Showing papers on "Effective porosity published in 2016"

Compaction and seepage properties of crushed limestone particle mixture: an experimental investigation for Ordovician karst collapse pillar groundwater inrush

Abstract: As mining activity rapidly develops to deeper and deeper ground, there have been an increasing number of groundwater inrush events occurring in China. The Ordovician limestone karst collapse pillar (KCP), which contains a lot of crushed rocks, usually functions as a channel for groundwater inrush. The factors, such as the geological conditions, groundwater flow and extraction percentage of coal seam in underground mines, are the main causes of recompaction and permeability change of crushed rock particles in KCP. The high permeability of rock will cause large volumes of groundwater flow, thus posing a great threat of loss of lives in the mines. It is important to take into account the compaction and seepage behaviors of the crushed rocks in Ordovician limestone KCP. The MTS815.02 system and a self-designed water flow apparatus were used to investigate the effect of particle size distribution on compaction and seepage behavior of crushed limestone particle mixture. The Reynolds number calculation of particle mixture shows that the seepage has been influenced by non-Darcy flow. Testing results indicate that the effective porosity of crushed limestone sample is strongly influenced by compaction and particle size distribution. The effective porosity decreases with the increase in compaction and decrease in larger particle size, respectively. Particle crushing during compaction is a main cause of size 0–2.5 mm materials, whereas some fine particles are washed away due to the effect of water seepage, which is a main cause of weight loss. Non-Darcy seepage properties of the crushed limestone are strongly influenced by compaction and particle size distribution. In general, during the compaction, the permeability decreases while the non-Darcy coefficient increases with the decreasing of effective porosity. The effective porosity, particle crushing and seepage properties of crushed limestone are not only related to compaction levels and mixture sizes but also to style of arrangement and initial pore structure.

Impact of particle transfer on flow properties of crushed mudstones

Abstract: Karst collapse pillar (KCP) is widespread in North China coalfields, where coal extraction above the Ordovician limestone aquifer is threatened by the abundant supply of water and a very high hydraulic pressure. KCP is composed of rock skeleton and fine fillings, which can be transferred under the effect of water flow, thus KCP usually functions as a channel for groundwater inrush. In order to investigate the mechanism of mining-induced groundwater inrush of KCP which was caused by fine fillings transfer, a self-designed particle transfer permeability testing system was used to test the crushed mudstones’ flow properties, which included fine filling particle transfer rate, porosity increase rate and permeability under the conditions of varying pore pressure, particle size mixture and compaction level (initial porosity). The tests indicate fine fillings transfer is the essential reason for mining-induced groundwater inrush (flow instability) of KCP. The flow properties changeability during fine particle transfer could be divided into four stages, i.e., initial flow stage, flow inrush stage, continued particle flow stage and stable flow stage, and flow inrush stage which is the key point to cause water inrush. Due to the crushing of edges and corners and the adjustment of the structure, the fluctuation of permeability–time relationship mainly distributed in the first two stages, which make a change to flow channel. Moreover, with the increasing of pore pressure, particle size mixture and initial porosity, the water inrush time and stable seepage time decreased more rapidly; the fine fillings, porosity and permeability increased gradually. The efficiency criteria analysis between measurement and calculation permeability shows an empirical equation can fit the relationship between porosity and permeability well, and not all of the pore structures were in flow; this means there was a part of pores that were invalid, but the effective porosity in flow can be treated as calculation value approximately.

Chapter 3: An SEM Study of Porosity in the Eagle Ford Shale of Texas—Pore Types and Porosity Distribution in a Depositional and Sequence-stratigraphic Context

