Pore Structure Of Sandstone Research Articles

Variations of Micropores in Oil Reservoir Before and After Strong Alkaline Alkaline-surfactant-polymer Flooding

Enhanced oil recovery (EOR) provides a significant contribution for increasing output of crude oil. Alkaline-surfactant-polymer (ASP), as an effective chemical method of EOR, has played an important role in advancing crude oil output of the Daqing oilfield, China. Chemical flooding utilized in the process of ASP EOR has produced concerned damage to the reservoir, especially from the strong alkali of ASP, and variations of micropore structure of sandstones in the oil reservoirs restrain output of crude oil in the late stages of oilfield development. Laboratory flooding experiments were conducted to study sandstones’ micropore structure behavior at varying ASP flooding stages. Qualitative and quantitative analysis by cast thin section, scanning electric microscopy (SEM), atomic force microscopy (AFM) and electron probe X-Ray microanalysis (EPMA) explain the mechanisms of sandstones’ micropore structure change. According to the quantitative analysis, as the ASP dose agent increases, the pore width and pore depth exhibit a tendency of decrease-increase-decrease, and the specific ASP flooding stage is found in which flooding stage is most affective from the perspective of micropore structures. With the analysis of SEM images and variations of mineral compositions of samples, the migration of intergranular particles, the corrosions of clay, feldspar and quartz, and formation of new intergranular substances contribute to the alterations of sandstone pore structure. Results of this study provide significant guidance for further application to ASP flooding.

Effect of quartz overgrowth precipitation on the multiscale porosity of sandstone: A (U)SANS and imaging analysis

We have performed a series of experiments to understand the effects of quartz overgrowths on nanometer to centimeter scale pore structures of sandstones. Blocks from two samples of St. Peter Sandstone with different initial porosities (5.8% and 18.3%) were reacted from 3days to 7.5months at 100 and 200°C in aqueous solutions supersaturated with respect to quartz by reaction with amorphous silica. Porosity in the resultant samples was analyzed using small and ultrasmall angle neutron scattering and scanning electron microscope/backscattered electron (SEM/BSE)-based image-scale processing techniques.Significant changes were observed in the multiscale pore structures. By 3days much of the overgrowth in the low-porosity sample dissolved away. The reason for this is uncertain, but the overgrowths can be clearly distinguished from the original core grains in the BSE images. At longer times the larger pores are observed to fill with plate-like precipitates. As with the unreacted sandstones, porosity is a step function of size. Grain boundaries are typically fractal, but no evidence of mass fractal or fuzzy interface behavior was observed suggesting a structural difference between chemical and clastic sediments. After the initial loss of the overgrowths, image scale porosity (>∼1cm) decreases with time. Submicron porosity (typically ∼25% of the total) is relatively constant or slightly decreasing in absolute terms, but the percent change is significant. Fractal dimensions decrease at larger scales, and increase at smaller scales with increased precipitation.

Pore-level foam generation and flow for mobility control in fractured systems

Abstract Pore-level foam generation, propagation, and sweep efficiency were visualized using silicon-wafer micromodels with an accurate representation of sandstone pore structure, grain shapes and sizes based on thin-section analysis. Foam generation by snap-off was observed both in the interior of the porous network (rectilinear snap-off) and at permeability discontinuities between fracture and porous matrix. Lamella creation by the two snap-off mechanisms identified here resulted in different foam textures. During foam injection for enhanced oil recovery, microvisual data revealed that the aqueous phase advanced as film flow along water-wet grains whereas discontinuous gas bubbles were located in the center of pores. Foam injection significantly enhanced sweep efficiency in fractured systems in terms of greater pore occupancy by gas and larger contact area with displaced fluid compared to continuous gas injection.

Reactive and Pore Structure Changes in Carbon Dioxide Sequestration

The importance of reactions involving CO2, brine, and rock formations into which CO2 is injected for CO2 sequestration in saline aquifers is understood. However, the pore-level changes that occur due to these reactions under flow conditions and their impact on the ultimate fate of CO2 in the repository have not received the same level of attention because of the perceived slowness of the carbonation reactions. In this paper we examine these reactive changes and their impact on the pore structure in sandstones and limestones at realistic aquifer pressure and temperatures, and under reactive flow conditions. The changes observed at the pore-level by direct porosity and microcomputer tomography measurements were complemented by the measurements of time-dependent effluent concentrations of target cations. It is observed that iron chemistry plays an important role in the dissolution and precipitation reactions in Berea sandstone. Illite dissolution leads to a peak in iron concentration in effluent brines. A higher level of dissolution and an increased porosity are observed near the inlet of the core. In limestones, consistent dissolution is observed throughout the experiment. Wormholes are also generated for experiments with larger total flow rates. Results show that reactive changes can cause significant pore-level changes over a short injection span during CO2 sequestration in saline aquifers with profound implications on injectivity and possibly major mechanical changes.

