Decline In Porosity Research Articles

Nano-silica and silica fume modified cement mortar used as Surface Protection Material to enhance the impermeability

In corrosion environment, corrosion ions can easily penetrate from the surface into the inside of the concrete due to the porous structure of the surface; in this case, concrete can inevitably suffer from the damage. In this study, an attempt to use nano SiO2 (NS) and silica fume (SF) modifying cement mortar as a Surface Protection Material (SPM) was made, in order to promote penetration resistance of the whole system. SPM was coated on the surface of matrix, and then interfacial bond strength between matrix and SPM was measured; shrinkage consistency was also considered; the chloride penetrability of the system was examined as well. To reveal the mechanism, effect of NS and SF on pore structure, Interfacial Transition Zone (ITZ), hydration process, and compressive strength of SPM were investigated. The results show that matrix coated with SPM on the surface has a good integrity, with excellent interfacial bond strength and little difference in shrinkage, and chloride diffusion coefficient of the system was considerably declined, in comparison with the matrix, showing an excellent penetration resistance. The mechanism behind is that SPM, which was modified with SF-NS, shows the excellent impermeability, and this kind of material existing on the surface can noticeably obstruct the chloride ions penetrating into the inside. In cement hydration process, SF and NS can not only consume a large amount of CH to form dense C-S-H, but also exert the grading filling effect, resulting in the decline in porosity, the increase in density, the improvement in microstructure of ITZ, and the enhancement in mechanical performance. The findings can provide useful experience for the design of the cement-based materials servicing in high corrosion environment.

Analysis of Effective Porosity and Effective Permeability in Shale-Gas Reservoirs With Consideration of Gas Adsorption and Stress Effects

Summary Nanoscale porosity and permeability play important roles in the characterization of shale-gas reservoirs and predicting shale-gas-production behavior. The gas adsorption and stress effects are two crucial parameters that should be considered in shale rocks. Although stress-dependent porosity and permeability models have been introduced and applied to calculate effective porosity and permeability, the adsorption effect specified as pore volume (PV) occupied by adsorbate is not properly accounted. Generally, gas adsorption results in significant reduction of nanoscale porosity and permeability in shale-gas reservoirs because the PV is occupied by layers of adsorbed-gas molecules. In this paper, correlations of effective porosity and permeability with the consideration of combining effects of gas adsorption and stress are developed for shale. For the adsorption effect, methane-adsorption capacity of shale rocks is measured on five shale-core samples in the laboratory by use of the gravimetric method. Methane-adsorption capacity is evaluated through performing regression analysis on Gibbs adsorption data from experimental measurements by use of the modified Dubinin-Astakhov (D-A) equation (Sakurovs et al. 2007) under the supercritical condition, from which the density of adsorbate is found. In addition, the Gibbs adsorption data are converted to absolute adsorption data to determine the volume of adsorbate. Furthermore, the stress-dependent porosity and permeability are calculated by use of McKee correlations (McKee et al. 1988) with the experimentally measured constant pore compressibility by use of the nonadsorptive-gas-expansion method. The developed correlations illustrating the changes in porosity and permeability with pore pressure in shale are similar to those produced by the Shi and Durucan model (2005), which represents the decline of porosity and permeability with the increase of pore pressure in the coalbed. The tendency of porosity and permeability change is the inverse of the common stress-dependent regulation that porosity and permeability increase with the increase of pore pressure. Here, the gas-adsorption effect has a larger influence on PV than stress effect does, which is because more gas is attempting to adsorb on the surface of the matrix as pore pressure increases. Furthermore, the developed correlations are added into a numerical-simulation model at field scale, which successfully matches production data from a horizontal well with multistage hydraulic fractures in the Barnett Shale reservoir. The simulation results note that without considering the effect of PV occupied by adsorbed gas, characterization of reservoir properties and prediction of gas production by history matching cannot be performed reliably. The purpose of this study is to introduce a model to calculate the volume of the adsorbed phase through the adsorption isotherm and propose correlations of effective porosity and permeability in shale rocks, including the consideration of the effects of both gas adsorption and stress. In addition, practical application of the developed correlations to reservoir-simulation work might achieve an appropriate evaluation of effective porosity and permeability and provide an accurate estimation of gas production in shale-gas reservoirs.

