k Mulli-Tamsa Vol. 10, No. 4, 2007, p. 332~344 sw œ w d w y y 1 *Á y 1 Á y 1 1w w Interpretation of Geophysical Well Logs from Deep Geothermal Borehole in Pohang Seho Hwang 1 *, In Hwa Park 1 and Yoonho Song 1 1 Groundwater & Geothermal Resource Division, Korea Institute of Geoscience & Mineral Resources sw w œ w d ww. d dd w, e sƒ, d. d d œ BH-1yœ g d w 3» š n d» n d d d w w. e d t, 3» n d,» n d z. œ BH-4yœ 1981.3 m 82.51 C d ù k d w œ BH-2yœ ƒ o z k d š w, 5~6 o C. /» y d, r d w w d, œ, e, t Abstract: Various geophysical well logs have been made along the four deep wells in Pohang, Gyeongbuk. The primary focus of geophysical well loggings was to improve understanding the subsurface geologic structure, to evaluate in situ physical properties, and to estimate aquifer production zones using fluid temperature and conductivity gradient logs. Especially natural gamma logs interpreted with core logs of borehole BH-1 were useful to discriminate the lithology and to determine the lithologic sequences and boundaries consisting of semi-consolidated Tertiary sediments and intrusive rocks such as basic dyke and Cretaceous sediments. Cross-plot of physical properties inferred from geophysical well logs were used to identify rock types such as Cretaceous sandstone and mudstone, Tertiary sediments, rhyolite, and basic dyke. The temperature log indicated 82.51 o C at the depth of 1,981.3 meters in borehole BH-4. However, considering the temperature of borehole BH-2 measured under stable condition, we expect the temperature at the depth in borehole BH-4, if it is measured in stable condition, to be about 5 or 6 o C higher. Several permeable fractures also have been identified from temperature and conductivity gradient logs, and cutting logs. Keywords: geophysical well logs, deep geothermal borehole, in situ physical properties, cross-plot of physical properties š ƒ w ƒ w wù w ƒwš.» ù y q w d w ù,, { y w w v ƒ. w» w wš,, p 2007 9 16 ; 2007 10 31 k *Corresponding author E-mail: hwangse@kigam.re.kr Address: Groundwater and Geothermal Resources Division Korea Institute of Geoscience and Mineral Resources 92 Kwahang-no, Yuseong-gu, Daejeon 305-350, Korea q w» w w ƒ w ƒ n œ. œ d, yw, t k w d dd w, e sƒ, d, w sƒ, f e j, f k y (Keys and Sullivan, 1979; Keys, 1982). œ w d œ œü f, ¾ w» w œ, m w 70 o C š œ y Áw w». sw w w 2003 l w 4 œ w d, 332
