Gas breakthrough experiments on fine-grained sedimentary rocks

The capillary sealing efficiency of fine‐grained sedimentary rocks has been investigated by gas breakthrough experiments on fully water saturated claystones and siltstones (Boom Clay from Belgium, Opalinus Clay from Switzerland and Tertiary mudstone from offshore Norway) of different lithological compositions. Sand contents of the samples were consistently below 12%, major clay minerals were illite and smectite. Porosities determined by mercury injection lay between 10 and 30% while specific surface areas determined by nitrogen adsorption (BET method) ranged from 20 to 48 m² g ^{? 1}. Total organic carbon contents were below 2%. Prior to the gas breakthrough experiments the absolute (single phase) permeability (k_abs) of the samples was determined by steady state flow tests with water or NaCl brine. The k_abs values ranged between 3 and 550 nDarcy (3 × 10^?21 and 5.5 × 10^?19 m²). The maximum effective permeability to the gas‐phase (k_eff) measured after gas breakthrough on initially water‐saturated samples extended from 0.01 nDarcy (1 × 10^?23 m²) up to 1100 nDarcy (1.1 × 10^?18 m²). The residual differential pressures after re‐imbibition of the water phase, referred to as the ‘minimum capillary displacement pressures’ (P_d), ranged from 0.06 to 6.7 MPa. During the re‐imbibition process the effective permeability to the gas phase decreases with decreasing differential pressure. The recorded permeability/pressure data were used to derive the pore size distribution (mostly between 8 and 60 nm) and the transport porosity of the conducting pore system (10^‐5–10^‐2%). Correlations could be established between (i) absolute permeability coefficients and the maximum effective permeability coefficients and (ii) effective or absolute permeability coefficients and capillary sealing efficiency. No correlation was found between the capillary displacement pressures determined from gas breakthrough experiments and those derived theoretically by mercury injection.     

Gas breakthrough experiments on pelitic rocks: comparative study with N2, CO2 and CH4

The capillary‐sealing efficiency of intermediate‐ to low‐permeable sedimentary rocks has been investigated by N₂, CO₂ and CH₄ breakthrough experiments on initially fully water‐saturated rocks of different lithological compositions. Differential gas pressures up to 20 MPa were imposed across samples of 10–20 mm thickness, and the decline of the differential pressures was monitored over time. Absolute (single‐phase) permeability coefficients (k_abs), determined by steady‐state fluid flow tests, ranged between 10^?22 and 10^?15 m². Maximum effective permeabilities to the gas phase k_eff(max), measured after gas breakthrough at maximum gas saturation, extended from 10^?26 to 10^?18 m². Because of re‐imbibition of water into the interconnected gas‐conducting pore system, the effective permeability to the gas phase decreases with decreasing differential (capillary) pressure. At the end of the breakthrough experiments, a residual pressure difference persists, indicating the shut‐off of the gas‐conducting pore system. These pressures, referred to as the ‘minimum capillary displacement pressures’ (P_d), ranged from 0.1 up to 6.7 MPa. Correlations were established between (i) absolute and effective permeability coefficients and (ii) effective or absolute permeability and capillary displacement pressure. Results indicate systematic differences in gas breakthrough behaviour of N₂, CO₂ and CH₄, reflecting differences in wettability and interfacial tension. Additionally, a simple dynamic model for gas leakage through a capillary seal is presented, taking into account the variation of effective permeability as a function of buoyancy pressure exerted by a gas column underneath the seal.     

Physical properties of carbonate fault rocks,fucino basin (Central Italy): implications for fault seal in platform carbonates