Grain composition and diagenesis of organic-rich lacustrine tarls, Triassic Yanchang Formation, Ordos Basin, China

The role of the primary detrital grain assemblage as a control on diagenetic pathways is reasonably well-understood in sandstones and limestones, but less so in mudrocks. We have documented diagenesis in mudstones from the Triassic Yanchang Formation that are dominated ([Formula: see text] by volume) by grains derived from outside the basin of deposition (terrigenous-argillaceous mudstones or tarls). Major extrabasinal grains are K-rich clay, quartz, plagioclase, K-feldspar, lithic fragments, and micas. In terms of the quartz-feldspar-lithic grain compositions, the silt fraction in these samples is classified as arkose. Grains of intrabasinal derivation include particulate organic matter, phosphatic debris, and rare carbonate allochems. The principal chemical diagenetic components in these mudrocks have strongly localized spatial distributions at micrometer to centimeter scales. Chemical diagenetic components include cone-in-cone structures, replacements of detrital feldspar, pore-filling precipitates within anomalously large pores, pore-filling solid hydrocarbon, and very minor quartz overgrowths associated with local packing flaws around silt-size detrital quartz grains. Matrix-dispersed intergranular cementation, as observed in well-known organic-rich marine mudstones, such as the Barnett Shale and the Eagle Ford Formation, is not observed in Yanchang Formation lacustrine mudstones. The authigenic features present are consistent with the thermal maturity of the units ([Formula: see text]) and are broadly similar to features observed in other mudstones that contain grain assemblages dominated by particles of extrabasinal derivation. The low porosity and the absence of significant amounts of intergranular cement indicate that compactional porosity loss and in-filling by migrated solid hydrocarbon were the major causes of porosity decline during diagenesis of Yanchang Formation mudrocks. Although the mudstones of the Yanchang Formation have a relatively high content of organic carbon and serve as source rocks in the Ordos Basin, the depositional grain assemblage is not conducive to creation of porosity, permeability, and mechanical properties that would make these mudrocks effective unconventional reservoirs.

Silica Diagenesis, Porosity Evolution, and Mechanical Behavior In Siliceous Mudstones, Mowry Shale (Cretaceous), Rocky Mountains, U.S.A.

Quartz in mudstones of the Mowry Shale (Cretaceous, Rocky Mountain region) takes several forms: extrabasinal detrital silt (some with transported overgrowths), replacement of skeletal debris, minor overgrowth on detrital quartz and replaced radiolaria, pore-filling in the intragranular pores of allochems, and authigenic microquartz that is dispersed through the clay-size matrix. The matrix-dispersed microquartz has CL character similar to the overgrowths with which it is intergrown, supporting the interpretation that it is authigenic and pore-filling. Radiolaria are widely observed in these mudrocks but are poorly preserved by partial mineral replacement (quartz, dolomite, calcite, pyrite). We interpret dissolved radiolaria, and possibly, now-cryptic diatoms, as the likely source of silica for the overgrowths and microquartz, through a reaction pathway that entailed the formation of opal-CT lepispheres that subsequently recrystallized to microquartz. A newly devised variation of the IGV (intergranular volume) concept, a parameter IGVm, is applied for partitioning the causes of porosity decline in mudstones. All of the samples examined contain authigenic microquartz, which averages around 40% of the rock volume. The large volume of microquartz, together with the volumes of surviving intergranular pores and pore-filling solid hydrocarbon, constrain the porosity to around 50% at the time of initial lepisphere formation. Despite the exceptional abundance of cement, overall porosity decline is compaction-dominated. The presence of significant volumes of cement has implications for mechanical rock properties in the Mowry Shale, which is expected to manifest brittle behavior where cements are most prominently developed.

