Low-porosity Carbonate Research Articles

APPLICATION OF MUD GAS DATA AND LEAKAGE PHENOMENA TO EVALUATE SEAL INTEGRITY OF POTENTIAL CO2STORAGE SITES: A STUDY OF CHALK STRUCTURES IN THE DANISH CENTRAL GRABEN, NORTH SEA

Depleted chalk oilfields and chalk structures in the Danish Central Graben, North Sea, are potential CO2storage sites. In most of these fields, the main reservoir is the Upper Cretaceous – Danian Chalk Group and the Eocene – Miocene mudstones of the Horda and Lark Formations constitute the primary seal. In a few fields, the reservoir is composed of the Lower Cretaceous Tuxen and Sola Formations. Here the main seal is assumed to be the Chalk Group which however has poor gas sealing characteristics; the Horda and Lark Formations constitute an efficient secondary seal although they are quite high in the section. This study documents a workflow that may help to evaluate the seal integrity of the structures from an integration of mud gas data from wells with seismic data. Mud gas data provide detailed information about the distribution and types of gas (biogenic or thermogenic) throughout the seal section and overburden. The presence of higher carbon number gases (C3–C5, propane to pentane) in the seal indicates migration of thermogenic gas into the thermally immature sealing mudstones; whereas the dominance of C1(methane) and partly C2(ethane) likely reflects the presence of in situ generated biogenic gas in the mudstones, thus indicating that there are no seal integrity issues. The vertical thermogenic gas migration front has been determined, and a “traffic light” indicator system has been used for seal integrity evaluation. Where no or minor migration of thermogenic gas into the primary seal has occurred and a primary seal >30 m thick is present, the seal is considered to have good matrix seal integrity (green). If some significant thermogenic gas migration has occurred into the primary seal but more than 30 m of primary seal is present above the thermogenic gas migration front, the seal integrity is reduced (yellow). In structures where thermogenic gas migration is recorded through the primary seal and into the overburden, seal integrity is considered to be poor (red). In areas where significant leakage of thermogenic gas has occurred into the seal, high density, low porosity carbonate beds frequently occur encapsulated within the sealing mudstones and are interpreted to be composed of methane‐derived authigenic carbonates (MDACs). Seismic data show that there is a convincing correlation between leakage as indicated from mud gas data and the presence of vertical wipe‐out zones (gas chimneys), bright zones (gas‐charged sediments or MDACs), and depressions (pockmarks). In general, potential CO2storage sites in the study area in tectonically inverted structures show good seal integrity, but this may locally be reduced and require additional analyses. Storage sites associated with salt diapirs generally show poor seal integrity and are likely to be poor candidates for CO2storage. In combination, mud gas and seismic data are therefore powerful tools to investigate (palaeo‐) leakage phenomena and provide support for seal integrity evaluation at local to regional scales.

How pressure-solution enables the development of deformation bands in low-porosity rocks

Occurrence of deformation bands developing with various orientations in a low-porosity bioclastic calcarenite in the Cotiella Massif (Spanish Pyrenees), questions about the mechanism responsible for their generation in low-porosity carbonates. Our work is based on the combination of a 2D and 3D petrographic and microstructural study, using electron backscattering diffraction microscopy and X-ray microtomography, with a diagenetic study, using cathodoluminescence microscopy and scanning electron microscopy images. Diagenetic and petrographic observations indicate early cementation, which led to significant porosity reduction prior to a sequence of deformation processes comprising the development of tectonic stylolites around which deformation bands eventually formed. Along the latter, quartz grains appear fractured with high strain patterns, showing a different appearance compared to the quartz along the stylolite. Moreover, the ratio of carbonate matrix versus quartz grains shows distinctive variations throughout the different stages of deformation. Based on these observations, we propose a conceptual model for the genesis of deformation bands in low-porosity rocks, accounting for the preferential dissolution of calcite with respect to quartz grains along the stylolite plane that resulted in local porosity development in between quartz grains. Consequently, quartz aggregation led to the classical deformation band mechanism of strain accumulation and breakage.