Lattice-Boltzmann simulation of microscale CH4 flow in porous rock subject to force-induced deformation

Accurate knowledge of the influence of rock deformation on the permeability of fluid flow is of great significance to a variety of engineering applications, such as simultaneous extraction of coal and gas, oil/gas exploitation, CO 2 geological sequestration, and underground water conservation. Based on the CT representation of pore structures of sandstones, a LBM (Lattice Boltzmann Method) for simulating CH 4 flow in pore spaces at microscale levels and a parallel LBM algorithm for largesize porous models are developed in this paper. The properties of CH 4 flow in porous sandstones and the effects of pore structure are investigated using LBM. The simulation is validated by comparing the results with the measured data. In addition, we incorporate LBM and FEM to probe the deformation of microstructures due to applied triaxial forces and its influence on the properties of CH 4 flow. It is shown that the proposed method is capable of visually and quantitatively describing the characteristics of microstructure, spatial distribution of flow velocity of CH 4 , permeability, and the influences of deformation of pore spaces on these quantities as well. It is shown that there is a good consistency between LBM simulation and experimental measurement in terms of the permeability of sandstone with various porosities.

Permeability Curves Characteristic Analysis of L Oilfield

The L oilfield Cretaceous (M-I-1), Jurassic Department (Ю-0-3) clastic pore types, including primary porosity, secondary porosity and cracks in three categories, their characteristics and the degree of development. Chalk Department of particles holes and grain dissolution porosity, an average of 53.2%, followed by argillaceous porous and contraction joints, while a small number of particles dissolved pore, showing a small amount of paste particles seam and tensile crack; Jurassic inter-granular holes and intra-granular dissolution porosity is developed, accounting for the porosity as high as 95%, while a small amount of argillaceous porous and granulizing hole and a very small amount of mold holes. L Oilfield Cretaceous and Jurassic reservoirs inter-granular pores, inter-granular dissolution pore, pore throat combination M-I-1, mainly to large - in the hole, micro-throat, Ю-0-3 large - in the hole Extra Coarse - rough throat-based thin throat, Ю-0-1, Jurassic sandstone pore structure better than the Cretaceous.

Micropore Structure Representation of Sandstone in Petroleum Reservoirs Using an Atomic Force Microscope

The pore structure of sandstone in an oil reservoir is investigated using atomic force microscopy (AFM). At nanoscale resolution, AFM images of sandstone show us the fine structure. The real height data of images display the three-dimensional space structure of sandstone effectively. The three-dimensional analysis results show that the AFM images of sandstone have unique characteristics that, like fingerprints, can identify different structural properties of sandstones. The results demonstrate that AFM is an effective method used to represent original sandstone in petroleum reservoirs, and may help geologists to appreciate the sandstone in oil reservoirs fully.

Effect of Pore Structure of Sandstone on Hydrophilic Characteristicis

Pore structure of sandstone has much influence on its hydrophilic characteristics. The results of pressured-mercury testing for sandstone are discussed in the paper with the help of fractal geometry theory. The effect of pore structure for sandstone is mainly discussed, combined the results of hydrophilic experiment. Results show that the pore structure of sandstone has single and multiple characteristics within some pore size. Four typical fractal characteristic are defined in the paper and it is found that characteristic of pore structure has much relation with its hydrophilic characteristics: hydrophilic rate of Sandstone with A or C-type fractal microstructure changes little and the double logarithmic hydrophilic relationship curve is always linear; however, the hydrophilic rate of ones with B and C-type fractal microstructure changes greatly and always taken as concave-convex.

Applying Authentic Sandstone Micromodel to the Study of Microanisotoropy of Chang 8 Sandstone in Jingchuan Area of Ordos Basin

The microanisotropy characteristic of sandstone reservoirs is one of the most important contents in reservoir geology. Based on the authentic sandstone micromodel experiments, the microanisotropy characteristic of Chang 8 sandstone of Baibao area in Ordos basin was fully analyzed. Chang 8 reservoirs belong to close sandstone. The authentic sandstone micromodel experiments audio-visually display the intense microanisotropy of Chang 8 sandstone in this area. The sandstone pore structure and diagenesis are main factors influencing Chang 8 sandstone microanisotropy. The dissolution resulted in the intense microanisotropy of Chang 8 low permeable sandstone, but this sandstone formed high permeable zone of Chang 8 sandstone in this area.

