porous media, the ratio of surface area to pore volume will be shown later to be porous. Some sediments have porosity ranging from to at deposition. The following relationships between porosity and textural properties apply to k/ µ), the permeability, k, which is a property of the porous medium and the viscosity . Results from this study confirm the complex relationship among permeability, porosity, specific surface area and irreducible water saturation of carbonate. measurements and to investigate relationships bet w een porosity, microstructure, .. Most studies have focused on sedimentary roc k s, and fe w measured permeability ¥, specific surface area s, characteristic bubble area A c, bubble.
Currently, most permeability values are determined from ex situ samples taken from boreholes. This method is very expensive and extremely slow, requiring up to several days or weeks to process. In addition, samples are often unavailable due to disturbance during drilling or contaminant considerations. So the permeability should be determined directly using geophysical logging tools Sturrock ; Tong et al. One of the used logging methods for determining permeability is nuclear magnetism resonance NMR logging tool Balzarini et al.
While the technique is available, it does have several disadvantages, such as the small depth of investigation, the high cost, the low signal-to-noise ratio Tong et al. When rock was submitted to an alternating electrical field at different frequencies, its resistivity and permittivity are complex quantities characterized by a dispersive behaviour, that is, the resistive and reactive components of the complex resistivity vary over the frequency spectrum.
The complex resistivity is a non-invasive technique and can work in downhole. It depends on microstructure of shaly sand cores and can be used to estimate the permeability, which is a determining factor for making production decision in petroleum industry and need to be measured in downhole. Compared with the nuclear magnetism resonance logging tools, the complex resistivity has several advantages, such as deep investigation depth and high signal-to-noise ratio.
Since the internal structure of the pore space affects the electrolytic charge transport and fluid flow in a similar way, the induced polarization IP can be used as an in situ permeability estimation method. It has several advantages compared to the NMR logging tools, such as high investigation depth and high signal-to-noise ratio Tong et al.
The time-domain IP for estimation of hydraulic properties has received much attention Titov et al. In most literatures, chargeability was used as the time-domain IP parameter. It is demonstrated that chargeability can be related smoothly and definitively to excess conductivity or intergranular permeability.
On the other hand, the chargeability also relates to the formation water resistivity which changes greatly and it is very difficult to be obtained for the water-flooding oil field, such as Daqing Oil Field.
In the frequency domain both real and imaginary parts of complex resistivity of earth are measured by using different frequencies. These are then used to estimate the hydraulic conductivity and cation exchange capacity of clays. Denicol and Jing had shown that the frequency dependence behaviour in the frequency range 10— kHzas characterized by the slope of impedance versus log fshows a correlation with permeability.
In most of these studies, frequency spectra of the complex electrical parameters were measured and frequency dependency of these parameters was used, but the porosity of the rocks had received little attention. On the other hand, most of the used frequencies in these studies are too high and the corresponding frequency spectra are very difficult to be measured in the logging tools.
In this paper, we use the frequency dependency of the imaginary part of the complex resistivity combined with the porosity data to estimate the permeability of shaly sand reservoir.
The frequency region is Hz—1.
These two physical characteristics complex quantity and dispersion or frequency dependence can be used as a means to estimate rock petrophysical properties, such as specific surface area and permeability Cerepi The complex electrical behaviour of a rock results from both its conductive and idelectric response in the presence of a varying AC electrical field; the former is related to the transport of free charges and the latter is associated with polarization phenomena at pore—grain interface.
The interface region between matrix and the fluid filling the pore space is of particular interest due to the existence of the ionic double layer. The concept of electrical double layer, which is always present whenever there is an interface, forms the theoretical basis for understanding the electrical properties of rocks, especially shaly sands. The volume of groundwater is a equivalent to a 55 meter thick layer spread out over the entire surface of the Earth.
It is an important resource for potable water, irrigation, and industry. Because it is largely hidden from view, it is often forgotten and subject to contamination by careless humans.
Groundwater is a primary agent of chemical weathering and is responsible for the formation of caves and sinkholes. The Groundwater System Groundwater resides in the void spaces of rock, sediment, or soil, completely filling the voids. The total volume of open space in which the groundwater can reside is porosity. Porosity determines the amount of water that a rock or sediment can contain. Porosity In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation.
Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well. Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space. Since cements tend to fill in the pore space, highly cemented sedimentary rocks have lower porosity.
In igneous and metamorphic rocks porosity is usually low because the minerals tend to be intergrown, leaving little free space. Highly fractured igneous and metamorphic rocks, however, could have high porosity Secondary porosity is porosity that developed after rock formation. Processes such as fracturing, faulting, and dissolution can create secondary porosity. Permeability is a measure of the degree to which the pore spaces are interconnected, and the size of the interconnections.
