Matrix Porosity Research Articles

Can Karst Conduit Models Be Calibrated? A Dual Approach Using Dye Tracing and Temperature.

Modeling flow and transport using both temperature and dye tracing provides constraints that can improve understanding of karst networks. A laminar flow and transport model using the finite element subsurface flow model simulated the conduit connection between a sinking stream and spring in central Pennsylvania to evaluate how conduit morphology might affect dye transport. Single and overly tortuous conduit models resulted in high concentrations as dye flowed back into the conduit from the matrix after dye injections ceased. A forked conduit model diverted flow from the main conduit, reducing falling limb dye concentration. Latin hypercube sampling was performed to evaluate the sensitivity of 52 parameter combinations (conduit hydraulic conductivity, conduit cross-sectional area, matrix transmissivity, matrix porosity, and dispersivity) for four conduit geometry scenarios. Sensitivity of arrival time for 50% of the dye indicated no parameter combinations which simulate falling limb dye concentrations for tortuous geometries, confirming the importance of the forked geometry regardless of other parameters. Temperature data from high-resolution loggers were then incorporated into the forked conduit model to reproduce seasonal spring temperature using variable sink inflow. Unlike the dye trace models, the thermal models were sensitive to other model parameters, such as conduit cross-sectional area and matrix transmissivity. These results showed this dual approach (dye and temperature) to karst network modeling is useful for (1) exploring the role of conduit and matrix interaction for contaminant storage, (2) constraining karst conduit geometries, which are often poorly understood, and (3) quantifying the effect of seasonal trends on karst aquifers.

Quantitative Seismic Interpretation Identifies Sweet Spots in Organic-Rich Mudrocks

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 197290, “Quantitative Seismic Interpretation Work Flow for Sweet-Spot Identification in Organic-Rich Mudrocks,” by Mustafa A. Al Ibrahim, Tapan Mukerji, SPE, and Allegra Hosford Scheirer, Stanford University, prepared for the 2019 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11–14 November. The paper has not been peer reviewed. Estimating the lateral heterogeneity of geochemical properties of organic-rich mudrocks is important for unconventional resource plays. Well-penetration data from emerging plays are sparse and so traditional geostatistical methods will not yield good results. In this study, the authors use a quantitative seismic interpretation work flow (QSI) based on a rock-physics template in estimating the uncertainty of the geochemical properties of organic mudrocks of the Shublik formation of the Alaskan North Slope. By combining the rock-physics template and the results of seismic inversion, multiple realizations of total organic content (TOC), matrix porosity, and brittleness index are generated. These parameters can be used for sweet-spot detection. Introduction Data for physical rock parameters are generally restricted to well locations. To extend data laterally, two general directions can be taken: a data-oriented approach and a physics-oriented approach. The data-oriented approach is powerful but requires a large amount of data commonly not available in emerging areas where well penetration is scarce. Physics-oriented approaches can be used to obtain the desired parameters used in decision-making; however, the results are highly dependent on the inputs, such as TOC and hydrogen index, and these typically are not defined regionally. The QSI is a powerful tool that can be used to estimate the lateral and vertical heterogeneity of rocks beyond wellbores. QSI combines both of the previously mentioned approaches by using well data to calibrate rock-physics models and then using statistical methods to derive relationships between rock-physics models and seismic data. In the complete paper, the authors demonstrate a work flow to apply the QSI to organic-rich mudrocks by incorporating a constructed rock-physics template for mudrocks. Joint estimation of rock properties is used to estimate the geochemical and mechanical properties important in determining sweet spots. Multiple possible realizations of Earth models are generated to capture the nonuniqueness associated with the inversion problem. Study Area The Triassic Shublik source rock is the main petroleum source in the North Slope of Alaska. The organic-rich calcareous interval is interpreted to be deposited during a marine upwelling period. Lateral and vertical heterogeneity in the Shublik formation has been documented with multiple facies identified.