Implementation of Seismic Inversion to Determine Porosity Distribution of Maysan in Amara Oil Field, Southern Iraq

The current research deals with studying the petrophysical properties represented by the porosity and its distribution on the level of all units of the top and bottom of the Kirkuk Formation Group. The study area is located in Maysan province in the south-eastern part of Iraq in the Amara field. The Kirkuk Group was deposited in the Tertiary Age. The post-stack method using seismic inversion and creating a relationship between seismic data was accomplished using Hampson-Russel software at well Am-1 and Seismic lines Ama 20 and 30. The research results indicate high porosity values on top of the formation with a decrease in acoustic impedance (Z) and, therefore, a reduction in the density. At the same time, low porosity values were indicated for the bottom of the formation with an increase in both acoustic impedance and density. The top of the formation contains clastic sediments with oil-sand of high porosity, which is considered hydrocarbon indication and low porosity carbonate materials at the bottom of the formation.

New insights into permeability determination by coupling Stoneley wave propagation and conventional petrophysical logs in carbonate oil reservoirs

The need to determine permeability at different stages of evaluation, completion, optimization of Enhanced Oil Recovery (EOR) operations, and reservoir modeling and management is reflected. Therefore, various methods with distinct efficiency for the evaluation of permeability have been proposed by engineers and petroleum geologists. The oil industry uses acoustic and Nuclear Magnetic Resonance (NMR) loggings extensively to determine permeability quantitatively. However, because the number of available NMR logs is not enough and there is a significant difficulty in their interpreting and evaluation, the use of acoustic logs to determine the permeability has become very important. Direct, continuous, and in-reservoir condition estimation of permeability is a unique feature of the Stoneley waves analysis as an acoustic technique. In this study, five intelligent mathematical methods, including Adaptive Network-Based Fuzzy Inference System (ANFIS), Least-Square Support Vector Machine (LSSVM), Radial Basis Function Neural Network (RBFNN), Multi-Layer Perceptron Neural Network (MLPNN), and Committee Machine Intelligent System (CMIS), have been performed for calculating permeability in terms of Stoneley and shear waves travel-time, effective porosity, bulk density and lithological data in one of the naturally-fractured and low-porosity carbonate reservoirs located in the Southwest of Iran. Intelligent models have been improved with three popular optimization algorithms, including Coupled Simulated Annealing (CSA), Particle Swarm Optimization (PSO), and Genetic Algorithm (GA). Among the developed models, the CMIS is the most accurate intelligent model for permeability forecast as compared to the core permeability data with a determination coefficient (R2) of 0.87 and an average absolute deviation (AAD) of 3.7. Comparing the CMIS method with the NMR techniques (i.e., Timur-Coates and Schlumberger-Doll-Research (SDR)), the superiority of the Stoneley method is demonstrated. With this model, diverse types of fractures in carbonate formations can be easily identified. As a result, it can be claimed that the models presented in this study are of great value to petrophysicists and petroleum engineers working on reservoir simulation and well completion.

Advancements towards DFKN modelling: Incorporating fracture enlargement resulting from karstic dissolution in discrete fracture networks

We present a multiscale approach to develop a deterministic Discrete Fracture Network (DFN) model including fracture enlargement due to karstic dissolution in low-porosity carbonate rocks. The result is a DFKN (Discrete Fracture and Karst Network) model from which it is possible to quantify, in a practical manner, the impact of fracture enlargement due to karstic dissolution on the fracture network transmissivity. We provide a case study using two large outcrops in the Jandaíra Formation, an intensely karstified carbonate platform of Turonian-Campanian age in the Potiguar Basin, Brazil. The study is based on scanlines collected both in field and from Unmanned Aerial Vehicle (UAV) imagery, allowing to cover three orders of magnitude in length and aperture of fractures. We found that if karstic dissolution is low, aperture-length data plotted on bi-logarithmic graphs can be fitted by straight lines, thus following the aperture-length power law typical of mechanical apertures. However, advanced stages of karstification modifies the aperture-length relationship due to the fact that dissolution causes similar fracture enlargement regardless of fracture length. As a result, the aperture-length curve gradually degenerates to a constant value as the degree of dissolution increases. This behavior is the key to quantitatively taking into account fracture enlargement, by simply adapting an aperture-length curve according to the known stage of karstic dissolution of the modeled carbonate rocks. We show that fracture enlargement can increase the transmissivity of a karstified fracture network by up to five orders of magnitude without changing its connectivity. Our approach can be useful to better understand the behavior of fluid flow in carbonate reservoirs with fracture networks affected by karstic dissolution.