We documented the porosity, permeability, pore geometry, pore type, textural anisotropy, and capillary pressure of carbonate rock samples collected along basin‐bounding normal faults in central Italy. The study samples consist of one Mesozoic platform carbonate host rock with low porosity and permeability, four fractured host rocks of the damage zones, and four fault rocks of the fault cores. The four fractured samples have high secondary porosity, due to elongated, connected, soft pores that provide fluid pathways in the damage zone. We modeled this zone as an elastic cracked medium, and used the Budiansky–O'Connell correlation to compute its permeability from the measured elastic moduli. This correlation can be applied only to fractured rocks with large secondary porosity and high‐aspect ratio pores. The four fault rock samples are made up of survivor clasts embedded in fine carbonate matrices and cements with sub‐spherical, stiff pores. The low porosity and permeability of these rocks, and their high values of capillary pressure, are consistent with the fault core sealing as much as 77 and 140 m of gas and oil columns, respectively. We modeled the fault core as a granular medium, and used the Kozeny–Carmen correlation, assigning the value of 5 to the Kozeny constant, to compute its permeability from the measured porosities and pore radii. The permeability structure of the normal faults is composed of two main units with unique hydraulic characteristics: a granular fault core that acts as a seal to cross‐fault fluid flow, and an elastic cracked damage zone that surrounds the core and forms a conduit for fluid flow. Transient pathways for along‐fault fluid flow may form in the fault core during seismic faulting due to the formation of opening‐mode fractures within the cemented fault rocks.     

Pressure distribution in a reservoir affected by capillarity and hydrodynamic drive: Griffin Field, North West Shelf, Australia

The effects of capillarity in a multilayered reservoir with flow in the aquifer beneath have characteristic signatures on pressure–elevation plots. Such signatures are observed for the Griffin and Scindian/Chinook fields of the Carnarvon Basin North West Shelf of Australia. The Griffin and Scindian/Chinook fields have a highly permeable lower part to the reservoir, a less permeable upper part, and a low permeability top seal. In the Griffin Field there is a systematic tilt of the free‐water level in the direction of groundwater flow. Where the oil–water contact occurs in the less permeable part of the reservoir, it lies above the free‐water level due to capillarity. In addition to these observable hydrodynamic and capillary effects on hydrocarbon distribution, the multi‐well pressure analysis shows that the gas–oil contacts in the Scindian/Chinook fields occur at different elevations. For both the Griffin and Scindian/Chinook fields the oil pressure gradients from each well are non‐coincident despite continuous oil saturation, and the difference is not attributable to data error. Furthermore, the shift in oil pressure gradient has a geographical pattern seemingly linked to the hydrodynamics of the aquifer. The simplest explanation for all the observed pressure trends is an oil leg that is still in the process of equilibrating with the prevailing hydrodynamic regime.     

Experimental study of the effect of variation in in‐situ stress on capillary residual trapping during CO2 geo‐sequestration in sandstone reservoirs

During a geo‐sequestration process, CO₂ injection causes an increase in reservoir pore pressure, which in turn decreases the reservoir net effective stress. Changes in effective stress can change all the reservoir and cap‐rock properties including residual saturations. This article presents the results of an experimental work carried out to understand the potential change in the volumes of residually trapped CO₂, while the porous medium tested underwent change in the net effective stress under in‐situ reservoir conditions of pore pressure and temperature. The experimental results obtained show that an initial 1725 psi (11.9 MPa) decrease in the net effective pressure caused 1.4% reduction in the volumes of residually trapped CO₂, while another 1500 psi (10.3 MPa) reduction caused a further 3.2% drop in the residual saturation of CO₂.     

Sub‐micron spongiform porosity is the major ultra‐structural alteration occurring in archaeological bone

Total pore volume and pore size distribution are indicators of the degree of post‐mortem modification of bone. Direct measurements of pore size distribution in archaeological bones using mercury intrusion porosimetry (HgIP) and back scattered scanning electron microscopy (BSE‐SEM) reveal a common pattern in the changes seen in degraded bone as compared to modern samples. The estimates of pore size distribution from HgIP and direct measurement from the BSE‐SEM images show remarkable correspondence. The coupling of these two independent approaches has allowed the diagenetic porosity changes in human archaeological bone in the >0.01 µm range to be directly imaged, and their relationship to pre‐existing physiological pores to be explored. The increase in porosity in the archaeological bones is restricted to two discrete pore ranges. The smaller of these two ranges (0.007–0.1 µm) lies in the range of the collagen fibril (0.1 µm diameter) and is presumably formed by the loss of collagen, whereas the larger pore size distribution is evidence of direct microbial alteration of the bone. HgIP has great potential for the characterization of microbial and chemical alteration of bone. Copyright © 2002 John Wiley & Sons, Ltd.     