Permeability and Porosity Evolution of Organic-Rich Shales from the Green River Formation as a Result of Maturation

Summary Source rocks (oil shale) were matured artificially via pyrolysis under geologically realistic triaxial stresses using a unique coreholder that is compatible with X-ray computed tomography (CT) scanning. This study focuses on characterization of porosity and permeability as well as the evolution of shale fabric during pyrolysis. Experiments were conducted using 1-in. diameter vertical and horizontal core samples from the Green River Formation. Prior to pyrolysis, the properties of the source rock were characterized [e.g., porosity, bulk density, mineralogy, and total organic carbon (TOC)]. Samples were then heated from room temperature to 350°C under a nitrogen environment to obtain conversion of organic matter (OM) to oil and gas via pyrolysis. Porosity and permeability after maturation were measured. Micro-CT visualization was applied to investigate the fracture network developed throughout the core. Scanning electron microscopy (SEM) images were also used to compare the shale fabric and porosity evolution (pre- to post-maturation) at greater resolution. In-situ observations reveal a decrease in the average CT number (i.e., density) within some volumetric regions of the cores after maturation. In these regions, OM (kerogen and bitumen) was converted into hydrocarbons. Changes in the source rock depend on the original TOC fraction, hydrogen index (HI), and temperature. The permeability prior to pyrolysis for vertical samples is in the undetectable to nanodarcy range. The permeability of all cores increased to the microdarcy range post-maturation. In particular, the permeability of the horizontal sample increased from 0.14 to 50 µd. This improvement in permeability occurred due to the generation of open porosity and fractures (dilation, generation, and/or drainage). Additionally, the porosity after Soxhlet extraction increased proportionally by 20% from pre- to post-pyrolysis depending on pyrolysis time and TOC fraction. Longitudinal deformation depends on the orientation of the sample with respect to the triaxial stress during pyrolysis. The deformation of vertically oriented samples with isostress conditions is larger than that of horizontally oriented samples with isostrain. The measured 3D in-situ porosity distribution indicates that OM has transformed into hydrocarbons by pyrolysis. The development of a fracture and matrix porosity system under stress provides an explanation for transport of hydrocarbon away from its point of origin.

Experimental and numerical study of latent heat thermal energy storage with high porosity metal matrix under intermittent heat loads

The potential mismatch between the ever-increasing demand for usable energy and characteristically intermittent supply from the renewable energy sources can be effectively reduced by latent heat thermal energy storage system (LHTES) using phase change material (PCM). The low thermal conductivity of PCM leads to thermal stratification in LHTES. In the present study, a lab-scale shell and tube LHTES with the metal matrix as thermal conductivity enhancer is designed for medium temperature solar applications (~200 °C) and the thermal performance of LHTES is evaluated for different operating conditions. A commercial-grade organic PCM and metal matrix are placed in the annulus, while a commercially available thermic oil used as a heat transfer fluid (HTF), flows through the finned internal tube. A simplified dynamic numerical model comprising of two energy equations is developed considering the absence of local thermal equilibrium between PCM and HTF. A newly developed correlation for interfacial heat transfer coefficient and the effective thermal conductivity of PCM and metal matrix are introduced in the model, which is validated against the experiment. The model is extended to study the effect of porosity of metal matrix on the dynamic performance of LHTES under several cycles of charging and discharging operations. It is found that at a porosity of 0.85, the fluctuation in HTF outlet temperature is less with an improvement in cumulative energy fraction. A significant saving of computational time can be achieved for large scale simulations of LHTES with metal matrix using the simplified dynamic model with reasonable accuracy.

