Carbonate Pore Research Articles

Characterization of interfacial interactions and emulsification properties of bicarbonate solutions and crude oil and the effects of temperature and pressure

Abstract An extensive study of fluid/fluid interactions of bicarbonate solutions and crude oil from Danish North Sea has been carried out. Interfacial tension (IFT) measurements, and oil/brine contact tests were conducted. Brine with salinity of 0.05 M was shown to be the optimum salinity in emulsification at basic pH of bicarbonate solution. ATR-FTIR spectra of oil and formed emulsions were recorded to characterize the nature of the emulsion. Information about the content of polar molecules and the water content of the emulsions was provided by the IR analysis. The emulsion structure and drop size which correspond to the optimum salinity were characterized using cryo-scanning electron microscopy (Cryo-SEM) images. The emulsion drop size was comparable to carbonate pore size. This suggests that flow diversion by emulsion formation is possible as a mechanism with an impact on oil recovery. In flow diversion mechanism, formed emulsions will plug fractures and bigger pores and redirect the injected brine to the non-flooded zones. IFT was measured at reservoir pressure (200 bar) and different temperatures (23, 60, 90 and 120 °C). Overall, the change in interfacial tension values in compared to formation water was up to 30% and rarely exceeded that. At each temperature, the IFT has been reduced to the lowest achievable value and no further reduction was observed by increasing the salinity. Furthermore, an increase in temperature led to additional reduction in IFT for each salinity.

Thermal history of potential gas reservoir rocks in the eastern Parnaíba Basin, Brazil

The Parnaiba Basin is a major intracratonic sedimentary basin in Brazil with unconventional petroleum systems as a potential natural resource formed by the influence of igneous intrusions. To constrain the thermal history of unexplored potential reservoir rocks in the eastern part of the Parnaiba Basin, sedimentary rocks near intrusions were analyzed by petrography and thermochronology (apatite fission-track [AFT] and zircon fission-track [ZFT] dating). Petrography shows grain dissolution and carbonate pore filling generated by thermal destabilization of feldspars. The AFT results indicate partial annealing by the last magmatic event in the basin, and ZFT results show ages of maximum paleotemperature compatible with the Sardinha magmatic event (120–130 Ma). In thermal history models, rocks close to intrusions experienced maximum paleotemperatures above 300°C, which is higher than what is considered favorable for reservoir rocks. Hydrothermal fluids modified the diagenetic evolution of the succession by dissolving and precipitating carbonate cement in pore spaces. In the studied area, at distances greater than 50 m (>164 ft) from the intrusion, the rocks were not substantially altered, and in terms of paleotemperature, they can be considered potentially viable reservoirs. The thermal history analysis of potential tight gas sandstone reservoirs affected by intrusive bodies is valuable for characterizing the thermal enhancement or destruction of the reservoir’s qualities.

Pore Fluid and Diagenetic Evolution of Carbonate Cements in Lacustrine Carbonate–siliciclastic Rocks: A Case from the Lower Cretaceous of the Erennaoer Sag, Erlian Basin, NE China

Lower Cretaceous lacustrine carbonate-siliciclastic rocks (including siltstones and mudstones) form a unique tight oil reservoir in the Erennaoer Sag in the Erlian Basin of northeastern China that is characterized by various carbonate cements and volcanic material. Based on integrated analysis of the petrology, mineralogy, and carbon and oxygen isotope geochemistry, the origin, compositions, and evolution of carbonate cements and pore fluids are determined. Five types of carbonate cements are identified, from oldest to youngest: penecontemporaneous micritic-crystalline ferroan dolomite-1; eogenetic micro-crystalline ferroan dolomite-2; sparry, micro-crystalline ferroan calcite-1; mesogenetic micro-crystalline non-ferroan dolomite-3; and calcite-2. The evolution of the pore fluids also controlled the compositions and distribution of the carbonate cements of the lacustrine mudstone during different periods. This may be advantageous to predict the distribution of high-quality reservoirs. The geochemistry of the paleo-pore fluids in these mixed lithological reservoirs are more complicated. Thus, the origin and distribution of pore fluids and their influences on carbonate cementation, fluid-rock interaction, and reservoir quality are complicated and certainly worth studying for hydrocarbon exploration. In addition, the evolution of the pore fluids of the Arshan Formation have experienced different salinity fluid environments under alkaline to weak acid conditions.

