Quantitative pore/rock type parameters in carbonates and their relationship to velocity deviations

Two significant questions in carbonate petrophysics are: (1) which objective, quantifiable, and parameterizable aspects of thin section pore geometry best explain commonly observed scatter in carbonate velocity-porosity cross plots; and (2) what is the importance of geometrical characteristics of the pore space on acoustic velocity relative to other physical properties? In this dissertation, quantitative description of the pore geometries of carbonate rocks and measurements of acoustic properties are used to develop an empirical link between general qualitative geological descriptions and a rigorous theoretical rock physics model.Comparison of petrophysical attributes of carbonate core plugs, geologic description, and quantitative geometric parameters derived from thin sections using digital image analysis demonstrate that: (1) geometric information captured by geological descriptions of pore type in carbonates explains aspects of scatter in velocity-porosity cross-plots; (2) four quantitative geometrical parameters that capture pore size, pore surface roughness, aspect-ratio, and pore network complexity statistically capture characteristics that quantitatively differentiate thin sections from each other; (3) two of these parameters (perimeter-over-area and dominate pore size) are related to the deviation of acoustic velocity from Wyllie\u27s time average equation (R 2 = 0.65 and R2 = 0.62 respectively); (4) comparing geometric characteristics and laboratory measurements of velocity and porosity with Extended Biot Theory (EBT) frame flexibility factors confirms the validity of EBT; (5) the two geometrical parameters (2D specific surface and dominate size) are strongly correlated to geometrical frame flexibility parameters derived from acoustic measurements ( R2 = 0.68 and R2 = 0.60 respectively); and (6) internal rock geometry influences acoustic velocity most strongly in high porosity carbonate rocks.The findings from this study imply that: (1) estimating porosity from acoustic data can be substantially improved by incorporating information on quantitative pore space geometry; and (2) in cases where good estimates of porosity are available (e.g. from well-log data or seismic with extensive well control) quantitative pore geometric characteristics can be estimated directly from acoustic data. These quantitative geometric characteristics of the pore space can be used to estimate permeability indirectly from acoustic data

Complex resistivity spectra and pore geometry for predictions of reservoir properties in carbonate rocks

Measurements of complex resistivity spectra (CRS) are performed on core plugs from a wide variety of carbonates in a log sweep from 0.1 to 100,000Hz inside a pressurized chamber at varying effective pressures up to 20MPa. We quantify the CRS curves by extracting the slope of both real and imaginary part of complex resistivity in the 10,000–100,000Hz range. Pore geometries are quantified with thin-section digital image analysis (DIA) from optical light microscopy. The dataset includes 330 carbonate core plug samples from twelve different study areas and hence includes a highly diverse range of carbonate rock types. This should make our results applicable to most carbonate rocks. Pore geometry parameters derived from DIA, such as Dominant Pore Size (DOMsize) and Perimeter over Area (PoA), correlate well with petrophysical properties such as cementation factor and permeability. However, when modeling those properties, higher correlation coefficients are achieved with CRS than with DIA parameters. Using CRS and model constants tuned to the sub-datasets, cementation exponents are predicted with R2=0.91 and permeability with R2=0.84. The correlation coefficient of a universal equation for all 330 samples is still high for cementation factors with R2=0.80, but less for permeability with R2=0.48. The results show that CRS in carbonates are directly related to permeability and formation factors, and greatly improve reservoir property estimates. This study also highlights the usability of low-frequency CRS data as a measure of flow and storage properties in carbonate rocks. The transfer of this methodology to wireline applications would result in more accurate and continuous permeability and cementation factor predictions from well logs. •Heterogeneous carbonate pore structure controls electrical and fluid flow properties.•Complex resistivity spectra (CRS) are directly linked to carbonate pore structure.•Permeability and cementation factors can be predicted from CRS.•Relationships established on core plugs can be transferred to wireline application.•Power law pore size distribution indicates fractal scaling in carbonate rocks

Indigenous microbial communities as catalysts for early marine cements: An in vitro study

Abstract Early marine cementation is a fundamental process for many characteristics of carbonates, like the stabilisation of steep slopes. The genesis of early cements is often attributed to physicochemical processes but there is evidence for microbial mediation. To elucidate the role of microbes and associated organic material, in vitro experiments were undertaken in the presence and absence of indigenous microbiota in ooids from Schooner Cays, Bahamas and compared with native grapestones from Joulter Cays, Bahamas. Microscopic examinations by stereomicroscopy, scanning electron microscopy and thin section analysis of in vitro incubations with native flora document rapid grain fusion, resulting in the formation of grapestones within 30–60 days. The initial binding of the grains is primarily facilitated by exudates of extracellular polymeric substances and microbial communities acting as catalysts in the formation of micritic bridges, cements and encrusted aggregates. In vitro grapestones are similar to native grapestones from Joulter Cays with intergranular areas infested with extracellular polymeric substances, microbes, micritic cements, amorphous calcium carbonate nanograins and micritised outer surfaces. These similarities suggest that incubations with native flora follow similar mineralisation mechanisms as in the natural environment. In contrast, sterilised grains remain loose with little crystal formation after 60 days and are devoid of microbes and organic exudates. Owing to the near absence of precipitates, abiotic precipitation is not the driving force promoting early cements. In contrast, grain fusion is microbially mediated via both a passive mechanism, where extracellular polymeric substances and cell surfaces function as templates for crystal nucleation and generation of micritic cements, and through an active mechanism by which biofilm heterotrophs and autotrophs induce chemical alterations of a local environment, facilitating precipitation. This study underscores that microbially mediated cementation can occur at fast rates and that firmground to hardgrounds and slope stabilisation take place shortly after deposition of carbonate grains

Complex resistivity spectra for estimating permeability in dolomites from the Mississippian Madison Formation, Wyoming

Dolomite rocks constitute many important reservoirs due to the porosity-preserving and connectivity-enhancing effects of dolomitization. This study explores the correlation between resistivity and pore structure in dolomite rocks from the Mississippian Madison Formation in Wyoming and proposes a novel approach for predicting permeability from complex resistivity spectra (CRS). Digital image analysis (DIA) on thin-sections is used to quantify the pore structure of 54 sucrosic dolomite samples. Pore-structure parameters derived from DIA are correlated to resistivity values of each plug and show that larger and simpler pore networks result in higher cementation factors. Analyses of CRS are performed on brine-saturated core plugs in a log sweep from 0.1– 100,000 Hz with a four-electrode setup at varying confining pressures. Results show that the frequency dispersion of CRS between 10 and 100 kHz is directly related to the porosity in these dolomites. The phase shift of CRS shows high variance in both low and high porosity samples with characteristic slopes βPhase and βAmplitude for frequencies between 10 and 100 kHz. Modifying an empirical model of Tong & Tao (2008) to include porosity and both βPhase and βAmplitude can predict permeability with a correlation coefficient of R2 = 0.84. •kHz-range complex resistivity spectra can be used to predict permeability.•Small and intricate pore networks facilitate electrical flow in dolomites.•Small pressure dependency of measurements suggests core data in dolomites is representative of reservoir conditions

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

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