Effect of brine salinity on the geological sequestration of CO2 in a deep saline carbonate formation

AbstractWhen supercritical CO2 is injected into a deep saline aquifer, the salinity of the brine can influence both the solubility of the injected CO2 in the brine and the subsequent aqueous geochemical reactions that occur. The objective of this study is to assess the effect of brine salinity on CO2 storage in deep saline carbonate formations. Specifically, the study aims to quantitatively assess the effect of brine salinity on (a) the amount of CO2 that can be sequestered in the aqueous phase, (b) the storage efficiency of the repository, (c) the change in pH of the residual brine after dissolution of CO2, and (d) changes in the mineralogy and porosity of the rock matrix that result from mineral dissolution and precipitation reactions induced by CO2 injection. Injection of CO2 into a model layered carbonate formation was simulated using the reactive‐transport code TOUGHREACT 3.3. The simulations consisted of 50 years of injection into a partially completed injection well, followed by 50 years of equilibration (no injection). Salinity of the brine was varied between 1% and 15% to determine the effects of salinity. Because the solubility of CO2 decreases as the salinity of the brine increases, an increase in brine salinity from 1% to 15% resulted in a 19% decrease in storage efficiency and a 25% decrease in the mass of CO2 dissolved in the aqueous phase. However, despite the strong effect of brine salinity on CO2 dissolution, there was almost no effect of salinity on the pH change in the residual brine: the chemical activity coefficient of the dissolved CO2 increases with brine salinity, counteracting the effects of decreased solubility. Consequently, brine salinity had little effect on mineral dissolution or precipitation, or on changes in formation porosity. The study therefore helps to choose sequestration sites depending upon the salinity of the brine: lower salinity results in more solubility trapping and better storage efficiency without noticeable differences in pH drop or mineral dissolution. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Impact of facies and diagenetic variability on permeability and fluid flow in an oolitic grainstone—Pleistocene Miami Oolite

AbstractThe Miami Oolite of South Florida is representative of a grainstone‐rich carbonate unit that has been surficially karsted, and therefore may be considered as an analogue for subsurface reservoirs/aquifers with ‘high’ permeability extremes. The deposit can potentially serve to improve a conceptual understanding of heterogeneity as imparted by shallow‐marine facies changes and early meteoric diagenetic modification. Reviewed here are recent studies of the Miami Oolite with the intent to emphasize those key aspects of the facies and early diagenesis that most impact permeability and fluid flow. The Miami Oolite displays the preserved morphology of a fossilized ooid sandbody, even though it has been subaerially exposed in a tropical climate since its deposition approximately 120 kyr bp during the last interglacial highstand. The depositional motif is one of a dip‐oriented, tidal bar belt of shoals and shallow channels fronted by a strike‐oriented barrier bar. The barrier bar comprises cross‐stratified grainstones and locally burrowed grain/packstones, while the tidal shoals and channels are more commonly burrowed pack/grainstones. Surficial karst features (dolines and stratiform caves) have been added during the ca 120 kyr of subaerial exposure, but of more significance is the associated solution‐enhancement of the widespread burrowed facies. Since the Miami Oolite is the uppermost portion of the Biscayne Aquifer, there is also an understanding of fluid flow through the deposit that sheds valuable insight on the larger scale, shallow subsurface plumbing. The pore system comprises matrix pores (interparticle and separate vugs) and touching‐vug macropores that are commonly associated with burrowed [Ophiomorpha] intervals. Ground‐penetrating radar, well and flow test data indicate that matrix porosity provides most of the groundwater storage, whereas the touching vug macropores account for the majority of flow. The dolines and shallow caves seem to be sufficiently spaced as to generally not be in direct connection, with the result that they are less important in terms of regional flow than the prevailing pore system.

