Gregor T. Baechle

Factors controlling elastic properties in carbonate sediments and rocks

Carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks. In particular, cementation and dissolution processes continuously modify the pore structure to create or destroy porosity. In extreme cases these modifications can completely change the mineralogy from aragonite/calcite to dolomite, or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock (Figure 1). All these modifications alter the elastic properties of the rock and, therefore, the sonic velocity. The result is a dynamic relationship among diagenesis, porosity, pore-type, and sonic velocity. The result is a wide range of sonic velocity in carbonates, in which compressional-wave velocity (VP) ranges from 1700 to 6600 m/s and shear-wave velocity (VS) from 600 to 3500 m/s.

Quantification of pore structure and its effect on sonic velocity and permeability in carbonates

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 rocks 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.

Changes of shear moduli in carbonate rocks: Implications for Gassmann applicability

In laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions. In particular, these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation. This change in shear modulus puts Gassmanns assumption of a constant shear modulus into question and also explains why velocities predicted with the Gassmann equation can be lower or higher than measured velocities.

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 500 m∕s higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity ( VS ) generally decreases, but nine samples show an increase of up to 230 m∕s . Water saturation decreases the shear modulus by up to 2 GPa in some samples and increases it by up to 3 GPa in others. The average increase in the shear modulus with water saturation is 1.23 GPa ; the average decrease is 0.75GPa . The VP ∕ VS ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have disti...

Estimating permeability of carbonate rocks from porosity and vp ∕ vs

We present a method for predicting permeability from sonic and density data. The method removes the porosity effect on the ratio vp ∕ vs of dry rock, and it addresses the specific surface as an indirect measure of permeability. We look at ultrasonic data, porosity, and the permeability of 114 carbonate core plugs. In doing so, we establish an empirical relationship between the specific surface of the solid phase (as calculated by Kozeny’s equation) and vp ∕ vs (linearly transformed to remove the porosity effect). One must view the specific surface derived by using Kozeny’s equation as an effective specific surface because Kozeny’s equation only holds for homogeneous rock with interconnected pores. The ratio vp ∕ vs of dry rocks, on the other hand, seems to be controlled by the true specific surface, pointing to an inherent limitation in the method. The 114 carbonate plugs originate in three geological settings and comprise 83 calcitic and 31 dolomitic samples. Their depositional texture varies from mud-do...

Modeling Velocity In Carbonates Using a Dual Porosity DEM Model

The differential effective medium theory is used to model the velocity of carbonates with two predefined end-member pore types and under dry and water saturated conditions. The dual porosity DEM takes into account input parameters derived from digital image analysis of thin sections. In particular the respective amount of microporosity and macroporosity and the aspect ratio of the macropores are incorporated. A conceptual aspect ratio of 0.1 for micropores and a measured aspect ratio of 0.5 for macropores is used as input parameters for the differential effective medium (DEM) model. The model predicts that the compliant micropores have a strong influence on the sonic velocity of porous carbonates because increasing concentrations of micropores reduce the rock stiffness. The model values are compared to high frequency (1MHz) laboratory velocity measurements. These velocity predictions with the dual porosity DEM model show significant better velocity prediction than empirical models, e.g. the Wyllie times average equation. We obtain a rootmean-square-error of 392 m/s when comparing predicted with measured velocity values. Our results also show that a differential effective medium model that uses measured input parameters from quantitative digital image analysis improves estimates of acoustic properties of carbonates.

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.

Effects of Pore Structure On 4D Seismic Signals In Carbonate Reservoirs

Carbonate reservoir rocks of different pore structure have quite distinct and detectable 4D seismic signatures. Using three defined pore structure types (PST), these seismic signatures are correlated with the permeability patterns of carbonate rocks. In both the high-permeability PST3 and intermediate-permeability PST2 regions, gas injection is preferred than water injection for effective 4D seismic monitoring. In the low-permeability PST1 region where lies most of the bypassed oil, both gas and water injection options result in similar 4D seismic signal magnitude. Fluid saturation changes under in-situ reservoir conditions can be better detected by offset domain 4D seismic analysis.

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 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.