Comparison of stress-dependent geophysical,hydraulic and mechanical properties of synthetic and natural sandstones for reservoir characterization and monitoring studies

Synthetic rock samples can offer advantages over natural rock samples when used for laboratory rock physical properties studies, provided their success as natural analogues is well understood. The ability of synthetic rocks to mimic the natural stress dependency of elastic wave, electrical and fluid transport properties is of primary interest. Hence, we compare a consistent set of laboratory multi-physics measurements obtained on four quartz sandstone samples (porosity range 20–25%) comprising two synthetic and two natural (Berea and Corvio) samples, the latter used extensively as standards in rock physics research. We measured simultaneously ultrasonic (P- and S-wave) velocity and attenuation, electrical resistivity, permeability and axial and radial strains over a wide range of differential pressure (confining stress 15–50 MPa; pore pressure 5–10 MPa) on the four brine saturated samples. Despite some obvious physical discrepancies caused by the synthetic manufacturing process, such as silica cementation and anisotropy, the results show only small differences in stress dependency between the synthetic and natural sandstones for all measured parameters. Stress dependency analysis of the dry samples using an isotropic effective medium model of spheroidal pores and penny-shaped cracks, together with a granular cohesion model, provide evidence of crack closure mechanisms in the natural sandstones, seen to a much lesser extent in the synthetic sandstones. The smaller grain size, greater cement content, and cementation under oedometric conditions particularly affect the fluid transport properties of the synthetic sandstones, resulting in lower permeability and higher electrical resistivity for a similar porosity. The effective stress coefficients, determined for each parameter, are in agreement with data reported in the literature. Our results for the particular synthetic materials that were tested suggest that synthetic sandstones can serve as good proxies for natural sandstones for studies of elastic and mechanical properties, but should be used with care for transport properties studies.     

Comparison of long-term and short-term uplift rates along an active blind reverse fault zone (Bajo Segura,Se Spain)

The seismic activity of the Guardamar-Torrevieja zone (Eastern Betic Cordillera, SE Spain) can be associated with the Bajo Segura fault zone, an E-W reverse blind fault with secondary NW-SE dextral faults. A high-precision levelling profile 30 kilometers long was set up and levelled in 1997 to monitor the vertical displacement of this active fault zone. This profile runs parallel to an older high-precision levelling line included in the Spanish first order levelling network measured by the Instituto Geográfico Nacional (IGN) in two different campaigns (1934 and 1976). The 1997 line was relevelled in 2003 and 10 new benchmarks were set up, both to increase benchmark density and to restore some of the 1997 benchmarks that had been destroyed. We have used the 1976 IGN and the 2003 measurement to construct a recent vertical movements profile with a significant time difference (27 years). This recent vertical movements profile shows that the vertical movements are very small, nearly equal to the error bars, with a 0.2 mm/year rise in the town of Guardamar, and a 0.2 mm/year subsidence of the southern part of the profile (Punta Prima) respect to the town of Torrevieja. These movements could be related to the activity of the Bajo Segura and the San Miguel faults respectively. Using geological markers we have deduced uplift rates of 0.1 mm/year during the last 3 million years. Therefore, these preliminary results indicate that geodetically (short-term deformation) determined uplift rates are similar to those estimated from geological markers (long term deformation).     

On the Structure and Metamorphism of the Roc de Frausa Massif (Eastern Pyrenees)