A Balance between Turning and Persistent Motion is Critical for Fast and Efficient 3-Dimensional Neutrophil Migration

While much research has been dedicated to identifying the cascade of specific biochemical processes involved in the recruitment of neutrophils, much less is known about the mechanical events driving their directed migration. In particular, it is still unclear how neutrophils generate the necessary traction forces to migrate across three-dimensional (3-D) extravascular spaces and the effects of matrix porosity on turning events and persistence during this process. In this study, we examine the effects of extracellular matrix properties on the mechanics of 3-D neutrophil motility in collagen gels. We embedded neutrophil-like differentiated human promyelocytic leukemia (dHL-60) cells in collagen matrices of different concentrations containing fluorescent micro-beads. To induce directed migration, we introduced the chemokine formyl-Methionyl-Leucyl-Phenylalanine (fMLP) to dHL-60 cells in a custom build device. We then used brightfield microscopy to track chemotaxing cells and both confocal and fluorescent microscopy techniques to image the movement of the embedded micro-beads and fluorescently labeled cells. We find that turning events, mediated by the Arp 2/3 complex, are crucial for fast 3-D neutrophil migration. Additionally, the Arp 2/3 complex and myosin II play an increasingly important role in 3-D migration as matrices become denser. Finally, we find that cellular force patterns vary in response to matrices with various degrees of pore size heterogeneities and cell's intrinsic abilities to engage in path-finding and contractility. The results from our study show that neutrophils migrating in 3-D environments employ distinct mechanical mechanisms that depend on the local structure of their mechanical environments. Our work has implications for understanding how neutrophils interact physically with their surrounding environment what conditions may be necessary for fast 3-D neutrophil migration.

Investigation of different production performances in shale gas wells using assisted history matching: Hydraulic fractures and reservoir characterization from production data

We investigated the difference in production performances of three shale gas wells with different completions and fracturing designs. We used assisted history matching technique to perform production match automatically and obtain multiple history matching solutions. The history matching algorithm used in this study is neural networks Markov chain Monte Carlo. In addition, we utilized embedded discrete fracture model technique to model hydraulic fractures instead of local grid refinement method to speed up the simulation runs. The history matching parameters consist of fracture half-length, fracture height, fracture conductivity, fracture width and fracture water saturation for hydraulic fractures properties and include matrix porosity, matrix permeability and relative permeability curves for matrix properties. We found that fracture width, fracture half-length, fracture conductivity and matrix permeability were significantly different in three shale gas wells while fracture height, fracture water saturation, porosity and relative permeability curves were less different. The information from this study will be used further to improve completion and fracturing design.

Study of polylactide matrices using x-ray microtomography

Three-dimensional matrices of biodegradable polymers are promising materials for regenerative medicine. They are widely used in restoring the integrity and functions of tissues and organs using bio-artificial tissue engineering structures. We present the results of studying the structure of porous bioresorbable polymer matrices for tissue engineering using X-ray microtomography. Samples were obtained by supercritical fluid plasticization of D,L-polylactide with subsequent foaming in cylindrical molds. The tomographic method makes it possible to construct a three-dimensional voxel model of the object under study and gain apart from the estimate on the integral matrix porosity (characteristic data obtained by traditional sorption procedures) additional information about the size and spatial distribution of pores thus providing a possibility of optimization of the process parameters for production of polylactide matrices required for specific biomedical applications of architectonics, as well as forecasting the processes of their bioresorption in enzymatic media. The experiments were carried out on a laboratory microtomograph (Mo anode, the scan time of the sample is 120 min, the detector pixel size is 9 μm). Tomographic reconstruction was performed by algebraic method. The binarization procedure required for calculation of the structural characteristics of studied matrices was implemented by the method with the choice of a global threshold. Calculations of the porosity and homogeneity of the porosity distribution in the bulk, as well as estimation of the specific surface area of pores revealed the isotropy of the spatial structure of polylactide matrices.