Low porosity usually results in low permeability, but high porosity does not necessarily imply high permeability. It is possible to have a highly porous rock with little or no interconnections between pores. A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability.
A thin layer of water will always be attracted to mineral grains due to the unsatisfied ionic charge on the surface. This is called the force of molecular attraction. If the size of interconnections is not as large as the zone of molecular attraction, the water can't move. Thus, coarse-grained rocks are usually more permeable than fine-grained rocks, and sands are more permeable than clays.
Aquifers An aquifer is a large body of permeable material where groundwater is present and fills all pore space. Good aquifers are those with high permeability such as poorly cemented sands, gravels, or highly fractured rock. An aquitard is a body of material with very low permeability.
In general, tightly packed clays, well cemented sandstones, and igneous and metamorphic rocks lacking fractures are good aquitards. Large aquifers can be excellent sources of water for human usage such as the High Plains Aquifer in sands and gravels or the Floridian Aquifer in porous limestones as outlined in your text.
Aquifers can be of two types: Unconfined Aquifers - the most common type of aquifer, where the water table is exposed to the Earth's atmosphere through the zone of aeration.
Confined Aquifers - these are less common, but occur when an aquifer is confined between layers of impermeable strata aquitards. The Water Table Rain that falls on the surface seeps down through the soil and into a zone called the zone of aeration or unsaturated zone also called the vadose zonewhere most of the pore spaces are filled with air. As it penetrates deeper it eventually enters a zone where all pore spaces and fractures are filled with water. This zone is called the saturated zone or phreatic zone.
The surface below which all openings in the rock are filled with water the top of the saturated zone is called the water table. The water table occurs everywhere beneath the Earth's surface. In desert regions it is always present, but rarely intersects the surface. In more humid regions it reaches the surface at streams and lakes, and generally tends to follow surface topography.
The depth to the water table may change, however, as the amount of water flowing into and out of the saturated zone changes. During dry seasons, the depth to the water table increases. During wet seasons, the depth to the water table decreases. Discontinuous aquitards and aquifers may exist in the subsurface. These arrest downward infiltration to the water table and form what are called perched water tables.
They overlie unsaturated material and may be confused with the main water table. Because they are smaller, they are more easily dewatered or contaminated.
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Movement of Groundwater Groundwater is in constant motion, although the rate at which it moves is generally slower than it would move in a stream because it must pass through the intricate passageways between free space in the rock. First the groundwater moves downward due to the pull of gravity. But it can also move upward because it will flow from higher pressure areas to lower pressure areas, as can be seen by a simple experiment illustrated below.
Imagine that we have a "U"-shaped tube filled with water. If we put pressure on one side of the tube, the water level on the other side rises, thus the water moves from high pressure zones to low pressure zones.
The same thing happens beneath the surface of the Earth, where pressure is higher beneath the hills and lower beneath the valleys The Earth's surface can be divided into areas where some of the water falling on the surface seeps into the saturated zone and other areas where water flows out of the saturated zone onto the surface.
Areas where water enters the saturated zone are called recharge areas, because the saturated zone is recharged with groundwater beneath these areas. Generally, recharge areas are greater than discharge areas. Groundwater movement is slow relative to that in surface streams. This is because it must percolate through pore openings and is further slowed by friction and electrostatic forces. For comparison, typical rates of flow are as follows: Local — Shallow flow occurs over short times and distances, whereas, deep long distance flow occurs over time scales of centuries.
The rate at which groundwater moves through the saturated zone depends on the permeability of the rock and the hydraulic head. The hydraulic head is defined as the difference in elevation between two points on the water table.
The hydraulic gradient is the hydraulic head divided by the distance between two points on the water table. The velocity, V, is of groundwater flow is given by: If we multiply this expression by the area, A, through which the water is moving, then we get the discharge, Q. It simply states that discharge is proportional to the hydraulic gradient times the permeability.
Note that like stream discharge, Q has units of volume per time i. Springs A spring is an area on the surface of the Earth where the water table intersects the surface and water flows out of the ground. Some springs occur when an aquitard intersects an aquifer at the surface of the Earth.
Such juxtaposition between permeable and impermeable rock can occur along geological contacts and fault zones see figure The waters are usually rich in dissolved minerals that often precipitate around the springs. They develop in two settings: Hot springs are distinctive geological features. If the surface through volcanic ash they become a viscous slurry called mudpots. If they precipitate dissolved minerals on cooling, they can form deposits like travertine made of calcite.
Hot springs can also produce a wide range of colors due to thermal sensitive bacteria that metabolize sulfur minerals.