Original Pore Space Research Articles

Thermodynamically consistent interfacial curvatures in real pore geometries: Implications for pore-scale modeling of two-phase displacement processes

In conventional Pore-network Modeling (PNM) approaches, fluid flow and transport are solved in a network of pore elements with idealized geometries. Such simplification can lead to inaccurate predictions when the original pore space features complex geometries. To overcome this limitation, this study introduces a novel workflow that integrates four key components: (i) an enhanced pore network extraction (PNE) platform capable of identifying and extracting pore cross-sections from high-resolution micro-computed tomography (micro-CT) images of the pore space, (ii) a computationally-efficient semi-analytical model that can faithfully predict capillary entry pressure and the corresponding fluid configuration of piston-like displacements using real two-dimensional cross-sections of pores, (iii) a PNM approach for two-phase flow modeling that utilizes the capillary entry pressure of pores predicted by the semi-analytical model, and (iv) an Artificial Intelligence (AI)-driven model that paves the way for future advancements in efficiently predicating fluid displacement properties in intricate pore structures. To validate this new workflow, we constructed various pore networks containing real pore and throat cross-sections over a diverse group of sandstone and carbonate rock samples. Subsequently, we simulate capillary pressure curves of Mercury Intrusion Capillary Pressure (MICP) and oil–water primary drainage displacements in Bentheimer and Berea sandstones, respectively, using both the conventional and enhanced PNM approaches. The latter demonstrated improved prediction accuracy compared to conventional methods. Next, primary drainage simulations are conducted for two carbonates, and the resulting capillary pressure curves from both PNM approaches are compared. In addition, we conduct an in-depth analysis of fourteen geometric features of the pore space, identifying key factors of hydraulic radius, circumradius, sphericity, and area, that significantly impact capillary entry pressure of pores. After that, we construct an Artificial Neural Network (ANN) to predict the capillary entry pressure of pores using their critical geometric features. This AI model, trained using data derived from the semi-analytical model, exhibits excellent predictive accuracy (with a R2 of 0.995 for the test data set) in estimating capillary entry pressure of pores. Overall, our newly proposed, integrated workflow represents a significant step forward in the field of digital rock technology (DRT), offering an accurate and efficient method for modeling fluid flow in rock samples with complex pore geometries.

Migration characteristics of constant elements in the process of coal dissolution by liquid CO2

The effect of liquid CO2 on coal dissolution plays a key role in improving coal bed methane (CBM) recovery and CO2 geological storage. In order to study the effect of liquid CO2–H2O reaction on the migration of mineral constant elements in coal, three kinds of coal samples (lignite, bituminous coal, and anthracite) with different degrees of metamorphism were selected as the research objects. The one-factor experiments of dissolution of liquid CO2 and water-bearing coal samples were carried out by simulating the temperature and pressure of the stratum. X-ray fluorescence spectroscopy (XRF) was used to test the changes in the relative content of constant elements in coal samples after liquid CO2 injection under different pressure conditions. Scanning electron microscopy (SEM) was utilized to observe the evolution pattern of the surface features of the coal before and after the dissolution of liquid CO2. A quantitative analysis of coal pore development characteristics was conducted based on low-temperature N2 adsorption/desorption experiments, and isothermal adsorption experiments were carried out before and after etching the coal samples to understand the adsorption characteristics of CH4 and CO2 by coal under different pressure conditions. The research results indicated that the migration ability of constant elements in coal showed a trend of first decreasing and then increasing with the increasing degree of coal metamorphism. Among the three selected coal samples, the migration ability of constant elements was found to be strongest in anthracite, followed by lignite and bituminous coal. Most of the element migration in coal was carried out through chemical reactions (hydrolysis, carbonation, redox), while some elements were primarily transported through physical means (such as the low-temperature impact of liquid CO2). The dissolution process expands the original pore space of coal and generates new pores, greatly increasing the coal's capacity for CO2 adsorption. These findings deepen our understanding of mineral dissolution in coal by liquid CO2 and provide important insights for achieving efficient and long-term coalbed CO2 geological storage and enhanced methane recovery techniques.

