No abstract

The microstructure of sedimentary rocks is studied by small-angle neutron scattering for length scales between 5 and 500 A. In limestones and dolomites, we find that the pore surfaces are effectively smooth above 50 A, but there is evidence for roughening on shorter length scales. In sandstones and shales, the pore surfaces show fractal character due to the presence of clay. The fractal dimension is nonuniversal. %e attribute these observations to impurity effects, which can lower the surface tension and maximize the surface area.Microgeometry of rocks is a subject of much scientific interest and practical importance. In the simplest model, one can view the rock as a random two-phase system. There is a pore space and a grain space, separated by a random interface. The disordered nature of this system suggests that it can be used to study fundamental statistical physics and to understand how randomness on a microscale can affect physical properties on a macroscale. This knowledge is important for extracting geological information from petrophysieal measurements.Two interesting recent studies suggest that the pore-grain interfaces in rocks are described by selfsimilar fractals with nonuniversal dimensions (2 & D & 3). In the first study, Avnir, Farin, andPfeifer examined surface-area-measurement data obtained by the molecular absorption technique. ' They showed that the area increases as the size of either the absorbent molecules or the absorbate particles decreases. In the second study, Katz and Thompson (KT) analyzed scanning electron microscope (SEM) and optical images of fractured sandstones, 2 from which they inferred a size distribution of "features. " They concluded that the number of features increases as the feature size decreases. The fractal interpretations of these two studies are not entirely consistent, however. For example, KT suggest that the lower limit of self-similarity is about 20 A in sandstones& awhile much of the absorption data extend down to 4 A. Unfortunately" there are no common samples in the two studies to make a firm comparison. KT also argue that the pore volume is a fractal with the same dimension D as the pore surface. This is an interesting idea not tested by the absorption experiments. In this paper, we report a detailed small-angle neutron-scattering (SANS) study on rocks to provide further understanding of their microgeometry. Specifically, we test the fractal and rough-surface ideas systematically in the three basic types of sedimentary rocks: sandstones, shales, and carbonates (limestones and dolomites).SANS is a powerful technique for the study of rocks or other porous materials, because neutrons can penetrate bulk samples to probe the virgin interior structure. Since the scattering amplitudes of common elements (carbon, oxygen, silicon, etc.) are comparable, for small wave vector q, the neutrons see the rock as a simple two-phase system. In the first Born approximation, the scattering cross section 1(q) is simply proportional to the Fourier transform of the geometric correla...

No abstract

Abstract--Sodium-saturated Wyoming bentonite was hydrothermally reacted at 150 ~ and 250~ for 30 to 180 days to determine smectite alteration rates that might be applied to nuclear-waste repository design. Na-Ca solutions deficient in K were used to determine the role of interlayer cations in the creation of high layer-charge in the smectites. The results provide insight into the mechanism and timing of various steps in the diagenetic alteration of smectite to illite. X-ray powder diffraction (XRD) analyses of the reacted clay showed little effect on the character of the 17-/~ reflection even after 180 days at 250~ Potassium saturation of these reacted clays and re-examination by XRD indicated collapse of some smectite layers, leaving at most only 60% expandable layers. The development of layer charge sufficient to cause collapse on saturation with a low hydration energy exchange-cation does not require K in the reacting fluid. Rate constants for the illitization reactions as determined by K-saturated collapse are between 1.0 x 10 -3 and 2.8 x 10-3/day with activation energies <3.5 kcal/mole. Ca in a Na-silicatebicarbonate solution slightly reduced the illitizafion rate constants. These rate constants are higher than expected from extrapolation of studies of beidellite-composition glasses at higher temperatures, but lower than values obtained in studies of natural clays in artificial sea water. The release of Si, A1, and Mg in the 150~ experiments suggests congruent dissolution of the smectite. In contrast, at 250~ the release of A1 was not stoichiometric with Si; as little as one half of the relative available Si was released. Rather than different mechanisms for dissolution at the two temperatures, the conclusion is that noncrystalline Al-rich phases formed at greater rates at higher temperatures. The cation-exchange capacities for several of these reacted smectites were significantly less than expected, suggesting a clogging of interlayer sites, perhaps by Al-complexes.