포항 심부 지열 시추공에 대한 물리검층 자료해석 Fig. 1. 2004). Geological map (1:50,000) of Pohang area (Song 333., et al 해석에 대한 것으로 연구지역의 지질개요, 자료취득 및 자료처 리, 자연감마선 검층을 이용한 층서해석, 원위치 물성 평가, 온 도/전기전도도변화율검층 자료를 이용한 투수성 구간 해석을 중심으로 기술한다. 지질 개요 한반도 동남부에 위치한 포항지역은 지체 구조적으로 환동 해알칼리화산지구(Lee, 1977)에 속하며 제3기 포항분지 중부 를 점유한다. Fig. 1은 포항 지질도폭으로 이 지역은 백악기 경 상누층군의 하양층군에 속하는 퇴적암류를 최기저층으로 하여 이를 분출하여 덮거나 관입한 제3기 에오세의 석질결정질응회 암, 흑운모화강암, 규장암, (용결)결정질응회암 등이 포항분지 의 기반암을 이루며 분포하고 있다. 물리검층 자료취득 및 처리 자료취득 물리검층 자료취득은 영국 Robertson Geologging사의 장비 를 이용하였으며 본체는 Micro Logger II이고 케이블 길이 2,000 m의 윈치를 이용하였다. 적용한 물리검층법은 공경 검층, 방사능검층(자연감마선검층, 감마-감마(밀도)검층, 중성 자검층), 음파검층, 전기(전자유도)검층, 온도/전기전도도검층 이다. Fig. 2는 물리검층을 수행한 심부 시추공의 위치도이며 BH1호공은 시추공 전 구간에 대한 코어링을 수행했으며 물리검 층 가능 심도는 1,091.4 m이다. 시추공 BH-2, BH-3 및 BH-4 에서는 약 100 m 간격으로 길이 3 m의 시추코어를 회수했으 며 물리검층 가능심도는 각각 1,501.3 m, 913.0 m, 1,996.0 m 이다. 물리검층은 기본적으로 시추설계에 따라서 일정 심도까 지 굴진이 종료되면 케이싱 삽입 전에 자료취득을 수행하였다. 코어링 시추공인 BH-1호공은 2003년 9월 2일에 첫 번째 물리 Fig. 2. Location map of boreholes for geophysical well logging. 검층 자료를 취득하였으며 시추공경이 작고 파쇄대 구간이 다 수 존재하여 나공 상태에서 측정이 이루어져야 하는 물리검층 자료의 취득에 많은 어려움이 있었다. 3기 반고결이암층 하부 구간은 손데의 직경이 작은 자연감마선검층만을 시추 Rod 내 에서 수행할 수 있었으며, 이때 Rod의 영향은 하부 나공 구간 에서 자료취득 후, 보정 값을 산출하여 제거하였다. 시추공 BH-2호공은 2003년 8월 27일부터 물리검층 자료를 취득하기 시작했으며 약 900 ~ 1,040 m는 시추공 유지가 매우 힘든 구 간으로 시추 종료 직후 48시간 동안 물리검층을 취득한 구간 이다. BH-3호공에 대한 물리검층 자료취득은 케이싱이 삽입되 기 전인, 2004년 10월 28말경에 천부 약 400 m 구간에 대한 1차 자료취득과 2005년 7월 초에 2차 자료취득을 하였으며 물 리검층 수행구간은 913.0 m이다. BH-4호공은 2,000 m 시추 가 종료된 2006년 8월에 물리검층을 수행하였으며 심도 1,700 m 하부는 고온/고압 환경에서 자료취득이 가능한 일부 검층만 을 수행하였다. 자료처리 물리검층 자료는 측정간격이 기본적으로 1 cm로 매우 방대 하여 자료처리를 완료한 후에 5 cm 간격으로 자료를 재정리
334 y yá yá y Fig. 3. Correction of steel casing effect for natural gamma log in borehole BH-2. w d s³ w vl w ( y (2000) 3 ). d ƒ ƒw f ƒ w» d z» w, š ƒ d t ƒ w š w. y q z d f ƒ w f n w» f z w f w w. Fig. 3 œ BH- 2yœ 1,001 ~ 1,002 m d w d w f w ùkü f w 30 ~ 35 API f (, œ, Ì )» d y w w. (, - ( ), ) d ³ (1998), y y (2000) x (2002) šw. d w d w d w d n 3» š n d dd w z. 3» š ü û d», d œ y d w. œ BH-4yœ d d» w w. Fig. 4 BH-1 BH-2yœ w d w œ 169 m š 1 m ü. d w g œ BH-1yœ w g d» d w wš» BH-2yœ w d w w. BH-1yœ y g d y d d ewù d ƒ w g z q. Table 1 2 œ BH-1 BH-yœ g r, š d w d ƒ w. p w sw» n (BH-1yœ 472.1 ~ 538.9 m, BH-2yœ 461.8 ~ 521.6 m) d 120 API Áw» n w š. BH-1yœ w 920 ~ 980 m w.» n mw 100 API w d yw w. ü û» BH-1yœ BH-2yœ ƒ ƒ 3 sw BH-2yœ, d w yw w w. BH-2yœ 1,440.7 ~ 1,492 m û» 1,270 ~ 1,286 m, 1,304 ~ 1,309 m, 1,316 ~ 1,324 m, 1,328 ~1,359 m, š 1,359 m w ùkù û. 1,200 m w» w d wì w ww ù». Fig. 5 BH-3 BH-4yœ w d w œ 128 m, BH-1yœ 1,040 m. 2 œ w d r w d ƒ Table 3 Table 4 w. 3» š d BH-4yœ 217.4 m, BH-3y œ 200.6 m¾ sw 360 m 3» š d sw œ BH-1 BH-2yœ 133~150 m ƒ., û 3» d ƒ ƒw. š œ BH-1 BH-2 3» š d w z sw. z» z n ƒ sw» œ BH-1 BH-2yœ w ¾ w. œ BH-4yœ y z» w d w» / d d w. BH- 1, BH-2 BH-3yœ n w BH-4yœ y BH-3yœ BH-4yœ ¾ q.