STUDY ON THE EVOLUTION LAW OF PORE STRUCTURE AND MECHANICAL PROPERTIES OF MUDSTONE UNDER THE EFFECT OF WATER-ROCK CHEMISTRY

The influence of different pH and concentration of water chemical solutions on the mechanical properties of mudstone is studied. The triaxial compression test and nuclear magnetic resonance (NMR) test of mudstone are carried out, and the root cause of the deterioration of mechanical properties of mudstone during the hydration damage process is explored, combining macroscopic and microscopic. The results show that hydrolysis and chemical ion exchange are responsible for the deterioration of mudstone shear strength parameters. The essence of mudstone hydration damage is that water chemistry changes the original pore space structure and pore size distribution inside. Part of the minerals dissolve and fall off under the influence of the water chemical solution, which expands the pore size and increases the porosity. The connection force between mineral particles decreases, macroscopically, the mudstone undergoes a softening evolution from brittleness to ductility, and the shear strength parameters deteriorate. The results show that mudstone is more sensitive to acidic solution than neutral and alkaline solution. The higher the pH of the alkaline solution or the higher the concentration of the neutral solution, the more significant the deterioration of mudstone. Under the influence of hydrochemical solution, the fractal dimension of mudstone gradually tends to 2 from 3, which indicates that the process of hydration damage reduces the complexity of mudstone pore structure. On this basis, the damage variable D<sub>n</sub> defined based on the change of porosity before and after hydration damage of mudstone, to a certain extent, quantitatively reflects the law of mudstone shear strength parameters accompanied by microscopic pore structure evolution. The shear strength parameters of mudstone decrease significantly with the increase of damage variable D<sub>n</sub>. The test analyzes the root cause of the deterioration of the macroscopic mechanical properties of mudstone during the hydration damage process from a microscopic point of view, and the conclusions obtained provide a good reference for the quantitative study of the effect of water chemistry on the mechanical properties of mudstone.

Diagenetic controls on the reservoir quality of tight reservoirs in digitate shallow-water lacustrine delta deposits: An example from the Triassic Yanchang Formation, southwestern Ordos Basin, China

Diagenesis is a very important factor to determine the quality of clastic reservoirs, especially in tight sandstone reservoirs. The tight reservoir of Triassic Yanchang Formation in southwestern Ordos Basin is the foremost hydrocarbon exploration target. The shallow-water lacustrine delta developed in Chang 8 reservoir of the study area is distributed in digitate shape, which are mainly composed of distributary channel, continuous mouth bar and together with natural levee above. The typical tight reservoir of Chang 8 member experienced a complex diagenetic evolution process, but the relationship among diagenetic alteration, deposits and lithofacies, as well as their control and influence on reservoir quality is still unclear. In this study, casting thin section, scanning electron microscope (SEM) and backscatter scanning electron microscope (BSE) combined with energy spectrum analysis, together with stable isotope analysis, X-ray diffraction analysis, and homogenization temperature test of fluid inclusions are used to analyze the reservoir characteristics including petrology, lithofacies, history and intensity of diagenesis. The control and influence of these characteristics on ultimate reservoir quality of digitate shallow-water lacustrine deltas are further evaluated. The results show that four lithofacies can be identified (namely, “i”, “ii”, “iii”, “iv”) with the decrease of grain size and increase of ductile components in the tight sandstones of digitate shallow-water lacustrine delta, respectively corresponding to preferential development positions in the deposits. There are similarities and differences on the diagenetic processes among various lithofacies. The strong mechanical compaction after burial primarily decrease the original pore space in all lithofacies, and the largest loss of pore space occurs in lithofacies iv sandstones (siltstone with wavy bedding). The chlorite is mainly in the morphology of pore-lining, but coating and rosette-like chlorite are rare. The ability to resist compaction in sandstones of lithofacies i and lithofacies ii is enhanced due to the moderate development of pore-lining chlorite, thus retaining partial pore space. Carbonate cements are abundant in relatively coarser-grained sandstones close to the sandstone-mudstone interface, and the occurrence of these cements is closely related to the adjacent mudstone, indicating that Fe and Mg ions necessary for carbonate cementation are most likely from adjacent mudstone where ions are released by mechanical compaction and subsequent hydrocarbon generation pressurization during the burial process. Weak carbonate cementation occasionally occurs at the architectural bounding surface far away from mudstone, which probably related to the transformation of clay minerals. Five typical diagenetic evolution patterns in digitate shallow-water lacustrine deltaic reservoir are summarized by considering the diagenetic processes with deposits and lithofacies. From pattern I to pattern V, the corresponding reservoir quality gradually deteriorates. The results in this study are of great help to deepen the understanding of diagenetic alteration process and reservoir heterogeneity of tight sandstones, and provide theoretical basis for exploration and sustainable development of similar lacustrine shallow-water lacustrine delta reservoirs, especially for bar fingers in shallow-water deltas.