Karst dissolution along fracture corridors in an anticline hinge, Jandaíra Formation, Brazil: Implications for reservoir quality

Abstract Folded, fractured, and karstified structures have been recognized in several carbonate reservoirs. However, they are rarely fully identified from seismic or well data and have been poorly described. The present study investigates the geological and structural controls that regional anticlines may exert on solution-enlarged fracture corridors and epigenic karst concentrations in folded-fractured carbonate units. We performed regional mapping based on 106 2D seismic lines, 51 well logs, unmanned aerial vehicle (UAV) imagery of four major outcrop sites (I, II, III and IV) and field investigations to parameterize fracture and karstic features (density, spacing, length, and aperture). The study area is Potiguar Basin, Brazil, where we identified the gentle NE-trending Apodi fold, ~10 km wide and ~20 km long. The fold formed along an inverted extensional fault during the latest Miocene-Quaternary stress field, and the envelope of the meandering trend of the Apodi River coincides with the NE-trending fold axis. The fracture pattern in the fold hinge zone consists of two orthogonal, syn-folding fracture sets: a NE-SW-striking hinge-parallel set and an orthogonal NW-SE-striking set. These fractures and superposed karstification are mostly concentrated in the hinge zone along fracture corridors in an area 1.5 km wide and 20 km long. We conclude that fracture corridors in anticline hinges are likely sites to have high fracture densities and wide apertures. These corridors provide reservoir spaces that are favorable for the formation of karst cavities, with significant increases in permeability and porosity along the fracture sets. These features act as important fluid pathways and storage areas where matrix porosity is low. The findings can be used for first-order prediction in karstified carbonate reservoirs.

Seasonal δ13CDIC sourcing and geochemical flux in telogenetic epikarst of south‐central Kentucky

AbstractTelogenetic epikarst carbon sourcing and transport processes and their associated hydrogeochemical responses are complex and dynamic. Carbon dioxide (CO2) transport rates in the epikarst zone are often driven by hydrogeochemical responses, which influence carbonate dissolution and conduit formation. This study examines the influence of land use on carbon sourcing and carbonate dissolution kinetics through a comparative analysis of separate, but similar, epikarst systems in south‐central Kentucky. The use of high‐resolution hydrogeochemical data from multiple data loggers and isotope analysis from collected water samples reflects the processes within these epikarst aquifers, which are estimated to contribute significantly to bedrock dissolution. Results indicate that, in an agricultural setting, long‐term variability and dissolution is governed by seasonal production of CO2. In a more urbanized, shallower epikarst system, land cover may affect CO2 transport between the soil and underlying bedrock. This concentration of CO2 potentially contributes to ongoing dissolution and conduit development, irrespective of seasonality. The observed responses in telogenetic epikarst systems seem to be more similar to eogenetic settings, which is suggested to be driven by CO2 transport occurring independent of high matrix porosity. The results of this study indicate site‐specific responses with respect to both geochemical and δ13CDIC changes on a seasonal scale, despite regional geologic similarities. The results indicate that further comparative analyses between rural and urban landscapes in other karst settings is needed to delineate the impact of land use and seasonality on dissolution and carbon sourcing during karst formation processes. © 2019 John Wiley & Sons, Ltd.

Multiscale ceramic matrix composite thermomechanical damage model with fracture mechanics and internal state variables

This work presents a multiscale damage model that accounts for ceramic matrix composite (CMC) length scale dependent behavior, including scale-specific brittle matrix damage initiation and propagation. Internal state variable theory is implemented to obtain the stress–strain constitutive relationship for damaged ceramic matrix material using damage variables which capture the effects of matrix cracking and nucleation and growth of matrix porosity. Matrix cracking is modeled using a damage variable determined using fracture mechanics and the self-consistent scheme. This methodology provides an effective way to model the effects of matrix cracks which initiate when stress intensity factors exceed the fracture toughness of the material, and whose growth is governed by crack growth kinetics. An additional source of material nonlinearity, matrix porosity, occurs as a result of material diffusion around grain boundaries and is related to the material entropy dissipation. These effects are captured using a porosity state variable that is explicitly related to the volumetric strain. The nonlinear predictive capabilities of the material model are demonstrated for monolithic silicon carbide, unidirectional (UD) carbon fiber/silicon carbide matrix (C/SiC) CMC, and five-harness satin (5HS) woven C/SiC CMC. The model predictions are in excellent agreement with experimental observations for 2D woven C/SiC CMCs from literature.

