Multiphase flow simulation through porous media with explicitly resolved fractures

Accurate simulation of multiphase flow in fractured porous media remains a challenge. An important problem is the representation of the discontinuous or near discontinuous behaviour of saturation in real geological formations. In the classical continuum approach, a refined mesh is required at the interface between fracture and porous media to capture the steep gradients in saturation and saturation‐dependent transport properties. This dramatically increases the computational load when large numbers of fractures are present in the numerical model. A discontinuous finite element method is reported here to model flow in fractured porous media. The governing multiphase porous media flow equations are solved in the adaptive mesh computational fluid dynamics code IC‐FERST on unstructured meshes. The method is based on a mixed control volume – discontinuous finite element formulation. This is combined with the P_N+1DG‐P_NDG element pair, which has discontinuous (order N+1) representation for velocity and discontinuous (order N) representation for pressure. A number of test cases are used to evaluate the method's ability to model fracture flow. The first is used to verify the performance of the element pair on structured and unstructured meshes of different resolution. Multiphase flow is then modelled in a range of idealised and simple fracture patterns. Solutions with sharp saturation fronts and computational economy in terms of mesh size are illustrated.     

Evidence of Stress Fracture in a Homo antecessor Metatarsal from Gran Dolina Site (Atapuerca,Spain)

We present the palaeopathological analysis of a right fourth metatarsal (ATD6‐124) recovered from the Atapuerca–Gran Dolina site (Spain). This fossil, ca. 1 Ma, belongs to Homo antecessor, the earliest known European hominin species. The metatarsal exhibits a proliferative lesion on its medial periosteal surface. Periosteal reaction can be the bone response to a wide number of injurious processes. We describe a lesion on the basis of macroscopic and microscopic analyses, including microtomography and scanning electron microscopy. Externally, the osteoblastic lesion presents a highly porotic and disorganised morphology. Internally, we observe a series of micro‐fractures on the compact bone that do not affect the medullary canal. We provide a differential diagnosis and suggest that the ATD6‐124 lesion could correspond to a pedal stress fracture, also known as fatigue or march fracture. Stress fractures have been related to a load increase and muscular fatigue. This type of fracture has been widely reported in the foot of soldiers and athletes, which are usually engaged in strenuous, excessive or prolonged locomotive activities. Despite its high frequency in these groups, stress fractures have not been reported as such in fossil collections, with the exception of a metatarsal belonging to the Sima de los Huesos site (Atapuerca). Copyright © 2013 John Wiley & Sons, Ltd.     

Bioarchaeological Analysis of a Royal Burial from the Oldest Maya Tomb in Palenque,Mexico

In 1957, a new discovery inside Temple XVIII‐A was made in Palenque, the Mayan archaeological site in Chiapas Mexico, a royal tomb of an original design which proved to be the oldest at the site. Two skeletons were found, the principal (PAL‐44) was at the centre showing red pigmentation on the surface of some bones, personal adornments and offerings, which indicate that he was an early ruler of Palenque. The second (PAL‐45), deposited at the entrance, not in a special mortuary position, without offerings, suggests it was of a lower social status. This research presents a new bioarchaeological analysis with the following results: (i) The result of a direct radiocarbon dating of PAL‐44 by accelerator mass spectrometry is 250–420 cal AD (LTL12759A:1696 ± 30 BP). This means that the personage was buried long before the first designated ruler of Palenque, who, according to the epigraphic record was enthroned in ad 431. (ii) With X‐Ray Fluorescence, the red pigment was identified as cinnabar (HgS). (iii) PAL‐44 is a young adult male with a severe idiopathic scoliosis with a double curvature that affected the ribcage and lower limbs. Such a three‐dimensional deformity resulted in changes to his body mechanics and induced motion asymmetry. Consequently, his health and mobility must have been significantly affected by such a severe condition. (iv) PAL‐45 is an adult female with signs of nutritional problems in childhood and entheseal changes probably related to her activity. Three ante‐mortem traumas are registered: rib fracture, frontal depressed circular fracture and a nasal septal deviation. This traumatic event took place two weeks, approximately, prior to PAL‐45's death. Copyright © 2013 John Wiley & Sons, Ltd.     

Legalized trauma

In the past, criminals were executed by a variety of means, including beheading or hanging, and both these methods may leave signs on the skeleton. It is not difficult to recognize individuals who have been beheaded because one or other of the cervical vertebrae will show evidence of the fatal blow, and there may also be cut marks on the base of the skull or the back of the mandible. Death from hanging, however, often leaves no marks on the skeleton, especially when carried out with a running noose and no drop. With a drop, dislocation of the sutures at the base of the skull may be noted and the classic ‘hangman's fracture’, which is a fracture dislocation of the second cervical vertebra, may be present. When there is no evidence of trauma, death from hanging may have to be inferred in skeletons which are found with the hands tied or with other signs that are discussed here.     

Applying forensic techniques to interpret cranial fracture patterns in an archaeological specimen

This paper presents some of the basic principles of bone fracture physics and outlines characteristics used by forensic anthropologists to distinguish gunshot trauma from blunt trauma. The fracture pattern of a cranium from a historic cemetery in Memphis, Tennessee provides an example of how the mechanism of trauma could be misinterpreted. The pattern of the perimortem fractures is indicative of gunshot trauma with a bullet entering one side of the vault and impacting the opposite side. Identification of radiating fractures, concentric heaving fractures, and observation of the direction of bevel on the fractured surfaces are essential for interpretation of the mechanism that produced the injury.     