Velocity-porosity relationships in hydrate-bearing sediments measured from pressure cores, Shenhu Area, South China Sea

Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks. Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project (GMGS4) expedition at Shenhu Area, north slope of the South China Sea. Sediments were cored above, below, and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results. Gamma density and <i>P</i>-wave velocity were measured in each pressure core before subsampling. Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments. The concentration of methane hydrates ranged from 0.3% to 32.3%. The concentrations of pore fluid (25.44% to 68.82%) and sediments (23.63% to 54.28%) were calculated from the gamma density. The regression models of <i>P</i>-wave velocity were derived and compared with a global empirical equation derived from shallow, unconsolidated sediments data. The results were close to the global trend when the fluid concentration is larger than the critical porosity. It is concluded that the dominant factor of <i>P</i>-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate. Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.

The impact of diagenesis on the reservoir quality of the early Cretaceous Lower Goru sandstones in the Lower Indus Basin, Pakistan

AbstractThe sandstone units of the Early Cretaceous Lower Goru Formation are significant reservoir for gas, oil, and condensates in the Lower Indus Basin of Pakistan. Even though these sandstones are significant reservoir rocks for hydrocarbon exploration, the diagenetic controls on the reservoir properties of the sandstones are poorly documented. For effective exploration, production, and appraisal of a promising reservoir, the diagenesis and reservoir properties must be comprehensively analyzed first. For this study, core samples from depths of more than 3100 m from the KD-01 well within the central division of the basin have been studied. These sandstones were analyzed using petrographic, X-ray diffraction, and scanning electron microscopic analyses to unravel diagenetic impacts on reservoir properties of the sandstone. Medium to coarse-grained and well-sorted sandstone have been identified during petrographic study. The sandstone are categorized as arkose and lithic arkose. Principal diagenetic events which have resulted in changing the primary characters of the sandstones are compaction, cementation, dissolution, and mineral replacement. The observed diagenetic processes can be grouped into early, burial, and late diagenesis. Chlorite is the dominant diagenetic constituent that occurs as rims, coatings, and replacing grains. The early phase of coating of authigenic chlorite has preserved the primary porosity. The recrystallization of chlorite into chamosite has massively reduced the original pore space because of its bridging structure. The current study reveals that diagenetic processes have altered the original rock properties and reservoir characteristics of the Lower Goru sandstone. These preliminary outcomes of this study have great potential to improve the understanding of diagenetic process and their impact on reservoir properties of the Lower Goru sandstone in the Lower Indus Basin and adjoining areas.

Tight Sandstone Reservoir Formation Mechanism of Upper Paleozoic Buried Hills in the Huanghua Depression, Bohai Bay Basin, Eastern China

Carboniferous-Permian petroleum resources in the Huanghua Depression of the Bohai Bay Basin, a super petroleum basin, are important exploration successor targets. The reservoir sedimentary environment of coal measures in the Upper Paleozoic buried hills is variable, and the structural evolution process is complicated, which restricts the optimization of targeting sections. Using the analysis and testing results of logging, thin section, porosity, mercury injection, hydrochemistry, and basin simulation, this study revealed the formation mechanism differences of tight sandstones in the Upper Paleozoic period in different buried hills. The results show that the sandstones are mainly feldspathic sandstone, lithic arkose, feldspathic lithic sandstone, and feldspathic lithic quartz sandstone. The quartz content varies between 25% and 70%, averaging 41%. Feldspar and debris are generally high, averaging 31% and 28%, respectively. Secondary dissolution pores are the main reservoir spaces, with 45% of the tested samples showing porosity of 5–10%, and 15% being lower than 5%. The pore radium is generally lower than 100 nm, and the sandstones are determined as small pore with fine throat and medium pore with fine throat sandstones by mercury saturation results. Frequent changing sedimentary environments and complex diagenetic transformation processes both contribute to the reservoir property differences. The former determines the original pore space, and the latter determines whether they can be used as effective reservoirs by controlling the diagenetic sequences. Combining tectonic movement background and different fluid history, the different formation mechanisms of high-porosity reservoirs are recognized, which are atmospheric leaching dominated (Koucun buried hills), atmospheric water and organic acid co-controlled (Wangguantun and Wumaying buried hills), and organic acid dominated (Nandagang buried hills) influences. The results can be beneficial for tight gas exploration and development in coal measures inside clastic buried hills in the Bohai Bay Basin.