A review of the nature and origin of limestone microporosity

Abstract Limestone microporosity is ubiquitous and extensively developed in most Phanerozoic limestones. From an economic perspective, microporosity is important because it contributes substantially to the carbonate pore system, which can host significant volumes of water and hydrocarbons. Therefore, determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations, reservoir characterization, and fluid flow simulations. From an academic standpoint, microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments, a fundamental process in carbonate diagenesis. Many types of micropores contribute to what has been referred to as microporosity, but the vast majority is hosted among low-magnesium calcite (LMC) microcrystals that are present in limestone matrix and allochems. Geochemical, textural, and mineralogical data from natural settings and laboratory experiments indicate that LMC microcrystals are diagenetic in origin. More specifically, these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high-magnesium calcite (HMC) minerals, and precipitation of LMC microcrystal cements. The stabilization process is inferred to take place in the meteoric, marine, and burial diagenetic realms. Although it has not been directly observed, carbon and oxygen isotopes, as well as trace element data suggest that LMC microcrystals form during burial diagenesis in marine-like fluids. Evidence suggests that porosity is not generated during this dissolution-precipitation process, but rather inherited from the precursor sediments. The final arrangement of the micropores in a limestone, however, depends on the precise diagenetic pathway. LMC microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size. The three main textural classes - granular (framework), fitted (mosaic), and clustered - have been recognized across a wide range of ages, depositional settings, burial depths, and precursor types, and are characterized by distinct petrophysical properties, such as porosity, permeability, and pore-throat size. Observations from modern sediments also support the hypothesis that LMC microcrystals develop from aragonite and HMC dominated lime mud. The origin of lime mud has been extensively studied but still highly debated. Of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the Phanerozoic. Microporous limestones comprised of LMC microcrystals are equally abundant during times of aragonite seas and calcite seas, which suggests that no special mineral precursor is required. Microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents. Unlike shallow marine limestones, deep marine sediments start out as mostly LMC therefore mineralogical stabilization is not a significant process in chalk diagenesis.

Pore Type and Porosity Distribution of Carbonate Reservoir Based on 3D Seismic Inversion in “P” Field Salawati Basin

Overall, carbonate rock has complex and more heterogeneous physical characteristic, compared to siliciclastic sedimentary rock. One parameter, which distinguishes carbonate rock and siliciclastic is pore structure/pore type. The heterogeneity and complexity of carbonate reservoir pore type are affected by the sedimentation process and the diagenesis process. Pore type classification is divided into three: interparticle, stiff, and crack. Therefore, carbonate pore type determination becomes important to enhance drilling success. This paper explains pore types prediction, porosity, and acoustic impedance on carbonate reservoir. The Differential Effective Medium (DEM) method to analyze carbonate reservoir pore type has been applied. DEM method generates bulk and shear modulus parameters to create carbonate Vp and Vs model based on pore type. We also do a 3D seismic inversion to create acoustic impedance distribution, porosity, and pore type. Afterward, we made cube porosity and pore type cube by using geostatistics method to provide a better result. Moreover, this study shows low impedance value correlates with high porosity value and enhancement of porosity value correlates with crack and interparticle pore type on “P” field, Salawati Basin.

