International Journal of Engineering & Technology IJET-IJENS Vol:09 No:10 67
1)
*)
Department of Petroleum Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti
Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia.
Abstract -Rock characterization and acoustic wave velocity analysis are very important stage in the petroleum reservoir characterization and seismic exploration. Meanwhile carbonate rocks are worthy of attention since they contain at least 40% of the world’s known hydrocarbon reserve and have some complexity in porosity, lithology facies and acoustic wave behavior. This paper present detail relationship between porosity and permeability, effect of pressure and temperature to the acoustic wave parameters such as compressional and shear wave velocities. Data collected includes petrography analysis,
S EM image, detail core description, and laboratory experimental of acoustic wave velocities measurements in variation of overburden pressure and temperature. S ome acoustic wave parameters were simulated as close as possible to the reservoir conditions. Based on the petrophysical data and acoustic wave measurement, the porosity is the main controlling factor of acoustic wave parameter. A plot of porosity versus velocity displays a clear inverse trend to porosity which an increasing of porosity resulting in decreasing of velocity. In addition, increasing of permeability will results in decreasing velocity value. The overburden pressure causes compaction, porosity reduction and increasing in velocity. This performance is slightly changed when temperature increase from 28.73
o C to
62.07 o C, generally both Vp and Vs value become lower. The results can be used for better seismic analysis performance, correspond to increase hydrocarbon discovery from the carbonate rock in the future.
Index Term -- Acoustic wave velocity; Carbonate rock;
Petrophysic properties; Pressure and temperature.
1.
I NTRODUCTION
With the rapid development in seismic exploration and petroleum reservoir characterization, detailed studies on acoustic wave velocity and its controls parameters such as pressure and temperature are getting more attention. There are
*
)
Corresponding author: Prof. Dr. Ariffin
Samsuri , Em ail address : ariffin@utm.my
T el : +60122105171
1) Permanent Address: Department of
Geological Engineering, Faculty of
Engineering, Gadjah Mada University,
Yogyakarta, Indonesia. Em ail address : j_setyowiyoto@yahoo.com
some researches focused on igneous rocks, sandstones, and unconsolidated carbonate sediments but few on carbonate rocks or core.
Carbonate rock result mainly from biochemical and biological processes in warm shallow marine and lacustrine environments and prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore type within carbonate rocks. It is volumetrically a most significant part of the geological record and possesses much of the fossil record of life on this planet. Their deposition involves a more complex suite of processes than many other sediment types [1].
They hold more than half of the world’s petroleum reserves.
However geophysical applications in carbonate reservoirs are less mature and abundant than those associated with siliciclastic reservoirs. It because carbonate reservoirs offer unique geophysical challenges with respects to reservoir characterization and are notoriously more difficult to characterize than siliciclas tic reservoirs [2].
Adding complexity to reservoir quality prediction is that carbonate which producing organism have evolved through time [3].
Carbonate diagenetic processes continuously modify the pore structure to create or destroy porosity. Cementation diagenetic processes for instance are prone to reduce porosity while dissolution will enlarge porosity. All these modifications will effect seismic wave velocity such as compressional wave velocity and shear wave velocity [4].
Pressure and temperature strong influence in determining the acoustic velocity in rocks. Reference [5] has measured Vp and
Vs on unconsolidated carbonate mud to completely lithified limestones under variable confining and pore-fluid pressures.
They reported that pure carbonate rocks show, unlike siliciclastic or shaly sediments, little direct correlation between acoustic properties (Vp and Vs) with age or burial depth of the sediments so that velocity inversions with increasing depth are common.
Reference [6] reported the effect of temperature and pressure on sonic wave velocities in sandstone. They showed that sonic velocity in the liquid saturated sandstone increases with increased pressure and decreasing velocity with increased temperature. Reference [7] investigated the effect of pressure on compressional and shear wave velocity in modern carbonate sediment and rock. They concluded that the wave velocities increase with increasing pressure. More over, reference [8] researched on the effect of pressure and
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International Journal of Engineering & Technology IJET-IJENS Vol:09 No:10 68 temperature to the acoustic wave velocity in marble and calcschist. They reported that the velocity includes compressional and shear waves will increase with increasing pressure and decrease with increased temperature.