Increasing of founding properties of secondary aluminium alloys

Purpose. To study the influence of metallurgical factors of production on casting properties of secondary aluminum alloy АК9М2. Methodology. For the experimental melts shaving amount in a charge, iron content and the quantity of modifier additive were chosen as independent variables. The components of modifier were being changed in the intervals of 25…40 % Na2CO3, 12…20 % SiC, 3…8 % Ti, the other – S. The microstructure of alloys was investigated under a light microscope, using the method of quantitative metallography. Influence analysis of certain parameters of alloys was conducted by mathematical statistics methods. The influence of shaving additions, iron and modifier amount on liquidity and porosity of the resulting alloys was studied. Findings. The paper shows that the increase of shaving content in the charge from 1 to 19 % and iron content in alloy from 0.66 to 2.34 % resulted in the decline of alloy liquidity on 30…35 %. Simultaneously the linear shrinkage reduction for 18…20 % and the porosity increase from 0.5 to 2.5 points were observed. The presented changes of alloy casting properties are conditioned by the amount of intermetallic phases of unfavorable form and its capacity for aeration. Increase of modifier additive from 0.02 to 0.15 % resulted in the liquidity increase on 10…15 %, the increase of linear shrinkage on 30…35 % and porosity decline from 2.5 to 0.5 points. At the same time a change of form of intermetallic phases and increase of their evenness were observed. Originality. The increase of iron concentration in silumin composition is accompanied by the decline of its liquidity. Thus, the rate of decline of alloy liquidity is proportional to the amount of dissolved iron. The character of iron influence is caused by formation of high temperature intermetallic compounds of the type Al3Fe, Al5SiFe, which promote the metal viscidity. Practical value. Practical use of the obtained scientific results would develop the technical solutions oriented to the quality indicator increase of silumin alloy.

A New Adsorption Based Correlation for Estimation of Membrane Bulk Porosity

In this work, a new simple correlation has been derived for the prediction of the bulk porosity decline during nanofiltration of a salty (ionic) solution using the PVD membrane. This model has been developed on the basis of the adsorption of the cations available in the feed onto the membrane. Therefore, three adsorption isotherms were tested and fitted to the equilibrium adsorption data, and the Freundlich isotherm was subsequently realized as the best choice. The porosity model included two adjustable parameters which can be determined theoretically and experimentally. Adsorption isotherms were combined for the determination of the first parameter and a novel method called “Flux test” was developed for the other parameter estimation. A reasonable agreement was observed between experimental- and model-predicted porosity. The model's ability of estimation indicates a little decrease at higher pressures due to approaching an unfavorable equilibrium adsorption condition and deviation from the isotherm's parameters from the actual equilibrium amounts to this situation.

Numerical simulation of profile control by clay particles after polymer flooding

A three-dimensional, two-phase, five-component mathematical model has been developed to describe flow characteristics of clay particles and flocs in the profile control process, in which the clay particle suspension is injected into the formation to react with residual polymer. This model considers the reaction of clay particles with residual polymer, apparent viscosity of the mixture, retention of clay particles and flocs, as well as the decline in porosity and permeability caused by the retention of clay particles and flocs. A finite difference method is used to discretize the equation for each component in the model. The Runge-Kutta method is used to solve the polymer flow equation, and operator splitting algorithms are used to split the flow equation for clay particles into a hyperbolic equation for convection and a parabolic equation for diffusion, which effectively ensures excellent precision, high speed and good stability. The numerical simulation had been applied successfully in the 4-P1920 unit of the Lamadian Oilfield to forecast the blocking capacity of clay particle suspension and to optimize the injection parameters.