Dual-layer catalyst layers for increased proton exchange membrane fuel cell performance

The use of a dual-layer cathode catalyst layer improves the performance of proton exchange membrane fuel cells. To generate single and dual-layer catalyst layers between the gas diffusion media and proton exchange membrane electrolyte, two commercial carbon-supported platinum catalysts with either a high microporosity carbon (Ketjen Black EC-300J) or a low porosity carbon (XC-72 Vulcan carbon) are utilized. We discovered that dual-layer cathode catalyst layers boost maximum power of fuel cells regardless of carbon porosity directionality, with either high surface area carbon at the proton exchange membrane electrolyte or gas diffusion media interface. At 25% relative humidity, a condition that generally inhibits fuel cell performance, a 20% performance boost is realized when using dual-layer cathode CLs. The electrochemical impedance spectroscopy results indicate that cathodes with dual-layer catalyst layer aid in water management due to lower kinetic activity losses of the Pt and improved hydration of the proton exchange membrane electrolyte.

Porosity Modeling of Huge Thick Carbonates: A Case Study of Lower Paleozoic Carbonates in the Tazhong Uplift, Tarim Basin, China

This study provides a solution to determine the porosity evolution of huge thick carbonates accurately using PetroMod version 2016.2 software with a cementation tool. The Lower Paleozoic carbonates from the Tazhong Uplift (Tarim Basin) were taken as examples. Results show that the errors of modeled porosity decreased from 256.3%–468.8% to 6.7%–12.5% for low-porosity carbonates and from 15.0%–22.9% to 2.9%–5.0% for high-porosity carbonates, indicating a better fit to the measured values after considering carbonate cementation. The calibrated model incorporating cementation exhibits typically low carbonate porosities and burial depths but high thermal conductivities, resulting in much lower formation temperatures, maturities, and hydrocarbon masses than the uncalibrated model. This study indicated that carbonate cementation plays a very important role in the reduction of carbonate porosity, and it is necessary to calibrate the thick-carbonate porosity model using the cementation tool. Although this method has certain feasibility and practicability and can be utilized widely to model huge thick carbonates, it has a limitation in recovering the increase in porosity caused by postdiagenesis.

Brittle-Ductile Transition and Hoek–Brown mi Constant of Low-Porosity Carbonate Rocks

Brittle-Ductile Transition and Hoek–Brown mi Constant of Low-Porosity Carbonate Rocks

Microstructural and main mechanical properties of novalac based carbon–carbon composites as the pyrolysis heating rate

In this study, the effects of pyrolysis heating rate on microstructural and main mechanical properties of Novalac-based carbon/carbon composites were investigated by CHNS, optical microscope, FE-SEM, BET N2 adsorption, XRD, Raman, FT-IR, wear analyzing, three-point bending test, tensile and Vickers micro-hardness tests. Firstly, PAN-derived carbon nanofibers (reinforcing agent) was synthesized using electrospinning followed by the functionalizing via the wet chemical oxidation to improve the strength of nanofiber bonding to the matrix of composites. Firstly, novalac resin (acting as a matrix), hexamethylenetetramine (hardener agent) and carbon nanofibers (reinforcing agent) were mixed and hot-pressed at 180 °C under the compression load of 40 kN to produce compressed CNFs-Novolac composites. Carbon/Carbon composites were obtained from the pyrolysis of CNFs-Novolac composites up to 1000 °C by the various heating rates under the compression press of 400 bar, finally. Structural and mechanical studies confirmed that the heating rates below or equal to 10 °C.min−1 resulted in the production of low porosity (≤17%) carbon composite with high carbon content (>90 wt%), high fracture strength (≥270 MPa), high toughness (≥9 MPa m1/2), high hardness (≥156 Hv), and low friction coefficient (<0.6).

