Significant Overpressure Research Articles

An Experimental Study of the Physical and Chemical Property Changes Occurring during the Smectite to Illite Reaction: ABSTRACT

Smectite dehydration and the formation of illite releases significant amounts of fluid deep within sedimentary basins. The release of water from smectite and the resulting physical property changes have been linked to the generation of significant overpressures and may also assist in the primary migration of hydrocarbons. Little quantitative data exists, however, as to how chemical and physical property changes may be coupled under in situ stress conditions. Presented here are the initial results from a one dimensional consolidometer system in which a K-smectite was reacted under differential stress conditions at temperatures up to 250[degrees]C. Sample volume, permeability and effluent fluid compositions are monitored through out the experiments. Partial collapse of the K-smectites occurred in all runs, with rapid dehydration apparently taking place at T close to 100[degrees]C. Initial samples were bilayer hydrates in the 0.5M KCI pore fluids used in the experiments. Dilution of the output fluid suggests the loss of two layers of water. Samples heated to <115[degrees]C and were still subsequently expandable, while the samples run at 250[degrees]C showed some irreversible collapse (illitization) after glycolation. Accelerated volumetric creep and permeability reductions occurred during heating increments, with over an order of magnitude reduction in permeability occurring during somemore » runs.« less

Role of overpressures and in situ stresses in fault-controlled hydrocarbon migration: a case study

Two wells (No. 1 and No. 2), separated by 2 km and situated in the upper prospective zone of a Tertiary basin in South-East Asiat, cross the same Mio-Pleistocene shaly-sandy series. From 1500 m downwards, these series are subject to significant overpressures related to an active margin. Comparison of well data, in terms of sonic compaction profiles, anomalous pressures and temperatures, has provided evidence of both a positive pressure and a temperature anomaly below a vertical depth of 1800 m in well 2. This anomaly is closely associated with the presence of liquid hydrocarbons in the 1900–2000 m reservoir interval. The confirmed deep source of these fluids and the amounts that have migrated during a short time interval can be explained only by the transitory dynamics of active transfers along faults and fractures. The episodic nature of this movement is the result of hydromechanical mechanisms in which the interactions between the high pressure regime and principal in situ stresses appear to be fundamental. The processes which induce open faulting and fluid pressure build-up in the direction of reservoirs with lower pressure regimes occur preferentially during phases of lateral tectonic stress relaxation, as soon as the effective minimum stress component becomes negative. The analysis of this case study gives a better understanding of the dynamic phenomena involving secondary hydrocarbon migration and caprock efficiency. The results obtained will help to optimize the overpressure prediction approach in comparable geological contexts.

Simulation of relativistic jets

Superluminal motion is often observed in extragalactic jets, but few simulations take into account the nature of relativistic fluids. Here, I have studied the behaviour of steady-state, supersonic jets propagating with a relativistic bulk velocity through a pressure gradient, similar to the observed atmospheres of extragalactic radio source hosts. The simulations were carried out in axisymmetry, using a Godunov-type fluid code in Lagrangian coordinates. The equation of state for a two-component ‘Synge gas’ was employed to close the system of conservation equations. It was found that initially hot jets (i.e. thermal energy of particles greater than rest energy) show large increases in bulk Lorentz factor during adiabatic cooling. In simulations where the jet experienced a period of significant over-pressure, and a resultant over-expansion, the familiar diamond-like structure of incident and reflected oblique shocks was seen. The final wavelength of the associated oscillations was revealed to be a function of initial Mach number and not temperature. It was also noted that, due to increased dissipation, the shock strength decays faster from one oscillation to the next as the initial temperature of the jet is raised.