Polyphasic hydrothermal and meteoric fluid regimes during the growth of a segmented fault involving crystalline and carbonate rocks (Barcelona Plain,NE Spain)

A polyphasic tectonic‐fluid system of a fault that involves crystalline and carbonate rocks (Hospital fault, Barcelona Plain) has been inferred from regional to thin section scale observations combined with geochemical analyses. Cathodoluminescence, microprobe analyses and stable isotopy in fracture‐related cements record the circulation of successive alternations of hydrothermal and low‐temperature meteoric fluids linked with three main regional tectonic events. The first event corresponds to the Mesozoic extension, which had two rifting stages, and it is characterized by the independent tectonic activity of two fault segments, namely southern and northern Hospital fault segments. During the Late Permian‐Middle Jurassic rifting, these segments controlled the thickness and distribution of the Triassic sediments. Also, dolomitization was produced in an early stage by Triassic seawater at shallow conditions. During increasing burial, formation of fractures and their dolomite‐related cements took place. Fault activity during the Middle Jurassic–Late Cretaceous rifting was localized in the southern segment, and it was characterized by hydrothermal brines, with temperatures over 180°C, which ascended through this fault segment precipitating quartz, chlorite, and calcite. The second event corresponds to the Paleogene compression (Chattian), which produced exhumation, folding and erosion, favouring the percolation of low‐temperature meteoric fluids which produced the calcitization of the dolostones and of the dolomite cements. The third event is linked with the Neogene extension, where three stages have been identified. During the syn‐rift stage, the southern segment of the Hospital fault grew by tip propagation. In the relay zone, hydrothermal brines with temperature around 140°C upflowed. During the late postrift, the Hospital fault acted as a unique segment and deformation occurred at shallow conditions and under a low‐temperature meteoric regime. Finally, and possibly during the Messinian compression, NW‐SE strike‐slip faults offset the Hospital fault to its current configuration.     

Fluid effect on hydraulic fracture propagation behavior: a comparison between water and supercritical CO2‐like fluid

The initiation of hydraulic fractures during fluid injection in deep formations can be either engineered or induced unintentionally. Upon injection of CO₂, the pore fluids in deep formations can be changed from oil/saline water to CO₂ or CO₂ dominated. The type of fluid is important not only because the fluid must fracture the rock, but also because rocks saturated with different pore fluids behave differently. We investigated the influence of fluid properties on fracture propagation behavior by using the cohesive zone model in conjunction with a poroelasticity model. Simulation results indicate that the pore pressure fields are very different for different pore fluids even when the initial field conditions and injection schemes (rate and time) are kept the same. Low viscosity fluids with properties of supercritical CO₂ will create relatively thin and much shorter fractures in comparison with fluids exhibiting properties of water under similar injection schemes. Two significant times are recognized during fracture propagation: the time at which a crack ceases opening and the later time point at which a crack ceases propagating. These times are very different for different fluids. Both fluid compressibility and viscosity influence fracture propagation, with viscosity being the more important property. Viscosity can greatly affect hydraulic conductivity and the leak‐off coefficient. This analysis assumes the in‐situ pore fluid and injected fluid are the same and the pore space is 100% saturated by that fluid at the beginning of the simulation.     

Permeability changes in simulated granite faults during and after frictional sliding

Thermal–hydrological–mechanical coupling processes suggest that fault permeability should undergo dynamic change as a result of seismic slip. In igneous rocks, a fault's slip surface may have much higher permeability than the surrounding rock matrix and therefore operate as a conduit for fluids. We conducted laboratory experiments to investigate changes in fracture permeability (or transmissivity) of a fault in granite due to shear slip and cyclic heating and cooling. Our experiments showed that high initial fracture transmissivity (>10^?18 m³) was associated with a high friction coefficient and that transmissivity decreased during slip. We propose that this reduction in transmissivity reflects the presence of gouge in fracture voids, increasing the area of contact in the fault plane and reducing the hydraulic aperture. In contrast, when initial fracture transmissivity was low (<10^?18 m³), we observed that friction was lower and transmissivity increased during slip. The high transmissivity and high friction may be explained by large areas of bare rock being in contact on the slip surface. Slip velocity had little influence on the evolution of permeability, probably because gouge produced at different slip velocities had similar grain size distributions, or because gouge leaked from the slip surface. Transmissivity decreased with increasing temperature in heating tests, probably due to thermal expansion increasing normal stress on the fracture. Frictional heating did not influence transmissivity during the shearing tests.     

A History of Depressed Skull Fractures from Ancient Times to 1800

The story of managing depressed fractures illustrates how knowledge of proven value does not always get handed down. Celsus was the first to describe sensible management for depressed fractures. As he wrote in Latin this was forgotten. Galen’s Greek writings survived forming the basis of management until the sixteenth century. In 1517, Hans von Gersdorff published a formidable illustrated surgical text. One illustration depicts an instrument for elevating depressed bone fragments. It looked dramatic but could not work and its defects were finally defined in the eighteenth century. Ambroise Paré used a bone punch just as we do today, but no later surgeon mentions this, though the instrument was well known. Elements of chance, fashion, emotionally powerful illustrations, and perhaps stubbornness had a profound effect on management delaying rational treatment for centuries.     

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.