sw œ w d w 335 Table 1. boundaries interpreted with natural gamma and core logs in borehole BH-1. Depth (m) ~ 359.1 Semi-consolidated mudstone (sandstone intercalated) 359.1 ~ 402.3 Dacite 402.3 ~ 418.4 Tuffaceous sediment 418.4 ~ 457.1 Lapilli Tuff 457.1 ~ 461.2 Tuffaceous sediment 461.2 ~ 469.7 Tuff 469.7 ~ 472.1 Tuffaceous sediment 472.1 ~ 538.9 Sandstone and mudstone intercalated 538.9 ~ 738.4 Rhyolite 738.5 ~ 739.8 Basic dyke 739.8 ~ 821.3 Rhyolite 821.3 ~ 828.5 Basic dyke 828.5 ~ 831.8 Rhyolite 831.8 ~ 865.2 Basic dyke 865.1 ~ 875.1 Rhyolite 875.1 ~ 1091.4 Sandstone and mudstone alternation (conglomerate & black shale intercalated) Table 2. boundaries interpreted with natural gamma, core, and cutting logs in borehole BH-2. Depth (m) Fig. 4. Interpretation of natural gamma logs in boreholes BH-1, and BH-2. Symbols,,,,,, and indicate semiconsolidated mudstone, tuff/tuffaceous sediment, crystal tuff/ tuffaceous sediment/basic dyke, sandstone, lapilli tuff, rhyolite, Cretaceous sandstone/mudstone, andesic volcanic breccia, respectively. Fig. 4 Fig. 5 dd w w w n ¾ d s w 3» š d û ¾. û z n sƒ wš z sƒ ƒw š. Fig. 4 Fig. 5 3» š d, z / z n d, z,, y z,, d, y ƒ w. Fig. 6 g d w w ( y, 2006). e w Fig. 7 ~ 10 4 œ w d Table 1 ~ 4» ƒ œ w e ~ 356.7 Semi-consolidated mudstone (sandstone intercalated) 356.7 ~ 423.7 Lapillli tuff 423.7 ~ 426.4 Tuffaceous sediment 426.4 ~ 450.6 Tuff 450.6 ~ 461.8 Tuffaceous sediment 461.8 ~ 521.6 Sandstone and mudstone (Tuffaceous sediment intercalated) 521.6 ~ 701.1 Rhyolite 701.1 ~ 708.0 Basic dyke 708.0 ~ 744.5 Rhyolite 744.5 ~ 775.7 Basic dyke 775.7 ~ 827.9 Rhyolite 827.9 ~ 830.6 Basic dyke 830.6 ~ 876.4 Rhyolite 876.4 ~ 1501.3 Sandstone and mudstone alternation (conglomerate & black shale intercalated) Table 5 ~ 7 w. e œ yƒ j ù yƒ j w t w w. ƒ œ d d w Table 5 ~ 7 w» ƒ œ p w w» w» w. (ƒ) BH-1yœ Fig. 7 BH-1yœ w d, œ d 250 m¾ œ 12 e¾ y œ œ y t w š. e d v w w d œ 3» š d
336 y yá yá y Table 3. boundaries interpreted with natural gamma, core, and cutting logs in borehole BH-3. Depth (m) ~ 206.1 Semi-consolidated mudstone (sandstone intercalated) 206.1 ~ 257.0 Crystal tuff 257.0 ~ 273.2 Tuffaceous sediment 273.4 ~ 292.6 Basic dyke 292.6 ~ 376.1 Crystal tuff 376.1 ~ 409.3 Basic dyke 409.3 ~ 428.3 Crystal tuff 428.3 ~ 474.7 Basic dyke 474.7 ~ 524.1 Lapilli tuff 524.1 ~ 628.7 Sandstone & mudstone (tuffaceous sediment intercalated) 628.7 ~ 632.1??? 