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Ida Lykke Fabricius - One of the best experts on this subject based on the ideXlab platform.
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Porosity and Sonic Velocity depth trends of Eocene chalk in Atlantic Ocean: Influence of effective stress and temperature
Journal of Petroleum Science and Engineering, 2014Co-Authors: Ahmed Awadalkarim, Ida Lykke FabriciusAbstract:Abstract We aimed to relate changes in porosity and Sonic Velocity data, measured on water-saturated Eocene chalks from 36 Ocean Drilling Program drill sites in the Atlantic Ocean, to vertical effective stress and thermal maturity. We considered only chalk of Eocene age to avoid possible influence of geological age on chalk compaction trends. For each depth, vertical effective stresses as defined by Terzaghi and by Biot were calculated. We used bottom-hole temperature data to calculate the time–temperature index of thermal maturity (TTI) as defined by Lopatin. Porosity and compressional wave Velocity data were correlated to vertical effective stresses and to TTI. Our porosity data showed a broader porosity trend in the mechanical compaction zone, and the onset of the formation of limestone at a shallower burial depth than the porosity data of the Ontong Java Plateau chalk show. Our porosity data do not show or at least it is difficult to define a clear pore-stiffening contact cementation trend as the Ontong Java Plateau chalk. Mechanical compaction is the principal cause of porosity reduction (at shallow depths) in the studied Eocene chalk, at least down to about 5 MPa Terzaghi׳s effective stress corresponding to a porosity of about 35%. This indicates that mechanical compaction is the principal agent of porosity reduction. Conversely, at deeper levels, porosity reduction is accompanied by a large increase in Sonic Velocity indicating pore-filling cementation. These deep changes are correlated with TTI. This indicates pore-filling cementation via an activation energy mechanism. We proposed a predictive equation for porosity reduction with burial stress. This equation is relevant for basin analysis and hydrocarbon exploration to predict porosity if Sonic Velocity data for subsurface chalk is available.
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Pore Radius and Permeability Prediction from Sonic Velocity
3rd EAGE Shale Workshop - Shale Physics and Shale Chemistry, 2012Co-Authors: Ernest Ncha Mbia, Ida Lykke FabriciusAbstract:Several authors have predicted permeability of shales either through laboratory measurements and or from field data using various empirical relations. A critical literature review by Mondol et al., (2008) on available permeability models, concluded that none of the existing models is ideal and all need to be calibrated and validated through a much larger permeability database of well-characterized mudstones. His results on smectite and kaolinite aggregates suggest that the permeability of smectitic clays may be up to five orders of magnitude lower than that of kaolinitic clays with the same porosity, density, Velocity or rock mechanical properties. Mari et al., (2011) described a methodology for obtaining a permeability log based on acoustic velocities Vp and Vs, porosity φ, P-wave attenuation and frequency, their calculation of the specific surface S of the formation was based on the relationship between porosity φ, Vp/Vs and S proposed by Fabricius et al. (2007). Fabricius (2011) indicate that pore radius and thus permeability of shale in the depth interval of mechanical compaction may be predicted from porosity and Sonic Velocity. In this work we are presenting the empirical equations developed from experimental data that can be used to predict pore radius and permeability of shale from Sonic Velocity data measured in the field.