Abstract: Although typically considered with a focus on high-resolution petrography, shale porosity should not be thought of as a stand-alone petrographic feature. Shale and mudstone porosity is the outcome of a long succession of processes and events that span the continuum from deposition through burial, compaction, and late diagenesis. For the Eagle Ford Shale this journey began with accumulation in intra-shelf basins at relatively low latitudes on a southeast-facing margin during early parts of the late Cretaceous. To understand the factors that generated and preserved porosity in this economically important interval, a scanning electron microscope study on ion-milled drill-core samples from southern Texas was conducted to understand the development of petrographic features and porosity and place them in stratigraphic context. The studied samples show multiple pore types, including pores defined by mineral frameworks (clay and calcite), shelter pores in foraminifer tests and other hollow fossil debris, and pores in organic material (OM). In many instances, framework and shelter pores are filled with OM that has developed pores due to maturation. Large bubble pores in OM suggest that hydrocarbon liquids were left behind in or migrated into these rocks following petroleum generation and that the bubbles developed as these rocks experienced additional thermal stress. These larger OM pores indicate deeper seated interconnection on ion-milled surfaces and in three-dimensional image stacks. The largest pores occur in the infills of foraminifer tests. The framework of crushed carbonate debris in planktonic fecal pellets shows intermediate levels of porosity, and the silicate-rich matrix that encloses framework components has the smallest average porosity. The distribution of pore types is not uniform. Our hypothesis is that facies association is an important factor that determines bulk porosity and influences reservoir performance. The observed variability in the attributes of the described distal, medial, and proximal facies associations is thought to translate into significant variability of rock properties such as total organic carbon and porosity. In turn, this variability should control the quality and distribution of the intervals that are optimum sources and reservoirs of hydrocarbons in the Eagle Ford Shale. The medial facies association most likely has the best porosity development when a favorable combination of more commonly abundant calcareous fecal pellets and organic material versus clay content is present. The systematic arrangement of facies associations into parasequences provides the basis for testing and predicting the best development of optimal reservoir facies within a sequence-stratigraphic framework in the Eagle Ford Shale.

Impacts of hydrothermal dolomitization and thermochemical sulfate reduction on secondary porosity creation in deeply buried carbonates: A case study from the Lower Saxony Basin, northwest Germany

Abstract: The role of deep-burial dissolution in the creation of porosity in carbonates has been discussed controversially in the recent past. We present a case study from the Upper Permian Zechstein 2 carbonate reservoirs of the Lower Saxony Basin in northwest Germany. These reservoirs are locally characterized by high amounts of carbon dioxide (CO2) and variable amounts of hydrogen sulfide (H2S), which are derived from thermochemical sulfate reduction (TSR) and inorganic sources. To study the contribution of these effects on porosity development, we combine petrography, stable isotope, and rare earth and yttrium (REY) analyses of fracture cements with Raman spectroscopy and δ13C analyses of fluid inclusions. It is shown that fluid migration along deep fault zones created and redistributed porosity. Fluid inclusion analyses of vein cements demonstrate that hydrothermal fluids transported inorganic CO2 into the reservoir, where it mixed with minor amounts of TSR-derived organic CO2. The likely source of inorganic CO2 is the thermal decomposition of deeply buried Devonian carbonates. The REY distribution patterns support a hydrothermal origin of ascending iron- and CO2-rich fluids causing dolomitization of calcite and increasing porosity by 10%–16% along fractures. This porosity increase results from hydrothermal dolomitization and dissolution by acids generated from the reaction of Fe2+ with H2S to precipitate pyrite. In contrast, hydrothermal dolomite cements reduced early diagenetic porosity in dolomitic intervals by approximately 17%. However, the carbonate dissolution in the predominantly calcitic host rock results in a net increase in porosity and permeability in the vicinity of the fracture walls, which has to be considered for modeling reservoir properties and fluid migration pathways.