Influence of humidification on deterioration of gas diffusivity in catalyst layer on polymer electrolyte fuel cell

The effect of water on polymer electrolyte fuel cell degradation was examined with humidity as a parameter. Polymer electrolyte fuel cells were subjected to long-term operation of 10 000 h to examine the relation between decline in cell voltage and degradation of the catalyst layers or gas diffusion layers. The diffusion overpotential increased during long-term operation at relatively high humidification of 81% RH, but only in the catalyst layer and not in the gas diffusion layer. At low humidification of 52% RH, the increase in diffusion overpotential was small, indicating that the increase was more likely to occur under high humidification. Post-analysis of the catalyst layer revealed that the membrane electrode assembly had increased diffusion overpotential during operation under high humidification, as a result of the sharp decline in porosity. The increase of diffusion overpotential in the catalyst layer was also investigated by the observation of the degradation due to the oxidation of the Pt–carbon supports. However, it was found that the oxidation of carbon support which had increased diffusion overpotential was small.

Threshold Concentration of Easily Assimilable Organic Carbon in Feedwater for Biofouling of Spiral-Wound Membranes

One of the major impediments in the application of spiral-wound membranes in water treatment or desalination is clogging of the feed channel by biofouling which is induced by nutrients in the feedwater. Organic carbon is, under most conditions, limiting the microbial growth. The objective of this study is to assess the relationship between the concentration of an easily assimilable organic compound such as acetate in the feedwater and the pressure drop increase in the feed channel. For this purpose the membrane fouling simulator (MFS) was used as a model for the feed channel of a spiral-wound membrane. This MFS unit was supplied with drinking water enriched with acetate at concentrations ranging from 1 to 1000 microg C x L(-1). The pressure drop (PD) in the feed channel increased at all tested concentrations but not with the blank. The PD increase could be described by a first order process based on theoretical considerations concerning biofilm formation rate and porosity decline. The relationship between the first order fouling rate constant R(f) and the acetate concentration is described with a saturation function corresponding with the growth kinetics of bacteria. Under the applied conditions the maximum R(f) (0.555 d(-1)) was reached at 25 microg acetate-C x L(-1) and the half saturation constant k(f) was estimated at 15 microg acetate-C x L(-1). This value is higher than k(s) values for suspended bacteria grown on acetate, which is attributed to substrate limited growth conditions in the biofilm. The threshold concentration for biofouling of the feed channel is about 1 microg acetate-C x L(-1).

The relationship between petroleum emplacement and carbonate reservoir quality: examples from Abu Dhabi and the Amu Darya Basin

The relative importance of petroleum emplacement in inhibiting diagenetic processes and preserving porosity and permeability in Lower Cretaceous, Thamama Group (Kharaib Formation) carbonate reservoirs of Abu Dhabi, UAE, and in Callovian-Kimmeridgian carbonate reservoirs of the Amu Darya Basin in Uzbekistan and Turkmenistan, has been evaluated by combining geologic, petrophysical and geochemical data. When petroleum emplacement is synchronous with and prior to significant burial cementation in carbonates, primary petroleum inclusions are trapped in the cements. The process appears to be characterised by steep intra-field porosity-depth trends within a more gradual regional decline in porosity with depth. This has profound implications for the prediction of porosity in carbonate reservoirs. Reservoir quality is better in grainstones and packstones compared to adjacent wackestones and lime mudstones in the Kharaib Formation because of preserved macroporosity (intergranular, vuggy, mouldic); the pore system in the finer units is dominated by micropores. These features indicate a primary textural control on porosity and permeability. Within the grainstones and packstones, macroporosity is variably filled by late equant sparry calcite cements. Porosity and permeability variations in grainstones and packstones at a reservoir scale are therefore controlled by the variation in amount of equant sparry calcite cement. This in turn depends on the timing of the precipitation of this cement relative to petroleum emplacement, as shown by fluid inclusion data. Where petroleum emplacement has occurred relatively early, at migration foci, prior to significant burial cementation by equant sparry calcite, reservoir quality is preserved. Where it has occurred after significant burial cementation, reservoir quality has been destroyed. In the Amu Darya sequences, primary macroporosity is commonly preserved down to depths of 11,000 ft (3.5 km) with differences in the porosity and permeability characteristics of grainstones being controlled by variations in the amount of early, probably freshwater, cement and the extent of associated dissolution. Small volumes of burial cements do occur, but they do not contain petroleum inclusions. Consequently, there is no firm evidence that petroleum emplacement has inhibited diagenesis in this area. This part of the study has shown that it is not always possible to obtain conclusive evidence from the diagenesis to pin down the processes responsible for the preservation of reservoir quality and that petroleum filling may not always be the primary cause. The relationships documented here show that the ‘race for space’ between diagenetic waters and petroleum is a major control on reservoir quality in the Thamama Group carbonate reservoirs, but is not so important for the Jurassic carbonates in the Amu Darya basin.