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Chalk porosity and Sonic Velocity versus burial depth: Influence of fluid pressure, hydrocarbons, and mineralogy
AAPG Bulletin, 2008Co-Authors: Ida Lykke Fabricius, Lars Gommesen, Anette Krogsbøll, D. OlsenAbstract:Seventy chalk samples from four formations in the overpressured Danish central North Sea have been analyzed to investigate how correlations of porosity and Sonic Velocity with burial depth are affected by varying mineralogy, fluid pressure, and early introduction of petroleum. The results show that porosity and Sonic Velocity follow the most consistent depth trends when fluid pressure and pore-volume compressibility are considered. Quartz content up to 10% has no marked effect, but more than 5% clay causes lower porosity and Velocity. The mineralogical effect differs between P-wave and shear Velocity so that smectite-bearing chalk has a high Poisson's ratio in the water-saturated case, but a low value in the dry case. Oil-bearing chalk has up to 25 units higher porosity than water-saturated chalk at similar depth but similar Velocity, probably because hydrocarbons prevent pore-filling cementation but not pore-structure stiffening cementation in this presumably water-wet chalk. These results should improve the modeling of chalk background Velocity for seismic inversion analysis. When describing the porosity-reducing process, pore-volume compressibility should probably be disregarded when correcting for fluid pressure because the cementing ions originate from stylolites, which are mechanically similar to fractures. We find that cementation occurs over a relatively short depth interval.
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how burial diagenesis of chalk sediments controls Sonic Velocity and porosity
AAPG Bulletin, 2003Co-Authors: Ida Lykke FabriciusAbstract:Based on P-wave Velocity and density data, a new elastic model for chalk sediments is established. The model allows the construction of a series of isoframe (IF) curves, each representing a constant part of the mineral phase contributing to the solid frame.The IF curves can be related to the progress of burial diagenesis of chalk, which is revised as follows:Newly deposited carbonate ooze and mixed sediments range in porosity from 60 to 80%, depending on the prevalence of hollow microfossils. Despite the high porosity, these sediments are not in suspension, as reflected in IFs of 0.1 or higher.Upon burial, the sediments lose porosity by mechanical compaction, and concurrently, the calcite particles recrystallize into progressively more equant shapes. High compaction rates may keep the particles in relative motion, whereas low compaction rates allow the formation of contact cement, whereby IF increases and chalk forms. Rock mechanical tests show that when compaction requires more than in-situ stress, porosity reduction is arrested.During subsequent burial, crystals and pores grow in size as a consequence of the continuing recrystallization. The lack of porosity loss during this process testifies to the absence of chemical compaction by calcite-calcite pressure dissolution, as well as to the porosity-preserving effect of contact cementation.At sufficient burial stress, the presence of stylolites indicates that pressure dissolution takes place between calcite and silicates, and depending on pore-water chemistry and temperature, pore-filling cementation may occur over a relatively short depth interval. Limestone and mixed sedimentary rock form, and porosity may be reduced to less than 20%. Isoframe increases to more than 0.6.In hydrocarbon reservoirs in North Sea chalk, relatively high porosity and high IFs are found. The reason may be that recrystallization and porosity-preserving contact cementation progress, whereas pore-filling cementation is small, probably because pressure dissolution along stylolites is arrested. Pressure dissolution may be arrested for two reasons: (1) the introduction of hydrocarbons causes a fall in effective burial stress, and (2) adsorption of polar hydrocarbons on the silicates may shield calcite from the silicates.
B. Tutmez - One of the best experts on this subject based on the ideXlab platform.
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Fuzzy and Multiple Regression Modelling for Evaluation of Intact Rock Strength Based on Point Load, Schmidt Hammer and Sonic Velocity
Rock Mechanics and Rock Engineering, 2006Co-Authors: M. Karakus, B. TutmezAbstract:Uniaxial Compressive Strength (UCS), considered to be one of the most useful rock properties for mining and civil engineering applications, has been estimated from some index test results by fuzzy and multiple regression modelling. Laboratory investigations including Uniaxial Compressive Strength (UCS), Point Load Index test (PL), Schmidt Hammer Hardness test (SHR) and Sonic Velocity (V_p) test have been carried out on nine different rock types yielding to 305 tested specimens in total. Average values along with the standard deviations (Stdev) as well as Coefficients of variation (CoV) have been calculated for each rock type. Having constructed the Mamdani Fuzzy algorithm, UCS of intact rock samples was then predicted using a data driven fuzzy model. The predicted values derived from fuzzy model were compared with multi-linear statistical model. Comparison proved that the best model predictions have been achieved by fuzzy modelling in contrast to multi-linear statistical modelling. As a result, the developed fuzzy model based on point load, Schmidt hammer and Sonic Velocity can be used as a tool to predict UCS of intact rocks.