The connectivity of pore space in mudstones: insights from high‐pressure Wood's metal injection,BIB‐SEM imaging,and mercury intrusion porosimetry

Study of the pore space in mudstones by mercury intrusion porosimetry is a common but indirect technique and it is not clear which part of the pore space is actually filled with mercury. We studied samples from the Opalinus Clay, Boom Clay, Haynesville Shale, and Bossier Shale Formations using Wood's metal injection at 316 MPa, followed by novel ion beam polishing and high‐resolution scanning electron microscopy. This method allowed us to analyze at high resolution which parts of a rock are intruded by the liquid alloy at mm to cm scale. Results from the Opalinus Clay and Haynesville Shale show Wood's Metal in cracks, but the majority of the pore space is not filled although mercury intrusion data suggests that this is the case. In the silt‐rich Boom Clay sample, the majority of the pore space was filled Wood's metal, with unfilled islands of smaller pores. Bossier Shale shows heterogeneous impregnation with local filling of pores as small as 10 nm. We infer that mercury intrusion data from these samples is partly due to crack filling and compression of the sample. This compaction is caused by effective stress developed by mercury pressure and capillary resistance; it can close small pore throats, prevent injection of the liquid metal, and indicate an apparent porosity. Our results suggest that many published MIP data on mudstones could contain serious artifacts and reliable metal intrusion porosimetry requires a demonstration that the metal has entered the pores, for example by Wood's metal injection, broad ion beam polishing, and scanning electron microscopy.     

A new apparatus for measuring elastic wave velocity and electrical conductivity of fluid‐saturated rocks at various confining and pore‐fluid pressures

Pore‐fluid pressure is a critical parameter that governs geodynamic processes including seismic activities. Its evaluation through geophysical observations provides us insights into these processes. The quantitative evaluation requires a thorough understanding of the influence of pore‐fluid pressure on geophysical parameters, such as seismic velocity and electrical conductivity. To study the influence of pore‐fluid pressure on these parameters, we have built a new apparatus with a pore‐fluid pressure control system, which is capable of simultaneously measuring elastic wave velocity and electrical conductivity. Our new apparatus employs two sets of plastic piston–cylinders for the electrical insulation and pore‐fluid pressure transmission. The pore fluid is electrically isolated from the metal work, and its pressure can be precisely controlled without significant contribution of the friction between the piston and cylinder. Our new apparatus was used for a simultaneous measurement of velocity and conductivity in a brine‐saturated Berea sandstone. Elastic wave velocity and electrical conductivity changed in response to the change in confining and pore‐fluid pressures, showing the usefulness of the new apparatus.     

Weakening processes in thrust faults: insights from the Monte Perdido thrust fault (southern Pyrenees,Spain)

The Monte Perdido thrust fault (southern Pyrenees) consists of a 6‐m‐thick interval of intensely deformed clay‐bearing rocks. The fault zone is affected by a pervasive pressure solution seam and numerous shear surfaces. Calcite extensional‐shear veins are present along the shear surfaces. The angular relationships between the two structures indicate that shear surfaces developed at a high angle (70°) to the local principal maximum stress axis σ₁. Two main stages of deformation are present. The first stage corresponds to the development of calcite shear veins by a combination of shear surface reactivation and extensional mode I rupture. The second stage of deformation corresponds to chlorite precipitation along the previously reactivated shear surfaces. The pore fluid factor λ computed for the two deformation episodes indicates high fluid pressures during the Monte Perdido thrust activity. During the first stage of deformation, the reactivation of the shear surface was facilitated by a suprahydrostatic fluid pressure with a pore fluid factor λ equal to 0.89. For the second stage, the fluid pressure remained still high (with a λ value ranging between 0.77 and 0.84) even with the presence of weak chlorite along the shear surfaces. Furthermore, evidence of hydrostatic fluid pressure during calcite cement precipitation supports that incremental shear surface reactivations are correlated with cyclic fluid pressure fluctuations consistent with a fault‐valve model.     