Extensive Well-Testing Operations Provide Insight Into Khuff Reservoirs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 203441, “Lessons Learned From Extensive Well-Testing Operations in Khuff Formations Offshore Abu Dhabi,” by Florian Hollaender, SPE, Schlumberger, and Mahmoud Basioni and Ahmed Yahya Al Blooshi, ADNOC, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually from 9-12 November. The paper has not been peer reviewed. An extensive appraisal campaign was performed in the Khuff reservoirs offshore Abu Dhabi, with multiple appraisal wells drilled in different fields. Those wells were evaluated using detailed logging campaigns and then subjected to well tests, usually through drillstem testing for targeted intervals. The interpretation of well tests, combined with advanced petrophysical analysis, formation-test data, and production logs, provided insight into the nature of the Khuff reservoirs. A wide range of responses was observed, from tight to highly productive, but not necessarily with clear previous indications of deliverability or inflow intervals. Overview of the Khuff Formations The key characteristics of the Khuff formations offshore Abu Dhabi have been well-documented in previous work and can be summarized by the following: Low porosity and permeability carbonate reservoirs, where natural fractures are critical contributors to flow Properties vary widely laterally, with significant uncertainty regarding connectivity Variations in stress and petrophysical properties can be significant and affected by diagenetic and tectonic history These reservoirs present significant challenges for development planning. Previous studies have shown that it can be difficult to relate production performance to standard petrophysical analysis directly and that the presence of fractures - in particular, critically stressed fractures - in the vicinity of the wellbore is an essential factor for production performance. Productivity also was found to vary by several orders of magnitude within the same reservoir depending on the field and lateral location of a given well. The presence of natural fractures has been recognized as a major contributor to flow in tight gas reservoirs; however, this raises several questions related to assessing formation potential. First, the nature of the fractures must be evaluated. Some will contribute to production, while others will remain sealed. Equally importantly, identifying zones with promising porosity developments is not a solid indicator of production expectations. Well-Test Observations With more than 20 drillstem tests performed in the Khuff reservoirs during a 4-year period, the first observation is the wide range of reservoir responses encountered, with an apparent lack of consistency within a given reservoir or field.

Fault zones in limestones: impact on karstogenesis and groundwater flow (Lez aquifer, southern France)

The Lez aquifer in southern France comprises low-porosity karstified limestones and provides drinking water for ~400,000 inhabitants. Population growth and climate change have increased the stress on the water resources. In order to provide long-term protection and to optimize the water supply, the hydrogeology of the Lez aquifer must be better characterized. This study focused particularly on the St-Clement major fault zone (12 km long with a 500-m normal throw) which was structurally characterized using accurate geological mapping of the area, outcrops analysis and geophysics tools. The research highlights and explains the close relationship between the fault and the karstic occurrences. Moreover, tracer tests and piezometric head variations in boreholes have shown (1) strong interconnection between the observed karstic formations and (2) the major role of St-Clement fault on mass and pressure transfers in the aquifer. At the reservoir scale, the other major faults of the Lez aquifer, such as Corconne-Matelles or Gourg Noir faults, have shown some common morphologic and dynamic characteristics, and suggest a similar hydrogeological functioning. This study then extends this model to a larger scale. It proposes that, in aquifers of low-porosity carbonates, fault zones control the development of the main karstic network which, in turn, controls the main groundwater flows. Thus, faults should be reconsidered in order to improve the vulnerability studies and the quality of karstic aquifer modelling. Therefore, this report can contribute to protecting the groundwater resource, improving yields and optimizing groundwater supply exploitation in this type of aquifer.