The referential grain size and effective porosity in the Kozeny–Carman model

Abstract: . In this paper, the results of permeability and specific surface area analyses as functions of granulometric composition of various sediments (from silty clays to very well graded gravels) are presented. The effective porosity and the referential grain size are presented as fundamental granulometric parameters expressing an effect of the forces operating on fluid movement through the saturated porous media. This paper suggests procedures for calculating referential grain size and determining effective (flow) porosity, which result in parameters that reliably determine the specific surface area and permeability. These procedures ensure the successful application of the Kozeny–Carman model up to the limits of validity of Darcy's law. The value of effective porosity in the referential mean grain size function was calibrated within the range of 1.5 µm to 6.0 mm. The reliability of the parameters applied in the KC model was confirmed by a very high correlation between the predicted and tested hydraulic conductivity values (R2 = 0.99 for sandy and gravelly materials; R2 = 0.70 for clayey-silty materials). The group representation of hydraulic conductivity (ranging from 10−12 m s−1 up to 10−2 m s−1) presents a coefficient of correlation of R2 = 0.97 for a total of 175 samples of various deposits. These results present new developments in the research of the effective porosity, the permeability and the specific surface area distributions of porous materials. This is important because these three parameters are critical conditions for successful groundwater flow modeling and contaminant transport. Additionally, from a practical viewpoint, it is very important to identify these parameters swiftly and very accurately.

A comprehensive simulation model of kerogen pyrolysis for the in-situ upgrading of oil shales

Abstract: Author(s): Lee, K; Moridis, GJ; Ehlig-Economides, CA | Abstract: Oil shale, which is composed of abundant organic matter called kerogen, is a vast energy source. Pyrolysis of kerogen in oil shales releases recoverable hydrocarbons. Here, we describe the pyrolysis of kerogen with an in-situ upgrading process, which is applicable to the majority of oil shales. The pyrolysis is represented by six kinetic reactions resulting in 10 components and four phases. Expanding the Texas AaM Flow and Transport Simulator (FTSim), which is a variant of the TOUGH +simulator (Moridis 2014), we develop a fully functional capability that describes kerogen pyrolysis and accompanying system changes. The simulator describes the coupled process of mass transport and heat flow through porous and fractured media and includes physical and chemical phenomena of reservoir systems. The simulator involves a total of 15 thermophysical states and all transitions between them and computes a simultaneous solution of 11 mass- and energy-balance equations per element. The simulator solves the equations in a fully implicit manner by solving Jacobian matrix equations with the Newton-Raphson iteration method. To conduct a realistic simulation, we account for geological structure of oil-shale reservoirs and physical properties of bulk-oil shale rocks by considering phases and components in the pores. In addition, we involve interaction between fluids and porous media, diverse equations of state (EOSs) for computation of fluid properties, and numerical modeling of fractured media. We intensively reproduce the field-production data of Shell Insitu Conversion Process (ICP) implemented in the Green River formation by conducting sensitivity analyses for the diverse reservoir parameters, such as initial effective porosity of the matrix, oil-shale grade, and the spacing of the natural-fracture network. We analyze the effect of each reservoir parameter on the hydrocarbon productivity and product selectivity. The simulator provides a powerful tool to quantitatively evaluate production behavior and dynamic-system changes during in-situ upgrading of oil shales and subsequent fluid production by thoroughly describing a reservoir model, phases and components, phase behavior, phase properties, and evolution of porosity and permeability.

Evaluation of Microstructure and Transport Properties of Deteriorated Cementitious Materials from Their X-ray Computed Tomography (CT) Images.

Abstract: Pore structure, tortuosity and permeability are considered key properties of porous materials such as cement pastes to understand their long-term durability performance. Three-dimensional image analysis techniques were used in this study to quantify pore size, effective porosity, tortuosity, and permeability from the X-ray computed tomography (CT) images of deteriorated pastes that were subjected to accelerated leaching test. X-ray microtomography is a noninvasive three-dimensional (3D) imaging technique which has been recently gaining attention for material characterization. Coupled with 3D image analysis, the digitized pore can be extracted and computational simulation can be applied to the pore network to measure relevant microstructure and transport properties. At a spatial resolution of 0.50 μm, the effective porosity (ψe) was found to be in the range of 0.04 to 0.33. The characteristic pore size (d) using a local thickness algorithm was found to be in the range of 3 to 7 μm. The geometric tortuosity (τg) based on a 3D random walk simulation in the percolating pore space was found to be in the range of 2.00 to 7.45. The water permeability values (K) using US NIST Permeability Stokes Solver range from an order of magnitudes of 10-14 to 10-17 m². Indications suggest that as effective porosity increases, the geometric tortuosity increases and the permeability decreases. Correlation among these microstructure and transport parameters is also presented in this study.