ANALYSIS OF PORE PRESSURE GENERATION AND DISSIPATION IN COHESIONLESS MATERIALS DURING SEISMIC LOADING

The earthquake loading of a shallow foundation resting on top of a cohesionless layer creates cyclic variations in the shear force and overturning moment acting on the supporting soil. These loads induce a tendency for volume change which, in turn, depending on the drainage conditions and material permeability, may cause in addition to a cyclic pore pressure variation a progressive pore pressure buildup. The paper develops an efficient and elegant way, based on a multiple time scale analysis, of solving this fully coupled problem. The theoretical solution is implemented in a finite element code and is applied to predict the pore pressure development and dissipation under a bridge pier foundation for which it was essential to limit the pore pressure increase.     

Pore‐scale investigation on stress‐dependent characteristics of granular packs and the impact of pore deformation on fluid distribution

Understanding the effect of changing stress conditions on multiphase flow in porous media is of fundamental importance for many subsurface activities including enhanced oil recovery, water drawdown from aquifers, soil confinement, and geologic carbon storage. Geomechanical properties of complex porous systems are dynamically linked to flow conditions, but their feedback relationship is often oversimplified due to the difficulty of representing pore‐scale stress deformation and multiphase flow characteristics in high fidelity. In this work, we performed pore‐scale experiments of single‐ and multiphase flow through bead packs at different confining pressure conditions to elucidate compaction‐dependent characteristics of granular packs and their impact on fluid flow. A series of drainage and imbibition cycles were conducted on a water‐wet, soda‐lime glass bead pack under varying confining stress conditions. Simultaneously, X‐ray micro‐CT was used to visualize and quantify the degree of deformation and fluid distribution corresponding with each stress condition and injection cycle. Micro‐CT images were segmented using a gradient‐based method to identify fluids (e.g., oil and water), and solid phase redistribution throughout the different experimental stages. Changes in porosity, tortuosity, and specific surface area were quantified as a function of applied confining pressure. Results demonstrate varying degrees of sensitivity of these properties to confining pressure, which suggests that caution must be taken when considering scalability of these properties for practical modeling purposes. Changes in capillary number with confining pressure are attributed to the increase in pore velocity as a result of pore contraction. However, this increase in pore velocity was found to have a marginal impact on average phase trapping at different confining pressures.     

Numerical model of pore‐pressure diffusion associated with the initiation of the 2010–2011 Guy–Greenbrier,Arkansas earthquakes

We model pore‐pressure diffusion caused by pressurized waste‐fluid injection at two nearby wells and then compare the buildup of pressure with the observed initiation and migration of earthquakes during the early part of the 2010–2011 Guy–Greenbrier earthquake swarm. Pore‐pressure diffusion is calculated using MODFLOW 2005 that allows the actual injection histories (volume/day) at the two wells to diffuse through a fractured and faulted 3D aquifer system representing the eastern Arkoma basin. The aquifer system is calibrated using the observed water‐level recovery following well shut‐in at three wells. We estimate that the hydraulic conductivities of the Boone Formation and Arbuckle Group are 2.2 × 10^?2 and 2.03 × 10^?3 m day^?1, respectively, with a hydraulic conductivity of 1.92 × 10^?2 m day^?1 in the Hunton Group when considering 1.72 × 10^?3 m day^?1 in the Chattanooga Shale. Based on the simulated pressure field, injection near the relatively conductive Enders and Guy–Greenbrier faults (that hydraulically connect the Arbuckle Group with the underlying basement) permits pressure diffusion into the crystalline basement, but the effective radius of influence is limited in depth by the vertical anisotropy of the hydraulic diffusivity. Comparing spatial/temporal changes in the simulated pore‐pressure field to the observed seismicity suggests that minimum pore‐pressure changes of approximately 0.009 and 0.035 MPa are sufficient to initiate seismic activity within the basement and sedimentary sections of the Guy–Greenbrier fault, respectively. Further, the migration of a second front of seismicity appears to follow the approximately 0.012 MPa and 0.055 MPa pore‐pressure fronts within the basement and sedimentary sections, respectively.     

