Low Effective Pressure Research Articles

Ultrasonic velocity and anisotropy of hydrocarbon source rocks

An experimental study of the physical properties of black, kerogen‐rich shales, also including maturation analysis, scanning electron microscope (SEM) observations, and physical modeling, revealed fairly peculiar petrophysical parameters. Specifically, these rocks have very low porosity and density, but most importantly, both P and S ultrasonic velocities normal to bedding are extremely low, whereas they are much higher parallel to bedding, giving rise to a strong anisotropy even at high confining pressures. We found that these parameters primarily reflect kerogen content, microstructure, and maturation level of these rocks. We found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales. These microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy, notably at low effective pressure. Our results show, that on a small scale, kerogen‐rich shales are transversely isotropic rocks and can be effectively modeled using the thin‐layer composite concept modified to account for the specific distribution of organic matter in the rock fabric.

Silicon oxycarbide glasses: Part I. Preparation and chemistry

Silicone polymers were pyrolyzed to form silicon oxycarbides that contained only silicon, oxygen, and carbon. The starting polymers were mainly methyl trichlorosilane with a small amount of dimethyl dichlorosilane. NMR showed that the polymers had a silicon-oxygen backbone with branching and ring units. When the polymer was heated in hydrogen, toluene and isopropyl alcohol, used in production of the polymer, were given off in the temperature range 150 °C to 500 °C. Substantial decomposition of the polymer itself began only above about 700°by evolution of methane. The network of silicon-oxygen bonds and silicon-carbon bonds did not react and was preserved; the silicon-carbon bonds were linked into the silicon-oxygen network. The silicon oxycarbide was stable above 1000 °C, showing no dimensional changes above this temperature. The interior of the silicon oxycarbide was at very low effective oxygen pressure because oxygen diffused slowly in it. There was also a protective layer of silicon dioxide on the surface of the silicon oxycarbide.

A zoned low P-high T complex at the level of anatexis—structural and plutonic patterns in metasediments of the Archean Yellowknife Supergroup, near Bathurst Inlet, N.W.T., Canada

Within metagreywackes and schists of the Archean Yellowknife Supergroup the development of an “anatectic stratigraphy” during the late stages of regional deformation has resulted in the formation of a complex with a metamorphicplutonic-structural zoning similar to that occurring in Phanerozoic low P-high T terrains. From bottom to top the “stratigraphy” consists of: (1) small-folded migmatite, grading with decreasing partial melting to (2) sillimanite schist, strongly strained by the emplacement of two-mica granites; (3) andalusite-cordierite schist, the site of large folds related to the emplacement of the complex. Each zone is characterized by a different structural style resulting from the interplay between metamorphic grade and deformation. For instance in the sillimanite zone, the structure was influenced by the extraction of syntectonic melt and its emplacement into fractures at low effective pressure. This zone was also the ductile envelope into which sills and plutons of two-mica granites derived from the lower migmatites migrated. The time taken for evolution of the complex has been constrained by U-Pb zircon and monazite dating of a member of the early tonalite-diorite suite and members of the two-mica granite suite. This shows the evolution of the complex was rapid, ca. 10 Ma, but possibly polyphase. The formation of a distinctively styled complex accompanied by syn-tectonic anatexis and plutonism, may be typical of lower crustal levels of the Yellowknife Supergroup and other low P-high T terrains.

