Pore Water Saturation Research Articles

Just Add Water and Salt: the Optimisation of Petrogenic Hydrocarbon Biodegradation in Soils from Semi-arid Barrow Island, Western Australia

We investigated the potential of soil moisture and nutrient amendments to enhance the biodegradation of oil in the soils from an ecologically unique semi-arid island. This was achieved using a series of controlled laboratory incubations where moisture or nutrient levels were experimentally manipulated. Respired CO2 increased sharply with moisture amendment reflecting the severe moisture limitation of these porous and semi-arid soils. The greatest levels of CO2 respiration were generally obtained with a soil pore water saturation of 50–70%. Biodegradation in these nutrient poor soils was also promoted by the moderate addition of a nitrogen fertiliser. Increased biodegradation was greater at the lowest amendment rate (100 mg N kg−1 soil) than the higher levels (500 or 1,000 mg N kg−1 soil), suggesting the higher application rates may introduce N toxicity. Addition of phosphorous alone had little effect, but a combined 500 mg N and 200 mg P kg−1 soil amendment led to a synergistic increase in CO2 respiration (3.0×), suggesting P can limit the biodegradation of hydrocarbons following exogenous N amendment.

A study of celestine equilibrium in marine sediments using the entire ODP/IODP porewater data base

The stability of celestine (SrSO 4) in marine sediments has been investigated through the calculation of its saturation index at the in situ temperature and pressure, using the entire ODP/IODP porewater composition data base (14,416 samples recovered from sediments collected during 95 ODP and IODP Legs) that has been thoroughly corrected for missing data and inconsistencies. The porewater in situ pressure has been obtained from depth data, and the in situ sample temperature has been calculated from the bottom seawater temperature and from the measured thermal gradient. When the latter is unavailable, a default value of 35 °C/km or an assumed gradient similar to that of a site located close to the study area has been used. Molarities have been converted to molalities through the calculation of the porewater densities, which have represented by an empirical function of the total dissolved solids load up to 150 g/L. Ocean bottom waters are largely undersaturated with respect to celestine. Yet sediment porewater saturation is not uncommon: it is reached in 83 boreholes (i.e. about 10% of all boreholes) drilled during 22 ODP/IODP Legs. Celestine equilibrium can be reached through two different non-exclusive causes: a strontium increase in porewater linked to the dissolution of Sr-rich aragonite or biogenic calcite and precipitation of Sr-poor calcite (carbonate recrystallization), or an increase in the strontium and sulfate porewater concentrations linked to a salinity increase due to the presence of brines. Sediments at most of the sites exhibiting celestine equilibrium share common characteristics such as a carbonate-rich lithology (typically higher than 80 wt.%) and a low organic carbon content (generally below 1 wt.%). These results indicate that modification of porewaters during burial diagenesis can easily lead to celestine saturation, especially in carbonate-rich sediments. We then briefly discuss former interpretations of the presence of celestine in ancient and recent marine sediments, as well as the consequences of the incorporation of celestine formation in diagenetic models using the porewater Sr content and Sr isotopic composition.

Experimental percolation of supercritical CO2 through a caprock

Abstract Leakage of CO 2 into the atmosphere is the most crucial concern for geological storage of anthropogenic CO 2 . Leakage routes could develop through existing wells and pipelines, but also by natural migration of CO 2 rich pore-fluid through the caprock and in fault zones. Therefore, a thorough geological characterization of the prospective formation identifying seal capacity, integrity and possible migration pathways must be performed prior to injection of CO 2 . A caprock is a low permeable confining layer trapping CO 2 stored in a reservoir rock. Although the caprock acts as a seal, its lower boundary will be in contact with CO 2 saturated pore water or even pure CO 2 . Chemical interaction between the pore fluid and the caprock may change its material properties. The aim of this study is to increase our understanding of the interaction between CO 2 and caprock, focusing on the microstructural properties of the rock. A flow through cell is used to flood a shale core (40 mm long and 38 mm diameter) with supercritical CO 2 at a temperature of 35 ∘ C and at pressures above 7.5 MPa. A pressure gradient is applied across the sample to obtain a breakthrough of CO 2 in the core. During flooding both axial and radial strain of the core are measured together with the acoustic velocities in the axial direction. The measurements are performed both for the brine saturated core and at various stages during flooding with CO 2 . The experimental results suggest flow of CO 2 along defined pathways within the shale. These pathways are likely to be controlled by the increasing pore pressure at the bottom of the sample which allows reopening of the cracks in the lower part of the sample, allowing CO 2 to enter the shale. Flow of CO 2 into the cracks in the lower part of the sample is followed by percolation of CO 2 in the upper part of the sample where the effective pressure is higher and crack reopening is less likely to occur.