Facile regulation of carbon framework from the microporous to low-porous via molecular crosslinker design and enhanced Na storage

Rational manipulation of the carbon framework from the microporous to nonporous via a molecular design approach is interesting but challenging. Herein, we report a versatile strategy for transforming the microporous carbon framework to the low porous one by an elaborate molecular crosslinker design in the polystyrene (PS) precursor. Direct coupling of benzene rings in PS via Scholl reaction yields hypercrosslinked PS-derived carbon with low porous framework, while insertion of methylene crosslinker into PS via a solvent knitting strategy leads to microporous carbon framework. The results show that methylene crosslinker functions as molecular-scale soft templates for facilitating micropores, whereas direct linking PS chains promotes aromatization and mitigates micropore formation during the pyrolysis. The distinct carbon frameworks derived from similar precursor and pyrolysis condition provide an intriguing platform for structure-property relationship study, as preliminarily exemplified by the application in Na ion storage. The low-porosity carbon shows higher initial Coulombic efficiency and superior capacity thanks to its low surface area and enhanced Na insertion into pseudo-graphitic microcrystal structure. The present protocol opens up new avenues towards flexible carbon framework porosity manipulation at molecular level and would trigger further efforts for low-porosity carbons in energy storage.

Faults in tight limestones and dolostones in San Vito lo Capo, Sicily, Italy: Internal architecture and petrophysical properties

Low-porosity carbonates in San Vito lo Capo underwent two episodes of faulting: extensional faulting in the late Miocene and strike-slip faulting in the Plio-Pleistocene. Limestones and dolostones reacted differently to these faulting events. The first extensional faulting event within the limestone formed localized faults with a single fault core surrounded by a damage zone. The fault rock was either dolomitized as deformation proceeded or resealed by calcite cement after brecciation. The dolomitized fault rocks were reactivated during the following strike-slip faulting, forming polyphase breccias. On the other hand, strain was distributed in dolostones throughout a wider area during the extensional faulting forming a pulverized fault zone. The pulverized rock experienced cataclastic deformation during the following strike-slip faulting forming anastomosing networks of cataclastic shear bands. Fault cores hosted in the limestone appear to have acted as flow conduits until they were cemented or hardened due to evolving cataclasis. The cataclastic shear bands in the dolostone are likely to form baffles to flow, at least on a local scale. The fracture spacing in the damage zone also varies significantly between the lithologies. In particular, damage zones in the limestone have a 5–10 cm fracture spacing whereas fracture spacing is 0.5–3 cm in the dolostone. It is likely that the differing mechanical and chemical properties of the dolostone and limestone were responsible for creating contrasting fault zone architectures.

Catalyst Particle Scale Analysis By Transport and Reaction Simulation on 3D Nano-Scale Images for PEMFC

Optimizing the structure of the catalyst layer is critical to improving the proton exchange membrane fuel cell (PEMFC) performance at low Platinum (Pt) loading. A cell with reduced cathode Pt-loading for the cost reduction suffers from a limited performance at high current density, and it is a barrier to practical applications of PEMFC [1]. In addition to Pt-loadings, the types of carbon supports affect cell performance [2]. In the prior work, the nano-scale scanning transmission electron microscopy computed tomography (STEM-CT) was applied to visualize the individual Pt particle distributions in 3D at two Pt-loadings on two types of carbon supports [3]. This imaging showed that with medium surface area, low porosity carbon support (e.g., Vulcan), a large majority of Pt particles are on the surface of the carbon support. In contrast, STEM-CT showed that with a high-surface-area carbon (HSC), a majority of Pt particles are within the porous carbon support. These internal Pt particles are not in contact with the acidic electrolyte, therefore it is unclear how protons are transported to the Pt surface and how the oxygen reduction reaction (ORR) occurs on these Pt particles. In addition, prior experimental results suggest that the utilization of inner Pt particles is improved at high relative humidity with HSC since the micropores in the carbon support are likely to be filled with condensed water [3]. Here, to investigate the characteristics of those catalyst materials, we developed a transport and ORR model on 3D structures obtained from the STEM-CT images. We solve the Poisson-Nernst-Planck equations to model the electromigration and surface charge effects on the proton concentration in the porous carbon supports with a continuum approximation. Figure 1 shows the ORR current density on the Pt particles in the HSC support at the electrode potential, 0.7 V. The simulation result suggests that negative surface charge on Pt particles may be necessary for a high proton concentration and activity of inner particles in water-filled micropores at high relative humidity. This model can be used to characterize the performance of the catalyst layers in different configurations. AcknowledgementThis work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Exceptional Water Desalination Performance with Anion-Selective Electrodes.

