15 research outputs found
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The role of macroporosity and microporosity in constraining uncertainties and in relating velocity to permeability in carbonate rocks
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Modeling velocity in carbonates using a dual-porosity DEM model
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Effects of microporosity on sonic velocity in carbonate rocks
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The Role of Macroporosity And Microporosity In Constraining Uncertainties And In Relating Velocity to Permeability In Carbonate Rocks
Velocity - porosity transforms and porosity - permeability transforms are frequently used for upscaling of rock properties from core and log scale to reservoir scale. Carbonate rocks often show a large scatter in the relationship between porosity and permeability. Hence further analyses are required in order to better predict permeability, which would result in more accurate reservoir modeling, and better reserve predictions. Incorporating image analysis enables us to reduce the uncertainties present in velocity and permeability scatter. Obtaining the microporosity by subtracting the macroporosity from the plug porosity leads to a better correlation with velocity than total porosity. The trend follows the Wyllie time average trend line. The deviation between measured total porosity and microporosity, the image macroporosity, is an excellent indicator of permeability in our dataset. Using the image macroporosity versus permeability trend reduces the uncertainty of permeability prediction by more than one order of magnitude. In our case, the microporosity is the dominant ineffective porosity for fluid flow. This reduction in uncertainty allows for better reservoir prediction and development
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Changes in dynamic shear moduli of carbonate rocks with fluid substitution
To assess saturation effects on acoustic properties in carbonates, we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5% to 30% under dry and water-saturated conditions. Complete saturation of the pore space with water causes an increase and decrease in compressional- and shear-wave velocity as well as significant changes in the shear moduli. Compressional velocities of most water-saturated samples are up to [Formula: see text] higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity [Formula: see text] generally decreases, but nine samples show an increase of up to [Formula: see text]. Water saturation decreases the shear modulus by up to [Formula: see text] in some samples and increases it by up to [Formula: see text] in others. The average increase in the shear modulus with water saturation is [Formula: see text]; the average decrease is [Formula: see text]. The [Formula: see text] ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have distinctly higher [Formula: see text] ratios. Grainstone samples with high amounts of microporosity and interparticle macro-pores preferentially show shear weakening, whereas recrystallized limestones are prone to increase shear strengths with water saturation. The observed shear weakening indicates that a rock-fluid interaction occurs with water saturation, which violates one of the assumptions in Gassmann’s theory. We find a positive correlation between changes in shear modulus and the inability of Gassmann’s theory to predict velocities of water-saturated samples at high frequencies. Velocities of water-saturated samples predicted by Gassmann’s equation often exceed measured values by as much as [Formula: see text] for samples exhibiting shear weakening. In samples showing shear strengthening, Gassmann-predicted velocity values are as much as [Formula: see text] lower than measured values. In 66% of samples, Gassmann-predicted velocities show a misfit to measured water-saturated P-wave velocities. This discrepancy between measured and Gassmann-predicted velocity is not caused solely by velocity dispersion but also by rock-fluid interaction related to the pore structure of carbonates. Thus, a pore analysis should be conducted to assess shear-moduli changes and the resultant uncertainty for amplitude variation with offset analyses and velocity prediction using Gassmann’s theory
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Effects of Porestructure On Sonic Velocity In Carbonates
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 Perimeter-over-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 2 of 0.49 (porosity alone) to R 2 of 0.78
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Factors controlling elastic properties in carbonate sediments and rocks
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