Gregor P. Eberli - One of the best experts on this subject based on the ideXlab platform.
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Effects of porestructure on Sonic Velocity in carbonates
Seg Technical Program Expanded Abstracts, 2012Co-Authors: Ralf J. Weger, Gregor T. Baechle, Jose Luis Masaferro, Gregor P. EberliAbstract:The presence of round pores generally causes a positive deviation from Wyllie’s equation (Anselmetti and Eberli 1993, 1999; Saleh and Castagna 2004). However, carbonates contain a large number of different pore types that are not related roundness alone. Three quantitative pore shape parameters derived from digital image analysis are introduced to capture the complicated pore structure of carbonates with the goal to enhance porosity prediction from Velocity. The first parameter that describes the roundness of the pores was first introduced by Anslemetti et al. (1998) and called γ. The second parameter Perimeterover-Area (PoA) captures the overall tortuosity of the pores system. The third parameter, Dominant Poresize, is a measure of the dominant pore size within the thin section. Out of these three parameters, PoA is the most dominant factor controlling Velocity at a given porosity with Dominant Poresize being second, while roundness alone is the least important factor of the three. We conclude that the roundness of individual pores is not as relevant as the simplicity of the pore system, i.e, the pore system with low tortuosity. Combining all three parameters and porosity in a multivariate linear regression increases correlation to Velocity from R of 0.49 (porosity alone) to R of 0.78.
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Quantification of pore structure and its effect on Sonic Velocity and permeability in carbonates
AAPG Bulletin, 2009Co-Authors: Ralf J. Weger, Gregor P. Eberli, Gregor T. Baechle, Jose Luis Massaferro, Yuefeng SunAbstract:Carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rock's physical properties. We introduce a digital image analysis (DIA) method that produces quantitative pore-space parameters, which can be linked to physical properties in carbonates, in particular Sonic Velocity and permeability. The DIA parameters, derived from thin sections, capture two-dimensional pore size (DomSize), roundness (), aspect ratio (AR), and pore network complexity (PoA). Comparing these DIA parameters to porosity, permeability, and P-wave Velocity shows that, in addition to porosity, the combined effect of microporosity, the pore network complexity, and pore size of the macropores is most influential for the acoustic behavior. Combining these parameters with porosity improves the coefficient of determination (R2) Velocity estimates from 0.542 to 0.840. The analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic Velocity at a given porosity than samples with small, complicated pores. Estimates of permeability from porosity alone are very ineffective (R2 = 0.143) but can be improved when pore geometry information PoA (R2 = 0.415) and DomSize (R2 = 0.383) are incorporated. Furthermore, results from the correlation of DIA parameters to acoustic data reveal that (1) intergrain and/or intercrystalline and separate-vug porosity cannot always be separated using Sonic logs, (2) P-wave Velocity is not solely controlled by the percentage of spherical porosity, and (3) quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates.
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effects of microporosity on Sonic Velocity in carbonate rocks
Geophysics, 2008Co-Authors: Gregor T. Baechle, Gregor P. Eberli, Arnout Colpaert, Ralf J. WegerAbstract:The elastic moduli of a rock are affected by three main factors: pore fluid, rock framework, and pore space. In carbonate rocks, the latter two factors are a function of the depositional environment and the diagenetic history. Cementation, recrystallization, and dissolution processes can change the mineralogy and texture of the original framework and thereby alter the original grain-to-grain contacts and/or occlude pore space. Dissolution processes can enlarge interparticle pore space or dissolve grains entirely, thereby increasing porosity. These diagenetic alterations and associated changes in the rock frame and pore structure result in a wide Velocity range at a given porosity.