Short‐range, overpressure‐driven methane migration in coarse‐grained gas hydrate reservoirs

Abstract: Two methane migration mechanisms have been proposed for coarse-grained gas hydrate reservoirs: short-range diffusive gas migration and long-range advective fluid transport from depth. Herein we demonstrate that short-range fluid flow due to overpressure in marine sediments is a significant additional methane transport mechanism that allows hydrate to precipitate in large quantities in thick, coarse-grained hydrate reservoirs. Two-dimensional simulations demonstrate that this migration mechanism, short-range advective transport, can supply significant amounts of dissolved gas and is unencumbered by limitations of the other two end-member mechanisms. Here, short-range advective migration can increase the amount of methane delivered to sands as compared to the slow process of diffusion, yet it is not necessarily limited by effective porosity reduction as is typical of updip advection from a deep source.

Physical properties of peat soils under different land use options

Abstract: Pristine peat soils are characterized by large porosity, low density and large water and organic matter contents. Drainage and management practices change peat properties by oxidation, compaction and mineral matter additions. This study examined differences in physical properties (hydraulic conductivity, water retention curve, bulk density, porosity, von Post degree of decomposition) in soil profiles of two peatland forests, a cultivated peatland, a peat extraction area and two pristine mires originally within the same peatland area. Soil hydraulic conductivity of the drained sites (median hydraulic conductivities: 3.3 × 10−5 m/s, 2.9 × 10−8 m/s and 8.5 × 10−8 m/s for the forests, the cultivated site and the peat extraction area, respectively) was predicted better by land use option than by soil physical parameters. Detailed physical measurements were accompanied by monitoring of the water levels between drains. The model ‘DRAINMOD’ was used to assess the hydrology and the rapid fluctuations seen in groundwater depths. Hydraulic conductivity values needed to match the simulation of observed depth to groundwater data were an order of magnitude greater than those determined in field measurements, suggesting that macropore flow was an important pathway at the study sites. The rapid response of depth to groundwater during rainfall events indicated a small effective porosity and this was supported by the small measured values of drainable porosity. This study highlighted the potential role of land use and macropore flow in controlling water table fluctuation and related processes in peat soils.

Impact of kinetic mass transfer on free convection in a porous medium

Abstract: We investigate kinetic mass transfer effects on unstable density-driven flow and transport processes by numerical simulations of a modified Elder problem. The first-order dual-domain mass transfer model coupled with a variable-density-flow model is employed to describe transport behavior in porous media. Results show that in comparison to the no-mass-transfer case, a higher degree of instability and more unstable system is developed in the mass transfer case due to the reduced effective porosity and correspondingly a larger Rayleigh number (assuming permeability is independent on the mobile porosity). Given a constant total porosity, the magnitude of capacity ratio (i.e., immobile porosity/mobile porosity) controls the macroscopic plume profile in the mobile domain, while the magnitude of mass transfer timescale (i.e., the reciprocal of the mass transfer rate coefficient) dominates its evolution rate. The magnitude of capacity ratio plays an important role on the mechanism driving the mass flux into the aquifer system. Specifically, for a small capacity ratio, solute loading is dominated by the density-driven transport, while with increasing capacity ratio local mass transfer dominated solute loading may occur at later times. At significantly large times, however, both mechanisms contribute comparably to solute loading. Sherwood Number could be a nonmonotonic function of mass transfer timescale due to complicated interactions of solute between source zone, mobile zone and immobile zone in the top boundary layer, resulting in accordingly a similar behavior of the total mass. The initial assessment provides important insights into unstable density-driven flow and transport in the presence of kinetic mass transfer.