Apparent gas permeability, intrinsic permeability and liquid permeability of fractal porous media: Carbonate rock study with experiments and mathematical modelling

This paper develops mathematical models for predicting apparent gas permeability, intrinsic permeability and liquid permeability of carbonates based on fractal capillary tube model. First, the modified Hagen–Poiseuille equation for the fluid flow in a single capillary tube with fractal characteristics of tortuosity was derived. With the assumption of fractal pore size distribution in carbonates, the intrinsic permeability of fractal capillary tube model was developed. Then, considering gas slippage effect and liquid boundary-layer effect, the mathematical models for predicting apparent gas permeability and liquid permeability were proposed, respectively. Finally, the proposed mathematical models for permeability prediction were validated with the experimental permeability data measured from 22 carbonate core plugs. High pressure mercury intrusion (HPMI) tests were utilized to obtain pore size distributions and pore structure parameters. The comparisons between model results and experimental results demonstrate that the developed mathematical models can effectively predict apparent gas permeability, intrinsic permeability and liquid permeability of carbonates. The influences of different pore structures on gas slippage effect and liquid boundary-layer effect were investigated. The research results demonstrate that gas slippage factor is the function of pore structure parameters and fluid properties, and it increases with increasing fractal dimension and decreasing tortuosity fractal dimension, and with carbonate pore size decreasing, the differences between liquid permeability and intrinsic permeability increase due to the increasing liquid boundary-layer effect.

CO2 sequestration by direct gas–solid carbonation of fly ash with steam addition

Abstract Mineral carbonation using alkaline industrial solid wastes is a promising CO2 sequestration technology. In this work, the effects of temperature, CO2 content, steam content, and reaction time on CO2 sequestration at atmospheric pressure were studied by the direct gas–solid carbonation of circulating fluid bed fly ash in a thermogravimetric analyzer and a fixed bed reactor system. Results indicate that increasing the temperature and the content of CO2 and H2O(g) can improve the CO2 sequestration efficiency of the circulating fluid bed fly ash. However, the effect of CO2 content is not as significant as that of temperature and H2O(g). The maximum CO2 sequestration capacity of 60 g CO2/kg fly ash with a maximum sequestration efficiency of 28.74% was achieved at 600 °C with 20% H2O(g) addition. The Brunauer–Emmett–Teller surface area and the pore volume decreased with the increase in average pore size after carbonation, due to the formation of a dense carbonate protective layer and pore blockage. With steam addition, the surface area and the pore volume increase, which contributes to the conversion of CaO to CaCO3. The production of Ca(OH)2 or transient Ca(OH)2 and the enhancement of CO2 molecular mobility account for the promotion of steam in the carbonation process. The promotion mechanism of steam in different stages of reaction and temperature were explored in detail. Given the rich reserves of fly ash in China, it shows a good application prospect to use the Chinese fly ash as a mineral carbonation feedstock, which may not only reduces CO2 emission, but also stabilizes the wastes.

Mechanism and Distribution of Calcareous Interbeds in Songtao Uplift and Its Periphery of Qiongdongnan Basin

Based on core and casting slice observation, well drilling and logging data, the source of carbonate materials, lithology together with electric properties, types, genesis mechanism and distribution of calcareous interbeds in Songtao uplift and its periphery of Qiongdongnan Basin have been thoroughly analyzed. Results show that typical features have been appeared from well logging curves, containing low gamma-ray, low acoustic travel time, low neutron value, high density, as well as bright white calcium nodules or bands in electrical imaging well-logging curves. Drilling results reveal that calcareous interbeds developed mostly in high position of paleostructures and their distribution was controlled by the combined effects of macroscopic and microscopic factors. Macroscopically, calcareous interbeds relate to paleogeomorphology together with the combination of sandstone and mudstone. They are also controlled microscopically by the source of carbonate cements and pore space. Under normal circumstances, with regard to the same sand, the closer to the mudstone and the thicker of mudstone, the more conducive to the formation of calcareous interlayer. Low compaction strength, high content of rigid particles, coarse grain size, well-sorted sandstone and large pore space during carbonate cementation are favorable for the development of calcareous interbeds.