GPR Attenuation and Scattering in a Mature Hydrocarbon Spill: A Modeling Study

Subsurface contamination by light nonaqueous‐phase liquids (LNAPLs) is a pressing environmental issue with many industrial sites having some degree of near‐surface pollution. Noninvasive, geophysical investigation methods, such as ground penetrating radar (GPR), have become increasingly popular, but their use has tended to be restricted to plume “mapping” and recent contamination events only. Mature LNAPL contamination, where the hydrocarbons undergo significant alteration and biodegradation, complicates the electrophysical properties of the subsurface, making GPR data interpretation more difficult. This is particularly true in shallow, coastal environments where LNAPLs form a disaggregated, smeared zone associated with the seasonally changing groundwaters. Therefore, any method that can improve data interpretation has a significant practical benefit in these complex environments. By using dielectric material property analysis, empirical mixing model evaluation, and three‐dimensional, finite‐difference time‐domain (FDTD) modeling, it has been possible to investigate the nature and spectral content of GPR signal attenuation and scattering within the vadose or smeared zone of a mature LNAPL‐contaminated site. Using FDTD models to simulate real subsurface conditions, a comparison between the GPR response of “clean” and contaminated environments has been made where the subsurface materials contain different LNAPL–porewater saturations and contaminant geometries. The results show that uniform mixtures, disaggregated scattering mixtures, and stratified pools of contaminated material all exhibit different, yet characterizable, temporal and spectral GPR responses and that the recorded data can provide new insights into the mode of contaminant distribution and saturation as long as the incident field of the GPR system can be determined accurately.

Quantitative interpretation of pore water O 2 and pH distributions in deep-sea sediments

A reaction-transport model is used to interpret 13 data sets of in situ O 2 and pH microelectrode profiles measured in deep-sea sediments from five oceanic regions. The model includes a mechanistic description of the major early diagenetic redox transformations, as well as solid and solute transport processes, pore water acid–base equilibria, and calcite dissolution. Four different dependencies of the calcite dissolution rate on the degree of pore water saturation are considered. The results of the systematic and data-driven approach indicate that consideration of two pools of reactive organic carbon is sufficient to reproduce the oxygen data; that the more reactive pool dominates the organic carbon deposition flux; and that suboxic degradation pathways play a non-negligible role at the majority of sites. Estimation of the calcite related parameters, however, yields only nine successful simulations of the pH profiles. The dissolution rate expressions with the higher rate orders tested (2 and 4.5) are generally more successful in reproducing the in situ pH profiles. While the pore water pH profiles cannot constrain the calcite deposition flux, robust estimates of the depth-integrated calcite dissolution rates can be obtained. These correlate positively with the benthic oxygen uptake fluxes, although sites overlain with undersaturated and supersaturated bottom waters exhibit separate trends.

Chemical weathering of a marine terrace chronosequence, Santa Cruz, California I: Interpreting rates and controls based on soil concentration–depth profiles

The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow argillic horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America. Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile b s is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux q h, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances ( q h = 0.025–0.17 m yr −1). During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as R = q h · m sol M total · 1 S v · b s · The product of q h and the ratio of solubilized to solid state feldspar ( m sat/ M total) define the weathering velocity. The weathering gradient b s reflects the kinetic rate of reaction where S v is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates ( R = 5–14 × 10 −16 mol m −2 s −1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.