Determination of Shale Volume and Distribution Patterns and Effective Porosity from Well Log Data Based On Cross-Plot Approach for A Shaly Carbonate Gas Reservoir

Abstract: Determination of shale volume distribution is one of the most important factors that has to be considered in formation evaluation, since existence of shale reduces effective porosity and permeability of the reservoir. In this paper, shale volume and distribution (dispersed, laminar and structural) and formation effective porosity are estimated from well log data and cross-plots. Results show that distribution of shale is mainly dispersed with few of laminar ones, and the quality of reservoir (effective porosity) decreases with depth resulting in low productivity of gas wells drilled in lower zones. Good agreement of estimated shale volumes and effective porosities from neutron-density cross-plot with the values determined from gamma ray log (CGR) and core analysis demonstrates the accuracy and applicability of these plots in determination of petrophysical parameters from conventional log data.

Porosity, mineralogy, and pore fluid from simultaneous impedance inversion

Abstract: We find that in a quartz/clay system, P- and S-wave impedances (IP and IS, respectively) each uniquely depend on a different linear combination of the total porosity and clay content. This implies that if the pore fluid is known, we can resolve these two seismically derived impedances for porosity and clay content. Key to such interpretation is rock-physics diagnostics that provide a theoretical rock-physics model to quantitatively explain well data by relating IP and IS to porosity, clay content, and pore fluid. The well data conditioned according to this model serve as input to simultaneous impedance inversion. Poisson's or the IP/IS ratio serves as the pore-fluid identifier. We give an example of such rock-physics-based interpretation of seismically derived impedances for rock properties based on well and seismic data from an offshore oil field.

Effect of diagenesis on the petrophysical properties of the Miocene rocks at the Qattara Depression, north Western Desert, Egypt

Abstract: This study focuses on the effect of diagenetic processes on the petrophysical characteristics of the Miocene rocks exposed east of the Qattara Depression, north Western Desert, Egypt. Several techniques were applied on the collected rock samples in order to determine their mineralogic composition and the diagenetic processes they have undergone. The petrophysical analyses are conducted on horizontal plugs representing the Miocene Formations in the study area. They mainly include porosity, permeability, and density. Petrographic analysis revealed that the Moghra Formation is composed mainly of quartzarenites with few shale and limestone intercalations. The Marmarica Formation, on the other hand, is composed mainly of sandy dolomicrite, sandy biodolomicrite, and sandy biomicrite facies. The main diagenetic processes encountered are neomorphism, dolomitization, dissolution, cementation, compaction, and replacement. Values of porosity, permeability, grain, and bulk densities for the studied plugs derived from the Moghra Formation range from 14.7 to 27.8 %, from 0.01 to 35.39 mD, from 2.51 to 2.79, and from 2.01 to 2.32 g/cm3, respectively, while they range from 2.9 to 40.8 %, from 0.002 to 14739.15 mD, from 2.67 to 2.8 g/cm3, and from 1.62 to 2.65 g/cm3 for the samples of Marmarica Formation. Both petrographical and petrophysical studies revealed primary and secondary origins of the sample porosity and permeability and that the studied sandstones can be considered as good hydrocarbon reservoirs. In addition, the studied carbonate rocks are characterized by high effective porosity and permeability due to the secondary enhancement through the dissolution of fossils and other components implying that their corresponding subsurface occurrences represent good reservoir rocks.

Study of threshold gradient for compacted clays based on effective aperture

Abstract: Compacted clay liner plays a crucial role in preventing leakage and migration of pollutants. Clay often appears in Non-Darcy phenomenon, one of which is characterized by having the threshold gradient. The threshold gradients exhibited by different clays are usually quite different. The reason for the discrepancy is currently unknown. To solve this problem, the mineral compositions of two clays were selected for test. The tests included threshold gradient, aperture and the pore water content. The test results showed that kaolin-based Clay A had no threshold gradient, whereas illite and Montmorillonite-based Clay B presented the threshold gradient that decreased with the increase of the porosity. For the samples mixed with Clay A and Clay B, the values of the threshold gradient increased with increasing the content of Clay B in the mixed samples. The reason is that the porosity and mineral content can affect the bound water content. If the value of pF is greater than 3.8, the bound water is gelatinous “immobile water”. Based on the water retention curves, the effective porosity can be obtained by subtracting the volume occupied by “immobile water” from the void of the soil particles. Then the modified effective aperture can be obtained by Hangen-Poiseuille law. The modified effective aperture will decrease with the decrease in the porosity and the increase of the content of illite and montmorillonite. This is the key reason why the different clays have different values of the threshold gradients.