Capacitive deionization (CDI) typically uses one porous carbon electrode that is cation adsorbing and one that is anion adsorbing. In 2016, Smith and Dmello proposed an innovative CDI cell design based on two cation-selective electrodes and a single anion-selective membrane, and thereafter this design was experimentally validated by various authors. In this design, anions pass through the membrane once, and desalinated water is continuously produced. In the present work, this idea is extended, and it is experimentally shown that also a choice for anion-selective electrodes, in combination with a cation-selective membrane, leads to a functional cell design that continuously desalinates water. Anion-selective electrodes are obtained by chemical modification of the carbon electrode with (3-aminopropyl)triethoxysilane. After chemical modification, the activated carbon electrode shows a substantial reduction of the total pore volume and Brunauer-Emmett-Teller (BET) surface area, but nevertheless maintains excellent CDI performance, which is for the first time that a low-porosity carbon electrode is demonstrated as a promising material for CDI.

Mapping of fracture zones and small faults using VSP and Cross Dipole Sonic in Eastern Siberia Carbonate Reservoirs, Yurubchansky Field, Russia

Zero offset VSP, two Walkaway lines, ultra sonic and cross dipole sonic were used to interpret fracture zones and small faults in the vicinity of a deviated well drilled through carbonate reservoirs of the Yurubchansky giant oil and gas field, Russia. The fractures and small faults are the main flow conduits and storage of the hydrocarbons within mainly low porosity carbonate reservoir of the Proterozoic age. The wells are only successful if they intersect these “sweet spots” in the reservoir. This is a challenge in developing the Yurubchansky field. The 3D seismic over the field has low resolution, strong heterogeneous reservoirs, varying degrees of anisotropy, multiples contamination and therefore cannot be used to map the “sweet spots” reliably.An incoherency attribute was used to guide fracture and fault interpretation. Two major fault zones and small faults were interpreted from VSPs. One of the fault zones within the reservoir corresponds to fractured core, intensive fracture zones interpreted from ...

Improving Porosity–Velocity Relationships Using Carbonate Pore Types

Acoustic impedance in carbonates is influenced by factors such as porosity, pore structure/fracture, fluid content, and lithology. Occurrence of moldic and vuggy pores, fractures and other pore structures due to diagenesis in carbonate rocks can greatly complicate the relationships between impedance and porosity. Using a frame flexibility factor ([Formula: see text]) derived from a poroelastic model to characterize pore structure in reservoir rocks, we find that its product with porosity can result in a much better correlation with sonic velocity ([Formula: see text] = [Formula: see text]) and acoustic impedance ([Formula: see text] = [Formula: see text], where A, B, C and D is 6.60, 0.03, 18.3 and 0.09, respectively for the deep low-porosity carbonate reservoir studied in this paper. These new relationships can also be useful in improving seismic inversion of ultra-deep hydrocarbon reservoirs in other similar environments.

Pd clusters supported on amorphous, low-porosity carbon spheres for hydrogen production from formic acid.