Tectonic overpressure in competent mafic layers and the development of isolated eclogites

Abstract Variation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions, whether this is mean stress or the normal stress acting across the reacting interface. The magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks. The magnitude of the mean stress difference across a non‐slipping interface between two materials with viscosity ratio &gt;c. 20:1 is effectively equal to the maximum shear stress for flow in the more viscous material. Progressive shortening of the interface results in a higher mean stress in the more competent material, whereas extension results in a lower mean stress. For high‐P/low‐T eclogite facies conditions, current experimental data indicate that clinopyroxene‐ and garnet‐rich layers of eclogite should be very strong and that pressure differences of up to 800 MPa (8 kbar) between competent layer and weaker matrix may be possible. Such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower (by a factor approaching the viscosity ratio). Extrusion of material between more rigid plates, which has been proposed as a regional mechanism of lateral ‘continental escape’for both the Alps and the Himalayas, should also be accompanied by a lateral gradient in effective pressure; otherwise extrusion could not occur. Maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width (e.g. around 20 times for a width‐to‐thickness ratio of 10). Tectonic overpressure in progressively shortened competent layers, particularly in regions of extrusion between more rigid plates, might help explain the occurrence of isolated layers and pods of low‐T eclogite (&lt;550°C) with estimated peak pressures markedly in excess of those in the surrounding matrix. It cannot explain the occurrence of isolated high‐T eclogites, because at temperatures &gt;c. 550°C, the dramatic weakening of clinopyroxene in the power‐law creep field precludes the development of significant overpressures in eclogite layers.

Simple Approximations for Estimating Quickly the Motion and Timing of Salt Diapir Rise, Overhang Development, and Associated Thermal Anomalies Using Present-Day Observations: Case History from the Gulf of Mexico and Danish North Sea

ABSTRACT The upward motion of salt through deposited and depositing sedimentary cover due solely to buoyancy forces can be split into several parts: 1. The critical thickness of sedimentary cover necessary to cause an underlying salt to become buoyant; 2. The critical thickness of sedimentary cover necessary for a salt diapir to reach the sediment mudline in the absence of an impeding pressure of competent sediments opposing salt rise, and in the absence of significant overpressure. Both differential impedance and differential overpressure will slow the rise of the salt to the mudline; 3. The effective speed of motion of the salt through the non-impeding sediments during the salt's buoyant ascent phase; 4. Current observed salt top depth below mudline versus non-impeded predicted salt top depth leading to (a) minimum estate of mechanical strength of competent resistive layers; and (b) an approximate estimate of buoyancy pressure of salt attempting to penetrate the resistive cover layer; 5. Uplift estimate of the overlying competent sediments, because of the buoyancy pressure, in relation to observed uplift, leading to an estimate of salt diapir rise speed since reaching the impeding formation; 6. Timing estimates of "mushroom cap" development of salt since emplacement of the resistive overlying layer, and an estimate of the lateral competence of sedimentary beds ahead of the mushroom salt sheet cap as a consequence of the observed mushroom extent. An estimate of mushrooming speed can also then be made. 7. An estimate of evolving thermal anomalies around the dynamical salt/sediment system as a consequence of high salt thermal conductivity. Such simple rough estimation methods are of importance in attempts to assess the local and regional factors influencing the dynamical, thermal and hydrocarbon retention factors in basinal sediments influenced by salt. Examples from the Gulf of Mexico and the Danish North Sea will be given to illustrate how to use both seismic and/or downhole data to perform the simple estimates.

Volatile fluid overpressure in layered intrusions and the formation of potholes

Potholes that develop in layered intrusions show morphologic similarities with sedimentary features known as pockmarks. By analogy with pockmark formation theory, it is suggested that the cumulate section below the pothole region develops significant overpressures as solidifying intercumulus liquid separates a volatile‐rich fluid phase. A fluid overpressure develops as fluid separates from the intercumulus liquid faster than the fluid can escape from the crystal pile. The increase in pore pressure expresses itself as a dome‐shaped swelling on the chamber floor and in the uppermost part of the crystal pile, which eventually fractures and results in the violent escape of fluid. The combination of fluidization of the cumulates and incongruent melting, caused by the depression of liquidus temperature as a result of volatile elements added to less evolved liquid and crystal assemblages, leads to the formation of the pothole. Excess sulphide precipitation should accompany the fluid‐liquid mixing event as S‐bear...