Abstract

The Roc de Frausa Massif, located at the Eastern Pyrenees, is formed by a stratoid Pre-Hercynian deformed granite (orthogneiss) interbedded with metasedimentary series. Hercynian granitoids (St. Llorenç — La Jonquera pluton) surround the southern and eastern part of the massif and Hercynian basic igneous rocks (Ceret stock) occupy the central part of it. The Pre-Hercynian granite and the sedimentary series were involved, during the Hercynian orogeny, in complex polyphasic tectonics and metamorphism. As a result, an ubiquitous penetrative foliation was developed during the earlier stages. This foliation was subsequently folded into a complex antiformal structural formed by a double dome : Roc de Frausa dome and Mas Blanc dome. Main lithological boundaries (gneiss — metasediments and metasediments — granitoids) are broadly parallel to the regional foliation, and they all display the dome geometry. Interference fold pattern between two late phases, an ealier one with NE-SW trending folds and a younger one with NW-SE trending folds is responsible for the dome geometry. Mylonitic deformation, with W-E to NW-SE orientations has been attributed to the last folding phase. Regional metamorphic climax and contact metamorphism, the last one resulting from Hercynian granitoid emplacement, preceeded the above mentioned late folding event, which developed under retrograde metamorphic conditions. Regional peak metamorphism is recognized by the static crystallization of cordierite + potassium feldspar. This paragenesis indicates pressure — temperature conditions of about 3.1 Kbar and 660 °C maximum. Contact metamorphism overprints the earlier regional metamorphism. Parageneses and thermal gradient of contact metamorphism around La Jonquera pluton are very similar to those related to regional metamorphism, whereas parageneses produced around Ceret stock present garnet + potassium feldspar. Geothermometry indicates metamorphic conditions locally higher for this paragenesis (around 700 °C).     

Second-order closure study of the covariance between chemically reactive species in the surface layer

A second-order modelling technique is used to investigate the influence of turbulence on chemical reactions. The covariance and variance equations for the NO-O₃-NO₂ system are developed as a function of the ratio of the timescale of turbulence ( _t ) and the timescale of chemistry (_Ch): the first Damköhler number ( _t /_Ch). Special attention is given to the calculation of the covariance between NO and O₃ normalized by the product of their means, the so-called intensity of segregation (I _S ). This parameter quantifies the state of mixing of two chemical species.The intensity of segregation is calculated as a function of the flux of NO and the first Damköhler number. The model results presented illustrate the importance of taking the effect of turbulence on chemical reactions into account for higher values of the NO flux, for values of the ratio O₃/NO larger than 12.5 and for values of the ratio _t /_CH larger than 0.1. For such cases, the effective reaction rates are slower than if the chemical species are assumed to be uniformly mixed.     

Variability of Coulter Counter Data and their Relationship with Other Oceano-graphical Parameters

Abstract. Coulter counter data (from the upper 100 m of water) gathered during a cruise in October, 1976 in the Spanish waters of the Western Mediterranean are analysed in this paper. The ANOVA shows that the main sources of variability are the geographical situation and the depth level, while the sampling method especially affects the results for the larger particles. Repeating the counting in the same sample also introduces some changes which have less importance in the overall sense. Correlations of the particle counts with chlorophyll a, total POC and PON, POC and PON above 200u.m, estimated POC based on the particle size spectra, and SECCHI disc visibility were calculated and are discussed in order to assess the kind of information given by seston counts obtained from two different sampling methods and expressed both in absolute values (ppm by volume) and as a percentage of volume for the different channels.
Some striking results arise which suggest that the use of a Coulter counter together with some kind of sample treatment might give an insight into the qualitative and quantitative characterization of the pelagic ecosystem.     

Petrographic and geochemical evidence for multiphase formation of carbonates in the Martian orthopyroxenite Allan Hills 84001

Martian meteorites can provide valuable information about past environmental conditions on Mars. Allan Hills 84001 formed more than 4 Gyr ago, and owing to its age and long exposure to the Martian environment, and this meteorite has features that may record early processes. These features include a highly fractured texture, gases trapped during one or more impact events or during formation of the rock, and spherical Fe‐Mg‐Ca carbonates. In this study, we have concentrated on providing new insights into the context of these carbonates using a range of techniques to explore whether they record multiple precipitation and shock events. The petrographic features and compositional properties of these carbonates indicate that at least two pulses of Mg‐ and Fe‐rich solutions saturated the rock. Those two generations of carbonates can be distinguished by a very sharp change in compositions, from being rich in Mg and poor in Fe and Mn, to being poor in Mg and rich in Fe and Mn. Between these two generations of carbonate is evidence for fracturing and local corrosion.