A Mechanism for Anomalous Decline in Radon Precursory to an Earthquake

Mechanisms for interpreting anomalous decreases in radon in ground water prior to earthquakes are examined with the help of a case study to show that radon potentially is a sensitive tracer of strain changes in the crust preceding an earthquake. The 2003 Chengkung earthquake of magnitude (M) 6.8 on December 10, 2003, was the strongest earthquake near the Chengkung area in eastern Taiwan since 1951. The Antung radon-monitoring station was located 20 km from the epicenter. Approximately 65 d prior to the 2003 Chengkung earthquake, precursory changes in radon concentration in ground water were observed. Specifically, radon decreased from a background level of 780 pCi/L to a minimum of 330 pCi/L. The Antung hot spring is situated in a fractured block of tuffaceous sandstone surrounded by ductile mudstone. Given these geological conditions, we hypothesized that the dilation of brittle rock mass occurred at a rate faster than the recharge of pore water and gas saturation developed in newly created cracks preceding the earthquake. Radon partitioning into the gas phase may explain the anomalous decrease of radon precursory to the 2003 Chengkung earthquake. To support the hypothesis, vapor-liquid, two-phase radon-partitioning experiments were conducted at formation temperature (60 degrees C) using formation brine from the Antung hot spring. Experimental data indicated that the decrease in radon required a gas saturation of 10% developed in rock cracks. The observed decline in radon can be correlated with the increase in gas saturation and then with the volumetric strain change for a given fracture porosity.

Acid volatile sulfide (AVS)

The sequestration of sulfur within sedimentary pyrite is a major sink in the global biogeochemical sulfur cycle and has consequent impacts on the related cycles of carbon and oxygen. In many sedimentary environments, a zone exists which releases H 2S if treated with acid. The materials that produce this sulfide are described as acid volatile sulfides or AVS and the sulfide which is evolved and collected by this treatment is called acid volatile sulfide (AVS-S). In this research review we show that AVS represents part of a complex and dynamic biogeochemical system which is not defined simply by the analysis of AVS-S. During the 40 years since R.A. Berner first defined the concept of acid volatile sulfide, a number of scientific myths have developed regarding the nature and composition of AVS and its role in the global sulfur cycle. We show that: • AVS is not equivalent to FeS and solid FeS phases have rarely been identified in marine sediments. AVS is a complex and variable component which does not represent any singular or simple group of sediment components. • The commonly used extraction methods do not quantitatively extract AVS minerals (mackinawite and greigite) without also significantly releasing sulfide from pyrite. • AVS varies geographically and AVS is not the same component or set of components wherever it is found. • The solubilities of FeS and Fe 3S 4 in neutral to alkaline systems are presently poorly constrained and pore water saturation state calculations cannot be used to support the presence of specific AVS minerals. • The colored zone in sediments does not reflect the distribution of AVS. • AVS is not always a minor fraction of total reduced sulfide (TRS) which dominantly occurs near the sediment–water interface. AVS minerals are not required precursors for pyrite formation. A better understanding of the sedimentary sulfur system, including its nature, formation and transformations, is a key component in understanding the evolution of the Earth and its future development. However, some basic parameters, such as the solubility and composition of the metastable iron sulfide solids, are still not constrained. Even the presence of these minerals in most sedimentary systems remains equivocal. Aqueous FeS clusters appear to be ubiquitous and appear to have significant geochemical and biological roles. The roles of organisms, especially in modifying rates and products, remain a key area of research. Although AVS may have served its purpose in the earlier exploratory days of the study of sedimentary sulfides, the concept is now probably more of a hindrance than a help in advancing this field. It is reasonable to question if it is not time to simply do away with the AVS concept altogether and get on with the business of trying to deal more directly with the behavior of the more important components known to fall within the group of sulfide species that contribute to sulfide that is volatilized when acid is added to a sediment.