Digital image reduction for the analysis of topological changes in the pore space of rock matrix

The paper presents an original algorithm for reducing three-dimensional digital images to improve the computing performance of persistence diagrams. These diagrams represent changes in pore space topology during essential or artificial changes in the structure of porous materials. The algorithm has linear complexity because during reduction, each voxel is checked not more than seven times. This check, as well as the removal of voxels, takes a constant number of operations. We illustrate that the algorithm’s efficiency depends on the complexity of the original pore space and the size of filtration steps. The application of the reduction algorithm allows the computation of one-dimensional persistence Betti numbers for models of up to 5003 voxels by using a single computational node. Thus, it can be used for routine topological analysis and the topological optimization of porous materials.

Study on the effect of chloride ion ingress on the pore structure of the attached mortar of recycled concrete coarse aggregate

In this study, based on the porous interface of recycled aggregates (RA), the effects of chloride ion (Cl−) ingress by drying-wetting cycles on the pore structure of recycled aggregate concrete (RAC) with a full replacement ratio under different temperatures conditions were investigated. Changes in the pore structure of the old and new mortars were determined by mercury intrusion testing. The surface morphology and morphological characteristics of the reaction products in the pores were observed via scanning electron microscopy. The results showed that the porosity, pore connectivity, fractal dimension and distribution of the pore size and pore volume were remarkably transformed by Cl− ingress. During the wetting phase, the products dissolved and migrated between the pores and crystallized during the drying phase, and the increase in temperature promoted this process. With the dry-wet cycles, the original pore space was gradually divided and filled, forming a large number of new pores with smaller pore sizes. The model of RA partition-interface Cl− ingress was presented based on the RA porous interface, and the mechanism of the effect of Cl− ingress by drying-wetting cycles on the pore structure of RAC was in-depth analyzed.

Pore size distribution in the tight sandstone reservoir of the Ordos Basin, China and their differential origin

The micro-nano pore throat system widely developed in tight sandstone is the essential difference between tight sandstone reservoir and conventional sandstone reservoir; it is also the main factor affecting the seepage characteristics and development effect of tight sandstone reservoir. This thesis analyzed the pore-throat types and distributions of the Chang 6 and the Chang 7 reservoirs in the Jiyuan area of the Ordos Basin and discussed the original causes by cast section, SEM, and constant-rate mercury injection. The results showed that the pore type of the Chang 6 reservoir is predominantly residual intergranular pores wherein the throat is mostly compacted. The pore of Chang 6 is more developed than the throat. The pore type of the Chang 7 reservoir is mainly feldspar solution pores, whereas its throat is mostly dissolution. The throat of Chang 7 is more developed than pore. The pore radius distribution range and the average value of the Chang 6 reservoir are similar to those of the Chang 7 reservoir. The distribution range and average value of the throat radius of the Chang 6 reservoir are obviously larger than that of the Chang 7 reservoir. Meanwhile, the physical property of the Chang 6 reservoir is better than that of the Chang 7 reservoir. The Chang 6 reservoir is located in delta sedimentary with shallow burial depth. The high chlorite membrane content and low compaction strength of the Chang 6 reservoir make it more conducive to maintaining the original sedimentary pore space; the dissolution further enlarges the pore space. The Chang 7 reservoir is located in a semi-deep lake sedimentary with large burial depth. The compaction and cementation of the Chang 7 reservoir are strong, and the original sedimentary pore space is massively compressed. Although dissolution strongly occurs in the later stage, the connectivity and seepage capacity of the throat of dissolution origin are lower than those of compaction origin. Dissolution can increase reservoir space; nevertheless, it cannot significantly improve seepage capacity. Therefore, there is a significant difference in permeability of the two successive layers.