632.1 ~ 770.4 Rhyolite 770.4 ~ 774.3 Basic dyke 774.3 ~ 901.1 Rhyolite 901.1 ~ 913.0 Sandstone & mudstone (conglomerate & black shale intercalated) Table 4. boundaries interpreted with natural gamma, core, and cutting logs in borehole BH-4. Depth (m) ~ 217.4 Semi-consolidated mudstone (sandstone intercalated) 217.4 ~ 252.7 Crystal tuff 252.7 ~ 258.0 Basic dyke 258.0 ~ 265.7 Crystal tuff 265.7 ~ 270.3 Basic dyke 270.3 ~ 279.6 Lapilli tuff 297.6 ~ 359.5 Crystal tuff 359.5 ~ 368.6 Basic dyke 368.6 ~ 443.3 Lapilli tuff 443.3 ~ 480.1 Sandstone 480.1 ~ 547.7 Lapilli tuff 547.7 ~ 1361.0 Sandstone & mudstone 1,361.0 ~ 1,996.0 Andesic volcanic breccia Fig. 5. Interpretation of natural gamma logs in boreholes BH-3 and BH-4. Symbols are the same of Fig. 4. ¾ w. 3» š n d w s ³ w 2.0 g/cm 3, 88 API, d 9~13 ohm-m ƒ ƒ w 4 ohm-m ¾ w w w. d ü d s 7 m, û,» w d. (ù) BH-2yœ Fig. 8 BH-2yœ w d 3» š n d BH-1yœ w û» w, œ» w, û œ yw.» n y s j» w s³ w. d 2004 8 ww œ (1,510 m) d w 70.15 o C œü» (specific electrical conductivity) 1,000 µs/cm. Table 5 s³ a) b) ƒƒ 3» š d c) d)» n ü w. ( ) BH-3yœ Fig. 9 BH-3yœ w d BH-1 BH-2 yœ y z sw z ƒ ƒw k q
sw œ w d w 337 Fig. 6. Detailed geology from core log of borehole BH-1, cutting logs of borehole BH-2, BH-3 and BH-4, and natural gamma logs of borehole BH-1, BH-2, BH-3 and BH-4 (Song et al., 2006). ƒwš œ w ƒw w. Table 6 ƒ s³ w a) 632.1 ~ 770.4 m, b) 774.3 ~ 901.1 m w. ( ) BH-4yœ BH-4yœ d ƒ ƒ 2,000 m 1,700 m w d d w 70 o C š ƒ /»,» d ww. Fig. 10 BH-4yœ w d 828 ~ 1,000 m œ y ù d ƒ w. œ d y ƒ w wš ƒ w ù q d Fig. 10 1,700 m ¾ ƒ w. q d d w k q (Vp) ƒw w» x z œü» 1,700 ~ 2,100 µs/cm š 1,981.3 m 82.51 o C. ( ) œ BH-3yœ d t w d d w w d,, w» y š w ƒ p ww. Fig. 11 BH-3yœ w d w ƒ t(cross-plot) w w sw t»w š w d t»w. Fig. 11(a) d w -œ t, û œ,» z 40 ~ 60 LPU 2.4 ~ 2.7 g/cm 3 s 3» š û 40 LPU œ š. œ 20 LPU z sww n d, y z. Fig. 11(b) - t ƒ wš n d s s û,» z û ƒ 2.6 ~ 2.1 g/cm 3.» û û. Fig. 11(c) -œ t û œ 80 ~ 100 API ew / û, û œ ew.» z û n œ,»
338 y yá yá y Fig. 7. Geophysical well logs in borehole BH-1. û œ» û œ š. 3» š œ 50 ~ 80 API ewš. 3 t d z -œ, -œ - t. d w d v w d, n q y, f e screen y. œ s k d w yw d ƒ wù œ r z œ wš
sw œ w d w 339 Fig. 8. Geophysical well logs in borehole BH-2. Table 5. Physical properties from geophysical well logs in borehole BH-2. Depth (m) Natural gamma (API) P-wave velocity (km/sec) Density (g/cm 3 ) Porosity (LPU)* Long normal (ohm-m) ~ 356.7 semi-consolidated mudstone 90 ~ 100 40 2.2 ~ 2.3 a) 2.6 b) 40 ~ 60 a) 23 b) 10 521.6 ~ 876.4 rhyolite 100 1.8 2.6 10 200 ~ 1,000 876.4 ~ 1,501.3 Cretaceous sandstone/mudstone 60 c) 5.1 2.5 ~ 2.7 8 70 20 d) *LPU Limestone Porosity Unit w.