The role of water in cooling ignimbrites

A summary of observational literature on ignimbrites provides the basis for the development of a two-dimensional numerical model of ignimbrite cooling processes. Factors include emplacement conditions, post-emplacement processes, and the nature and timing of interactions with water during cooling. The model uses the multiphase finite element heat and mass transfer (FEHM) code, which has been enhanced to handle conditions up to 1500 °C. The instantaneous emplacement of a 750 °C ignimbrite with internal gas pressures of up to 0.5 MPa (lithostatic) has a great effect on the variably saturated substrate. A water table present within a few tens of meters of the base of the ignimbrite produces a region of high pressure and temperature that exists for about 20 years, driving vapor upward through the ignimbrite as diffuse flow and in gas escape structures and enhancing cooling at the base of the ignimbrite. Variations in initial gas pressure between atmospheric and lithostatic conditions have little effect on the thermal evolution. The results of the numerical modeling of 20- and 40-m-thick ignimbrites indicate that, even for moderate pore water saturations in the substrate, vaporization and resultant pressurization may exceed lithostatic confining pressures in the upper substrate and basal ignimbrite, and explosive pressure release may occur, resulting in the development of discrete fumarole conduits or phreatic explosions. The likelihood for explosive pressure release appears to be greater when the nominal ignimbrite thickness is on the order of the depth of a buried valley. The pressure buildup is enhanced by the geometry of the ignimbrite–substrate interface, especially at convex corners such as on the edges of a buried valley. The boiling zones at the top and bottom of a cooling ignimbrite involve the development of a heat-pipe, which provides an efficient means of transporting heat from the superheated tephra out tens of meters into the ambient environment. The predicted temporal evolution of temperature, pressure, and vapor flow in a 40-m ignimbrite support the conceptual model of degassing, welding and compaction, devitrification, and alteration occurring concomitantly in the first several years after emplacement and driven in part by production and migration of meteoric steam. This vapor flowing through the ignimbrite matrix at 5×10 −5 kg s −1 in the first 10 years enhances devitrification in any part of the ignimbrite above the base in nonwelded deposits. In the case where welding occurs, lower permeability limits the diffuse flow of gas upward through the ignimbrite from the region of boiling and pore pressurization at the base, and enhanced devitrification in the basal parts of the ignimbrite may occur where pore vapors circulate in abundance. Immediately above the welded zone, a devitrified horizon may develop where the upper boiling/condensation zone and perched meteoric infiltration results in enhanced saturations.

Environmental controls over methyl halide emissions from rice paddies

This paper examines primary controlling factors that affect methyl halide emissions from rice paddy ecosystems. Observations of four cultivars under multiple growth conditions during studies in commercial fields and the University of California, Irvine, greenhouse lead to the conclusion that daily emissions of methyl halides are primarily determined by the growth stage of the rice plant, with the exception that methyl chloride emissions show no clear seasonal pattern. Methyl chloride emissions appear to be more from the paddy water and/or soil as opposed to the plants; however, in soils with high chloride content, these emissions appear to peak during the reproductive phase. Strong secondary influences include air temperature, soil halide concentration, and soil pore water saturation. The cultivars studied had statistically separate seasonally integrated emissions. Irradiant light and aboveground biomass appear to have little effect on emissions. Emissions of methyl chloride, methyl bromide, and methyl iodide are estimated to be 3.5, 2.3, and 48 mg/m2/yr, or 5.3, 3.5, and 72 Gg/yr, from rice paddies globally.

Role of sulfide oxidation in dolomitization: Sediment and pore-water geochemistry of a modern hypersaline lagoon system

Dolomite precipitation has been reported in sediments of marginal-marine hypersaline lagoons near Rio de Janeiro, Brazil, and has been related to microbial sulfate reduction. Our field study of pore-water and sediment geochemistry in two lagoons reveals only modest degrees of SO4 reduction and even H2S oxidation. Significant differences in dolomite contents, salinity, and carbon-sulfur cycling occur in the two lagoons; pore waters from the most dolomite-rich sediments exhibit significantly elevated SO4/Cl ratios relative to seawater. Moreover, the O isotope composition of SO4 in all lagoonal waters has evolved significantly from seawater values. This suggests active sulfur recycling via H2S oxidation, which provides acid that decreases pore-water saturation state for high-Mg calcite and aragonite, thermodynamically favoring the precipitation of dolomite. The open hydrogeochemical nature of these lagoons maintains SO4 concentrations below gypsum saturation, allowing dolomite to form from fluids with normal marine Mg/Ca.