Characterization of compact carbonate pore–throat network systems in the Xiagou Formation in Qingxi Sag, Jiuquan Basin, China

The Xiagou Formation is the main tight oil reservoir in the Qingxi Depression of the Jiuquan Basin; however, its poor physical properties have restricted tight oil exploration and development. Therefore, itis important to gain a better understanding of its micropore structure. The results show that the lithology of the Xiagou Formation reservoir is dominated by gray argillaceous dolomite and dolomitic mudstone. The pore types are primarily composed of elliptical and irregular intergranular pores. Intercrystalline and organic-matter-hosted pores are mostly isolated and have poor connectivity. Moreover, NMR spectroscopy results show that the T2spectra of the rock samples in the study area are predominantly unimodal or bimodal in distribution, with pore sizes principally ranging between 0.01 and 0.69 μm. Using CT scanning, we obtained a three-dimensional (3D) pore–throat stereogram. At the micrometer scale, the pore throats differed in size and exhibited shelf-like, banded, and spheroidal shapes. At the nanometer scale, we observed a greater number of nanometer-scale micropores. These pores were shelf-like and spheroidal in shape and were distributed inside or at the mineral grain (crystal) surface. The nanometer-scale spheroidal micropores showed poor connectivity and were isolated in 3D space, with most serving as reservoir spaces. However, the nanometer-scale short-shelf-like micropores showed connectivity with the micrograde shelf-like micropores, were adjacent to the isolated spheroidal nanometer-scale micropores, and acted as both pore throats and pores. The larger throat-to-pore ratio coefficient indicates a greater correlation between the pore and throat, which further results in a higher concentration of pores. The larger average pore volume leads to higher porosity and, together with the higher average coordination number, enables better pore connectivity, thereby yielding a better potential reservoir and penetration capacity.

An integrated approach to quantify geologic controls on carbonate pore types and permeability, Puguang gas field, China

Accurate estimation of permeability for reservoir simulation and production is challenging in carbonate rocks due to the diversified pore structures resulting from deposition and diagenetic modification. A significant amount of residue gas is expected in the Puguang field, China. We use a shear-frame flexibility factor [Formula: see text] from a rock-physics model as an index to quantify its spatial variation of pore structure and constrain the estimation of permeability in this field. The pore-structure index [Formula: see text] is established and used to classify various permeability-porosity trends at well locations where core and log data are available. It is found that when [Formula: see text], the pore type is dominated by intercrystalline pores with large pore-throat sizes and high connectivity, the permeability-porosity relation is [Formula: see text]; when [Formula: see text], the pore type consists of isolated moldic pores, the permeability-porosity relation is [Formula: see text], where [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] are constants, which are 80, 18, 3, 0.004, 5.5, and 1.6, respectively, for the studied gas reservoir. Rock-physics-based seismic inversion is then applied to quantify the spatial variation of pore type and permeability. The inversion results indicate that regional stratigraphy has a paramount control on the distribution of pore type and permeability. Moldic pores ([Formula: see text]) are widely distributed near the unconformities, whereas the large intercrystalline porosity and microporosity are distributed below and above the unconformities due to the sea-level regression and transgression, respectively. It is concluded that production problems may occur in porous and permeable intercrystalline-porosity zones and the exploration of residual gas remains in the high porosity yet less permeable moldic-porosity zones.

A textural and diagenetic assessment of the Zechstein Limestone carbonates, Poland using the transverse Nuclear Magnetic Resonance relaxometry