340 y yá yá y Fig. 9. Geophysical well logs in borehole BH-3. Table 6. Physical properties from geophysical well logs in borehole BH-3. Depth (m) Natural gamma (API) Density (g/cm 3 ) Porosity (LPU) P-wave velocity (km/sec) Long norma/ Short normal (ohm-m) Induction resistivity (ohm-m) 0 ~ 206.1 semi-consolidated mudstone 64 1.65 65 206.1 ~ 257.0 tuff 104 1.84 30.6 20/25 257.0 ~ 273.2 tuffaceous sediment 119 1.75 26.5 70/69 273.2 ~ 292.6 basic dyke 33 1.62 55 1.03 14/18 292.6 ~ 376.1 crystal tuff 127 2.29 20.4 1.73 71/99 376.1 ~ 409.3 basic dyke 33 2.49 43 1.52 12/20 409.3 ~ 428.3 crystal tuff 133 2.62 12 3.78 57/65 40 428.3 ~ 474.7 basic dyke 30 2.65 26 4.03 39/47 29 474.7 ~ 524.1 lapilli tuff 100 2.67 16 4.12 35/42 25 524.1 ~ 628.7 Cretaceous sandstone/mudstone 77 2.56 22 3.29 38/47 26 632.1 ~ 901.1 rhyolite 90 2.71 10 4.74 290/617 a) 140 a) 118/211 b) 118 b) 901.1 ~ 913.0 Cretaceous sandstone/mudstone 140 2.75 23.5 3.70 35/42 23
sw œ w d w 341 Fig. 10. Geophysical well logs in borehole BH-4. Table 7. Physical properties from geophysical well logs in borehole BH-4. Depth (m) Natural gamma (API) P-wave velocity (km/sec) Long normal (ohm-m) crystal tuff 217.4 ~ 252.4 133 3.14 31 297.3 ~ 359.5 137 2.89 65 sandstone 443.3 ~ 480.1 37 4.28 66 270.3 ~ 279.3 102 2.79 102 lapilli tuff 368.6 ~ 443.3 120 3.41 120 480.1 ~ 547.7 95 3.37 39 Cretaceous sandstone/mudstone, 547.7 ~ 1,361.0 78 4.01 65 andesic volcanic breccia 1,361.0 ~ 1,996.0 48 4.68 204
342 y yá yá y Fig. 11. Cross-plots for geophysical well logs in borehole BH-3. (a) gamma-gamma density vs. limestone porosity, (b) natural gamma vs. gamma-gamma density, and (c) natural gamma vs. limestone porosity. w z f e yw /» d. BH-1yœ ùœ k œ x d ww w BH-3 yœ z w» w /» d y û r. BH-2yœ w /» d z» ww 2004 8 29 w š BH-4yœ 2,000 m z 2006 9 1 w w sw d. /» d œ BH-2yœ d» y n d w» y w. Fig. 12 œ BH-2, BH-3 BH-4yœ w /» y d ùkü y d y 3 o C/100 m. 3 œ» 2,150 µs/cm» ww BH-2yœ, 800 ~ 1,200 µs/cm». d d w BH-2, BH-3 BH-4yœ ƒƒ 1,502.6 m 69.49 o C, 912.27 m 44.40 o C, 1,981.3 m 82.51 C o ùkû. y d d BH-2yœ BH-4yœ» y y n q ƒ w w. d BH-2yœ, 600 ~ 690 m, / 940 ~ 980 m, 1,170 ~ 1,190 m, 1,265 ~ 1,300 m, 1,450 m š BH-3yœ y z 485 m, 660 m 880 m w BH-4yœ / 920 ~ 960 m, 1,008.0 m, 1,014.8 m, 1,251.0 m, 1,321.7 m, 1,342.8 m, 1,453.3 m, 1,511.3 m, 1,738.5 m, 1,879 ~ 1,970 m. 3 œ n q w ƒ œ BH-4yœ. Fig. 13 d BH-2yœ z w BH-4yœ d ùkü. Fig. 13 950 ~ 1,000 m w ù 1,000 m w BH-4yœ ƒ BH-2yœ. x w q 1,500 m BH-2yœ 69. 