Double effects of biochar in affecting the macropore system of paddy soils identified by high-resolution X-ray tomography

Biochars are widely used to improve soil macropore structures. However, the size-dependent effects of biochars in affecting macropore structure still remain unclear. In this study, the modification of soil macropore structure following biochar addition was investigated by high-resolution X-ray tomography (CT) and advanced data analytical methods. Quantification of soil macropore structure (>52 μm) was based on the intact soil cores (5 cm in diameter and 7 cm in height) collected from biochar-amended paddy soil. The treatments were: (a) control (CK), (b) rice straw biochar (RSB), (c) corn straw biochar (CSB), and (d) bamboo biochar (BB). The application rate of biochar was 25 Mg ha−1 (w/w). Results revealed that the biochars affected the soil macropores through both “occupying effect” and “expansion effect”. The “occupying effect” means that the biochar particles occupy the original pore space of the soil and reduce the soil porosity, while the “expansion effect” means that the biochar particles produce additional pore space in the soil matrix and increase the soil porosity. For all the biochar treatments in our study, the “occupying effect” was dominated. Therefore, the connected- and isolated porosity of biochar-treated soils were significantly lower than those of CK. The size-dependent effects of biochar in modifying soil macropores were observed. For RSB treatment, the “expansion effect” predominated at the size of <1200 μm and >1800 μm, while both the “occupying effect” and “expansion effect” were observed at the size of 1200–1800 μm. For CSB and BB treatments, the “expansion effect” predominate only at the size of 600–1200 μm; whereas both the “occupying effect” and “expansion effect” were observed at the size of <600 μm and >1800 μm. Our results indicated that the effects of biochar on soil macroporosity were dependent on the combined effects the “occupying effect” and “expansion effect”. In conclusion, all the biochar types in this study have adverse effects in increasing the soil macroporosity.

Failure criteria for porous dome rocks and lavas: a study of Mt. Unzen, Japan

Abstract. The strength and macroscopic deformation mode (brittle vs. ductile) of rocks is generally related to the porosity and pressure conditions, with occasional considerations of strain rate. At high temperature, molten rocks abide by Maxwell's viscoelasticity and their deformation mode is generally defined by strain rate or reciprocally by comparing the relaxation timescale of the material (for a given condition) to the observation timescale – a dimensionless ratio known as the Deborah (De) number. Volcanic materials are extremely heterogeneous, with variable concentrations of crystals, glass–melt, and vesicles (of different sizes), and a complete description of the conditions leading to flow or rupture as a function of temperature, stress and strain rate (or timescale of observation) eludes us. Here, we examined the conditions which lead to the macroscopic failure of variably vesicular (0.09–0.35), crystal-rich (∼ 75 vol %), pristine and altered dome rocks (at ambient temperature) and lavas (at 900 °C) from Mt. Unzen volcano, Japan. We found that the strength of the dome rocks decreases with porosity and is commonly independent of strain rate; when comparing pristine and altered rocks, we found that the precipitation of secondary mineral phases in the original pore space caused minor strengthening. The strength of the lavas (at 900 °C) also decreases with porosity. Importantly, the results demonstrate that these dome rocks are weaker at ambient temperatures than when heated and deformed at 900 °C (for a given strain rate resulting in brittle behaviour). Thermal stressing (by heating and cooling a rock up to 900 °C at a rate of 4 °C min−1, before testing its strength at ambient temperature) was found not to affect the strength of rocks.In the magmatic state (900 °C), the rheology of the dome lavas is strongly strain rate dependent. Under conditions of low experimental strain rate (≤ 10−4 s−1), ductile deformation dominated (i.e. the material sustained substantial, pervasive deformation) and displayed a non-Newtonian shear thinning behaviour. In this regime, the apparent viscosities of the dome lavas were found to be essentially equivalent, independent of vesicularity, likely due to the lack of pore pressurisation and efficient pore collapse during shear. At high experimental strain rates ( ≥ 10−4 s−1) the lavas displayed an increasingly brittle response (i.e. deformation resulted in failure along localised faults); we observed an increase in strength and a decrease in strain to failure as a function of strain rate. To constrain the conditions leading to failure of the lavas, we analysed and compared the critical Deborah number at failure (Dec) of these lavas to that of pure melt (Demelt = 10−3–10−2; Webb and Dingwell, 1990). We found that the presence of crystals decreases Dec to between 6.6×10−4 and 1×10−4. The vesicularity (φ), which dictates the strength of lavas, further controls Dec following a linear trend. We discuss the implications of these findings for the case of magma ascent and lava dome structural stability.