As a consequence of carbonate pore system ambiguity, only two basic carbonate textures, i.e. mudstone and completely contrasting grainstone have thus far been found to be separable using Nuclear Magnetic Resonance (NMR) relaxometry. Therefore, a new methodology of NMR-derived carbonate facies and diagenetic alteration recognition was prepared and experimentally launched. As many ancient carbonates have often experienced a meteoric exposure stage, this approach focuses on such conditions. Especially the paleoreef carbonate reservoirs are of the main interest of the authors. The Wielichowo Reef (western Poland), developed on an uplifted pre-Zechstein horst structure - the Wolsztyn Ridge - was chosen for the study object. The reef is representative for its region because it is characterized by both high textural and diagenetic variability, often repeatable in case of other objects present on the Wolsztyn Ridge. Consequently, twenty one Zechstein Limestone (Ca1) carbonate core samples from the chosen object were qualitatively studied using a low-field, 50 [mT] NMR spectrometer. Corresponding thin section samples were carefully investigated using both polarizing and electron microscopy, cathodoluminescence and sample staining to obtain a full overview of their petrography and enable further comparisons between NMR and the remaining data. As shown by the obtained results, various carbonate facies can be distinguished on the basis of few criteria. These include: (1) the changeability of the signal's amplitude, (2) the values of clay-bound (MCBW), capillary (BVI) and freely moving (FFI) water, and (3) – the width of the main T2 peak, defined as the first peak occurring directly after the BVI, whose average amplitude exceeds or equals at least 75% of the maximal average amplitude encountered within MCBW or BVI systems. Similarly, it seems possible to separate dissolution vugs from other pore systems. The wider the main peak, the higher the probability of finding irregularly-shaped vugs, as diameter size variances account for elevating signal's amplitude, corresponding to various relaxation times. Furthermore, fractures and increased cementation have a significant impact on final T2 distribition's appearance, as well - hence making the low-field NMR an invaluable tool in the evaluation of carbonate reservoirs influenced by the meteoric exposure.

Microstructures and deformation mechanisms in Opalinus Clay: insights from scaly clay from the Main Fault in the Mont Terri Rock Laboratory (CH)

Abstract. The Main Fault in the shaly facies of Opalinus Clay is a small reverse fault formed in slightly overconsolidated claystone at around 1 km depth. The fault zone is up to 6 m wide, with micron-thick shear zones, calcite and celestite veins, scaly clay and clay gouge. Scaly clay occurs in up to 1.5 m wide lenses, providing hand specimens for this study. We mapped the scaly clay fabric at 1 m–10 nm scale, examining scaly clay for the first time using broad-ion beam polishing combined with scanning electron microscopy (BIB-SEM). Results show a network of thin shear zones and microveins, separating angular to lensoid microlithons between 10 cm and 10 µm in diameter, with slickensided surfaces. Our results show that microlithons are only weakly deformed and that strain is accumulated by fragmentation of microlithons by newly formed shear zones, by shearing in the micron-thick zones and by rearrangement of the microlithons.The scaly clay aggregates can be easily disintegrated into individual microlithons because of the very low tensile strength of the thin shear zones. Analyses of the microlithon size by sieving indicate a power-law distribution model with exponents just above 2. From this, we estimate that only 1 vol % of the scaly clay aggregate is in the shear zones.After a literature review of the hypotheses for scaly clay generation, we present a new model to explain the progressive formation of a self-similar network of anastomosing thin shear zones in a fault relay. The relay provides the necessary boundary conditions for macroscopically continuous deformation. Localization of strain in thin shear zones which are locally dilatant, and precipitation of calcite veins in dilatant shear fractures, evolve into complex microscale re-partitioning of shear, forming new shear zones while the microlithons remain much less deformed internally and the volume proportion of the µm-thick shear zones slowly increases. Grain-scale deformation mechanisms are microfracturing, boudinage and rotation of mica grains, pressure solution of carbonate fossils and pore collapse during ductile flow of the clay matrix. This study provides a microphysical basis to relate microstructures to macroscopic observations of strength and permeability of the Main Fault, and extrapolating fault properties in long-term deformation.