A biological source of bubbles in sandy marine sediments

Gas in sediments, even in small quantities, will modify the propagation and scattering of sound. Shallow water littoral environments are often sufficiently well lit by sunlight to support healthy populations of benthic and epibenthic marine microalgae. Photosynthesis in marine algae produces oxygen. Oxygen saturation levels as high as 600% have been measured in the pore water of a sandy sediment at 1 mm depth, decreasing to 100% saturation at ca. 4.5 mm. Bioturbation and physical processes routinely mix materials at the surface of the seabed, including algae, to depths of at least a few cm. Mixing times and depths vary with the sediment type and the species and abundance of organisms present, but time scales of minutes to hours are common. While light is rapidly attenuated with depth in sand, measurements show that it can penetrate to depths of a few mm. Physical and biological mechanisms are suggested which could produce gas bubbles in oxygen saturated pore water. Laboratory measurements of sound scattering from marine algae on a sand surface suggest a possible method for in situ bubble detection in shallow marine environments. [Work supported by ONR.]

Modelling of Lithology and Hydraulic Conductivity of Shallow Sediments from Resistivity Measurements Using Schlumberger Vertical Electric Soundings

The vertical electric soundings (VES) technique has been effectively used for several purposes related to investigations of surface soils and shallow sediments. These include detection of subsurface lithology, structures, stratification, and dipping layers; evaluation of physical properties that govern fluid flow and electric-current conduction; delineation of aquifers; assessment of engineering-site properties (such as pore-water saturation and clay content); and mapping various environmental problems (such as saline-water intrusion, groundwater contamination, and leachate transport through porous media). In this study, Schlumberger VES-measurements, along with borehole and experimental data, were employed to image laminations, layering, and lithology of surface soils and subsurface aeration and saturated zones of glacial sediments; and to model the electric resistivity and hydraulic conductivity variations with depth and lithology. It is shown that the VES-technique can provide a substantial improvement in the resolution of thin laminations within thick layers and can also provide a good assessment of the hydraulic conductivity, water flux, and water movement. Numerous references, dealing with a wide range of VES-applications for several purposes, were reviewed and cited in this study.

Modelling of Lithology and Hydraulic Conductivity of Shallow Sediments from Resistivity Measurements Using Schlumberger Vertical Electric Soundings

The vertical electric soundings (VES) technique has been effectively used for several purposes related to investigations of surface soils and shallow sediments. These include detection of subsurface lithology, structures, stratification, and dipping layers; evaluation of physical properties that govern fluid flow and electric-current conduction; delineation of aquifers; assessment of engineering-site properties (such as pore-water saturation and clay content); and mapping various environmental problems (such as saline-water intrusion, groundwater contamination, and leachate transport through porous media). In this study, Schlumberger VES-measurements, along with borehole and experimental data, were employed to image laminations, layering, and lithology of surface soils and subsurface aeration and saturated zones of glacial sediments; and to model the electric resistivity and hydraulic conductivity variations with depth and lithology. It is shown that the VES-technique can provide a substantial improvement in the resolution of thin laminations within thick layers and can also provide a good assessment of the hydraulic conductivity, water flux, and water movement. Numerous references, dealing with a wide range of VES-applications for several purposes, were reviewed and cited in this study.