The objective of this study includes detail relationship among pressure, temperature, and petrophysisc parameters such as porosity and permeability to the acoustic wave parameter, i.e compressional and shear wave velocity in Miocene carbonate core taken from around 2800 meter deep.
2.
M ETHOD AND D ATA C OLLECTION
Twenty one carbonate core samples have been prepared for analysis of detail core description, petrophysic, petrography, and Scanning Electron Microscope (SEM). Twelve cylindrical core-plug samples from those were analyzed in the Wave
Inversion and Subsurface Fluid Imaging Research Laboratory to obtain the value of compressional wave velocity (Vp) and shear wave velocity (Vs) in variation of overburden pressure, pore pressure and also temperature. Some acoustic wave parameters were simulated as close as possible to the reservoir conditions.
The samples were cleaned in methanol and dried in a vacuum oven at 85 o
C for period of twenty-four hours and than saturated with brine/formation water of 16,271.67 mg/liter. The acoustic velocity measurement on the carbonate samples have been performed under brine saturated conditions at frequencies of about 10 Hz, the overburden pressure range from 50 – 460 bar, the pore pressure range from 40-400 bar, and temperature range from 28-57 o
C. These procedures were run in
Wave Inversion and Subsurface Fluid Imaging Research
Laboratory, Institute Technology of Bandung.
Petrographic analysis was undertaken on all the cores which had been impregnated with araldite resin to maintain the existing natural porosity and staining for carbonate minerals with solution of Alizarin Red-S. The carbonate coloration given by this staining is as follows; pink color for calcite, bluish pink to blue for ferroan calcite, dark blue to greenish blue for ferroan dolomite and unstained for dolomite.
In order to obtain an understanding of diagenetic fabrics, particularly clay and micrite, and their roles with respect to reservoir quality, SEM-EDX analysis was also conducted. The samples were cleaned using organic solvents and ultrasound treatment, then were broken to create fresh surface and mounted on10 mm Cu-stub. They were air brushed free of dust and other contaminants, placed under vacuum overnight to remove most remaining volatile, and electrostatically coated with both carbon and gold alloy.
3.
R ESULT AND D ISCUSSION
3. 1 Rock Characterization
Detail descriptions of the carbonate core samples include rock texture, sedimentary structure, composition and fossil content had been analyzed. Supported by integrated petrography and
Scanning Electron Microscopy (SEM) analysis, it has identified seven carbonate rock types.
3.1.1 Bedded Large Forams Grainstone
Large forams grainstone in general is a grayish white in colour.
Inclined parallel bedding indicated by changes in sediment grain size may represent considerable periods of time when there was little deposition, and then tilted due to endogenic uplifting force. The grain size ranges from 0.52mm – 1.8mm, dominantly point type grain contact, moderately sorted and mostly abraded (rounded). It is composed mainly of skeletal grains such as large forams and red algae, and associated with minor amount of echinoid, bryozoans, brachiopods, and indeterminate bioclast. Pore system is dominated by vuggy porosity, some intercrystalline and intragranular pore ty pes.
Fig. 1, detail petrography analysis shows a grainstone mainly consist of large forams (C-I, 5-6; C-G, 2-4; K-L, 8-9) and less of red algae (F-G, 8-9; J-M, 6; A-B, 4-5).
3.1.2 Cross-Bedded Large Foram Grainstone
The carbonate rock of this type in general is light grey to grey, commonly grainstone texture. Cross bedding sedimentary structures were observed in this rock. This sedimentary structure indicated that there are changes of flow velocity or depth during their deposition. The grains size range 0.22 mm -
3.75 mm, point type grain contact, and moderately sorted and mostly abraded (rounded). This rock contains commonly large forams, and less of red algae, echinoderms, small benthonic forams, planktonic forams, and bryozoan. Moldic pore type is dominant, mostly filled by mosaic calcite cement type which is overgrowth on some echinoderms grains. Diagenetic processes include micritization of grains; also fill intraparticle voids and cause reducing porosity.