Extraction and identification methods of micro-fractured characteristics information in pore space of porous media based on morphological theory

Natural micro-fractures in ultra-tight formations, such as shales and coal seams, provide key information about pore structure and continuous channel in it. How to identify the geometrical and topological properties of micro-fractured structure and matrix pore space and the correlation between them are the major contributors which can lead to strong calculation results during the following seepage flow simulation. Through characterizing the pore space included micro-fractures (i.e., connectivity capability of pore space, extension of fracture, etc.), we can therefore describe and develop pore-fracture structure and the suitable models to gain better understanding of the roles of micro-fractures on the drainage of hydrocarbons from matrix pores. In this work, we proposed a new skeleton model to distinguish fractures from pore space via extraction of surface points set of fracture. In the procedure of points set extraction, we improved the classic “medial axis based” shrink method to “medial surface-based” method for new fracture description through introducing a new set of skeleton points (i.e., surface points and edge points of the fracture), one of which describes its aperture and the other is used for collecting connectivity information and determining the extension ranges of the fracture. The new skeleton model can show more comprehensible forms of the real connected junction instead of the former ideal model, voxel-thickness medial surface extracted can also satisfy demands of the classic skeleton extraction model and preserve the topology of the original pore space included micro-fractures in the meantime. New points set classification method mentioned above is determined by considering the difference of their topological properties. Through calculating and collecting their topological number one by one, we can obtain a new skeleton model formed with medial axis and surface. Among them, the simple points set was composed of values of topological number T 6 = T 26 =1 in the 3×3×3 direct neighborhood system as before and would be deleted in the process of shrinking pore space in the certain order of the distance values of the space. The surface points forming medial surface was composed of the points close to the center of the micro-fractures in all directions. An object point was defined as surface point if no background voxels continuously existed between any two neighbor voxel that shares a “face” in its 3×3×3 neighborhood system, which means its topological number T 6 >1. Edge points represented as the junction between fractures and matrix if they are always the one of the 26 neighbors of surface points. Moreover, in the process of obtaining new skeleton model, characteristic parameters and connectivity of micro-fractures can then be easily got via statistics and calculation. Through combining Euclidean distance maps and geometric transformation, we can easily calculated the parameters of width, thickness, orientation, and inclination angle of micro-fractures. Connectivity location information from edge points would play the part of following simulation of flow interaction between fractures and the matrix. As a contrast, ideal and real fracture models were used to verify feasibility of our new methods, all results showed good effectiveness and accuracy. The study will lead to more realistic pore space models and help to extend the applicability to a wider range of porous media especially for the study of multi-scale pore space representation. This work was inspired by challenges in developing a fast and accurate method for micro-scale modeling in micro-fractured porous media, and potentially applicable for flow simulations in the tight porosity samples. Overall, our new methods improved the level of micro-fracture characterization representation of the pore space including fractures for the following flow simulation.

Cristobalite in the 2011–2012 Cordón Caulle eruption (Chile)

Cristobalite is a low-pressure high-temperature polymorph of SiO2 found in many volcanic rocks. Its volcanogenic formation has received attention because (1) pure particulate cristobalite can be toxic when inhaled, and its dispersal in volcanic ash is therefore a potential hazard; and (2) its nominal stability field is at temperatures higher than those of magmatic systems, making it an interesting example of metastable crystallization. We present analyses (by XRD, SEM, EPMA, Laser Raman, and synchrotron μ-cT) of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011–2012 eruption of Cordon Caulle (Chile). Cristobalite was not detected in pyroclasts, negating any concern for respiratory hazards, but it makes up 0–23 wt% of lava samples, occurring as prismatic vapour-deposited crystals in vesicles and/or as a groundmass phase in microcrystalline samples. Textures of lava collected near the vent, which best represent those generated in the conduit, indicate that pore isolation promotes vapour deposition of cristobalite. Mass balance shows that the SiO2 deposited in isolated pore space can have originated from corrosion of the adjacent groundmass. Textures of lava collected down-flow were modified during transport in the insulated interior of the flow, where protracted cooling, additional vesiculation events, and shearing overprint original textures. In the most slowly cooled and intensely sheared samples from the core of the flow, nearly all original pore space is lost, and vapour-deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix. Holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite. Vapour deposition and devitrification act concurrently but semi-independently. Both are promoted by slow cooling, and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies. Our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions, with insight afforded by the fundamentally different emplacement geometries of flows and domes.

Imaged-based multiscale network modelling of microporosity in carbonates

Abstract Diagenetic changes such as cementation or dissolution have a strong control on carbonate pore structure, and often disconnect the original intergranular pore space. Spatial distribution of submicron porosity (microporosity) that develops in the process, as well as its influence on flow properties, is difficult to image and characterize. Yet, a petrophysically rigorous pore-scale model that accounts for submicron porosity interconnectivity would help in the understanding and development of carbonate reservoirs dominated by microporosity. We present algorithms to geometrically match pore–throat networks from two separate length scales that can be extracted directly from three-dimensional (3D) rock images, or be constructed to match the relevant measured properties. We evaluate the combined influence of cementation and dissolution using a Bentheimer Sandstone sample, as well as image-identified microporosity on flow transport properties in an Estaillades Limestone sample.