Gas relative permeability and pore structure of sandstones

Abstract Accurate quantification of gas relative permeability is important in several engineering problems including gas storage and production, drying and wetting processes, oil production etc. In this study, experimental curves of gas relative-permeability for three different kinds of sandstone were defined using a modified version of the pulse-decay method. In order to understand the role played by the porous structure in defining the nonwetting phase permeability, the porous networks of the rocks tested were identified by two complementary methods: mercury porosimetry and sorption techniques. The results obtained show two distinct types of behaviour which are closely dependent on the pore structure. The pores located near the peak of the mercury intrusion curve seem to control the variation in gas relative-permeability. In order to clarify the influence of wetting fluid viscosity on the nonwetting phase relative permeability, each sample was subjected to two series of tests using two different saturating fluids. Our results show that, for a viscosity ratio μnw/μw≪1, the gas relative-permeability remains unchanged even if the wetting fluid viscosity is twenty times higher than that of water. A simple empirical relation (the Brooks and Corey equation) was used to correlate the gas relative-permeability with the degree of saturation. The comparison shows that this correlation accurately predicts the nonwetting-phase relative permeability.

Electrochemical study of the pore structure of sandstone and similar media

Three methods are discussed by which the internal geometry of a porous insulator may be investigated electrochemically. By adopting a simple model, the pore geometry is characterized by two parameters: the tortuosity and the porosity. In each method, a cylindrical sample of the porous medium is soaked with an aqueous electrolyte solution and its lower face is brought into contact with a mercury pool electrode. The other electrode is also a mercury pool, but in the first technique it is remote, whereas in the second and third methods it floods the upper surface of the cylindrical sample. The first technique employs a d.c. potential step, whereby the concentration of a probe ion is depleted at the lower electrode. The attempt of diffusion to replenish this surface concentration leads to a faradaic current whose time dependence is analyzed to provide estimates of the tortuosity and porosity parameters. The second method measures the frequency-dependent cell impedance. The pore structure parameters are estimated from the equivalent circuit of the cell. No faradaic process is involved in the third method, which measures the frequency- dependent conductance of the cell as a means of characterizing the pore structure. These methods are applied successfully to sandstone and fritted glass samples.

BAYU NUCLEAR MAGNETIC RESONANCE STUDY: COMMERCIAL APPLICATION OF APPLIED RESEARCH AND FIELD TECHNIQUES IN A GAS-CONDENSATE FIELD

Nuclear Magnetic Resonance (NMR) technology was applied to assist reservoir characterisation of the gas-bearing sandstones in the Bayu-Undan Field, Zone of Cooperation, Area A Blocks 91–12 and 91–13. NMR logs were run in several wells and laboratory NMR measurements on water-saturated core plugs were used to determine the sandstone pore structure and characterise effective porosity and permeability in the gas-bearing sands.These sandstones exhibit a range of pore size behaviour, ranging from those sandstones with no clay-bound water to those with a significant proportion of their porosity associated with micropores. Mean relaxation times tend to be slower than expected for the observed pore sizes in these sands by comparison with global analogs. Surface relaxivity is lower than the world-wide average for sandstones, and reflects a network of unusually clean pore- walls.Comparison between core permeability values and the log-estimated permeabilities over a four-order of magnitude range produces a correlation coefficient of greater than 0.7 in a single well.Integration of field- and laboratory-based NMR data proved that permeability and pore-size distribution models could be integrated with conventional core analysis results. These datasets are continuous over the reservoir interval and once calibrated are easily integrated into reservoir simulation models.

Pore geometry and pressure dependence of the transport properties in sandstones : Geophysics V56, N4, April 1991, P436–446

The model of the pore geometry of sandstones proposed in this paper is based on three‐dimensional qualitative observations of the pore structure of sandstones reported in the literature. Three categories of pores can be distinguished: large spherical pores at four‐grain vertices (or nodal pores); tube‐like throats at three‐grain edges; and narrow sheet‐like throats at two‐grain faces. As a first step, the model is limited to rocks with a low clay content and, therefore, pores and throats are supposed to be free of clay. In this model, the sheet‐like and tube‐like throats are in parallel and, because of their different shapes, respond differently to pressure. The transport properties can thus be split into two parts: a strongly pressure dependent part associated with the sheet‐like throats and an approximately constant part associated with the tube‐like throats and the equidimensional nodal pores. Following these remarks, a scheme to estimate the volume fraction of each category of pores using the pressure...