5 o C, BH-4yœ 63.7 C o 5.8 C o š. ƒ w š w BH-4yœ 1,981.3 m d 82.51 o C q. 2,000 m w w yw d 3,000 m e d 120 o C, 30 MPa š Áš /» d l 2007 w» ƒ w q.
sw œ w d w 343 Fig. 12. Fluid temperature, specific conductivity and temperature gradient logs in boreholes BH-2, BH-3, and BH-4. sƒ w 4 œ d w, e sƒ d, w w. n 3» š d d yp» d d w w. d g z w BH-1yœ» d w wš k œ w w r wì ww. BH-1yœ BH-2yœ w dd dz n d w w swš ew. d w 4 œ d w w, BH-1, BH-2, BH-3yœ w d BH-4yœ 3 œ y y. 3» š d BH-3 BH-4yœ ew BH-1 BH-2yœ ew û ¾ swš. e w š, /y z, / ü š ü,», n d û.» w d, š d,», y z û» w, š d ü,, z, n dü, y ƒ» w. œ BH-3yœ w ƒ d w y q w
344 y yá yá y w» y». w w Ì. š x Fig. 13. Fluid temperature logs in BH-2 from relatively stable borehole condition and BH-4 from unstable condition.,», š /, z. d 3 œ BH-2yœ w d ƒ ƒ d w ƒ w BH-2, BH-3 BH-4y œ ƒƒ 1502.6 m 69.49 o C, 912.27 m 44.40 o C, 1,981.3 m 82.51 o C. 3 œ» 2,150 µs/cm» ww BH-2y œ 800 ~ 1,200 µs/cm. y d d BH-2yœ BH-4yœ y» yƒ y n q ƒ w w. BH-4yœ 1,981.3 m 82.51 o C ù k d w BH-2yœ w, q. y, y, ³, ½ y, y y, yw, š,,,, y, 2000, k, w š 1999-R-TI02-P-01, p111. y,,, ½x,, š,,, y, k,,,,,»,,, k, xx,, x, y y, y, ½,, ½ ³,,,, ½m«,,,, 2004,, w š -04( )-01, p226. y,,, ½x,, š, y, k,,,,, y y, ³,, ½m «,,,, ½», x,,, 2006,, w š OAA2003001-2006(4), p190. x, y y, ½xk, ³,, 2002, œ d w œ d, w œwz w tz, 123-128. ³, y y, yw, y, 1998, w y w d t, w KR-98(C)-10, p65. y y,, ³,, 2000, œ d, w œwz, w y wz, w k wz œ w tz, 298-300. Keys, W. S., and Sullivan, K., 1979, Role of borehole geophysics in defining the physical characteristics of the Raft River geothermal reservoir, Idaho, Geophysics, 26, 1116-1141. Keys, W. S., 1982, Borehole geophysics in geothermal exploration, in Developments in Geophysical Exploration Method -3, Fitch, A. A., eds, Applied Science Publishers. Lee, D. S., 1977, Chemical composition of petrographic assemblages of igneous and related rocks in south Korea, J. Korea. Inst. Mining Geol., 10, 75-92.