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The effects of rock texture and pore type on Sonic Velocity in dolomite
GEO 2008, 2008Co-Authors: Shouwen Shen, Gregor P. EberliAbstract:In order to assess the controlling parameters for Velocity in dolomites, the Sonic velocities of 129 dolomite samples with different porosities were measured and their petrologic textures were determined using microscopy. The results revealed that Sonic Velocity is a function not only of total porosity but also of the pore type and rock texture. The measured velocities showed an inverse correlation with porosity, but departures from the general trends of correlation can be as high as 1,500 meter/second. These deviations can be explained by the occurrence of different crystal shapes, pore type, rock type and crystal size. When crystal shape and pore type were combined to classify the dolomite many relationships became apparent. Seven texture combination types were distinguished in the study samples. Rocks with texture combinations of anhedral and moldic (A M) have relatively high velocities, whereas those with mostly euhedral shapes and inter-crystalline pore types (E I) have relative low velocities. Rock types partially explained the variations of velocities. Generally, grainstones have relatively high velocities, whereas mudstones have relative low ones. Breccias have the lowest velocities. Crystal size itself is very poorly correlated to Sonic Velocity except that large crystal sizes do not have slow velocities. However, if the crystal size of each combination-type is evaluated, the correlation improves. Because total porosity, together with pore type and rock texture, control Sonic Velocity in dolomite, it is possible to predict the Velocity from these parameters. The highlight of this research is an empirical formula that predicts the Velocity of dolomite.
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oomoldic carbonates pore structure and fluid effects on Sonic Velocity
Seg Technical Program Expanded Abstracts, 2008Co-Authors: Gregor T. Baechle, Gregor P. Eberli, Austin Boyd, Jean Marie Degrange, Layaan AlkharusiAbstract:Summary In order to model the effect of oil/gas production or CO2 injection at the seismic scale, we have to understand the effects of pore structure, pressure and fluid changes on Velocity at the laboratory scale. To reach this goal, we measured carbonate rocks with a suite of miscible fluids, simulating the entire range of reservoir fluid moduli from light to heavy oils. In our experiments, compressional Velocity (Vp) and shear wave Velocity (Vs) are simultaneously measured at a frequency of 1MHz and under increasing effective stress from 3 MPa to 30 MPa. We observe large variations in velocities between 3200 m/s and 6500 m/s and a large scatter in the P-wave Velocity – porosity relationship. The P-wave Velocity shows up to 2000m/s difference at a given porosity. The Velocity increases between 250 and 750m/s as pressure incresases from 3 to 30MPa. The bulk of the samples show increasing Vp/Vs ratios with pressurization, up to values between 1.7 and 1.84. The ratio of normalized bulk versus shear modulus ranges from 0.7 to 0.9. Twenty-one oomoldic carbonate samples with nearly spherical pores show a weak correlation between Velocity and porosity under dry conditions. We attribute the weak correlation between Velocity and porosity in rocks with similar pore geometry to variations in inter-crystalline porosity in the rock frame. This finding questions the assumption that spherical pores have a dominant effect on Velocity. Four oomoldic samples were chosen for fluid substitution and saturated “in-situ” with seven different pore fluids. Significant effects of fluid changes on Velocity are observed. A linear correlation exists between bulk modulus and fluid modulus (r 2 > 0.97). In contrast, shear modulus changes correlated with the viscosity of the fluids: the lower the fluid viscosity, the lower the shear modulus. Our results question common hypotheses for modeling pore-structure effects on acoustic properties in carbonates; (a) P-wave Velocity is controlled by the percentage of spherical porosity, and (b) the P-wave Velocity in oomoldic rocks is insensitive to fluid and pressure changes because of high stiffness of the rock frame. These findings imply that one has to be cautious in relating rock-physics model parameters to volumetric dominant pore types.
Peter Leary - One of the best experts on this subject based on the ideXlab platform.