Saturation effects on horizontal and vertical motions in a layered soil–bedrock system due to inclined SV waves

A study of the effects of pore-water saturation on the horizontal and vertical components of ground motions in a multi-layered soil–bedrock system due to inclined SV waves is presented. Both the soil and the rock are modeled as a partially water-saturated porous medium which is characterized by its degree of saturation, porosity, permeability, and compressibility. An efficient formulation is developed for the computation of the two-dimensional ground motions, which are considered as functions of the angle of incidence, the degree of saturation, the frequency, and the geometry of the system. Numerical results for both the half-space model and the single-layered model indicate that the effect of saturation may be significant, and is dependent on the angle of the incidence. Even a slight decrease of full saturation of the overlying soil may cause appreciable difference in the amplitudes of ground motions in both the horizontal and vertical components and the amplitude ratios between the two components at the ground surface, implying that one may need to carefully take into account the saturation conditions in the interpretation of field observations.

Effects of Pore-Water Saturation on Seismic Reflection and Transmission from a Boundary of Porous Soils

Recent analysis of array observations indicated that pore-water saturation of soils may strongly affect vertical site amplification. A study is therefore motivated to investigate the saturation effect on seismic reflection from a boundary of porous soils. The problem described herein corresponds to a P - or SV -wave incident at the interface between rock formation and overlying sand. The sand is modeled as a partially water-saturated porous material while the rock is approximately regarded as ordinary one-phase solids. Preliminary results show that in either P - or SV -wave incidence, even a slight decrease of the complete saturation may lead to a substantial influence on both reflected and transmitted waves as well as the amplitude ratios between horizontal and vertical motion at the interface, and this influence is dependent on the angle of incidence. The underlying suggestion is that we may need to carefully consider the saturation effect in the interpretation of field observations, especially in such situations that partial saturation may very probably take place.

Interpretation of Seismic Vertical Amplification Observed at an Array Site

Strong ground motions were recorded at Port Island, Kobe, by a borehole array during the 1995 Hyogo-ken Nanbu (Kobe) earthquake. These records indicate that, while the horizontal peak accelerations were reduced as seismic waves traveling from the bottom to the surface, motions in the vertical direction were significantly amplified at the surface, with peak value of 1.5 to 2 times larger than the horizontal components. Some studies have discussed local site effects on the horizontal motions, whereas the available study on the vertical amplification is limited. In this article, we present a possible mechanism to explain the observed vertical amplification in detail. We consider that the observed large vertical amplification is mainly associated with incomplete saturation of near-surface soils, which causes substantial amplification of P waves but does not affect the propagation of S waves. Based on the concept of homogeneous pore fluid and Biot's theory of two-phase media, we discuss the characteristics of P -wave velocity, Poisson's ratio and degree of saturation in shallow soil layers, and show evidence of incomplete saturation of near-surface soils at the array site. We analyze a simple model to theoretically investigate the effects of saturation on vertical-motion amplification. The results show that the degree of saturation may produce substantial influence on the amplification, both amplitude and frequency content. Using a solid-fluid coupled finite element procedure, we perform a simulation of the observed vertical motions at the array site by including the effects of saturation. The results demonstrate the mechanism of large amplification caused by incomplete saturation of near-surface layers. The present study indicates that vertical-component motions may significantly be affected by pore-water saturation of soils, suggesting that we may need to carefully examine the condition of saturation in the study of vertical site amplification. Manuscript received 24 May 1999.