Gas Explosions in Long Closed Vessels

Abstract The majority of experimental data on which current vent design practice is based, has been obtained in compact vessels of length lo diameter ratio (L/D) less than 3. This leads to great uncertainty when designing vents for larger L/D vessels. The need for more information on explosions in long enclosures has been noted in recent critical reviews and official guides. In this work methane/air explosions were investigated in closed vessels of large L/D with the aim of providing experimental data relevant to the explosion protection of such vessels. A 76mm diameter tube of an L/D of 21.6 and a 162 mm diameter tube of variable L/D (6.2-18.4) were employed. High rates of pressure rise, associated with fast flame speeds and significant overpressures, characterized the very early stages of these explosions, indicating the need for fast and effective pressure relief within the initial 10% of the total explosion time, Lower flame speeds and lower rates of pressure rise succeeded the fast initial phase. Small changes in the spark position were found to have a significant effect on the initial flame speeds and rates of pressure rise. The rates of pressure rise measured in this work did not scale in accordance with the ‘cube root’ law and therefore, the Ka concept cannot be used for large L/D vessels. Different scaling trends were determined on a tentative basis, and the need for more data was pointed out. Recent literature on this type of explosion has concentrated on phenomena occurring in the later stages, such as the tulip flames, pressure wave effects, acoustic instabilities and transition to turbulence. These later stages or the explosion arc irrelevant, from a safety point of view, unless the vessel can withstand overpressures of 1.5 to 2 times the initial pressure, generated in the earlier stages. Thus, the present work has shown that the very early phases are the most important, and should dominate vent design considerations because of the high rates of pressure rise and high overpressures measured in this period.

Hydrologic constraints on the genesis of the Upper Mississippi Valley mineral district from Illinois Basin brines

Mississippi Valley-type deposits of the Upper Mississippi Valley mineral district probably formed during a period of regional ground-water flow across the Illinois basin initiated by uplift of the Pascola arch in post-Early Permian and pre-Late Cretaceous time. Numerical modeling of this inferred paleohydrologic regime shows that district temperatures attained by this process depend on flow rates through the basin, heat flow along flow paths, and presence of structures to cause convergence and upwelling of fluids. Predicted flow rates and timing of mineralization agree with previous estimates. Modeling results also offer explanations of banding in district mineralization and district silicification patterns. Modeling of ground-water flow due to sediment compaction during basin subsidence, however, shows that this process was not responsible for mineralization. Fluids displaced from the deep basin by compaction-driven flow moved too slowly to avoid conductive cooling to the surface before reaching the district. Episodic dewatering events are unlikely to have occurred, because the basin did not develop significant overpressures during subsidence. Results of the compaction-driven flow modeling probably also apply to the Michigan and Forest City basins. Study results suggest that exploration strategies for Mississippi Valley-type deposits should account for tectonic histories of basin margins distant from targets.

Petrogenesis of the copper-bearing breccia pipes, Redbank, Northern Territory, Australia

Copper-bearing breccia pipes in the Redbank area intersect an interbedded sequence of igneous and dolomitic sedimentary rocks which have undergone various degrees of metasomatism.The steeply inclined breccia pipes are of small size and cylindrical form and typically show in situ brecciation. The breccia matrix and associated veins consist essentially of various proportions of microbreccia, dolomite, quartz, chlorite, celadonite, hematite, K-feldspar, apatite, and chalcopyrite, with minor barite, rutile, galena, and pyrobitumen. K-metasomatism is most intense in the vicinity of the breccia pipes and associated veining, and there is mineralogical and textural evidence indicating that fluids enriched in K, Cl, P, Mg, Ce, La, CO 2 , and H 2 O were introduced at the time of breccia-pipe formation. Carbonate and sulfide minerals from brecciated and metasomatized rocks at lower stratigraphic levels have isotopic compositions consistent with magmatic hydrothermal derivation. However, the delta 13 C values of the great bulk of the dolomite in the breccia pipes indicate remobilization of sedimentary carbonate. Furthermore, the sulfur isotope ratios of the main sulfide mineralization, which occurs near the top of the brecciated sequence, are variable and generally enriched in 34 S relative to the minor amounts of sulfide at lower levels.It is concluded that the breccia pipes formed by explosive release of fluids following the buildup of significant over-pressure in a postulated carbonated, K-rich trachytic magma at depths of roughly 2 to 3 km beneath the surface. This was accompanied by intense metasomatism and precipitation of some carbonate and sulfide minerals which partly infilled the open spaces. The thermal gradients and fracturing caused extensive circulation of connate brine (and possibly descending sea water). The brine remobilized sedimentary and magmatic hydrothermal components in and around the pipes and copper mineralization in the Redbank area mainly precipitated from this brine.