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deep borehole log evidence for fractal distribution of fractures in crystalline rock
Geophysical Journal International, 1991Co-Authors: Peter LearyAbstract:Summary Sonic Velocity and electrical resistivity logs run to a depth of 3.5 km in crystalline rock near the San Andreas fault at Cajon Pass in southern California correlate over scale-lengths both small (sub-metre) and large (tens to hundreds of metres). No such correlations are seen with the more lithologically sensitive natural gamma intensity log. The correlation between the Sonic Velocity and electrical resistivity logs suggests that a non-lithologic property of the crystalline rock controls fluctuations. In situ fracture intensity is a logical candidate for the controlling rock property. The fluctuations of the individual Sonic Velocity and electrical resistivity logs are examined with the Hurst rescaled range parameter over borehole log intervals 1.5 m < L < 1500 m. For log fluctuations arising from a scale-invariant physical process the Hurst rescaled range scales with data interval as LH, 0 < H < 1. A purely random sequence of in situ fractures produces a scaling exponent H= 0.50. Fluctuations in the Cajon Pass Sonic Velocity and electrical resistivity logs yield H˜ 0.70 evidence that in situ fracture sets tend to occur in clusters rather than at purely random intervals. The tendency for fracture clustering over log intervals 1.5 m < L < 1500 m suggests that fracture formation is a fractal process independent of length-scale in which larger fracture intervals form from clustering of numerous smaller fracture intervals. Seismic reflectivity derived from the borehole Sonic Velocity log is also scale independent over the range of data intervals 1.5m < L < 1500 m with a Hurst exponent H= 0.21. If we associate fracture clustering with crustal fault formation, the Cajon Pass borehole Sonic Velocity and electrical resistivity logs predict that crustal faults scale fractally with fractal dimension D˜ 2.30. The equivalent b-value for earthquake size distribution is b˜D/2˜ 1.15. On this hypothesis observed b-values < 1.15 indicate a tendency for earthquakes to cluster on existing (weak?) faults. A scale-independent mechanics for crystalline rock fracture formation in which larger scale fractures form as clusters of smaller scale fractures provides a mechanical link between the small pervasive stress aligned flaws, cracks and microfractures which can impart anisotropy to crystalline rock and the larger scale fractures associated with finite strain and faulting. Thus in crustal regions of active but low strain, fracture anisotropy is observed to be aligned with the inferred maximum principal stress, while in actively faulted crust, fracture anisotropy is observed to be more nearly fault parallel as if the fractures are aligned by the finite strain faulting process.
M. Karakus - One of the best experts on this subject based on the ideXlab platform.
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Fuzzy and Multiple Regression Modelling for Evaluation of Intact Rock Strength Based on Point Load, Schmidt Hammer and Sonic Velocity
Rock Mechanics and Rock Engineering, 2006Co-Authors: M. Karakus, B. TutmezAbstract:Uniaxial Compressive Strength (UCS), considered to be one of the most useful rock properties for mining and civil engineering applications, has been estimated from some index test results by fuzzy and multiple regression modelling. Laboratory investigations including Uniaxial Compressive Strength (UCS), Point Load Index test (PL), Schmidt Hammer Hardness test (SHR) and Sonic Velocity (V_p) test have been carried out on nine different rock types yielding to 305 tested specimens in total. Average values along with the standard deviations (Stdev) as well as Coefficients of variation (CoV) have been calculated for each rock type. Having constructed the Mamdani Fuzzy algorithm, UCS of intact rock samples was then predicted using a data driven fuzzy model. The predicted values derived from fuzzy model were compared with multi-linear statistical model. Comparison proved that the best model predictions have been achieved by fuzzy modelling in contrast to multi-linear statistical modelling. As a result, the developed fuzzy model based on point load, Schmidt hammer and Sonic Velocity can be used as a tool to predict UCS of intact rocks.