Remote Sensing of the Ross Ice Streams and Adjacent Ross Ice Shelf, Antarctica

In the first few seasons of the Antarctic Siple Coast project, the University of Wisconsin has concentrated on radar and seismic studies. Highlights of the results to date include the delineation of ice streams A, B, and C and the ridges in between, determination of the surface elevations over the area, discovery of a much more advanced grounding line than previously recognized and recognition of a broad, flat, barely grounded “ice plain” just inside the grounding line. Complex zones between and adjoining some of the ice streams, characterized by an interspersal of undisturbed ice and crevassed patches, give the impression of being transformed from sheet flow into stream flow in a process of ice stream expansion. An indicated negative mass balance for ice stream B could be the result of this “activation” process. Ice stream C, currently stagnant, exhibits terraces and reversals of surface slope, associated with zones of strong, steady basal radar reflections. These features suggest that subglacial water has been trapped by reversals in the hydraulic pressure gradient.Low seismic P-wave and S-wave velocities in a meters-thick layer immediately below the ice strongly indicate a saturated sediment of such high porosity (~40%) and low effective (differential) pressure (~50 kPa, or 0.5% of the glaciostatic pressure) that it must be too weak not to be deforming. We presume this deforming layer to be a dilated till. Its base exhibits ridges and troughs parallel to the flow direction that resemble glacial megaflutes. We believe that at our site on the upper part of ice stream B the ice stream moves principally by deforming its bed. Analysis of seismographic recordings of micro-earthquakes that occur at the glacier bed shows that the micro-earthquakes are both small in energy and infrequent. This implies that virtually none of the energy of ice stream motion is dissipated by brittle fracture at the bed. If our models are correct, the subgiacial deforming till becomes increasingly soft down-glacier, and/or the ice becomes decoupled from the till by intervening water, until on the “ice plain” basal drag is less important than longitudinal stresses in the dynamic balance. Our models also imply that the “ice plains” rest on “till deltas” that have been formed by the deposition of till carried along beneath the ice streams, and that the till deltas, and the grounding lines that bound them, are currently advancing in front of the active ice streams.

Remote Sensing of the Ross Ice Streams and Adjacent Ross Ice Shelf, Antarctica

In the first few seasons of the Antarctic Siple Coast project, the University of Wisconsin has concentrated on radar and seismic studies. Highlights of the results to date include the delineation of ice streams A, B, and C and the ridges in between, determination of the surface elevations over the area, discovery of a much more advanced grounding line than previously recognized and recognition of a broad, flat, barely grounded “ice plain” just inside the grounding line. Complex zones between and adjoining some of the ice streams, characterized by an interspersal of undisturbed ice and crevassed patches, give the impression of being transformed from sheet flow into stream flow in a process of ice stream expansion. An indicated negative mass balance for ice stream B could be the result of this “activation” process. Ice stream C, currently stagnant, exhibits terraces and reversals of surface slope, associated with zones of strong, steady basal radar reflections. These features suggest that subglacial water has been trapped by reversals in the hydraulic pressure gradient.Low seismic P-wave and S-wave velocities in a meters-thick layer immediately below the ice strongly indicate a saturated sediment of such high porosity (~40%) and low effective (differential) pressure (~50 kPa, or 0.5% of the glaciostatic pressure) that it must be too weak not to be deforming. We presume this deforming layer to be a dilated till. Its base exhibits ridges and troughs parallel to the flow direction that resemble glacial megaflutes. We believe that at our site on the upper part of ice stream B the ice stream moves principally by deforming its bed. Analysis of seismographic recordings of micro-earthquakes that occur at the glacier bed shows that the micro-earthquakes are both small in energy and infrequent. This implies that virtually none of the energy of ice stream motion is dissipated by brittle fracture at the bed. If our models are correct, the subgiacial deforming till becomes increasingly soft down-glacier, and/or the ice becomes decoupled from the till by intervening water, until on the “ice plain” basal drag is less important than longitudinal stresses in the dynamic balance. Our models also imply that the “ice plains” rest on “till deltas” that have been formed by the deposition of till carried along beneath the ice streams, and that the till deltas, and the grounding lines that bound them, are currently advancing in front of the active ice streams.

Remote Sensing of the Ross Ice Streams and Adjacent Ross Ice Shelf, Antarctica

In the first few seasons of the Antarctic Siple Coast project, the University of Wisconsin has concentrated on radar and seismic studies. Highlights of the results to date include the delineation of ice streams A, B, and C and the ridges in between, determination of the surface elevations over the area, discovery of a much more advanced grounding line than previously recognized and recognition of a broad, flat, barely grounded “ice plain” just inside the grounding line. Complex zones between and adjoining some of the ice streams, characterized by an interspersal of undisturbed ice and crevassed patches, give the impression of being transformed from sheet flow into stream flow in a process of ice stream expansion. An indicated negative mass balance for ice stream B could be the result of this “activation” process. Ice stream C, currently stagnant, exhibits terraces and reversals of surface slope, associated with zones of strong, steady basal radar reflections. These features suggest that subglacial water has been trapped by reversals in the hydraulic pressure gradient. Low seismic P-wave and S-wave velocities in a meters-thick layer immediately below the ice strongly indicate a saturated sediment of such high porosity (~40%) and low effective (differential) pressure (~50 kPa, or 0.5% of the glaciostatic pressure) that it must be too weak not to be deforming. We presume this deforming layer to be a dilated till. Its base exhibits ridges and troughs parallel to the flow direction that resemble glacial megaflutes. We believe that at our site on the upper part of ice stream B the ice stream moves principally by deforming its bed. Analysis of seismographic recordings of micro-earthquakes that occur at the glacier bed shows that the micro-earthquakes are both small in energy and infrequent. This implies that virtually none of the energy of ice stream motion is dissipated by brittle fracture at the bed. If our models are correct, the subgiacial deforming till becomes increasingly soft down-glacier, and/or the ice becomes decoupled from the till by intervening water, until on the “ice plain” basal drag is less important than longitudinal stresses in the dynamic balance. Our models also imply that the “ice plains” rest on “till deltas” that have been formed by the deposition of till carried along beneath the ice streams, and that the till deltas, and the grounding lines that bound them, are currently advancing in front of the active ice streams.