Experimental Studies on Wear Resistance & Mechanical Properties of Aluminium Hybrid Composites (LM25/Al<sub>2</sub>O<sub>3</sub>/Graphite)

This paper involves the fabrication of hybrid Aluminium matrix composite samples of different compositions using Stir casting technique and testing the prepared hybrid composite test specimens for analysing their wear and mechanical properties. LM25 Al alloy was used as matrix material reinforced with constant 9 wt.% of Al₂O₃ with particle size of 6-23 μm and different wt. percentages of Graphite (particle size of 24-94 μm) as 3%, 4% and 5% to prepare hybrid composite samples of three different compositions. Mechanical properties, for examples, tensile strength, hardness, compression strength and impact strength were analysed for both base LM25 Al. alloy and composite samples. Wear loss and coefficient of friction of composite samples also were studied using Pin-on-disc apparatus. Microstructure images of the hybrid composite samples were examined to study dispersion of dual reinforcement particles in the matrix material, porosity and shrinkage cavities developed during the casting of the composite samples using Optical Microscope. Results revealed that Tensile strength, Hardness and Wear properties of the hybrid composite sample, having 9 wt.% of Al₂O₃ and 4 wt.% of Graphite particles, were found to be superior to those of re-cast LM25 Al. alloy. This study is mainly focussed on achieving the improved wear resistance and mechanical properties of the hybrid aluminium matrix composite materials for meeting the desired properties of the products by exploring newer compositions of the dual particles reinforced hybrid composites.

Damage analysis of semi-flexible pavement material under axial compression test based on acoustic emission technique

Semi-flexible pavement (SFP) material has been promoted to apply in pavement engineering mainly for its excellent performance in rutting resistance. But its mechanical behavior is complicated due to its heterogeneity and interlocking structure. According to the present study, the damage process of SFP (with and without fiber), as well as its porous asphalt (PA) matrix, under uniaxial compression test was detected employing acoustic emission (AE) technique. The heterogeneity and the interlocking effect were varied by changing the matrix porosity and the gradation, respectively. The stress-strain curves reveal that cement grouting could decrease the material’s failure strain distinctly, while the addition of fiber made no effect on the results. Nevertheless, both the cement grouting and the addition of fiber caused medium and high amplitude (50–65 dB) AE signals during the AE detection. Besides, the Rise Angle (RA) value and energy distribution both illustrate that there was increasing trend in the amounts of shear cracking in SFP during compression, which was different from asphalt mixtures. And the addition of fiber could restrain the shear cracking to improve the crack resistance. Quantificationally, heterogeneity was characterized by standard deviation of compositions’ modulus and an interlocking index was extracted from a modulus micromechanical model to characterize the interlocking effect. The Weibull distribution was adopted to characterize the damage evolution, where the cumulative AE energy was chosen as the damage variable. The result shows the two Weibull parameters m, n had obviously positive correlation with the matrix porosity and the interlocking index, respectively.

Experimental Investigation of the Effect of Salt Precipitation on the Physical and Mechanical Properties of a Tight Sandstone

Salt precipitation in rocks is a strongly coupled physico-chemical process, which has implications on natural gas extraction efficiency. The paper examines the effect of salt precipitation on porosity, permeability, permeability-stress sensitivity and elastic wave velocities of the rock matrix in tight sandstone with high salinity. Results show that the porosity and permeability decrease significantly after salt precipitation, and the permeability-stress sensitivity was aggravated by the salt precipitation. Following salt precipitation, the elastic wave velocities decreased significantly, the dynamic Young’s modulus of the rock decreased, and the dynamic Poisson’s ratio increased. Microstructural analysis suggests that following salt precipitation, the smaller pores and pore throats become the main seepage channels. The crystalline salt was deposited in the pores, the micro-fractures and on the surface of clay minerals, which not only reduces porosity and permeability, but contributes to the development of secondary cracks that weaken the rock matrix.