3.1.3 Red Algal Packstone
Generally, the red algal packstone to floatstone is grey in colour. Minor discontinous thin laminae of detrital clay and carbonaceous materials are present in this rock. The grain size ranges from 0.3 mm - 3.75 mm, mostly abraded. Grain to grain contact is dominated by floating type and some of them are point type. Composition of the rock is predominantly red algae and larger forams. Other grain constituents are minor amount of echinoderms, brachiopods, coral debris and indeterminate bioclasts. The porosity type is predominantly mouldic and interparticle pores which are mostly filled by calcite cement type. Detail petrography analysis as shown in Fig. 2 reveals a packstone mainly consist of red algae (D-I, 6-7; A-C, 6-9) and some of large forams (A-F, 3-4; C-I, 4-5).
Bioclastic Grainstone
In general the rock type is light grey in colour, common grainstone texture. The rock shows grains -supported fabric, grain size range from 0.8mm – 3.2mm, moderate sortation, abraded and point type grains as shown in Fig. 3. Petrog raphy analysis as presented in Fig. 4 reveal that the main composition of this carbonate is indeterminate bioclasts grains / fragments that is underwent neoformism diagenetic changed into calcitesparite and micrite. Other components are mollusk fragments and benthic forams. Petrography analysis reveals that the forming of calcite-sparite and micrite due to neomorfism diagenetic process (A-D, 4-9; G-M, 1-9; photo A). Calcite
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International Journal of Engineering & Technology IJET-IJENS Vol:09 No:10 69 cement (G-M, 1-5; photo B) and calcite-sparite of carbonate mud (A-M, 1-9; Foto B) are present in the rocks as pore filling of intercrystalline pore types.
3.1.5 Mollusc Corraline Rudstone
This rock type is dark grey in colour, has grain -supported fabric, moderate sortation, grain size range from 1.8mm – 6.2mm
, mostly abraded and point type grains. The main composition of this rock is molusc and corral fragments and benthic forams.
Other components are forams and undetermined bioclasts.
Calcite-sparite and micrite of carbonate mud distributed in the rock as pore filling of vuggy and intercrystalline porosity are formed by recrystallization process.
3.1.6 Corraline Rudstone
In general this rock type is dark grey in colour, has grain supporting fabric, grain size range from 0.14mm – 5.71mm, poor to moderate sortation and point type grain, and dominantly the grains were abraded. Other components are brachiopods, red algae, and benthic forams. Porosity is dominated by mouldic and vuggy pore types. Some of them filled by carbonate mud and grain that are underwent micritization process.
3.1.7
Red Algae Floatstone
This rock type is dark grey in colour, has grain size ranging from 0.5mm – 11.42mm, poor sortation, and dominantly the grains were abraded and floating in the mud carbonate, predominantly consists of red algae fragments and undetermined bioclastics. Other components are micritization of forams. Carbonate mud and calcite sparite are underwent micritization and fill some of the porosity that is dominated by mouldic and intercrystalline pore type.
3.2 Correlation between Porosity and
Permeability
Correlation between porosity and permeability as shown in Fig.
5 show that porosity is directly proportional to the permeability. The increasing of porosity results in increasing permeability. All of the carbonate samples studied show heterogeneity in porosity and permeability related to the preburial factors of depositional texture and diagenesis process, including the compaction and creation of mouldic or vuggy porosity by leaching [5].
3.3 The Effect of Pressure on the Acoustic Wave Velocity
Fig. 6 shows the effect of overburden pressure to the compressional wave velocity. Generally, the velocity increases with increasing pressure. From the graph it can be analyzed that velocity drastically increases with pressure (3650 m/s to
3900 m/s) in the low pressure range (50 bar to 200 bar), because the thinnest pores close at low pressures and the compacted rocks will have higher acoustic velocity. Further increasing of pressure in the higher pressure range has less effect on the velocities because cracks may have already been closed [4].
