Effective porosity

About: Effective porosity is a research topic. Over the lifetime, 1199 publications have been published within this topic receiving 26511 citations.

A Method of Determining the Permeability and Effective Porosity of Unconfined Anisotropie Aquifers

Abstract: The equations of unsteady flow toward a partially penetrating well in an unconfined aquifer of finite thickness are solved by linearization. It is assumed that the aquifer is nondeformable and that the effective porosity at the water table is constant. It is also assumed that the aquifer is anisotropic (the principal axes of the permeability tensor being horizontal and vertical, respectively), that the pumping discharge is constant, and that the drawdowns are small. The vertical component of the flow velocity is not neglected. The solution is, therefore, equally valid in the vicinity and at large distances from the pumping well. In the latter case, it coincides with the Theis solution. A method of matching the theoretical solution with drawdown measurements during pumping tests is outlined. By the matching method the horizontal and vertical hydraulic conductivities and the effective porosity of the aquifer may be determined. The method is illustrated by analyzing data from pumping tests carried out in three anisotropic aquifers.

Ecological optimality in water-limited natural soil-vegetation systems: 2. Tests and applications

Abstract: The long-term optimal climatic climax soil-vegetation system is defined for several climates according to previous hypotheses in terms of two free parameters, effective porosity and plant water use coefficient. The free parameters are chosen by matching the predicted and observed average annual water yield. The resulting climax soil and vegetation properties are tested by comparison with independent observations of canopy density and average annual surface runoff. The climax properties are shown also to satisfy a previous hypothesis for short-term optimization of canopy density and water use coefficient. Using these hypotheses, a relationship between average evapotranspiration and optimum vegetation canopy density is derived and is compared with additional field observations. An algorithm is suggested by which the climax soil and vegetation properties can be calculated given only the climate parameters and the soil effective porosity. Sensitivity of the climax properties to the effective porosity is explored.

Porosity gradients in North Sea oil-bearing sandstones

Abstract: The relationship between porosity and burial depth of sandstone reservoirs is complex. It depends on primary porosity, which is related to grain-size and depositional process and also on the porosity gradient (the change in porosity with depth) which, in most cases, is linear. This relationship may be expressed as: φ d =φ p − G.D, where φ d is the porosity at depth D below surface, φ p is the primary depositional porosity, and G is the porosity gradient in % porosity per 1000 ft (305 m). Porosity gradients decrease with increasing quartz content, abnormal pressures, and in the presence of hydrocarbons. They increase with increasing geothermal gradient. There are at present insufficient data to quantify these relationships. Empirically derived porosity gradients for North Sea Rotliegendes, Jurassic and Palaeocene reservoir sandstones are 2.2, 2.6 and 2.9 per cent per 1000 ft (305 m) respectively. When calibrated with the porosity spectra of the various facies, graphs can be drawn showing the depth: porosity windows within which economic reservoirs may be found.

Influence of Depth, Temperature, and Geologic Age on Porosity of Quartzose Sandstone

Abstract: The porosity of sedimentary rocks tends to decrease with increasing age and depth of burial. Shales lose water and compact regularly and readily during burial; sandstones behave much less predictably, particularly at shallow depths. Laboratory determinations of the crushing strength of quartz suggest that porosity might persist to very great depths. Other experiments have shown, however, that the solubility of quartz in water increases as temperature and pressure increase and that the crushing strength decreases greatly with rising temperature in the presence of water solutions. Natural sandstones should therefore compact and lose porosity with increasing depth and rising temperature. Experiments further suggest that loss of porosity is a function of time. With sufficient data it should be possible to predict the maximum porosity to be expected in well sorted, clean, quartz sandstone saturated with formation water at various depths and temperatures. Such sandstones have been investigated experimentally. As in natural sandstones, the experimental consolidation of quartz sands occurs by two distinct processes--compaction and cementation; both processes are accelerated by high temperatures, moving water solutions, and large pressures equivalent to large depths of burial. Porosity, depth, and temperature data have been assembled for Ordovician, Pennsylvanian, Eocene, Oligocene, Miocene, and Pliocene sandstones which are reservoirs for oil and gas. All, without exception, show reductions in both maximum and average porosity with increasing depth. Rather unexpectedly, the bounding curve of maximum porosity approximates a straight line. In areas with relatively high temperatures (high thermal gradient), sandstone porosities tend to be lower than for similar sandstones in areas with lower thermal gradients. In general, also, the younger sands have higher porosities at equivalent depths, but higher temperatures in younger rocks may reverse this relation. Theoretical consideration and porosity-depth curves for natural sandstones agree in suggesting that areas of high thermal gradient are generally less favorable for persistence of porosity with depth. The observed curves provide a basis for estimating maximum expectable porosities of untested sandstones in areas where geologic age and thermal gradient can be approximated.