statinary electric wave in the nnpermeable frmatin. The Stneley wave als generates a radiating seismelectric wave at the interface between the prus medium and nnprus frmatin. Cmparisn f seismelectric signals with the acustic waves shws that it is easer t identify a fracture with seismelectric signals than acustic wavefrms. The seismelectric lgging is an effective new lgging methd t explre fractures and fractured znes in brehles. ACKNOWLEDGMENTS We thank Dr. M. W. Haartsen, Mr. O. V. Mikhailv, Prf. Steven R. Pride, and Prf. T. R. Madden fr their valuable suggestins and useful discussins. This study was supprted by the Brehle Acustics and Lgging/Reservir Delineatin Cnsrtia at M.LT. and by the Department f Energy grant #DE-FG2 93ERl

7 Seismelectric Measurements in a Brehle REFERENCES Bckris, J. and A.K.N. Reddy, 197, Mdern Electrchemistry, Plenum Press. Butler, K., R. Russell, A. Kepic, and M. Maxwell, 1996, Measurement f the seismelectric respnse frm a shallw bundary, Gephysics, 61, Cerda, C.M. and N.-C. Kiry, 1989, The use f sinusidal streaming flw measurements t determine the electrkinetic prperties f prus media, Cllids and Surfaces, 35, Haartsen, M.W., 1995, Cupled electrmagnetic and acustic wavefield mdeling in pr-elastic media and its applicatin in gephysical explratin, Ph.D. thesis, MIT. Mikhailv, a.v., M.W. Haartsen, and M.N. Tks6z, 1997a, Electrseismic investigatin f the shallw subsurface: Field measurements and numerical mdeling, Gephysics, 62, Mikhailv, a.v., J.H. Queen, and M.N. Tks6z, 1997b, Using brehle electrseismic measurements t detect and characterize fractured (permeable) znes, 67th SEG Annual Internatinal Meeting Expanded Abstracts, SS4.8, Pride, S.R. and M.W. Haartsen, 1996, Electrseismic wave prperties, J. Acust. Sc. Am., 1, Thmpsn, A.H. and G.A. Gist, 1993, Gephysical applicatins f electrkinetic cnversin, The Leading Edge, 12, Zhu, Z. and M.N. Tks6z, 1996, Experimental studies f seismelectric cnversin in fluid-saturated prus medium, SEG 66th Annual Internatinal Meeting Expanded Abstracts, RP1.6, Zhu, Z. and M.N. Tks6z, 1997, Experimental studies f electrkinetic cnversin in fluid-saturated brehle mdels, 67th SEG Annual Internatinal Meeting Expanded Abstracts, BH3.13,

8 Zhu and Tksz Electrde Lucite (r Slate) Lucite (r Slate) Acustic Surce [a] ( Electrde ( Glued Sand Slate (r Lucite) Acustic Surce [b] Figure 1: Diagram fr measuring the seismelectric field in the brehle mdel with a water-filled fracture (a) and a glued-sand layer (b). The aperture f the fracture is.2 mm. The thickness f the sand layer is 1 mm. The diameter f the brehle is 12.7 mm. An electric square pulse f 1 V generates the acustic surce. 9-8

9 Seismelectric Measurements in a Brehle [a] (j) 15 [tl E l1) C 1.g "iii Q.. QJ > "w 1;5 5 a: "C 1:3.!!! w (j) 15 [tl E l1) C 1 Q '" Q.. iii > 'w QJ 5 a: "S! ;;; ::J «.1 v -... [b]... v -.V",-_. VA.. -.\J..\... A vv.1 Figure 2: Seismelectric signals (a) and acustic wavefrms (b) recrded in the Lucite Lucite brehle mdel (Figure la). Trace 7 is at the fracture with an aperture f.2 mm. The amplitude f the seismelectric signals (a) is nrmalized by 2/LV. The acustic waves are nrmalized by the maximum amplitude f all wavefrms

10 Zhu and Tksz 1 ' Ul.8 <tl "" c: ::s.. Ul.6 c. u E. <tl U ].4 N.- Cii E E'!!!2... Ul z.\. I...\.. ;... :-:.,. I -.-,:.. '-- I''-- --l' -I..'...J Aperture f the fracture (mm) -. Figure 3: The relatinship between the aperture f a fracture and the amplitude f the seismelectric signal generated by the Stneley wave in the Lucite-Lucite brehle mdel. 9-1

11 Seismelectric Measurements in a Brehle [a] 2l "' ""E E < ':, "'.. Q;. 5 t::=::;;;;;;;;;;;;::a a:." " ỤS!! w.1 [b].2 15 " ""E E '" C'l c 1 iii.. <D. <D " 5 <D a:. U) ::J "« ,.1.2 Figure 4: Seismelectric signals (a) and acustic wavefrms (b) recrded in the slate Lucite brehle mdel (Figure la). The acustic surce is lcated in the slate sectin. Trace 7 is at the fracture which has an aperture f.2 mm. The amplitude f the seismelectric signals (a) is nrmalized by 12!'V. The acustic waves are nrmalized by the maximum amplitude f all wavefrms. 9-11

12 Zhu and Tksz [aj 15.y J ""E E '1"l C'l t -;; 1 i 'iii - m> 'CiJ al a: ()." t5.!j! w [bj.2 15 () ""E E '1"l.. 'iii '" c 1 -.2: () cr: : -: '".: A.. - :t- ::.1.2 Figure 5: Seismelectric signals (a) and acustic wavefrms (b) recrded in the slateslate brehle mdel (Figure 1a). Trace 7 is at the fracture, which has an aperture f.2 mm. The amplitude f the seismelectric signals (a) is nrmalized by 12/LV. The acustic waves are nrmalized by the maximum amplitude f all wavefrms. 9-12

13 Seismelectric Measurements in a Brehle [a] W 15 u '" '"E E t C"J c 1.g ';.. :;; > 'ijj u 5 OJ :. Ui => u «[b] A &. -:J:j.1 15 t /-.._----j i'" E t C"J CD -;; 1r '.g ';.. :;; > 'ijj al : u.;:: U.!!! w -.2 <. [c] E 12.. e "> 6 " () 4 " ().. U 2,.,:. 1,..1 2 ':--:-I-..a..---l Nrmalized amplitude Figure 6: The acustic waves (a) and electric signals (b) in the slate-sand-slate sandwiched brehle mdel (Figure 1b). The acustic waves (a) are nrmalized by the maximum amplitude f all wavefrms. The amplitude f the seismelectric signals (b) is nrmalized by 12/LV. Figure 6c shws the amplitude f the electric signals nrmalized by the amplitude f the Stneley wave at each trace I

14 Qj' u 15 '" <:: E t C") <2- c 1.g 'Vi.. > 'w u 5 <D ::." u; 5 u «[b] Zhu and Tksiiz LaJ k v "'"'"- <:: E t C") cd g 'Vi.. Ci:i.'" 5 <D ::.g t5 <D W c 8 U) c. "C u UJ 6 4' [c] l ' ĒE 12 '" '" CD 1' L..-...J-_L..-..I...---I Nrmalized Amplitude Figure 7: The acustic waves (a) and electric signals (b) in the Lucite-sand-Lucite sandwiched brehle mdel (Figure Ib). The acustic waves (a) are nrmalized by the maximum amplitude f all wavefrms. The amplitude f the seismelectric signals (b) is nrmalized by 12JLV. Figure 7c shws the amplitude f the electric signals nrmalized by the amplitude f the Stneley wave at each trace. ( 9-14