Amorphous, low-porosity carbon spheres on the order of a few micrometers in size were prepared by carbonization of squalane (C30H62) in supercritical CO2 at 823 K. The spheres were characterized and used as catalysts' supports for Pd. Near-edge X-ray absorption fine structure studies of the spheres revealed sp(2) and sp(3) hybridized carbon. To activate carbons for interaction with a metal precursor, often oxidative treatment of a support is needed. We showed that boiling of the obtained spheres in 28 wt % HNO3 did not affect the shape and bulk structure of the spheres, but led to creation of a considerable amount of surface oxygen-containing functional groups and increase of the content of sp(2) hybridized carbon on the surface. This carbon was seen by scanning transmission electron microscopy in the form of waving graphene flakes. The H/C atomic ratio in the spheres was relatively high (0.4) and did not change with the HNO3 treatment. Palladium was deposited by impregnation with Pd acetate followed by reduction in H2. This gave uniform Pd clusters with a size of 2-4 nm. The Pd supported on the original C spheres showed 2-3 times higher catalytic activity in vapor phase formic acid decomposition and higher selectivity for H2 formation (98-99%) than those for the catalyst based on the HNO3 treated spheres. Using of such low-porosity spheres as a catalyst support should prevent mass transfer limitations for fast catalytic reactions.

Porous-grain-upper-boundary model and its application to Tarim Basin carbonates

Most of the carbonates in the Tarim Basin in northwest China are low-porosity and low-permeability rocks. Owing to the complexity of porosity in carbonates, conventional rockphysics models do not describe the relation between velocity and porosity for the Tarim Basin carbonates well. We propose the porous-grain-upper-boundary (PGU) model for estimating the relation between velocity and porosity for low-porosity carbonates. In this model, the carbonate sediments are treated as packed media of porous elastic grains, and the carbonate pores are divided into isolated and connected pores The PGU model is modified from the porous-grain-stiff-sand (PGST) model by replacing the critical porosity with the more practical isolated porosity. In the implementation, the effective elastic constants of the porous grains are calculated by using the differential effective medium (DEM) model. Then, the elastic constants of connected porous grains in dry rocks are calculated by using the modified upper Hashin-Shtrikman bound. The application to the Tarim carbonates shows that relative to other conventional effective medium models the PGU model matches the well log data well.

Mercury porosimetry as a tool for improving quality of micro-CT images in low porosity carbonate rocks

The combined use of Hg-impregnation and microCT scanning is a powerful tool for detecting structural and textural features in low porosity massive carbonate rocks. Hg-impregnation of carbonate rocks can improve extremely the quality and resolution of microCT images, because of the high density contrast between Hg and the surrounding rock. The success of the combined used of these two techniques depends mainly on two different rock characters:1) Shape of the pores and characteristics of the porous system. Ink-bottle shape pores and tortuous complex porous systems prevent Hg from flowing out of the sample at the end of Hg-porosimetry test, producing reliable microCT images of the porous system itself. On the other side low tortuous cylindrical pores allow Hg flowing out, resulting in not completely true microCT images.2) Effective rock porosity. Hg-impregnation of carbonate rocks with very low porosity (<1%) and tortuous or ink-bottle pores improves significantly the analysis and quantification of their porous system. On the contrary, for carbonate rock with higher porosity (>4%), although with ink-bottle pores, microCT images are undetectable, as the relatively high quantity of Hg entrapped in the sample creates artefacts (metal effect) that completely obliterate the rock structure. Intermediate porosity (about 2%) can behave either as low porosity (<1%) or as higher porosity (>4%) depending on pore shape. A prevalence of tortuous ink-bottle pores results in undetectable microCT images, due to widespread metal effect. A small amount of cylindrical pores, reducing the quantity of entrapped Hg, results in a good quality of microCT images.The resolution of the technique depends on the rock porous system. For low porosity (<1%) rock, due to high density contrast between Hg and rock, pores with radius one order of magnitude smaller than the voxel size are clearly imaged by microCT and pores with radius two order of magnitude smaller than the voxel size are still detectable. Hg-impregnated pores smaller than the voxel increase the voxel density, as a function of the relative volume of Hg and rock. As a consequence, pores smaller than the voxel size appear as large as the voxel itself and bigger pores appear larger than they really are (partial volume effect). Although Hg-impregnation improves 3D qualitative analysis of porous system and its relationships with rock texture, quantification of the porous system through segmentation of microCT images is strongly affected by intrinsic error in pore dimensions, caused by partial volume effect. Quantification error is a function of the shape of the porous system, being lower for rounded pores and higher for complex tortuous pore system.