THE SPATIOTEMPORAL EVOLUTION OF U.S. CARBON DIOXIDE EMISSIONS: STYLIZED FACTS AND IMPLICATIONS FOR CLIMATE POLICY

We characterize the evolution of U.S. carbon dioxide (CO₂) emissions using an index number decomposition technique which partitions the 1963–2008 growth of states’ energy‐related CO₂ into changes in five driving factors: the emission intensity of energy use, the energy intensity of economic activity, the composition of states’ output, per capita income and population. Compositional change and declining energy intensity attenuate emissions growth, but their impacts are offset by increasing population and income. Despite absolute interstate divergence in both emissions and their precursors, states’ emission‐ and energy intensities—and ultimately, CO₂—appear to be stochastically converging. We assess the implications of these trends using a novel vector autoregression (VAR) emission forecasting technique based on our index numbers. The resulting emission projections are comparable to, but generally exceed, those forecast by the 2010 EIA Annual Energy Outlook.     

Faults and fault properties in hydrocarbon flow models

The petroleum industry uses subsurface flow models for two principal purposes: to model the flow of hydrocarbons into traps over geological time, and to simulate the production of hydrocarbon from reservoirs over periods of decades or less. Faults, which are three-dimensional volumes, are approximated in both modelling applications as planar membranes onto which predictions of the most important fault-related flow properties are mapped. Faults in porous clastic reservoirs are generally baffles or barriers to flow and the relevant flow properties are therefore very different to those which are important in conductive fracture flow systems. A critical review and discussion is offered on the work-flows used to predict and model capillary threshold pressure for exploration fault seal analysis and fault transmissibility multipliers for production simulation, and of the data from which the predictions derive. New flow simulation models confirm that failure of intra-reservoir sealing faults can occur during a reservoir depressurization via a water-drive mechanism, but contrary to anecdotal reports, published examples of production-induced seal failure are elusive. Ignoring the three-dimensional structure of fault zones can sometimes have a significant influence on production-related flow, and a series of models illustrating flow associated with relay zones are discussed.     

Wettability alteration of caprock minerals by carbon dioxide

One of the critical factors that control the efficiency of CO₂ geological storage process in aquifers and hydrocarbon reservoirs is the capillary‐sealing potential of the caprock. This potential can be expressed in terms of the maximum reservoir overpressure that the brine‐saturated caprock can sustain, i.e. of the CO₂ capillary entry pressure. It is controlled by the brine/CO₂ interfacial tension, the water‐wettability of caprock minerals, and the pore size distribution within the caprock. By means of contact angle measurements, experimental evidence was obtained showing that the water‐wettability of mica and quartz is altered in the presence of CO₂ under pressures typical of geological storage conditions. The alteration is more pronounced in the case of mica. Both minerals are representative of shaly caprocks and are strongly water‐wet in the presence of hydrocarbons. A careful analysis of the available literature data on breakthrough pressure measurements in caprock samples confirms the existence of a wettability alteration by dense CO₂, both in shaly and in evaporitic caprocks. The consequences of this effect on the maximum CO₂ storage pressure and on CO₂ storage capacity in the underground reservoir are discussed. For hydrocarbon reservoirs that were initially close to capillary leakage, the maximum allowable CO₂ storage pressure is only a fraction of the initial reservoir pressure.     

The politics of repressing environmentalists as agents of foreign influence

Theoretically, this article reveals the long-term risk for local non-governmental organisations (NGOs) of participating in transnational advocacy networks (TANs), accepting money from foreign sources and throwing ‘boomerangs’ internationally—a strategy used by local NGOs to seek international allies to pressure repressive and unresponsive states at home. Focusing primarily on the suppression of environmental NGOs that oppose natural-resource extraction, this article examines three cases—Russia, India and Australia—to illuminate the consequences of this trend for local civil society and TANs. It also documents a global trend towards states depicting local NGOs with international linkages as subversive agents of foreign interests, justifying legal crackdowns and the severing of foreign funding and ties. State framing of NGOs as agents of foreign interests is repressing local environmental activism, depoliticising civil society and weakening international NGO alliances—a conclusion with far-reaching consequences for the future of TANs, local NGOs and environmental activism.     