Experimental study on influencing factors of fracture propagation in fractured carbonate rocks

Carbonate reservoirs show poor porosity and permeability in matrix, it is necessary to improve the permeability channel through reservoir reconstruction. The influence of different fracturing parameters on fracture propagation of carbonate rock is studied, which is of great significance to hydraulic fracturing design of carbonate rock. In this paper, a series of hydraulic fracturing experiments were performed under different conditions, the fracturing pump pressure curve, radial deformation characteristics and fracture morphology were studied and analyzed. The results show that: (1) The rock-acid reaction between gelled acid and carbonate rock produces etching wormholes, changes the pore structure and weakens the mechanical parameters of carbonate rocks, which is conducive to reducing fracturing pressure and increasing fracture complexity; (2) The types of fracturing fluids and the viscosity significantly affect fracturing rupture pressure and the width of the fracture. Under the same in-situ stress and flow rate conditions, the fracturing rupture pressure of slick water is the lowest and that of guar gum is the highest; (3) Composite fracturing can improve the complexity of fractures in carbonate reservoir, for carbonate rocks with fracture developed near wellbore, complex fracturing fractures can be made by slick water fracturing, and main fracturing surfaces can be eroded by gelled acid to improve the width and increase the roughness. (4) To solve the problem of high fracturing pressure under high in-situ stress, gelled acid can be used for pretreatment to reduce fracturing pressure, and then non-reactive fluid, slick water or guar gum can be used for fracturing. Our research findings provide a reliable basis for deep fractured carbonate reservoir reconstruction.

Upscaling of Elastic Properties in Carbonates: A Modeling Approach Based on a Multiscale Geophysical Data Set

AbstractLinking ultrasonic measurements made on samples, with sonic logs and seismic subsurface data, is a key challenge for the understanding of carbonate reservoirs. To deal with this problem, we investigate the elastic properties of dry lacustrine carbonates. At one study site, we perform a seismic refraction survey (100 Hz), as well as “sonic” (54 kHz) and ultrasonic (250 kHz) measurements directly on outcrop and ultrasonic measurements on samples (500 kHz). By comparing the median of each data set, we show that the P wave velocity decreases from laboratory to seismic scale. Nevertheless, the median of the sonic measurements acquired on outcrop surfaces seems to fit with the seismic data, meaning that sonic acquisition may be representative of seismic scale. To explain the variations due to upscaling, we relate the concept of representative elementary volume with the wavelength of each scale of study. Indeed, with upscaling, the wavelength varies from millimetric to pluri‐metric. This change of scale allows us to conclude that the behavior of P wave velocity is due to different geological features (matrix porosity, cracks, and fractures) related to the different wavelengths used. Based on effective medium theory, we quantify the pore aspect ratio at sample scale and the crack/fracture density at outcrop and seismic scales using a multiscale representative elementary volume concept. Results show that the matrix porosity that controls the ultrasonic P wave velocities is progressively lost with upscaling, implying that crack and fracture porosity impacts sonic and seismic P wave velocities, a result of paramount importance for seismic interpretation based on deterministic approaches.

A Direct Compression Matrix Made from Xanthan Gum and Low Molecular Weight Chitosan Designed to Improve Compressibility in Controlled Release Tablets.