The effect of overburden pressure to the shear wave velocity is relatively similar to compressional wave velocity. Fig. 7 demonstrates shear velocity drastically increases (1840 m/s to
1940 m/s) even in the lower pres sure range (50 bar to 200 bar).
At higher
3.4 The Influence of Pressure and Temperature on Acoustic
Wave Velocity
Fig. 8 shows that Vp increases range from 3660 m/s to 4100 m/s, and Vs also increases slowly with range from 1840 m/s to
2020 m/s when overburden pressure increase from 50 Bar to
460 Bar. The overburden pressure causes compaction, porosity reduction and increasing in velocity. This performance is slightly changed when temperature increase from 28.47
o
C to
57.10 o
C, generally both Vp and Vs value become lower. The Vp increases with range from 3480 m/s to 3820 m/s and Vs values range from 1780 m/s to 1950 m/s (Fig. 9).
Fig. 10 shows that when pore pressure increase from
40 Bar to 400 Bar, Vp decreased with range from 3950 m/s to
3600 m/s, and Vs also decreased slowly with range from 2015 m/s to 1850 m/s. This behavior changed when temperature increase from 28.47
o
C to 57.10 o
C. Generally both Vp and Vs value become lower. The Vp decreased with range from 3910 m/s to 3480 m/s and Vs values slightly decreased from 1965 m/s to 1780 m/s (Fig. 11).
3.5 The Effect of Porosity and Permeability to the Acoustic
Wave Velocity
Velocity is strongly dependent on the rock-porosity [4] .
A plot of porosity versus compressional wave velocity (Vp), as shown in Fig. 12 displays a clear inverse trend; an increase in porosity from (5% to 20%) will resulting a decrease in velocity from 4500m/s to 2000m/s. Increasing porosity will create a mount of pore space that cause slow of acoustic velocity [9].
For the shear wave velocity (Vs), as illustrated in Fig. 13 also demonstrated a clear inverse trend; an increase in porosity (5% to 20%) will resulting a decrease in velocity (2300m/s to
1000m/s).
The same phenomenon also occurs in the correlation between permeability and acoustic wave velocity. Fig. 14 shows an increase in permeability (1.8mD to 10.2mD) will cause a decrease in velocity from 4600m /s to 2000m /s. For the shear wave velocity (Vs ), as illustrated in Fig. 15 also demonstrated a clear inverse trend; an increase in permeability (5% to 20%) will caused a decrease in velocity from 2300mD to 1000mD.
C ONCLUSION
The porosity and permeability are the main factor in determining acoustic wave velocity in carbonate rocks. An increase in porosity and permeability will decrease in velocity both compressional and shear waves. Velocity is also strong influenced by pressure and temperature. Increasing overburden pressure will result in increasing of velocity, on the other hand increasing of pore pressure produce decreasing of velocity and increasing temperature will also resulting in decreasing of velocity.
A CKNOWLEDGEM ENT
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R EFERENCES
[1] M. E. T ucker, V. P. Wright, and J. A. D. Dickson, Carbonate
Sedim entology , Blackwell Science, UK, 2001.
[2] W. Dong, A. T ura, and G. Sparkman, An introduction - Carbonate geophysics. T he Leading Edge Journal. Society of Exploration
Geophysics (SEG), 2003, pp. 637 – 638.
[3] J. L. Wilson, Carbonate Facies in Geologic History , Springer-Verlag,
Newyork, 1975, pp. 471.
[4] Z. Wang, and A. Nur, Aspects of Rockphysics in Seism ic Reservoir
Surveillance , Reservoir Geophysics, Edited by Robert E. Sheriff,
SEG, T ulsa, Oklahoma, 1992.
[5] F. S. Anselmetti, and G. P. Eberli, Rocks and Rock Properties:
Control on Sonic Velocity in Carbonates.
T he Leading Edge
Journal. Society of Exploration Geophysics (SEG), 1993 .
[6] S. A. Mobarak, and W. H. Somerton, The Effect of Tem perature and
Pressure on Wave Velocities in Porous Rock , Fall Meeting
Alternate, SPE 3571, 1971.