Description and reconstruction of the soil pore space using correlation functions

In this paper a method for the description and reconstruction of the soil pore space using correlation functions has been examined. The reconstruction procedure employed here is the best way of verification of the potential descriptor of the soil pore space. Thin sections representing eight major types of pore space in zonal loamy soils and parent materials of the Russian Plain with pores of different shapes and orientations have been chosen for this study. Comparison based on the morphological analysis of the original pore space images and their correlation function reconstructions obtained using simulated annealing technique indicates that this method of reconstruction adequately describes the isometric soil pore space with isometric dissected, isometric slightly dissected, and rounded pores. The two-point correlation functions calculated with the use of the orthogonal method proved to be different for the examined types of soil pore space; they reflect the soil porosity, specific surface, and pore structure correlations at different lengths. The results of this study allow us to conclude that the description of the soil pore space with the help of correlation functions is a promising approach, but requires more development. Further directions of the development of this method for describing the soil pore space and determining the soil physical processes are outlined.

Transient N2O accumulation and emission caused by O2 depletion in soil after liquid manure injection

Emission of nitrous oxide (N2O) from soils is the net result of N2O-producing and consuming processes within the soil, and studying the regulation of these processes in the real soil environment is essential to the understanding of the factors governing N2O emission. In this study, microscale distributions of O2 and N2O in the soil were investigated to describe how N2O production within, and emission from, soils are regulated by anoxic volumes created by injection of liquid manure. An application device simulating field injection methodology was developed and liquid pig manure was injected at a depth of 5 cm into boxes containing soil. Microsensors with <0.12 mm tip diameter were used to measure high-resolution vertical N2O and O2 concentration profiles though the centre of the horizontally positioned soil-manure core and up to 4 cm laterally away from the centre. Both microsensor measurements and N2O emission rate determinations, with a closed chamber, were performed daily. Injected manure filled the original air-filled pore space of a 6-cm-wide cylindrical core and created anoxia. Nitrous oxide was detected in the anoxic part of the core, indicating N2O production by denitrification in the entire anoxic volume. Although anoxia was present in the core during all 3 days of the experiment, a peak rate of net N2O production was detected after 1 day, with a maximum N2O accumulation of 500–700 Pa in the core. Comparison of the cumulated N2O net production and emission revealed a delay of N2O emission, as N2O was trapped inside the saturated core.

Retention of High Permeability During Shallow Burial (300 to 500 M) of Carbonate Grainstones

Abstract A progressive decrease in matrix permeability occurs with burial in the Paleogene limestones of west-central Florida, yet grainstones with permeabilities > 400 md persist below burial depths of 300 m. Why some grainstones retain high permeability during passage through the shallow-burial realm was investigated using 85 dominantly foraminifera and micritized-grain grainstones from the Eocene Avon Park Formation. Twenty-seven variables representing diagenesis, stratigraphic packaging, depositional energy, grain types, and pore-network attributes were quantified. Permeabilities range from 23 to 7,160 md and thin-section porosities range from 14% to 36%. Grain size (lower coarse to upper fine sand) covaries positively with sorting (poor to very good). The samples are from three different systems tracts within the Avon Park, and sampled grainstone bodies ranged in thickness from 0.5 to 3.5 m. Evidence of diagenesis include mechanical and chemical burial compaction, pre-compaction cements, and dissolution. Original interparticle porosity loss due to compaction (COPL) and cementation (CEPL) range from 0 to 35.3% and 1.6 to 26.2% respectively. Most original intraparticle porosity is occluded, but most moldic porosity, which ranges from 1.2% to 12%, remains open. Multivariate regression reveals that 61% of the variance in permeability is attributed to grain size, CEPL, COPL, and systems tract (a proxy for relative abundance of compactable foraminifera versus robust micritized grains). Large grain size means a large initial permeability; all other regression components reflect diagenetic reductions in permeability. Observed ranges in CEPL and COPL values mean they can reduce permeability by more than three orders of magnitude whereas the other regression parameters reduce it by only one order of magnitude. Retention of high (> 400 md) permeability can occur by (1) cementation of ~ 40% of the original interparticle pore space, which inhibits subsequent compaction yet preserves high permeability, or (2) a combination of cementation and compaction that destroys less than two thirds of the original interparticle pore space. These relations, derived from originally calcite-rich sediments, provide a quantitative reference by which to evaluate permeability preservation during shallow burial in numerical simulation of diagenesis and reservoir development. Capillary-pressure data also support the hypothesis that pore geometries, and hence permeability, respond differently to compaction versus cementation. Cement-dominated samples exhibit a dominance of porosity behind intermediate-sized pores at higher permeabilities than compaction-dominated samples. Compaction also does not yield a large change in the amount of porosity behind pore throats smaller than 1 μ at any permeability above 20 md, whereas cementation-dominated diagenesis generates significant amounts of pore space behind those small throats once permeability goes below 100 md. These differences arise because cementation converts the original interparticle pores to a system of interparticle polyhedral and sheet-like pores connected by progressively smaller pore throats between cement crystals, whereas compaction merely constricts the original interparticle porosity behind smaller intergranular pore throats.