Fluid pressure response to undrained compression in saturated sedimentary rock

The pore pressure response of saturated porous rock subjected to undrained compression at low effective stresses are investigated theoretically and experimentally. This behavior is quantified by the undrained pore pressure buildup coefficient, [Formula: see text] where [Formula: see text] is fluid pressure, [Formula: see text] is confining pressure, and [Formula: see text] is the mass of fluid per unit bulk volume. The measured values for B for three sandstones and a dolomite arc near 1.0 at zero effective stress and decrease with increasing effective stress. In one sandstone, B is 0.62 at 13 MPa effective stress. These results agree with the theories of Gassmann (1951) and Bishop (1966), which assume a locally homogeneous solid framework. The decrease of B with increasing effective stress is probably related to crack closure and to high‐compressibility materials within the rock framework. The more general theories of Biot (1955) and Brown and Korringa (1975) introduce an additional parameter, the unjacketed pore compressibility, which can be determined from induced pore pressure results. Values of B close to 1 imply that under appropriate conditions within the crust, zones of low effective pressure characterized by low seismic wave velocity and high wave attenuation could exist. Also, in confined aquifer‐reservoir systems at very low effective stress states, the calculated specific storage coefficient is an order of magnitude larger than if less overpressured conditions prevailed.

Frame modulus reduction in sedimentary rocks: The effect of adsorption on grain contacts

The adsorption of vapor on the internal surface of a sandstone reduces its frame moduli. A theoretical model has been developed which predicts the weakening as a function of fluid chemistry and effective pressure. The model is based on well established contact theory and surface chemistry. Frame moduli are directly proportional to the stiffness of the grain contacts. The contact stiffness is dependent on surface energy. We show how the adsorption of various chemicals onto quartz surfaces lowers the surface energy, thereby weakening the contact stiffness. The predicted variation of frame moduli with the partial pressure and chemical species is qualitatively consistent with recent experimental results. The calculations suggest that the chemical effect is only significant at low effective pressures. At high pressures, the elastic loading dominates the contribution due to surface energy.

Evolution of herbicide-resistant weeds.

The first generation of species-selective phenoxy herbicides went into use at the same time as modern antibiotics, chlorinated hydrocarbon insecticides and new fungicides and rodenticides. No resistance has appeared to the still widely used phenoxy herbicides. However, resistance has developed, in many isolated areas around the world, to s-triazine herbicides and, in a few small areas, to bipyridillium herbicides. The parameters of population genetics that govern herbicide resistance and those that govern resistance of microorganisms and fungi to other xenobiotics are the same: generations, selection pressure and fitness. Even though generation times are longer with plants, a sufficient number has passed for resistance to be apparent. The only special factor that controls the development of herbicide resistance in weeds is the spaced germination of seed throughout many seasons. The reason that resistance has not developed to the phenoxy herbicides is probably because of a low effective selection pressure; germination of susceptible seeds occurs late in the season after the herbicide is biodegraded. The highly persistent triazines, and the monthly used, highly ephemeral bipyridillium, paraquat, have exerted much stronger selection pressures. Different modes of tolerance and resistance seem to have evolved in the same species. Crop and herbicide rotation can considerably delay the possibility of resistance development until it is effectively precluded.