Simulating dispersion in porous media and the influence of segmentation on stagnancy in carbonates

Understanding the transport of chemical components in porous media is fundamentally important to many reservoir processes such as contaminant transport and reactive flows involved in CO2 sequestration. Carbonate rocks in particular present difficulties for pore-scale simulations because they contain large amounts of sub-micron porosity. In this work, we introduce a new hybrid simulation model to calculate hydrodynamic dispersion in pore-scale images of real porous media and use this to elucidate the origins and behaviour of stagnant zones arising in transport simulations using micro-CT images of carbonates. For this purpose a stochastic particle model for simulating the transport of a solute is coupled to a Lattice-Boltzmann algorithm to calculate the flow field. The particle method incorporates second order spatial and temporal resolution to resolve finer features of the domain. We demonstrate how dispersion coefficients can be accurately obtained in capillaries, where corresponding analytical solutions are available, even when these are resolved to just a few lattice units. Then we compute molecular displacement distributions for pore-spaces of varying complexity: a pack of beads; a Bentheimer sandstone; and a Portland carbonate. Our calculated propagator distributions are compared directly with recent experimental PFG-NMR propagator distributions (Scheven et al., 2005; Mitchell et al., 2008), the latter excluding spin relaxation mechanisms. We observe that the calculated transport propagators can be quantitatively compared with the experimental distribution, provided that spin relaxations in the experiment are excluded, and good agreement is found for both the sandstone and the carbonate. However, due to the absence of explicit micro-porosity from the carbonate pore space image used for flow field simulations we note that there are fundamental differences in the physical origins of the stagnant zones for micro-porous rocks between simulation and experiment. We show that for a given micro-CT image of a carbonate, small variations in the parameters chosen for the segmentation process lead to different amounts of stagnancy which diffuse away at different rates. Finally, we use a filtering method to show that this is due to the presence of spurious isolated pores which arise from the segmentation process and suggest an approach to overcome this limitation.

Experimental simulation of dissolution law and porosity evolution of carbonate rock

Experiments of acetic acid (initial 0.2%) with porous dolostone, fractured-porous-vuggy dolostone, porous limestone and fractured limestone were done in a continuous flow diagenesis simulation system to find out the controlling factor of dissolution and dissolution effect. The results show that the dissolution quantity of carbonate rock inversely proportional to temperature and directly proportional to pressure, and the temperature effect is greater than the pressure effect. Therefore, relatively shallow burial and lower temperature environment is more beneficial to the formation of large scale carbonate dissolution pores. Quantitative comparison of porosity volume and permeability variation, and evolution of pores inside the rock before and after the experiment show that pore structure has apparent control over the carbonate dissolution and pore evolution. After dissolution, porous dolomite with homogeneous pore distribution saw rise in pore volume (matrix pore volume) and permeability, and remained as pore type in terms of reservoir space; porous limestone, with significant heterogeneity in original pores and texture, saw significant increase in pore volume and permeability, but the increased pores were fracture type, so its reservoir space turned into fracture-pore type; dissolution increased the permeability of fracture-pore dolomite and fracture limestone remarkably by 2-3 orders of magnitude; and the pores increased were mainly along dissolution fractures, turning the reservoir space into fracture-cave type.

Effects of changes in rock microstructures on permeability: 3‐D printing investigation

AbstractRocks are naturally heterogeneous; two rock samples with identical bulk properties can vary widely in microstructure. Understanding how the microstructure and bulk properties of rocks then evolve during experiments and computations simulating diagenesis is inherently a multiscale problem. The advent of modern 3‐D printing has provided an unprecedented opportunity to link those scales by combining the strengths of digital and experimental rock physics. In this study, we take a computerized tomography‐scanned model of a natural carbonate pore space then iteratively digitally manipulate, 3‐D print, and measure the flow properties in the laboratory. This approach allows us to access multiple scales digitally and experimentally and test hypotheses about how changes in rock microstructure due to compaction and dissolution affect bulk transport properties in a repeatable manner.