[7] G. P. Eberli, G. T . Baechile, F. S. Anselmetti, and M. L. Incze,
Factors Controlling Elastic Properties in Carbonate Sedim ents and
Raocks , T he Leading Edge, 2003, pp. 654-660.
[8] R. Punturo, H. Kem, R. Cirrincione, P. Mazzoleni, A. Pezzino, P- and S-wave velocities and densities in silicate and calcite rocks from the Peloritani Mountain, Sicily (Italy): The effect of pressure, tem perature and the direction of wave propagation , Journal of
T ectonophysics, Vol. 409, Elsevier, 2005, pp. 55 -72.
[9] J. H. Schon, Physical Properties of Rocks: Fundam entals and
Principles of Petrophysic, Vol. 18, Elsevier Science Ltd. Oxford.
U.K, 1996.
A
B
International Journal of Engineering & Technology IJET-IJENS Vol:09 No:10 70
Fig. 4. Petrography analysis of bioclastic grainstone.
12
10
8
6
4
2
0
0 y = 0.5661x - 2.3622
R
2
= 0.8342
5 10 15 por vs. perm
Linear (por vs. perm)
20 25
Porosity (%)
Fig. 5. Correlation between porosity and permeability.
4200
4100
4000
3900
3800
3700
3600
3500
0 y = 221.59Ln(x) + 2719.2
R
2
= 0.9092
100 200 300
Vp (m/s)
Log. (Vp (m/s))
400
Overburden pressure (bar)
Fig. 6. Effect of overburden pressure to the compressional wave
2050 velocity.
2000
1950
1900
1850
1800
0 y = 82.469Ln(x) + 1498.7
R
2
= 0.9591
100 200 300
Vs (m/s)
Log. (Vs (m/s))
400
Overburden pressure (bar)
Fig. 7. Effect of overburden pressure to the shear wave velocity.
500
500
4200
4100
4000
3900
3800
3700
3600
3500
3400
0 100 200 300
Vp at 28.47 degC
Vp at 57.10 degC
400 500
Overburden pressure (bar)
Fig. 8. T he effect of overburden pressure to compressional wave velocity in different temperatures.
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International Journal of Engineering & Technology IJET-IJENS Vol:09 No:10 71
2050
2000
1950
1900
1850
1800
1750
Vs at 28.47 degC
Vs at 57.10 degC
0 100 200 300 400 500
Fig. 9. T he effect of overburden pressure to shear wave velocity (Vs) in different temperatures.
4000
3900
3800
3700
3600
3500
3400
0
Vp at 28.47 degC
Vp at 57.10 degC
100 200 300 400 500
Fig. 10. T he effect of pore pressure to compressional wave velocity in different temperatures.
2050
2000
1950
1900
1850
Vs at 28.47 degC
1800
Vs at 57.10 degC
1750
0 100 200 300 400 500
Fig. 11. T he effect of pore pressure to shear wave velocity in different temperatures.
5250
4500
3750
3000
2250
1500
750 y = -147.15x + 5400.5
R
2
= 0.9126
Vp (m/s)
Linear (Vp (m/s))
0
0 5 10 15 20
Porosity (%)
Fig. 12. Cross plot between porosity and compressional wave velocity
2500
2000
1500
1000
500 y = -61.423x + 2584.7
R
2
= 0.7969
Vs (m/s)
Linear (Vs (m/s))
0
0 5 10 15
Porosity (%)
20 25
Fig. 13. Cross plot between porosity and shear wave velocity.
5000
4000
3000
2000
1000 y = -221.25x + 4525.9
R
2
= 0.7104
.
Vp (m/s)
Linear (Vp (m/s))
0
0 2 4 6 8
Permeability (mD)
10 12
25
Fig. 14. Correlation between permeability and compressional wave velocity.
2500
2000
1500
1000
500 y = -105.25x + 2288.8
R
2
= 0.8057
Vs (m/s)
Linear (Vs (m/s))
0
0 2 4 6 8 10 12
Permeability (mD)
Fig. 15. Correlation between permeability and shear wave velocity.
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