Responsibilities for materials reliability and safety : G. P. Smedley. Metallurgist, Materials Technologist p. 505 (September 1979)

Variations of the mechanical and transport properties of cracked and/or porous rocks under isotropic stress depend on both the confining pressure (Pc) and the pore-fluid pressure (Pp). To a first approximation, these rock properties are functions of the differential pressure, Pd = Pc − Pp; at least for low differential pressures. However, at higher differential pressures, the properties depend in a more complicated way upon the two pressures. The concept of effective pressure, Pe, is used to denote this variation and it is defined as Pe(Pc,Pp) = Pc − n(Pc,Pp)Pp. If n = 1 (and therefore, is independent of Pc and Pp), the effective pressure is just the differential pressure.We have used an asperity-deformation model and a force-balance equation to derive expressions for the effective pressure. We equate the total external force (in one direction), Fc, to the total force on the asperities, Fa, and the force of the fluid, Fp, acting in that same direction. The fluid force, Fp, acts only on the parts of the crack (or pore-volume) faces which are not in contact. Then, the asperity pressure, Pa, is the average force per unit area acting on the crack (or grain) contacts Pa = Fa/A=Fc/A−Fp/A= Pc − (1 −Ac/A)Pp, where A is the total area over which Fc acts and Ac is the area of contact of the crack asperities or the grains. Thus, the asperity pressure, Pa, is greater than the differential pressure, Pd, because Pp acts on a smaller area, A−Ac, than the total area, A. For elastic asperities, the area of contact Ac and the strain (e.g., crack and pore openings) remain the same, to a high degree of approximation, at constant asperity pressure. Therefore, transport properties such as permeability, resistivity, thermal conductivity, etc. are constant, to the same degree of approximation, at constant asperity pressure. For these properties, the asperity pressure is, very accurately, the effective pressure, Pc.Using this model, we find that the dynamic (undrained) elastic modulus (Mcr) of saturated cracks (rocks) at low effective pressure is given by Mcr = (1 − PpA′f)Ma + (1 − Af)Mf = −wdPc/dw, where Ma is the (dry-matrix or crack-) asperity modulus, Mf is the fluid's modulus, Af is the fractional area of contact (Af = Ac/A), A′f =dAf/dPa and w is a measure of the crack or pore openings. This simple model accounts for the dependence of the rock modulus (and elastic velocity) on: (1) the elastic properties of the fluid, (2) the elastic properties of the dry rock, and (3) the pore-fluid and confining pressures. Explicit expressions depend upon the choice of the asperity- (or grain-) deformation models and their contact distribution functions. The effective pressures for transport properties are different than the ‘effective pressure’ for the mechanical properties. Calculated results based on the ‘bed-of-nails’ model having power-law (or fractal) asperity-height distribution functions can be fitted quite well to experimental data with a minimum of fitting parameters.

Subglacial Shearing and Crushing, and the Role of Water Pressures in Tills from South-East Iceland

Till exposed at the retreating margin of Breidamerkurjökull has a two-layer structure; an upper horizon of high void ratio and with a high concentration of fines, and an underlying, denser, less fine-grained horizon with a platy structure. It is suggested that the upper horizon has undergone continuous subglacial shearing in which dilation and interparticle crushing have occurred, the latter having produced the high concentration of fines, and that much grain crushing generally attributed to glacial transport may well be post-depositional in origin. The lower horizon is thought to have lain below the level at which shear stresses imposed by the overriding glacier were attenuated to values below the strength of the till, and thus to have undergone compaction only.Under normal circumstances, the frictional strength of the till will rise under increasing ice loads whereas the basal shear stress will remain constant, so that till deformation will be largely a marginal phenomenon. However the effective pressure in the till is reduced by the existence of a subglacial water pressure.Equations are developed which show how the effective pressure in subglacial beds depends not only on the shape and size of the glacier but also on the compressibility of the beds and their hydraulic transmissibility at zero effective pressure. A low hydraulic transmissibility gives a low effective pressure, which is conducive to deformation of subglacial sediments.Implications for glacier surging, drumlin formation and till fabric orientation are discussed.

Cytochemical evidence for the leakage of acid phosphatase through ultrastructurally intact lysosomal membranes.

Fixation under ‘improper’ conditions of in vitro cultivated cells results in an extensive diffusion of the lysosomal enzyme acid phosphatase because of the influence of a low effective osmotic pressure. In the present investigation, advantage was taken of this predictable diffusion in order to establish whether or not leakage of acid phosphatase could take place through ultrastructurally ‘intact’ lysosomal membranes.