Pore geometry and pressure dependence of the transport properties in sandstones

The model of the pore geometry of sandstones proposed in this paper is based on three‐dimensional qualitative observations of the pore structure of sandstones reported in the literature. Three categories of pores can be distinguished: large spherical pores at four‐grain vertices (or nodal pores); tube‐like throats at three‐grain edges; and narrow sheet‐like throats at two‐grain faces. As a first step, the model is limited to rocks with a low clay content and, therefore, pores and throats are supposed to be free of clay. In this model, the sheet‐like and tube‐like throats are in parallel and, because of their different shapes, respond differently to pressure. The transport properties can thus be split into two parts: a strongly pressure dependent part associated with the sheet‐like throats and an approximately constant part associated with the tube‐like throats and the equidimensional nodal pores. Following these remarks, a scheme to estimate the volume fraction of each category of pores using the pressure dependence of the transport properties and a simple capillary model is developed. This approach was applied to data from the literature and appeared to succeed and fail when it was expected to (however, one exception was noted). For 15 of the 27 sandstones considered, the volume fraction and the size of the throats and pores predicted agreed in order of magnitude with microstructure data gathered in the literature. For several of the 12 remaining sandstones, independent microstructure data showed that basic assumptions of the model were not satisfied. Eight of these sandstones were associated with high clay content, and it was speculated that the last four might contain semimacroscopic fissures.

Tight Gas Sands-Permeability, Pore Structure, and Clay

Summary Gas permeability has been measured on a suite of cores from the Spirit River tight gas sand of western Alberta and on two samples from the Cotton Valley formation of east Texas. Using nitrogen as the mobile fluid, we have measured permeability as a function of partial water saturation at in-situ levels of pore pressure and confining pressure. Samples from both locations show strong dependence of permeability (k) on effective pressure and on the degree of water or brine saturation. The validity of Darcy's law in the micro darcy range has been verified in a dry Spirit River sample. Extensive thin-section, x-ray diffraction, and scanning electron microscope (SEM) studies have been conducted. The primary clays in Spirit River and Cotton Valley cores are chlorite and illite. In one sample we measured k vs. saturation first with distilled water and then with a 2% KCl brine solution and saw no significant change in permeability behavior. By observing the effects of pressure, partial saturation, and salinity on permeability in these samples, we can deduce several important characteristics of the pore structure and can evaluate the relative importance of clay content. Introduction Conventional well logging and well stimulation methods have had to be modified substantially to be successful in tight sandstones. It is now clear that there are some fundamental differences in rock/water/gas interactions in these formations as compared with "normal" gas reservoirs. These differences result primarily from significant pore structure alterations as the rock undergoes compaction and diagenesis. In this paper the effect of tight sandstone pore structure on gas permeability is presented in terms of the response to effective pressure and partial water saturation. Hopefully, some of these results will facilitate the development of well evaluation and stimulation techniques. We measure nitrogen permeability with a pulse-decay technique that allows independent control of pore pressure, confining pressure, and partial saturation. The core sample is about 2 in. (5 cm) in diameter and is 2.4 to 2.8 in. (6 to 7 cm) long. It is jacketed, placed in a confining pressure vessel, and subjected to hydrostatic loading of up to 1.000 bar (100 MPa). The pore fluid is nitrogen at pressures of up to 700 bar (70 MPa). Permeability is measured by applying a 1 -bar (0.1 -MPa) pore pressure differential across the sample and by observing the rate of pressure decay as gas flows through the core. This pulse decay can be used to calculate permeability. A complete description of the experimental method is given in Ref. 1. We have performed several tests to ensure the accuracy and repeatability of all measurements, and one such test is described in this paper. The primary advantage of the pulse-decay method is the speed and simplicity of data acquisition as compared with steady-state experiments. Sample Characterization Ten of the samples tested were vertical cores from the Spirit River formation in the Deep Basin of western Alberta. The other two samples were horizontal cores from the Cotton Valley formation of east Texas. The mineralogy of these samples as determined from thin section analysis is shown in Table 1. Samples are designated by a number corresponding to depth in feet preceded by "SR" for Spirit River and "CV" for Cotton Valley. SEM and X-ray diffraction studies indicate that chlorite and illite in approximately equal proportions are the major clay constituents in Cotton Valley samples and that illite is the major clay in Spirit River cores. The porosities shown in Table 1 were determined by careful weighing of dry and fully saturated samples. JPT P. 2708^