Preliminary Characterization of Pottery by Cathodoluminescence and SEM–EDX Analyses: An Example from the Yeha Region (Ethiopia)

Yeha was a political and cultural centre of the Ethio‐Sabaean culture (D’MT) in northern Ethiopia and southern Eritrea. Part of the archaeological research deals with pottery of local, regional and imported origin. The research—investigations and examinations—tries to classify local pottery in two ways: first, by analysing the mineralogy of the temper using light microscopy and cathodoluminescence; and, second, by analysing the main element composition of the clay matrix using a scanning electron microscope with energy‐dispersive X‐ray detection (SEM–EDX). The cathodoluminescence shows that the temper material has differences in the colours of feldspars with a similar mineralogical composition. The results demonstrate that the pottery was produced by using local material that originates from various sources in the Yeha region and that the same raw materials were used in different types of pottery.     

Mesolithic in South‐Eastern Norway

This article discusses the reliability of shore‐line displacement curves based on pollen analysis in the Oslo Fjord area. The conclusion is that only small parts of the curves ‐ in the late Atlantic period ‐ are fairly reliable for the purpose of dating Mesolithic coastal sites.

Twelve Mesolithic settlement sites from Østfold, south‐eastern Norway are classified morphologically. The author suggests a chronological lineal model with four succeeding phases: 1. The Fosna culture, 2. Late Boreal/early Atlantic group, 3. The N?stvet culture, 4. Late flint‐point using group. A connection between the Fosna culture and early Maglemose culture is claimed.

A study of the ecological adaptation in the four phases is based on topographical conditions, on the distribution and situation of settlement sites, and on animal bones from three Mesolithic sites in south‐eastern Norway. Hypotheses on seasonal migrations are suggested.     

Soluble Salts Amplify the Effect of Small Heterogeneities in the Structure of Porous Building Materials During Drying

Salt crystallization is a major cause of degradation in old buildings. One of the issues that stills need clarification is regarding the influence of the salts on the capillary absorption and subsequent drying of porous building materials. This article presents an experimental study that included capillary absorption and evaporative drying tests on two types of material (lime mortar and ceramic brick) using pure water or saturated solutions of six salts (sodium chloride, sodium sulfate, sodium nitrate, sodium carbonate, potassium nitrate, or potassium carbonate). The results of capillary absorption agree only roughly with the linear relationship, predicted by theory, between sorptivity and the square root of the ratio between viscosity η and surface tension σ of the solution (σ/η)^1/2. This poor agreement is probably due to material heterogeneity. The drying dynamics was regular and showed little dispersion between specimens, but only for the uncontaminated materials. Indeed, the drying dynamics of the salt contaminated materials was often irregular or diverged among similar specimens, and the same happened with their salt decay patterns. The main conclusion is that soluble salts can amplify the effects on drying of the small structural heterogeneities that porous building materials normally depict.     

SHEAR MODULUS AND DAMPING OF CLAYEY SANDS

A series of cyclic triaxial tests on clayey sands was carried out and attempts were made to evaluate the strain dependency of shear modulus and damping. Although the strain dependency of the shear modulus of clayey sands was similar to that of clay in the low strain range, G/G ₀ became close to that of clean sand in the high strain range. The damping ratio was less for the clayey sand containing more fines, especially in the medium to large strain range. It was shown that the change in the effective confining stress with loading cycles in the undrained shear test needed to be considered, particularly in the large strain range. The consideration can be made by normalising G with G ₀' instead of using G ₀. When an initial shear stress was applied to the specimen, the amount of residual strain, which was induced with cycles, had to be added to the normal strain amplitude in order to determine the shear modulus. If the effects of excess pore pressure and the amount of residual strain were taken into account properly, it was suggested that the shear modulus of clayey sand in irregular loading could be evaluated reasonably well by using the average G/G ₀—γcurve obtained by cyclic loading tests on isotropically-consolidated specimens.