Multiscale pore structure characterization of middle east carbonates

This paper focuses on the measurement of pore-scale geometrical and topological properties of heterogeneous Middle East carbonates using scanning electron microscopy (SEM), image analysis, and mercury porosimetry. Quantitative pore network descriptors including porosity, pore body and throat sizes, coordination number (number of throats per pore), shape factor (pore area to perimeter ratio), aspect ratio (pore body to throat size ratio), and their distributions are determined from SEM-based images of thin sections at multiple resolutions as well as physical core measurements. Image analysis revealed pore bodies in 2D images whereas their connecting throat sizes are derived from mercury intrusion data. To compensate for pore sizes that vary over several orders of magnitude, SEM images are collected at 75× and 750× with pixel resolutions of 2.63 and 0.077µm, respectively. Mosaics (75X) reveal abundant evidence of textural and structural differences within and between the carbonate samples. SEM pore-size distributions generated by merging data from both 75× and 750× images incorporate microporous features as small as 0.077µm and macropores as large as 310µm into a single multiscale pore system. A pore-throat classification method was developed relating mercury volume invaded through throats of a given size to the corresponding pore area as derived from SEM data. Four pore-throat classes were identified with unique pore- and throat-size distributions ranging from 0.077 to 310.34µm and 0.007–23.703µm, respectively. The average coordination number ranges mostly from 3 to 5, and irregular triangle-shaped cross-sections dominate the carbonate pore system. The pore to throat aspect ratio is generally large (>3), indicating the potential for significant residual oil saturation. These pore system parameters and their distributions provide the basis for future pore network modeling studies and allow for improved understanding of pore structure and network framework and their influence on multiphase transport properties and, accordingly, secondary and enhanced oil recovery efficiency in carbonate reservoirs.

Laboratory measurements of the effect of fluid saturation on elastic properties of carbonates at seismic frequencies

ABSTRACTA significant portion of the world's hydrocarbon reserves are found in carbonate reservoirs, yet analysis of the petrophysical properties of these reservoirs is associated with a number of challenges. Some of these challenges stem from physical and chemical interactions between the carbonate rock matrix and pore fluids, which can affect elastic properties of the rock. Hence, the study of the pore fluid effects on the elastic properties of carbonates is important for understanding a change of the field performance properties of а carbonate reservoir caused by fluid movements during hydrocarbon extraction in producing fields. In this laboratory study, we investigate the applicability of Gassmann's model for predictions of the elastic moduli of water‐ and hydrocarbon‐saturated Savonnières limestone and the influence of partial water saturation on elastic and anelastic properties of the rock. We present the results of two sets of laboratory experiments on the Savonnières oolitic limestone where we: (i) evaluate the effect of full water and n‐decane saturation on elastic moduli and attenuation at seismic (0.1 Hz–120 Hz) and ultrasonic (0.5 MHz) frequencies; and (ii) quantify the dependence of elastic moduli and extensional attenuation on water saturation at two seismic frequencies of 1 Hz and 10 Hz. We demonstrate that the change in the bulk modulus of limestone fully saturated either with n‐decane or water is in agreement with Gassmann's fluid substitution theory, whereas the shear modulus is noticeably reduced. The measurements with partial saturation show that the bulk modulus decreases with increasing water saturation to a lesser extent than the Young's and shear moduli. Our results show that extensional attenuation in the samples with closed boundaries is insignificant under dry and fully saturated conditions but is influenced greatly by the liquid content when saturation is between 0 and 20% or 95% and 100%.

Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas

Molybdenum (Mo) isotope variations recorded in black shales provide important constraints on marine paleoredox conditions. However, suitable shales are not ubiquitous in the geologic record. Moreover, reliable reconstruction of Mo isotope records from shales requires deposition from a water column containing very high concentrations of sulfide—a condition which is both rare and difficult to verify with certainty when examining preserved sediments. The utility of Mo isotopic records could be improved if reconstructions were possible using alternative lithologies, such as marine carbonates, which are more abundant in the geologic record.Here, we focus on the role of early diagenesis in determining the Mo isotopic composition preserved in shallow-water carbonate sediments from four push cores collected in different shallow-water depositional environments in the Bahamas. In contrast with carbonate primary precipitates, which generally contain <0.2ppm Mo, we find that carbonate sediments deposited under fully oxic shallow bottom water can experience extensive syndepositional authigenic Mo enrichment (1 to >10ppm Mo). The extent of this authigenic enrichment appears to be driven by high concentrations of hydrogen sulfide in the porewaters. In cores with the least authigenic Mo enrichment and lowest pore water sulfide, Mo isotopes are ~1–1.2‰ lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2–2.5‰), even under oxic bottom water conditions. However, the sensitivity of bulk carbonate δ98Mo to syndepositional diagenetic conditions potentially complicates interpretation of a carbonate Mo isotope paleoredox proxy. Robust reconstruction of seawater Mo isotopic composition from carbonates will thus require the ability to place constraints on early diagenetic conditions of pore waters at the time of deposition. We show that in order to record seawater Mo isotope values, carbonate pore waters must contain 50–100μM H2Saq, which is achieved only in organic- and sulfide-rich carbonate sediments.