Numerical Classification of Sandstone Pore Structure: ABSTRACT

Numerical Classification of Sandstone Pore Structure: ABSTRACT

Correlation Between Pore Structure of Sandstones and Tertiary Oil Recovery

An excellent correlation has been attained between pore structure and additional recoveries obtained in tertiary surfactant floods for 15 different sandstones. A reasonably good correlation exists between the reciprocal of mean neck diameter (as obtained from mercury porosimetry) and the additional recoveries obtained in tertiary surfactant floods. THEORY AND EXPERIMENT When conducting laboratory tertiary surfactant floods on a variety of different sands, sometimes great differences are found in the tertiary oil recoveries - the reasons for which are often not well understood. In the present study it has been demonstrated that, for the set of conditions used, the observed differences in tertiary oil recoveries could be correlated with the pore structure of the samples. The problem of how pore structure might influence tertiary oil recovery has been discussed by Dullien et al. In the present study, 15 outcrop sandstones have been successfully ranked on the basis of their pore structure in the order of increasing amounts of tertiary recoveries expected under identical conditions of flooding.

A Scanning Electron Microscope Study of the Pore Structure of Sandstone

The scanning electron microscope can be used to produce, in a way that is not possible with optical techniques or with casts of pore space, a quasi three-dimensional representation of the pore structure of sandstone, making it possible to determine the pore structure of sandstone, making it possible to determine the number of interconnected flow channels. Introduction Efforts have been made for many years to observe pore structure of sedimentary rocks on a microscopic pore structure of sedimentary rocks on a microscopic scale. A better description of the pore structure in reservoir rock would aid in the development of relations between flow through tube network models and flow in consolidated reservoir rock. The unique qualities of the scanning electron miscroscope (SEM) make it the ideal tool to use in a study of pore structure. Previous methods of studying pore structure included plastic casts of pore space and optical microscopy. There is now available a new method to observe three-dimensional flow channels in reservoir rock: a scanning electron microscope having variable magnification from 20 x to 20,000 x and approximately 1,000 times the depth of focus of the conventional light microscopes. This microscope also can obtain stereoscopic images, and can measure and diagram flow channels so that it is possible to construct a network of tubes that will model consolidated reservoir rock. Theory of the Scanning Electron Microscope The scanning electron microscope (SEM) functions by projecting a beam of electrons through magnetic focusing lenses at a specimen and recording secondary electrons excited by the primary beam. The amount of secondary electron emission portrays the topography of the specimen because more secondary electrons are emitted from high points than from low points. A schematic of the SEM is shown in Fig. 1. points. A schematic of the SEM is shown in Fig. 1. The property of the SEM that is of most importance in this study is its great depth of focus. The depth of focus results from using electrons rather than light. In the SEM, no electron trajectory is inclined more than 0.5 degrees from the optic axis, in contrast with the more than 60 degrees in optical microscopy (Fig. 2). The major problem encountered with the SEM is the build-up of a space charge on insulating specimens under bombardment by the 20 kev primary beam. To prevent charge build-up, insulating specimens must prevent charge build-up, insulating specimens must be coated with a conducting film to drain away the space charge. Sample Preparation The technique used to prepare rocks for examination with the SEM is the most critical part of obtaining micrographs. To obtain information about the pores of a rock, a surface must be prepared in such a way that the grains are cut through without being pulled from the rock matrix. This is difficult because the grains are stronger than the matrix. The problem was solved by application of techniques used to make thin-sections. Rock samples 5 cm in diameter x 5 cm thick were mounted in polyester resin, with care being taken not to allow the resin to enter the pores. JPT P. 543

Pore Structure in Sandstones by Compressible Sphere-Pack Models

Porosity and flow measurements made on packs of rubber spheres under compression indicate that ideal sphere packs do not model the flow properties of consolidated sandstones. Packs of rubber cubes have plate-like pore spaces. Under compression brine-saturated packs of rubber cubes display the same rapid increase in electrical conductivity observed when brine-saturated sandstones are compressed. This observation leads to the hypothesis that pore spaces in consolidated sandstones are plate-like, that is, they have two large and one small dimension. Compression of consolidated sandstone causes collapse of these plate-like pores.