Yves Géraud

Analysis of the internal structure of rocks and characterization of mechanical deformation by a non-destructive method: X-ray tomodensitometry

Abstract Tomodensitometry is a new non-destructive method which reveals the internal structure of rocks through analysis of X-ray images without any damage to the sample. The scanner gives a radiological density which is a function of gravimetric density and mineralogy. For limestones and sandstones, an attempt is made to quantify the relation between radiological density and porosity. Spatial variations in the values of the radiological density measured on rock core specimens that are deformed and fractured in a triaxial cell are then related to the intensity of experimental volumetric deformation.

Transport properties of pyroclastic rocks from Montagne Pelée volcano (Martinique, Lesser Antilles)

The hydraulic and electrical properties of pyroclastic rocks have been investigated in laboratory on a representative sampling of Montagne Pelee (Martinique, France) deposits with renewed interest in geophysical applications. This sampling covers all the lithologic units of this volcano: lava dome and lava flows, pumices from ash-and-pumice fall and flow deposits, lava blocks from block-and-ash flow and Peleean “nuees ardentes” deposits, scoriae from scoria flow deposits. The connected porosity varies over a wide range from 3 to 62%. The unconnected porosity is important only on pumices where it can reach 15%. The permeability covers more than 5 orders of magnitude, ranging from 10−16 to 35 × 10−12 m2. The higher values are obtained on lava blocks and the scoriae, even if these rocks are less porous than the pumices. The formation factor ranges from 7 to 1139. The transport properties of these rocks are slightly correlated with porosity. This indicates that these properties are not only controlled by the connected porosity. To connect the transport properties to the textural characteristics of the pore network of pyroclastic rocks, different models, based on geometrical considerations or percolation theory, were tested. The pore access radius distribution and the tortuosity control the transport properties of pyroclastic rocks. Consequently, the models (electric and hydraulic) based on the concept of percolation (e.g., the models of Katz and Thompson), apply better than the equivalent channel model of Kozeny-Carman. In addition, the difference in transport properties observed on lava blocks and pumices confirms that the mechanisms of degassing and vesiculation are different for these two types of rock.

Porosity and Thermal Conductivity of the Soultz-sous-Forêts Granite

Abstract — The success of the Soultz-sous-Forêts Hot-Dry-Rock project depends on the ability to maintain fluid circulation in a fractured granite. Fractures represent the main fluid pathways. To understand the behavior of this granite in respect to thermal fluid-rock interaction the important aspects are (1) the porous network around these fractures and (2) the thermal conductivity of the rock. This granite is altered and composed of different weathered facies. Variations of porosity and thermal conductivity take place in regard to the alteration and fracturing of the granite. Two types of porosity measurements were performed, mercury injection and water porosity on two samples sizes. The two methods give similar porosity values between 0.3% and 10%. Thermal conductivity measurements were performed in two perpendicular directions to look at anisotropy with two methods at different scale and value ranges from 2.3 to 3.9 W.m−1.K−1. Optical scanning provides us with a good knowledge of local increase of thermal conductivity due to sealed fracture or quartz-cemented matrix. The relationship between porosity and thermal conductivity is not obvious and has to be studied in details, and results show three cases: (1) a relationship between conductivity and porosity (increase of conductivity with a decrease of porosity), (2) a relationship between conductivity and sealed fractures (increase of conductivity related to an increase of fracture density), (3) and a combination of the two previous ones. The results are carefully compared for different types of granite: alterated, fractured or both. These first results indicate that parameters such as thermal conductivity are linked to the porous medium, the structure and the mineralogy of the rock.

Porosity network of a ductile shear zone

The permeability of ductile shear zones is assumed to be significantly higher than that of surrounding undeformed rocks, although lower than that of fracture systems. This assumption, which results indirectly from geochemical mass balance calculations, is checked in this paper by the tomography of the porosity network in a metre-wide deformation zone. Combined techniques, with different resolutions ranging between 1 mm and 1 μm, are used on oriented thin sections and drilled cores: nuclear magnetic resonance (NMR) imaging, X-ray tomodensitometry (medical scanner), mercury injection porosimetry and scanning electron microscopy (SEM). We show that porosity ranges between 1% at the margins and 8% in the median part, with throat diameters between 0.8 and 0.01 μm. The porosity network is heterogeneous, mixed (cracks and tubes) and anisotropic. Cracks are common in the less deformed samples, whereas tubes prevail in the more strained median part. The porosity network is anisotropic, especially in the high strained samples, where tubes are preferentially oriented in the c-planes. This shape duality is expected to influence the transfer properties of the studied rocks. Estimates of related permeability are about 5 × 10−16 and 5 × 10−15 m2 in the shear zone, and about 10−17 m2 in the less deformed granite. We discussed the occurrence of such a contrast at depth between the shear zone and the granite.