Multiscale characterization of pore structure in carbonate formations: Application to the Scurry Area Canyon Reef Operators Committee Unit

Carbonate formations consist of a wide range of pore types with different shapes, pore-throat sizes, and varying levels of pore-network connectivity. Such heterogeneous pore-network properties affect the fluid flow in the formation. However, characterizing pore-network properties (e.g., effective porosity and permeability) in carbonate formations is challenging due to the heterogeneity at different scales and complex pore structure of carbonate rocks. We have developed an integrated technique for multiscale characterization of carbonate pore structure based on mercury injection capillary pressure (MICP) measurements, X-ray micro-computed tomography (micro-CT) 3D rock images, and well logs. We have determined pore types based on the pore-throat radius distributions obtained from MICP measurements. We developed a new method for improved assessment of effective porosity and permeability in the well-log domain using pore-scale numerical simulations of fluid flow and electric current flow in 3D micro-CT core images obtained in each pore type. Finally, we conducted petrophysical rock classification based on the depth-by-depth estimates of effective porosity, permeability, volumetric concentrations of minerals, and pore types using an unsupervised artificial neural network. We have successfully applied the proposed technique to three wells in the Scurry Area Canyon Reef Operators Committee (SACROC ) Unit. Our results find that electrical resistivity measurements can be used for reliable characterization of pore structure and assessment of effective porosity and permeability in carbonate formations. The estimates of permeability in the well-log domain were cross-validated using the available core measurements. We have observed a 34% improvement in relative errors in well-log-based estimates of permeability, as compared with the core-based porosity-permeability models.

Pore space characterization in carbonate rocks — Approach to combine nuclear magnetic resonance and elastic wave velocity measurements

Pore space features influence petrophysical parameters such as porosity, permeability, elastic wave velocity or nuclear magnetic resonance (NMR). Therefore they are essential to describe the spatial distribution of petrophysical parameters in the subsurface, which is crucial for efficient reservoir characterization especially in carbonate rocks. While elastic wave velocity measurements respond to the properties of the solid rock matrix including pores or fractures, NMR measurements are sensitive to the distribution of pore-filling fluids controlled by rock properties such as the pore-surface-to-pore-volume ratio. Therefore a combination of both measurement principles helps to investigate carbonate pore space using complementary information.In this study, a workflow is presented that delivers a representative average semi-axis length of ellipsoidal pores in carbonate rocks based on the pore aspect ratio received from velocity interpretation and the pore-surface-to-pore-volume ratio Spor as input parameters combined with theoretical calculations for ellipsoidal inclusions. A novel method to calculate Spor from NMR data based on the ratio of capillary-bound to movable fluids and the thickness of the capillary-bound water film is used. To test the workflow, a comprehensive petrophysical database was compiled using micritic and oomoldic Lower Muschelkalk carbonates from Germany. The experimental data indicate that both mud-dominated and grain-dominated carbonates possess distinct ranges of petrophysical parameters. The agreement between the predicted and measured surface-to-volume ratio is satisfying for oomoldic and most micritic samples, while pyrite or significant sample heterogeneity may lead to deviations. Selected photo-micrographs and scanning electron microscope images support the validity of the estimated representative pore dimensions.