Porosity/depth trends in reservoir sandstones: assessing the quantitative effects of varying pore-pressure, temperature history and mineralogy, Norwegian Shelf data

AbstractEmpirical porosity/depth trends from the Norwegian Continental Shelf are discussed in the light of models on compactional processes during burial. Linear best-fit lines are useful as a first approximation, illustrating the general decline in porosity with depth. Porosity increases non-linearly with pore-pressure, and notably higher than normal porosity is found in very high overpressure reservoirs. Hydrostatic pressure sandstones from the Haltenbanken area are on average more porous than equivalents from the northern Viking Graben. This may be related to the burial history, as generally more subsidence occurred in Late Pliocene and Pleistocene in the Haltenbanken basins. The porosity/depth data show considerable scatter, and this is to a large degree related to the initial mineralogical composition. Correlated with mineralogical analyses, the reservoir sandstones show a trend towards lower porosity with increasing clay content at shallow burial, probably due to more extensive mechanical compaction. At depths >3.0–3.5 km, the porosity loss is highest in sandstones with low clay contents. This may be explained by a lower rate of quartz cementation in sandstones with abundant intergranular clays or clay coatings around quartz grains. These data show that pre-burial mineralogy is a very important factor in porosity prediction of sandstones, in many cases more important than the degree of overpressure.

Effects of subsurface drainage, cultivation, and stubble retention on soil porosity and crop growth in a high rainfall area

Wheat, triticale, and rapeseed growth and yield were studied under various tillage (conventional, deep ripping, direct drilling) and stubble-handling (burnt, retained) regimes with and without drainage at Hamilton in south-western Victoria from 1985 to 1987. Grain yield was increased from about 2 to >4 t/ha by drainage in both years; however, effects of other treatments, although significant, were much less. Soil structure (as measured by fractional air-filled porosity at -5 J/kg) deteriorated during winter and recovered during spring and summer. A laboratory experiment showed that this variation in soil structure resulted from saturation per se and redrying. In the field, the decline in porosity was most pronounced with cultivation and the absence of drainage, but overall, the effects of stubble retention and tillage treatments were small. There was a significant positive relationship between yield and porosity on undrained areas, but not where drains were present. Drainage reduced soil structural decline during winter, while stubble retention reduced the decline in porosity in the cultivated-undrained treatment in 1987.

Synchronous oil migration and cementation in sandstone reservoirs demonstrated by quantitative description of diagenesis

We present a model that explains the patterns of sandstone burial diagenesis in certain oil reservoirs, in which petroleum migration and burial cementation were synchronous. The coincidence of these two processes controls the chemistry and distribution of major burial cement phases across the field, which in turn controls the distribution of reservoir quality, causing a rapid decline of porosity and permeability with depth. Such a rapid poroperm deterioration is observed in many North Sea sandstone oilfields; we highlight the Magnus Sandstone Member of the Magnus Oilfield, northern North Sea as a type example of such a reservoir. The two most significant elements of the synchronous cementation and migration model are that burial cementation in the reservoir occurs over a restricted time interval, probably less than 10 Ma and that rapid and widespread fluid circulation is not invoked to explain the concentrations of cements observed. We speculate that cementation takes place at, and in a series of zones below, the oil-water contact which descends as oil fills the reservoir, with little change to the bulk chemistry of the reservoir formation waters through time.