Comparison between connected and overall porosity of thermally stressed granites

Abstract The structural evolution of four granites, heated from 20 to 700°C, was examined after cooling. The granites have four typical initial structures, which can be sub-divided into two groups: rocks with high connected porosity; comprising cracks with low aspect ratio and alteration pores (type A), pores and a few cracks (type B), or cracks (type C); and rocks with low connected porosity, with cracks (type D). The connected porosity was measured by mercury injection, and the total porosity by X-ray computerized tomography. At low temperature ( T ), the connected porosity decreases with increasing temperature, particularly for the material with high crack density and connected porosity (A and B). The total porosity increases for B, C and D, and decreases for the material with low aspect ratio cracks (A). Above 300°C, both porosities increase. The data show that the thermal history of a granite can be decomposed into two phases. In the first stage the connected porosity decreases with increasing temperature, cracks are initiated at low temperature and they form unconnected networks. In the second stage, the connected and total porosities increase, the previously initiated cracks form a connected network by opening, and they interact with newly created cracks.

Pore-throat characterization in highly porous and permeable sandstones

Several methods have been developed to characterize the pore spaces in sandstone reservoirs using data on the pore-throat-size distribution obtained from mercury injection tests. The Winland equation, the threshold pressure, the displacement pressure, and Pittmans equation are mostly used for this purpose to delineate the stratigraphic traps and seals. This study examines the reliability of these methods applied to the highly permeable Nubia sandstones in their type section in southern Egypt. These sandstones are composed mainly of siliceous sandstones and constitute the main Paleozoic–Cretaceous aquifers and reservoirs in Egypt. Routine core analysis and mercury injection tests were conducted to delineate the pore network characteristics for these rocks. The relationships between helium porosity and the uncorrected air permeability from the routine core analysis, and the various parameters derived from mercury injection–capillary pressure curves were established using multiple regressions. This study indicates the high reliability of the displacement pressure at 10% mercury saturation and also reveals the apex of Pittmans hyperbole at 45% mercury saturation as a complexity apex at which the pore network becomes highly chaotic. Despite the great benefits of such types of measurements, they are not commonly used because of their high cost. This study introduces a series of empirical equations for constructing a partial pore-aperture-size distribution curve from routine core analysis for the highly permeable rocks.

Porosity and fluid flow characterization of granite by capillary wetting using X-ray computed tomography

Abstract The porosity and transfer properties of a very low porosity material (granite) are measured. A new procedure is defined using a capillary test and X-ray computed tomography (CT) scanning. Injected volumes are very low, i.e. a few cm3 for a sample volume of 1 dm3, using a fluid/rock ratio lower than 0.1%. This technique allows monitoring of the anisotropy of fluid flow during the test. Flow along the injection direction is higher than along the perpendicular direction. Saturation depends on the specific saturation of each mineral zone. Multiscale analysis allows defining the flow conditions as being controlled at both the mineral and the sample scale. Results indicate the specific role for various constituting parts of the material. High speed flow occurs in the crack network of K-feldspar, while the storage function is localized in the reaction zone forms by quartz and muscovite.

Late Ordovician tunnel valleys in southern Jordan

Abstract The Upper Ordovician glacial record of southern Jordan (Ammar Fm.) essentially consists of palaeovalley infills and of a subordinate time-transgressive fluvial to shallow-marine succession overstepping both the palaeovalleys and interfluvial areas. Valley size (depth, 60–160 m; width, 1–3 km), steep (20–50°) margins, internal organization and depositional facies point to an origin as tunnel valleys. The tunnel valleys are infilled by either fluvioglacial sandstones or fluviodeltaic coarsening-upward successions including fine-grained clayey sediments. Re-occupation of previous valleys is evident in places. At least three generations of tunnel valleys are inferred from cross-cutting relationships, although they most probably only reflect temporary standstills and minor re-advances related to the overall recession following the main glacial advance recorded in Saudi Arabia. Petrophysical measurements indicate that higher permeabilities are located in the glacially related strata (1.5–3 darcy in fluvioglacial infills), with a somewhat reduced porosity (22–28%) relative to the preglacial sandstones owing to a higher clay content, probably of diagenetic origin. Sandstone amalgamation, however, gives the fluvioglacial sandstones a high reservoir quality.

Porosity microstructures of a sandstone affected by a normal fault

Normal faults are part of the elements that control fluid flows in sedimentary basins. They can play the role of a barrier or a drain [Hippler, 1993]. These pathways are anisotropic. The aim of this study is to determine the fluid pathways and to characterise the pore network and its role in the transfer properties. Petrophysics, petrographics, geochemical and fluid inclusion studies allow us to characterise a Buntsandstein sandstone affected by a normal fault. This sandstone has a fluviatile origin, field evidenced by fluviatile channels, but also by some clay layers. The fault is located in the north east of France, in the Rhine Graben. The vertical displacement is about 3 meters, and the dip is 70o east. The fractured zone is composed of three compartments (the hanging wall and the footwall separated by a gouge) divided by three main faults (fig. 1). Oriented samples were taken from the three blocks and were studied following the procedure figure 2.

Reconstitution du réseau poreux d'un échantillon décomprimé. Mise en évidence d'une anisotropie de connectivité

The porosity network of samples cored in highly stressed granitic material is investigated by mercury injection. In addition to threshold and porous volume classically measured with these techniques, we obtained information on heterogeneity and anisotropy of the porous network. Anisotropy of the crack network connectivity is inferred as the relationship between porous volume measured on classic sample and porous volume measured on a sample partly covered by resin. Mercury injection into samples partly covered by resin is possible only along the core axis.