The effect of carbonate cementation on permeability heterogeneity in fluvial aquifers: An outcrop analog study

Spatial variations in permeability have been widely recognized as a dominant control on solute transport and petroleum migration. Outcrop studies are one means of improving our understanding of the nature of permeability heterogeneity and the underlying geologic controls. To date, these studies have either neglected the importance of cementation or investigated only the modern cemented permeability without explicitly considering how cementation has affected permeability. Two outcrops of channel sand deposits in the Plio-Pleistocene Sierra Ladrones Formation, New Mexico are investigated to quantify (1) the effect of cementation on permeability at a given location, and (2) the patterns of cementation and their impact on permeability heterogeneity. We analyzed approximately 400 spatially located samples (∼ 10 cm 3 each) from the two outcrops. Cementation is typically weak to moderate, filling less than 60% of the original pore space. Permeability and cement content are bimodally distributed and negatively correlated. Permeability reduction is investigated by measuring modern permeability, minus-cement permeability, minus-cement porosity, and volumetric cement content. Permeability is reduced locally by up to three orders of magnitude. By combining models of permeability for uncemented sediments and cemented sediments, we derive a model of cemented permeability ( k c) that depends primarily on three factors: uncemented permeability ( k u), original porosity, and cement fraction. Our results suggest that accounting for porosity reduction only may result in an overestimation of k c. Conversely, accounting for porosity reduction, increased specific surface area, and increased tortuosity appears to result in an underestimation of k c. Accounting for porosity reduction and increased specific surface area provide the best estimate of k c. Cementation is highly variable spatially and is poorly correlated with lithofacies. Variogram analysis of cementation and permeability indicates that the spatial distribution of cementation is the dominant control on the permeability correlation structure. The spatially variable cementation also results in an order-of-magnitude increase in the variance of log-permeability. Applying stochastic models of effective flow and transport properties to the results of this study suggest that cementation has a relatively small effect on effective permeability but can potentially increase dispersivity by two orders of magnitude.

Kela-2: a major gas field in the Tarim Basin of west China

Kela-2, a large gas field developed in a sub-thrust anticline within the Kuqa depression of the Tarim Basin and with reserves of 7.55×10 12 SCF, represents the largest unsegmented gas pool in China. Detailed petrographic, geochemical, structural and fluid inclusion studies show that the field is a result of a favourable interrelationship of geological events, involving several periods of compressive structuring, multiple periods of oil and gas migration and a complex diagenetic history. The Kela-2 discovery well encountered abundant oil and gas shows over a 448 m gas column within Cretaceous to Lower Tertiary Bashijiqike and Baxigai Formation sands. Gas is trapped in a large, heavily faulted, northeast-east striking fold with a décollement horizon developed along a gypsiferous horizon. The structure, with a closure of 55 km 2 , was initiated in the Late Tertiary compressive phase and was accentuated during Quaternary Xiyu compression. The source of the gas is Triassic and Jurassic coals, which communicate with the reservoir vertically through faults. It is possible through fluid inclusion studies to identify two periods of secondary oil migration and two periods of re-migration within the reservoir. The reservoirs were deposited in a continental environment, within which secondary porosity has been developed by leaching of carbonate cement that infilled original primary pore spaces. The gypsiferous mudstones of the Lower Tertiary provide a common seal to both reservoirs, which are abnormally pressured.