Laboratory studies of stick-slip and their application to earthquakes

Stick-slip on pre-existing faults has been suggested as one source of crustal earthquakes. Current laboratory studies of stick-slip are reviewed to note factors which determine whether sliding is stable or unstable in laboratory samples, point out ways in which the laboratory experiment may not model the situation in the earth, and emphasize areas in which further laboratory study is needed. The most important factors which determine whether sliding will be unstable (stick-slip and earthquake-producing) or stable (stable sliding or fault creep) include mineralogy, porosity, effective confining pressure, temperature and thickness of fault gouge. In general, stable sliding is enhanced by high temperature, low effective pressure, high porosity, thick gouge, and the presence of even small quantities of minerals like serpentine and calcite. It is still not clear just how well the laboratory experiment models a seismic region in the earth. This is in part because both the detailed geometry of natural seismic faults and factors like gouge thickness are not well-known. Also, it has not yet been possible in the laboratory to study the characteristics of a fault in which slip is limited to part of the surface, as is the case in a typical seismic area. The laboratory experiment also does not correctly model the real situation in which normal stress, temperature, and effective pressure may vary along the fault surface. In addition to design of experiments in which the above features are included, work should be done both on the theory of stick-slip and on a better understanding of the physical characteristics of rocks responsible for observed differences in sliding behavior.

Strength of Oil Well Cements at Downhole Pressure-Temperature Conditions

Abstract Triaxial compression tests with independently applied external confining pressures and internal pore pressures show that the ultimate compressive strengths of representative oil well cements are nearly linear functions of effective pressure the difference between external and internal pressures on the jacketed cylindrical specimens (to 15,000 psi). The strengths are little affected by the test temperature to 350F (not to be confused with the curing temperature). At an effective pressure of 15,000 psi, strengths are in the range of 30,000 to 50,000 psi, comparable to those of sedimentary rocks under similar conditions. The cements become very ductile even under low effective pressures; permanent shortenings of 30 per cent or more are attainable without rupture. Introduction Since the pioneering work of Richart, Brandtzaeg and Brown on the failure of cement under combined compressive stresses, it has been recognized that ultimate compressive strength is greatly enhanced by the application of confining pressure. More recently, McHenry showed that the strength of concrete was a linear function of the effective pressure (the difference between the external confining pressure on a jacketed specimen and the internal fluid pore pressure) at least for a range of 0 to 1,500 psi. The effect of temperature had not been investigated, and no previous systematic triaxial compression testing of materials used for oilwell cementing seems to have been done. The present work was suggested by the late J. M. Bugbee who stated that "consideration of the common application of high-pressure hydraulic fracturing to the initial completion or recompletion of wells, and the large pressure drawdowns in some producing wells, particularly those in abnormally high-pressure gas-condensate reservoirs, raises the question of what is a suitable cement strength for various completions. The intuitive belief exists that cement strength need be no greater than formation strength. Tests should, however, be conducted at downhole conditions."The ultimate compressive strengths of rocks penetrated by the borehole must rise several fold with increasing depth. This marked enhancement of strength is due to the influence of the effective pressure, the total weight per unit area of the overburden less the hydrostatic pore pressure. (The effect of temperature due to the geothermal gradients is relatively small for depths to 30,000 ft.) A significant comparison of the strengths of rocks and cements at downhole conditions requires knowledge of the confined compressive strengths of cements as well. EXPERIMENTAL PROCEDURES The theory and technique of triaxial compression testing are fully discussed in earlier reports. Briefly, cylindrical specimens 1-in. long and 0.5-in. in diameter are jacketed in thin copper tubes of negligible strength, placed in the test chamber and subjected to an external confining pressure of kerosene and loaded axially by the piston at a strain rate of 1 per cent per minute. Pore pressures of water (or kerosene) are applied independently through the hollow piston and are maintained constant during the shortening of the specimen. Tests at sensibly 0 pore pressure are arranged so that any free water in the cement can escape to the atmosphere. (If egress of water were denied, pore pressure would rapidly attain the value of the external confining pressure because of reduction of pore space.) The test chamber can be heated for high-temperature experiments. Unless other-wise noted, the cement samples were air dried for about a week. Recorded during a test are pore and confining pressures, shortening and axial differential force (total force less the product of the confining pressure and the area of the piston). SPEJ P. 341ˆ