Models of burial diagenesis for deep exploration plays in Jurassic fault traps of the Central and Northern North Sea

With increasing burial depth, intergranular porosity declines in a predictable manner according to theoretical and empirically established mathematical relationships. In basins with simple structural and thermal histories, average porosity can often be represented as an exponential function of depth. There are, however, common significant deviations from such a simple model of porosity evolution. Four mechanisms are recognized by which enhanced porosity may be generated or maintained during burial: development of overpressures due to restricted pore fluid escape; inhibition of mechanical compaction by selective cementation; inhibition of cementation as a result of hydrocarbon accumulation; or generation of secondary porosity due to either grain or cement dissolution together with simultaneous removal of reaction products. Secondary porosity may be generated either near the surface by the ingress of dilute meteoric-derived waters or at depth by the generation of ‘aggressive’ pore fluids. Deep burial diagnesis may encompass both closed and open system porosity generating reactions. In closed systems, increase in temperature concomitant with burial overcomes kinetic barriers enabling dissolution of metastable detrital minerals. By contrast, in more open systems, movement of fluids from either mudstones or evaporites to either overlying or adjacent sandstones introduces chemical disequilibrium that may cause mineral dissolution. Rapid, intermittent movement of fluids along active faults may additionally introduce temperature disequilibrium to further facilitate mineral dissolution in the proximity of faults. Over geological periods of time, however, faults are barriers to fluid flow and provide a mechanism for generating overpressure. In the Central and Northern North Sea basins, an upper diagenetic regime is separated from a lower diagenetic regime by a regional cover of Cretaceous mudstones and chalks. The Cretaceous sediments provide a regional seal which often coincides with the development of overpressure above deep graben centres reflecting restricted pore water escape. Fluid and solute movement between these two regimes is only possible where faults or gas chimneys penetrate the Cretaceous seal. In the upper regime, porosity loss in Tertiary sediments broadly conforms to the predicted decline of porosity with depth. In the lower regime, Jurassic sandstones on structural highs are characterized by extensive secondary porosity resulting from feldspar grain and carbonate cement dissolution, best preserved where oil accumulation rapidly follows porosity generation. Enhanced porosity can be expected either where sandstones interdigitate with mudstones or in graben centre plays adjacent to faults. This contrasts with graben margin plays where faults are the locus of pronounced cementation and complex diagenetic sequences are developed. In the absence of either oil accumulation or overpressure, secondary porosity either compacts or is occluded by a characteristic deep burial authigenic mineral assemblage. The present heat flow in the North Sea can be interpreted in terms of a residual thermobaric fluid flow from basin centres to basin margins which was probably initiated during the early Tertiary, coincident with the onset of rapid thermal relaxation subsidence.

Evolution of Porosity and Permeability in the Lower Cretaceous Travis Peak Formation, East Texas (1)

Porosity and permeability decrease significantly with depth in low-permeability, gas-bearing Travis Peak sandstones on the western flank of the Sabine arch, east Texas. The Travis Peak Formation is approximately 2000 ft (610 m) thick and is buried to depths of 6000-10,000 ft (1830-3050 m). Reservoirs consist chiefly of sandstones of fluvial and paralic origin. Thin-section and core-analysis data from a total of 3680 ft (1122 m) of core from 26 wells indicate that both depositional environment and diagenetic history influenced the present distribution of porosity and permeability. Porosity in clean sandstones decreases from an average (geometric mean) of 16.6% at 6000 ft (1830 m) to 5.0% at 10,000 ft (3050 m). Permeability is reduced four orders of magnitude over the same depth range, from an average (geometric mean) of 10 md at 6000 ft (1830 m) to 0.001 md at 10,000 ft (3050 m). Loss of porosity by mechanical compaction took place mainly within the first 3000 ft (900 m) of burial, prior to significant cementation. The observed decline in porosity and permeability with depth between 6000 and 10,000 ft (1830 and 3050 m) results from (1) increasing quartz cement, (2) decreasing secondary porosity, and (3) increasing overburden pressure that closes narrow pore throats. At any given depth, average permeability is 10 times higher in clean fluvial than in clean paral c sandstones. Clean fluvial and paralic sandstones contain similar volumes of quartz cement, but paralic sandstones are finer grained and contain an average of 7% more total cement.