Methods for estimating petrophysical parameters from well logs in tight oil reservoirs: a case study

Abstract: Estimating petrophysical parameters from well logs plays a significant role in the exploration and development of tight oil resources, but faces challenges. What's more, the methods for petrophysical parameters from well logs are paid little attention at present. In this paper, the typical tight oil reservoirs of Northwest China are used as an example. Based on the characteristics of mineralogy and fluids in the study field, the rock is assumed into five components which are clays, quartz and feldspar, carbonates, kerogen and pore fluids (porosity). The sum of kerogen content and porosity is defined as the apparent porosity. Then, two porosity log response equations are established. Once the clay content is determined by an individual method, the quartz and feldspar content, carbonate content and apparent porosity are calculated through the established equations. The kerogen content is the difference of the apparent porosity and porosity from nuclear magnetic resonance (NMR) logs. This paper also presents a new approach that combines the complex refractive index method (CRIM) and pseudo Archie method to compute saturation from dielectric logs, which avoids selection for the dielectric constants of each of the minerals. The effectiveness and reliability of these methods are verified by the successful application in the study of the target tight oil play in Northwest China.

Effect of pore geometry on Gassmann fluid substitution

Abstract: Although there is no assumption of pore geometry in derivation of Gassmann’s equation, the pore geometry is in close relation with hygroscopic water content and pore fluid communication between the micropores and the macropores. The hygroscopic water content in common reservoir rocks is small, and its effect on elastic properties is ignored in the Gassmann theory. However, the volume of hygroscopic water can be significant in shaly rocks or rocks made of fine particles; therefore, its effect on the elastic properties may be important. If the pore fluids in microspores cannot reach pressure equilibrium with the macropore system, assumption of the Gassmann theory is violated. Therefore, due to pore structure complexity, there may be a significant part of the pore fluids that do not satisfy the assumption of the Gassmann theory. We recommend that this part of pore fluids be accounted for within the solid rock frame and effective porosity be used in Gassmann’s equation for fluid substitution. Integrated study of ultrasonic laboratory measurement data, petrographic data, mercury injection capillary pressure data, and nuclear magnetic resonance T2 data confirms rationality of using effective porosity for Gassmann fluid substitution. The effective porosity for Gassmann’s equation should be frequency dependent. Knowing the pore geometry, if an empirical correlation between frequency and the threshold pore-throat radius or nuclear magnetic resonance T2 could be set up, Gassmann’s equation can be applicable to data measured at different frequencies. Without information of the pore geometry, the irreducible water saturation can be used to estimate the effective porosity.

An Experimental Study on the Slippage Effect of Gas Flow in a Compact Rock

Abstract: Gas flow in small pore throats in compact rocks is usually affected by the gas slippage effect due to the dense structure and low porosity of the rocks. In this study, permeability and porosity of two granitic gneiss specimens under different pore and confining pressures are measured. Petrographic studies are also performed using X-ray diffraction, optical microscopy, and scanning electron microscopy coupled with an energy-dispersive spectrometer. Test data indicate that the gas flow in the compact rock does not follow Darcy’s law due to the effect of gas slippage, and the measured permeability needs to be corrected by the gas slippage effect. The test results show that the gas slippage effect increases subsequently when the pore pressure is low, which leads to the measured permeability higher than the absolute permeability. The influence of confining pressure on the impact rate of the slippage effect appears to approach an upper limit symptomatically. It is found that a power law describes well the relationship between the absolute permeability and the effective porosity. A correlation of the slippage factor and the absolute permeability is provided. When the confining pressure is high and the pore pressure is low, the flows are slip flow and transitional flow and traditional fluid dynamics N–S equations are not applicable and Knudsen’s diffusion equations should be used.