Pore Fluid Salinity Research Articles

Brine and gas recovery from geopressured systems

A series of parametric calculations was run with the geopressured—geothermal simulator MUSHRM to assess the effects of important formation, fluid and well parameters on brine and gas recovery from geopressured reservoir systems. The specific parameters considered are formation permeability, pore-fluid salinity, temperature and gas content, well radius and location with respect to reservoir boundaries, desired flow rate, and possible shale recharge. It was found that the total brine and gas recovered (as a fraction of the resource in situ) were most sensitive to formation permeability, pore-fluid gas content and shale recharge.

Regional Variations in Physical and Chemical Properties of South Louisiana Oil Field Brines: ABSTRACT

ABSTRACT Knowledge of in-situ physical and chemical properties of pore fluids in sedimentary basins is essential in understanding problems of fluid migration and diagenetic reaction. We have completed a study of the regional variations in pore fluid temperature, pressure, and salinity for the entire Tertiary section of South Louisiana to depths of 4 to 6 km. From this information it is possible to calculate values for hydraulic head and in-situ pore fluid densities and viscosities, parameters necessary in quantifying rates and directions of subsurface fluid migration. It is also possible to determine such chemical properties as methane solubility. Maximum salinities, typically in the range of 120-150 g/1, are found in the lower part of the hydropressured zone, where salt is introduced by dissolution of salt diapirs. Hydraulic head increases from essentially zero in the hydropressured zone to values exceeding 3 km above mean sea level deep in the geopressured section. Fluid densities increase with depth to maximum values approaching 1.07 g/cm at depths of approximately 2 km. Densities decrease with further depth as a result of increasing temperature and decreasing pore water salinity. The entire lower portion of the pore fluid column is thus gravitationally unstable, a factor favoring fluid overturn. Fluid viscosities systematically decrease from 1 cp at the surface to less than 0.2 cp at depth. As a result of these variations in density and viscosity, pore waters will flow five times more efficiently at depth. Methane solubility decreases upward from over 50 SCF/bbl in deep overpressured sediments to 10 SCF/bbl at the base of the hydropressured zone to less than 5 SCF/bbl at shallow depth. Significant exsolution of methane should thus occur as a result of upward migration of pore fluid or molecular diffusion of dissolved methane.

Effect of Geothermal Pore-Pressure Conditions and Natural Gas Composition on In Situ Natural Gas Hydrate Occurrences, North Slope, Alaska: ABSTRACT

The factors controlling the distribution of natural gas hydrates (solid compounds composed of natural gas and water) in the earth include mean annual ground temperatures, geothermal gradients, subsurface pressure conditions, gas composition, and pore-fluid salinity. A thorough analysis of the effect of these parameters on thickness and depth of hydrate stability zones has been conducted. A thermodynamic model has been used to compute depth and thickness of zones of stability of gas hydrates in 34 representative wells on the North Slope. Several well logs in these depth ranges have been analyzed to determine hydrate zone thickness, porosity, and hydrate saturation. In well log analysis, the hydrate presence has been indicated by the following evidence: increase in acoustic velocity, strong resistivity deflection, small spontaneous potential deflection, gas shown on mud log, oversized caliper increase in the neutron porosity, separation of long normal from short normal, and decrease in drilling rate. In several of these wells, multiple zones of hydrates have been detected. In the Prudhoe Bay and Kuparuk fields, hydrates are expected to occur primarily in six stratigraphic horizons, mostly in an unconsolidated unit characterized by a poorly sorted sandstone and conglomeratic lithology. Detailed examination of the neutron porosity and sonic velocity responses within one hydrate horizon in six wells in Kuparuk field indicates an average porosity of 44% and hydrate saturation of 93%. Such information is extremely relevant to quantification of gas hydrate deposits. End_of_Article - Last_Page 665------------

Geochemical inputs for hydrological models of deep-lying sedimentary units: Loss of mineral hydration water

Hydrological models that treat phenomena occurring deep in sedimentary piles, such as petroleum maturation and retention of chemical and radioactive waste, may require time spans of at least several million years. Many input quantities classically treated as constants will be variables on this time scale. Models sophisticated enough to include transport contributions from such processes as chemical diffusion, mineral dehydration and shale membrane behavior require considerable knowledge about regional geological history as well as the pertinent mineralogical and geochemical relationships. Simple dehydrations such as those of gypsum and halloysite occur at sharply-defined temperatures but, as with all mineral dehydration reactions, the equilibrium temperature is strongly dependent on the pore-fluid salinity and degree of overpressuring encountered in the subsurface. The dehydrations of analcime and smectite proceed by reactions involving other sedimentary minerals. The smectite reaction is crystallographically complex, yielding a succession of mixed-layered illite/smectites, and on the U.S.A. Gulf of Mexico coast continues over several million years at a particular stratigraphic interval.