Intergranular Volume (IGV) Decline Curves for Evaluating and Predicting Compaction and Porosity Loss in Sandstones

In contrast to conventional empirical porosity prediction schemes based on porosity decline with depth or thermal maturity, development of a conceptually sound porosity prediction algorithm requires quantitative assessment with depth (or thermal maturity) of two independent processes: compaction (as monitored by intergranular volume or IGV) and cementation. IGV is the sum of intergranular porosity, matrix, and cement as measured in thin section. For this study, in order to evaluate the effects on porosity of compaction alone, a plot of IGV versus depth was constructed using data from greater than 350 relatively uncemented ({lt}5% cement) sandstones from a wide variety of depths, ages, and geographic locations. Because texture and composition affect the initial IGV, the following additional restrictions were applied to the data set: {ge}0.1 mm grain size, well to moderately well sorted, {lt}5% matrix, {lt}15% feldspar grains, and {lt}5% ductile grains. The resulting IGV decline curve reveals that sands compact mechanically and IGV declines rapidly from 40 to 42% at the surface to about 28% at 5000 ft. Between 5000 and 8000 ft, decline continues slowly until the framework stabilizes at around 26%. No further significant IGV decrease is observed to the depth limits of the data set at 22,000more » ft. The most significant implications of this curve for porosity prediction in sandstones are that (1) primary porosity can be preserved to great depths in the absence of cement, suggesting that reported volumes of secondary porosity in sandstones are exaggerated, (2) intergranular pressure solution plays a minor role in localized quartz cementation, and (3) cement prediction is a major key to porosity prediction.« less

Porosity Trends of Nonreservoir and Reservoir Sandstones, Anadarko Basin, Oklahoma

The porosity of nonreservoir sandstones in Caddo County, Oklahoma, is determined using compensated-neutron and formation-density logs. The authors preliminary data set represents more than 3,000 net ft of Pennsylvanian and Permian age sandstones from 12 well locations. These porosity data and the average porosities of sandstone oil and gas reservoirs within the Anadarko basin of Oklahoma are each compared to a broad, composite set of porosity data from numerous basins that represent sandstones in general, and they are also compared to each other. The porosity of nonreservoir sandstones in Caddo County declines predictably as a power function of increasing thermal maturity for vitrinite reflectance (R{sub 0}) of 0.5 to 1.3%. The rate of porosity decrease with increasing thermal maturity is more rapid than that of the average porosity-R{sub 0} trend of the composite set, but is still within the porosity-R{sub 0} envelope of sandstones in general. Hydrocarbon reservoir sandstones of the Anadarko basin, however, follow a different pattern. Their rate of porosity loss is much slower than that of both sandstones in general, and nonreservoir sandstones of Caddo County. This slow rate of porosity decline with increasing R{sub 0} could be due to inhibiting effects of early hydrocarbon emplacement onmore » diagenesis and (or) to the bias of economic selection. In any case, as R{sub 0} increases beyond about 1%, the porosity of Anadarko basin reservoir sandstones is anomalously high compared to both nonreservoir Anadarko basin sandstones and sandstones in general.« less

Losses of Porosity and Permeability in Lithic Sands Assessed with Compaction Experiments: ABSTRACT