The subject of our research is the optimization of direct compression (DC), controlled release drug matrices comprising chitosan/xanthan gum. The foregoing is considered from two main perspectives; the use of low molecular weight chitosan (LCS) with xanthan gum (XG) and the determination of important attributes for direct compression of the mixtures of the two polymers. Powder flow, deformation behaviour, and work of compression parameters were used to characterize powder and tableting properties. Compression pressure and LCS content within the matrix were investigated for their influence on the crushing strength of the tablets produced. Response surface methodology (RSM) was applied to determine the optimum parameters required for DC of the matrices investigated. Results confirm the positive contribution of LCS in enhancing powder compressibility and crushing strength of the resultant compacts. Compactibility of the XG/LCS mixtures was found to be more sensitive to applied compression pressure than LCS content. LCS can be added at concentrations as low as 15% w/w to achieve hard compacts, as indicated by the RSM results. The introduction of the plasticity factor, using LCS, to the fragmenting material XG was the main reason for the high volume reduction and reduced porosity of the polymer mixture. Combinations of XG with other commonly utilized polymers in controlled release studies such as glucosamine, hydroxypropyl methylcellulose (HPMC), Na alginate (ALG), guar gum, lactose and high molecular weight (HMW) chitosan were also used; all the foregoing polymers failed to reduce the matrix porosity beyond a certain compression pressure. Application of the LCS/XG mixture, at its optimum composition, for the controlled release of two model drugs (metoprolol succinate and dyphylline) was examined. The XG/LCS matrix at 15% w/w LCS content was found to control the release of metoprolol succinate and dyphylline. The former preparation confirmed the strong influence of compression pressure on changing the drug release profile. The latter preparation showed the ability of XG/LCS to extend the drug release at a fixed rate for 12 h of dissolution time after which the release became slightly slower.

Modeling matrix acidizing in naturally fractured carbonate reservoirs

Natural fractures, often developed in carbonate reservoirs, have significant influence on flow fields, thus affecting wormhole propagation in matrix acidizing. They are vital factors to be considered in modeling of matrix acidizing and field treatment designs. This paper conducts a research on wormholing behavior under radial flooding conditions in naturally fractured carbonates. Firstly, statistical natural fracture models are established by using the method of Monte Carlo, based on which generate natural fracture distributions. Then the natural fractures are projected onto the matrix porosity distribution to generate a porous media of matrix and natural fractures. Secondly, Coupling the porosity distribution and natural fracture distribution with a two-scale continuum wormhole model forms a new wormhole propagation model with natural fracture effect. Finally, based on the model, extensive numerical simulation is conducted to investigate wormholing characteristics in presence of natural fractures. Parameter sensitivity analysis is done to determine the effect of the natural fracture parameters on wormhole propagation behavior. This study reveals the mechanism of wormholing in presence of natural fractures, identifies the effect of natural fractures, and provides theoretical basis for matrix acidizing designs in naturally fractured carbonates.

Zeolite and fly ash in the composition of oil well cement: Evaluation of degradation by CO2 under geological storage condition

The performance of cement class G used in cementation of oilfield wellbores with addition of pozzolans was evaluated under geological carbon storage conditions. Two commercial synthetic zeolites types (4A-1 and 4A-2) and fly ash from a coal-fired plant were used as pozzolanic materials in amounts of 5 and 10% in weight replacing the cement. After curing, the cement samples were submitted to degradation tests in CO2-saturated water at 15 MPa and 90 °C for 7 and 14 days. The cement chemical degradation by CO2 was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and compressive strength tests. The chemically altered layer thickness was averaged 3.46 mm for standard cement after 14 days of exposure to CO2. On the other hand, cement systems with 10% wt. of pozzolanic material varied from 1.70 to 5.50 mm depending on the type of pozzolan and level of cementitious matrix porosity related to pozzolanic particle clustering. In general, 4A-1 zeolite presented better performance in terms of resistance to CO2 attack and higher compressive strength after 14 days when compared to 4A-2 zeolite. The results showed that the addition of fly ash improved the compressive strength of the samples but increased the chemically altered layer due to CO2 diffusion. SEM and XRD analyses showed that the portlandite was consumed and carbonation occurred in the chemical modified layer due to cement reaction with aqueous CO2. Most cement systems with and without pozzolanic material exhibited no expressive loss on compressive strength after being exposed to CO2-rich environment up to 14 days. On the contrary, some cement systems with 4-A1 zeolite and fly ash exhibited a mechanical resistance increase due to the carbonation process.