BackgroundAltered enteroendocrine hormone responses are widely believed to underlie the beneficial effects of bariatric surgery in type 2 diabetes. While elevated postprandial glucagon-like peptide-1 (GLP-1) is considered one of the mediators, increased postprandial glucagon levels have recently been implicated.ObjectivesWe investigated hormonal responses in lean patients after prophylactic total gastrectomy (PTG), as a model of Roux-en-Y gastric bypass without the confounding effects of obesity or massive weight loss.SettingUniversity hospital, United Kingdom.MethodsTen participants after PTG and 9 healthy volunteers were recruited for oral glucose tolerance tests. Plasma glucose, insulin, GLP-1, peptide YY, glucose-dependent insulinotropic-polypeptide, glucagon, oxyntomodulin, glucagon(1-61), and glicentin levels were assessed using immunoassays and/or mass spectrometry.ResultsPTG participants exhibited accelerated plasma glucose appearance, followed, in 3 of 10 cases, by hypoglycemia (<3 mM glucose). Plasma GLP-1, peptide YY, glucose-dependent insulinotropic-polypeptide, glicentin, and oxyntomodulin responses were elevated, and glucagon appeared to rise in PTG participants when measured with a glucagon-specific enzyme-linked immunosorbent assay. We revisited the specificity of this assay, and demonstrated significant cross-reactivity with glicentin and oxyntomodulin at concentrations observed in PTG plasma. Reassessment of glucagon with the same assay using a modified protocol, and by liquid chromatography-mass spectrometry, demonstrated suppression of glucagon secretion after oral glucose tolerance tests in both PTG and control cohorts.ConclusionsCare should be taken when assessing glucagon levels in the presence of elevated plasma levels of other proglucagon products. Substantial elevation of GLP-1 and insulin responses after PTG likely contribute to the observed hypoglycemia, and mirror similar hormone levels and complications observed in bariatric weight loss patients.

CO2/CH4 exchange in a sandstone-hosted methane hydratereservoir was executed in the field, following several years of laboratoryexperimentation. Reservoir simulation and laboratory data informedfieldtrial design, including use of a cell-to-cell model that included correctliquid/vapor/hydrate phase behavior of methane, carbon dioxide, nitrogen, andwater. Most concepts for producing methane from hydrate deposits rely ondepressurization, heating, or chemical melting. These techniques resultin dissociation of hydrate into its water and gas constituents. Effectiveexchange of CO2 for CH4 in the crystalline hydratelattice, without dissociation, was long deemed an improbable recovery strategybecause experimental results on bulk hydrate samples indicated very slowreaction kinetics. Recent laboratory tests documented enhanced exchangekinetics and efficiency, attributed to the increased surface area present inporous media. A series of laboratory tests ranging from simple gas-richsystems to more complex gas-deficient / water-rich systems guided the design ofa field test program. Ignik Sikumi #1 was drilled in 2011 on the AlaskaNorth Slope, designed specifically for testing CO2/CH4exchange in hydrate-bearing sandstones. Ignik Sikumi #1 was drilled vertically with chilled oil-based mud to a depthof 2600ft. Four hydrate-bearing sandstones were encountered, andpetrophysical evaluation indicated the Sagavanirktok " C Sand" hosted thehighest hydrate saturations. These sandstones occur in the subsurface atreservoir conditions similar to temperatures and pressure conditions of labtests. Reservoir modeling with conventional simulators and in-housecell-to-cell models guided both equipment design and test parameters. Anticipated low injection rates and cryogenic injectant required the design ofspecialized pumping equipment. Operations at Ignik Sikumi #1 re-commencedin December 2011. Following perforation, over 200,000 scf of mixedCO2/N2 gas was injected. A short unassisted flowperiod was followed by extended production testing via jet pumping. Results from the production test will be shown. CO2/CH4 exchange is a novel approach to recovermethane from sandstone-hosted hydrates. Field trial has validatedlaboratory results and reservoir simulations, and has proven thatCO2 can be injected into naturally occurring sandstone-hostedhydrates. Subsequent flowback/drawdown testing produced injectants(nitrogen, carbon dioxide, and tracer gases) methane, water, and very finesand.

Abstract--Mixed-layer illite/smectite (I/S) clays in lower Eocene sediments of the Texas Gulf Coast decrease in expandability with depth as found in previous studies of clay diagenesis, but at the same depth the 1/S clays from sand laminae tend to be more expandable than clays from adjacent shales. The I/S clays with greater expandability from sands collapse to phases with very low expandabilities after saturation with K. This non-ideal smectite behavior indicates that many of the expandable layers in I/S clays from sand laminae possess an illite-level negative layer charge on the structure. The Greene-Kelley Li-saturation test of the I/S clays also reveals that a considerable portion of the increased layer charge deficiency is created by tetrahedral substitution. The illitization of individual smectite layers in an I/S clay is envisioned to be a two step process instead of the single step rapid transformation implied in many earlier studies. The first rate-limiting step is the creation of sufficiently large negative layer charge in the lattice, primarily by AI for Si substitution in the tetrahedral sheet. The second rate-limiting step is the supply of K to the highcharged expandable layers. The abundance of high-charged expandable layers in I/S clays from sand laminae suggests that the K supply is influenced more by competing ions in the interstitial waters than by the absolute activity of K.

No abstract