Compaction experiments with Recent lithic sands indicate that in addition to grain rearrangement, ductile grain deformation can significantly reduce porosity and permeability in sandstones. Compacted artificial mixtures of quartz and potentially labile rock fragments demonstrate, as expected, that the lithic grain content can control the amount of compaction and porosity loss at depth. Using a triaxial deformation apparatus, simulated overburden experiments were conducted with a Recent lithic sand from the Eagle River, Alaska, containing approximately 55% argillaceous sedimentary rock fragments. Additional experiments were conducted with artificial mixtures of quartz grains and varying percentages of shale fragments. Water-saturated samples were compacted at a constant strain rate of 10/sup -4/ under maximum uniaxial stress levels ranging from 1500 to 9000 psi (equivalent to burial depths of 3000 to 18,000 ft). Porosity loss due to compaction was calculated from volume reduction. Vertical permeability for each experiment was measured in situ at the maximum simulated overburden stress level. Results indicate that the porosity of these lithic sands initially declines almost linearly with increasing overburden pressure. At higher pressures (in excess of 5000 psi) the porosity decline gradient is greater for sands with a greater percentage of lithic fragments. This relationship may be attributed tomore » continued porosity reduction due to mechanical deformation of ductile grains. Thin-section analysis of the Eagle River sands indicates that deformation of labile grains is important in reducing porosity to 7% and vertical permeability to 3.2 md at a simulated overburden pressure of 9000 psi.« less

Diagenesis of Sandstone Reservoirs of Pattani Basin, Gulf of Thailand

Miocene sandstones from gas fields in the Gulf of Thailand's Pattani basin provide an example of rapid decline in porosity and permeability with increasing burial depth. This decline results from rapid burial diagenesis that is related to very high geothermal gradients in the basin. Porosity loss with depth in these subquartzose sandstones results from mechanical compaction and from progressive cementation by quartz overgrowths, kaolinite, and illite. Quartz overgrowths increase with depth, indicating continuous or episodic silica cementation. Kaolinite occurs as a pore-filling cement and is abundant between the depths of 1,980 and 3,050 m (6,500 and 10,000 ft). Minor cements include calcite, ankerite, mixed-layer illite-smectite, siderite, pyrite, chlorite, and barite. > The best porosity and permeability in Pattani basin reservoirs are generally associated with large intergranular pores in sandstones between 915 and 1,980 m (3,000 and 6,500 ft). At greater depths most interparticle pores have been occluded, and porosity is mainly secondary in origin. Skeletonized feldspars indicate progressive dissolution with increasing burial depth. In low-permeability sandstones from deeper zones (2,285-3,050 m or 7,500-10,000 ft), porosity is mainly restricted to dissolution voids within detrital feldspars. These secondary pores are usually partly filled by authigenic kaolinite and illite, and their pore apertures are generally smaller (1-15 µm) than intergranular pore apertures (10-75 µm). However, favorable reservoir properties may occur locally at de th where large feldspars have been leached from coarse-grained sandstones.

Reduction and prediction of sandstone reservoir potential, Jurassic , North Sea

Porosity, permeability, mineralogical and depth data for two North Sea Jurassic sandstone sequences were analysed. Both sequences show statistically significant negative correlations between present burial depth and porosity. The influence of secondary porosity creation is subordinate to that of the general decline in porosity. For a given burial depth , sequence A is, on average, a little more porous (about 3%) than B. However, for a given porosity sequence A displays a permeability 1—3 orders of magnitude greater than B. The large permeability difference between A and B is a function of authigenic mineralogy. The only significant cement within the reservoir intervals of sequence A is quartz . Sequence B contains authigenic clays, quartz and subordinate carbonate. The abundant authigenic clay in B severely reduced permeability. In both instances, the cements are products of burial and were precipitated from pore waters expelled from shales during compaction. The expelled pore waters were both acidic and rich in solutes; a product of reactions between maturing organic matter, clays and iron oxides. The difference in authigenic mineralogy between the sequences was caused by the reaction between pore waters and sandstones with different detrital mineral compositions. Thus the present reservoir quality is a product of burial and of the reactions between evolving pore fluids and minerals in the sandstone.