Influence of porefluid salinity on instability of sensitive marine clays: A new approach to an old problem

AbstractThe geographic association between earthflow occurrence, sensitivity of the sediment and salinity of the interstitial fluid has been examined in an area of exposed marine (Champlain Sea) deposits in the Lachute region northwest of Montreal, Quebec. Mapping of earth resistivity, as a means of calculating porefluid salinity, revealed that areas of large earthflows (>20 ha) coincided with sediment of very low porefluid salinity. Earthflow scars were absent from intervening areas of higher salinity (>5 gl−1). Mapping of the thickness of the clay deposit by seismic refraction showed, furthermore, that low porefluid salinity occurred only in areas of relatively thin (<25 m) deposits resting upon more permeable drift or rock. Finally, laboratory tests confirmed that high sensitivity was restricted to areas of low porefluid salinity. The study thus provides a geomorphological verification of the importance of ‘salt depletion’ as a prerequisite for earthflow occurrence, and, by implication, presents mapping of porefluid salinity as a method of delineating zones which should be safe from landslide retrogression in this type of sediment.

Electrical Conductivity Models of the Crust

Compositional crustal models (continental, subcontinental, oceanic) are used as a basis for a construction of the electrical conductivity distribution within the crust. In addition, the procedure incorporates other parameters, such as pressure, porosity, pore fluid salinity, and resistivity. Calculation is made in three steps. First, 'dry' models based on mineral conduction alone are constructed. Second, electrolytic conduction through the pore fluids is estimated from the Archie's Law. Both results are then combined to produce the final, 'wet' models. These appear to agree with generalized conductivity profiles for the stable continental areas, as well as with the published values for the oceanic crust. Magnetotelluric response curves calculated for the final models give, for each crustal type, a band of possible responses which agree well with theoretical and in situ results.

Chemical Composition of Oceanic Water During Tertiary Time; Evidence from Pore-Water Studies on JOIDES Drill Cores: ABSTRACT

End_Page 858------------------------------Chemical analyses have been performed on pore solutions from more than 25 drillholes in the North and South Atlantic Ocean. Several holes penetrated Tertiary-Mesozoic sediments and bottomed in basalt. The data indicate that in the central areas of the Atlantic Ocean the salinity and chlorinity of pore fluids approach the values for bottom waters and vary less than about 1-2% with depth, with a few exceptions. Diagenetic changes in major inorganic ions are relatively minor, regardless of depth, lithology, or proximity to basalt bottom. Significant effects include chiefly loss of magnesium, partial loss of sulfate, and corresponding increases in alkalinity. Nearer the continents, fluid compositions range over somewhat wider limits, especially in the direction of lower salinity. With due allowance for molecular diffusion and other disturbing effects, the data offer no evidence that the oceans varied appreciably in either chloride, salinity, or ionic composition during Tertiary time. End_of_Article - Last_Page 859------------

The effect of pressure on the electrical resistivity of water-saturated crystalline rocks

Electrical resistivity of eight igneous rocks and two crystalline limestones was measured at pressures to 10 kb. The rocks were saturated with tap water or salt solution, and the pore pressure was maintained near zero. The dependence of resistivity on temperature, porosity, and pore fluid salinity suggested that conduction was primarily electrolytic throughout the entire pressure range, even though the porosity of some rocks was less than 0.001. Resistivity increased with increasing pressure. The average increase over the 10-kb range amounted to a factor of 250. The changes of resistivity with pressure parallel changes of compressibility with pressure, being rapid over the first 2 kb and tapering off more gradually at higher pressures. The data suggest that the electrical conduction of these rocks consists of (1) conduction along cracks, below a few kilobars pressures, and (2) volume and surface conduction along a network of pores which persist throughout the entire pressure range. Surface conduction of the rocks saturated with tap water was 10 to 20 times greater than the volume conduction of the pores. The dependence of conductivity on porosity for all the samples saturated with saline solution followed the same empirical law that is observed for porous sedimentary rocks, σ(rock) = σ(solution) × η2.