Petrophysic properties of rock and acoustic wave velocity analysis are very important stage in the reservoir characterization and seismic exploration. Meanwhile the 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 to the acoustic wave parameters such as compressional and shear wave velocities. Data were collected to obtain detail relationships between acoustic wave velocity and petrophysical data of the carbonate rocks, such as: petrography analysis, SEM image, detail core description, and laboratory experimental of acoustic wave velocities measurements in variation of overburden pressure and temperature. Some acoustic wave parameters were simulated as close as possible to the reservoirs condition. Based on the petrophysical data and acoustic wave measurement, the total 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 results can be used to get the better seismic analysis performance, correspond to increase hydrocarbon discovery from the carbonate rock in the future.
Key words: petrophysic properties, acoustic wave velocity, carbonate rock.
1. Introduction
Physical properties of rock such as porosity and permeability are very important in the interpretation of geophysical data especially in petroleum industry [1]. Furthermore, rock physics as a highly interdisciplinary field involving geology, geophysics / acoustic, geochemistry, well logging, core analysis, and petroleum engineering. It emphasizes that nearly all-major petroleum companies conduct research in rock physics [2].
Meanwhile, carbonate rock is volumetrically a most significant part of the geological record and possess much of the fossil record of life on this planet. Their deposition involves a more complex suite of processes than many other sediment type [3]. 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 siliciclastic reservoirs [4]. Adding complexity to reservoir quality prediction is that carbonate which producing organism have evolved through time [5, 6].
The objective of this study includes detail relationship among petrophysisc parameters such as porosity and permeability to the acoustic wave parameter, i.e compressional and shear wave velocity.
2. Literature Review
The complexity of porosity in carbonate is the result of many factors, which include the variable dimension of sedimentary carbonate particles, the variability of skeletal pores, partial to total occlusion of pores by internal sediment or cement, creation of secondary pores (fabric selective or fabric independent), and of highly variable dimensions, dolomitization, and recrystallization [7]. However, Choquette and Pray [8] have divided porosity type into three groups that are fabric selective (e.g. intergranular, intragranular, intercrystalline, mouldic, fenestral, shelter, and framework porosity types), not fabric selective (e.g. fracture, channel, vug, cavern, and stylolitic porosity types), and fabric control or not (e.g. breccia, boring, burrow, and shrinkage porosity types).
Most porosity in carbonate rocks is diagenetic in origin, and as result it has some complexity in porosity, which will be controlled by the original facies type and later diagenetic processes [3]. 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 (Vp) and shear wave velocity (Vs).
Anselmetti and Eberli [9] have 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.
3. Method and Data Collection
Twenty one carbonate core samples have been done 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 reservoirs condition
The samples were cleaned in methanol and dried in a vacuum oven at 85oC for period of twenty-four hours and than saturated by brine/formation water with NaCl of 16,271.67 mg/l. 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 28-57oC. 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 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.
4. Rock Characterization
Detail descriptions of the carbonate core samples include rock texture, sedimentary structure, composition and fossil content. By supporting of integrated petrography and Scanning Electron Microscopy (SEM) analysis, it has identified seven carbonate rock type i.e.:
4.1. Bedded Large Forams Grainstone
Large forams grainstone in general is 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 types.
4.2. Cross-Bedded Large Foram
Grainstone
The carbonate rock 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.
4.3. Red Algal Packstone
Red algal packstone to floatstone in general is grey in colour. Minor discontinous thin laminae of detrital clay and carbonaceous materials are present in this rock. The grain size ranges 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.
Bioclastic Grainstone
In general the rock type is light grey in colour, common grainstone texture. The rock shows grains-supported fabric, grain size range 0.8mm - 3.2mm, moderate sortation, abraded and point type grains as shown in Figure 1. Petrography analysis as presented in Figure 2 reveal that the main composition of this carbonate is indeterminate bioclasts grains / fragments that is underwent neoformism diagenetic changed into calcite-sparite 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 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.
Figure 1: Bioclastic Grainstone
A
B
Figure 2: Petrography analysis of the bioclastic grainstone
4.5. Mollusc Corraline Rudstone
This rock type, dark grey in colour, has grain-supported fabric, moderate sortation, grain size range 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.
Corraline Rudstone
In general the rock type is dark grey in colour, has grain supporting fabric, grain size range 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.
Red Algae Floatstone
In general the rock 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. That composition 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 porosity that is dominated by mouldic and intercrystalline pore type.
5. Result and Discussion
5.1 Correlation between porosity and
permeability
Correlation between porosity and permeability as shown in Figure 3 show that porosity is directly proportional to the permeability. The increasing of porosity results in increasing permeability. All of the samples carbonate 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 [9].
Figure 3: Correlation between porosity and permeability.
5.2 The effect of pressure on the acoustic wave velocity
Figure 4 shows the effect of overburden pressure to the compressional wave velocity. Generally, the velocity increase with increasing pressure. From the graph it can be analyzed that velocity increase with pressure very fast (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 pressure in the higher pressure range has less effect on the velocities because cracks may have already been closed [10]. The effect of overburden pressure to the shear wave velocity is relatively similar to compressional wave velocity. Figure 5 demonstrate shear velocity increase very fast (1840 m/s to 1940 m/s) also in the lower pressure range (50 bar to 200 bar). At higher pressure, the velocities are slightly more gradually flat.
Figure 4: Effect of overburden pressure to the compressional wave velocity (Vp) on brine saturated T.8-8 carbonate core.
Figure 5: Effect of overburden pressure to the shear wave velocity (Vs) on brine saturated T.8-8 carbonate core.
5.3 The effect of porosity and permeability to the acoustic wave velocity
Velocity is strongly dependent on the rock-porosity [10, 11]. A plot of porosity versus compressional wave velocity (Vp), as shown in Figure 6 display a clear inverse trend; an increase in porosity from (5% to 20%) produces 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 [1]. For the shear wave velocity (Vs), as illustrated in Figure 7 also demonstrated a clear inverse trend; an increase in porosity (5% to 20%) produces a decrease in velocity (2300m/s to 1000m/s).
The same phenomenon also occurs in the correlation between permeability and acoustic wave velocity. Figure 8 show an increase in permeability (1.8mD to 10.2mD) produces a decrease in velocity from 4600m/s to 2000m/s. For the shear wave velocity (Vs), as illustrated in Figure 9 also demonstrated a clear inverse trend; an increase in permeability (5% to 20%) produces a decrease in velocity from 2300mD to 1000mD.
Figure 6: Cross plot between porosity and compressional wave velocity (Vp)
Figure 7: Cross plot between porosity and shear wave velocity (Vs)
Figure 8: Correlation between permeability and compressional wave velocity (Vp)
Figure 9: Correlation between permeability and shear wave velocity (Vs)
6. Conclusions
Based on the research we conclude that porosity and permeability are the main factor in determining acoustic wave velocity in carbonate rocks. An increase porosity and permeability produces a decrease in velocity both compressional and shear waves. In contrast, the increasing of overburden pressure results in increasing compressional and shear wave velocities.
Acknowledgments
We wish to thank Pertamina for their permission to publish these data and Laboratory of Wave Inversion and Subsurface Fluid Imaging Research, Institut Teknologi Bandung for seiscore analysis, and we also extend our thanks to IRPA Malaysia.