Water Block Effects In Low Permeability Gas Reservoirs

Abstract We assert that the effects of fluid invasion on formation permeability in reservoirs with ultra-low water saturations has been over-dramatized, perhaps because the mechanisms of permeability reduction as a result of fluid invasion are not well understood. It has been proposed that some reservoirs contain less than irreducible water, Swirr (based on capillary pressure), and as such, have additional pore volume which contributes to hydrocarbon permeability. Consequently, introduction of fluids during drilling and completion could result in more permeability loss in these reservoirs, than in reservoirs which contain water saturations controlled by capillary pressure. This paper demonstrates, with actual laboratory data and with Poiseuille's theory, that for water-wet reservoirs (which most gas reservoirs are, since gas is strongly non-wetting) areas of porosity where bound water exists (or any other wetting fluid) contribute little to the overall permeability. Several permeability measurements performed on clastic, limestone and dolomite core samples with and without connate water saturation demonstrate that minimal permeability reduction occurs due to the presence of an irreducible water saturation. Even in extremely low permeability cores less than 0.5 md, the effect of irreducible water saturation on gas permeability was only a 20% reduction. These results suggest that permeability damage is only slightly worse in desiccated gas reservoirs than in conventional gas reservoirs. Furthermore, permeability impairment as a result of fluid invasion is more a function of effects on macroporosity than microporosity. Introduction With the increased activity in natural gas exploitation in Canada, much attention has been drawn to the potential for formation damage as a result of the introduction of drilling and completions fluids, particularly in low permeability gas reservoirs. A perception exists that damage may be greater in reservoirs which are initially at ultra-low water saturations; i.e., lower than what might be predicted from capillary pressure measurements. This thinking differs from the general consensus that microporosity, which is the major portion of pore volume controlling water saturation in a water-wet reservoir, contributes little to permeability. This paper reviews various literature views on the relationship of pore size and water saturation with respect to contributions to permeability. Laboratory permeability measurements on cores from four formations are presented and compared to other published data, to demonstrate the effect of water saturation on effective permeability, and how it relates to capillary pressure and pore throat size. Finally, a technique will be presented, based on Pouseille's theory, showing how to calculate the contribution to permeability as a function of pore throat size using air/mercury capillary pressure data. Literature Review Thomeer(1) and Swanson(2) developed empirical techniques for predicting absolute permeability from capillary pressure data. They recognized the insignificant contribution to permeability of smaller pores compared to the contribution of larger pores. Swanson's method of characterizing capillary pressure curves uses the inflection point (point A in Figure 1) on the lower portion of the curve to predict permeability. He argued that this inflection point represents the maximum product of pore throat size times effective flow area.

Streaming potential in porous media: 2. Theory and application to geothermal systems

Self‐potential electric and magnetic anomalies are increasingly being observed associated with hydrothermal fields, volcanic activity, and subsurface water flow. Until now a formal theoretical basis for predicting streaming potential of porous materials has not been available. We develop here a model giving both the macroscopic constitutive equations and the material properties entering these equations. The material properties, like the streaming potential coupling coefficient, depend on pore fluid salinity, temperature, water and gas saturations, mean grain diameter, and porosity. Some aspects of the model are directly tested with success against laboratory data. The streaming potential increases with temperature, grain size, and gas saturation, and decreases with salinity. At the scale of geological structures the model provides an explanation for the presence of kilometer‐scale dipolar self‐potential anomalies in geothermal systems and volcanoes. Positive self‐potential anomalies are associated with fluid discharge areas, whereas negative self‐potential anomalies are associated with fluid recharge areas. Self‐potential anomaly maps determined at the surface of active hydrothermal fields appear to be a powerful way of mapping the fluid recharge and discharge areas. In the case of free convection the vorticities of the convection pattern generate a magnetic field. The greater these vorticities, the greater the associated magnetic field. It follows that hydrothermal systems act as natural geobatteries because of the flow of pore fluids in the subsurface of these systems.

Critical Condensate Saturation in Porous Media

The understanding of gas and condensate flow in porous media is critical to the optimum exploitation of gas–condensate reservoirs. Critical condensate saturation and relative permeabilities are the key parameters for the evaluation of possible recovery strategies. This work is aimed at developing a mechanistic network model for the critical condensate saturation in which phase trapping and connectivity in the pore corners are critically examined. Porous media are modeled by networks of pore bodies interconnected by pore throats. Bodies and throats are characterized by their connectivity, shapes, and radii distributions. Pore-level laws are identified from micromodel experiments with near-critical fluids. A nonzero critical condensate saturation can be obtained in the absence of contact angle hysteresis due to the converging–diverging nature of the throats. The critical saturation at which the condensate flows is found to be a function of pore geometry, water saturation, and interfacial tension (or the Bond number). The modified sphere-pack model underpredicts the critical condensate saturation of typical sandstones. The cubic model adequately predicts the critical saturation and its experimentally observed trends.