The value of pore water pressure?
I'm having trouble with this problem. From a consolidated-drained triaxial test, the effective stress parameters for a fully saturated clay are obtained as c’=35 kN/m2 and φ’=30°. In a separate, unconsolidated undrained triaxial test on a specimen of the same clay, the chamber pressure σ3 was 100 kN/m2 and the principal stress difference (σ1- σ3) at failure was 160 kN/m2. What was the value of pore water pressure in units of kN/m2, of this specimen at failure?
How can I calculate a pore pressure gradient?
There are tools that measure the pore pressure at chosen depths. Examples are the RFT, RDT, MDT, FMI toolsAfter recording the open hole log in your well, select pressure points at the depths of interest, the tool will measure the pressures at the selected depths, if you measure the pore pressure at two different depths within the same body, you can calculate the gradient of the pore pressure between these two points. The resultant value will give you an idea about the type of the formation fluid.Say P1=1000 psi at 5000 ftP2=1001 psi at 5004 ftThe Gr. will be (1001–1000)÷(5004/5000)= 0.25 psi/ftThis is oil gradient, it means there could be oil in this body in which the two points of pressure were measured.Generally speakingIf the Gr. Is from 0.2 till 0.41 it is oilIf it from 0.43 and above, it is waterIf it 0.0xx it is gas.Double check is got from the interpretation of the open hole log itself. And also from analog with the previous surroudning wells production history.DST testsPVT samplingLFA
How do hydrostatic and osmotic pressure work?
in general : -Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity. -Oncotic pressure is a form of pressure exerted by proteins in blood plasma that usually tends to pull water into the circulatory system there is an equilibrium between the capillary and interstitium, as you(should) know the fluid moves in and out the capillaries,there are 4 primary forces that determine movement of fluid between capillaries and the interstitium : 1.Capillary hydrostatic pressure (This pressure drives fluid out of the capillary ) 2.Capillary oncotic pressure (this pressure tends to pull the fluid from interstitum to the capillary) 3.Interstitial hydrostatic pressure (this pressure drives fluid out of the interstitum) 4.Interstitial oncotic pressure (tends to pull fluid from the capillary to the interstitium) so basically we are talking about forces and the movement of fluid depends on the value of this forces. Capillary hydrostatic pressure(a) =15-30 mmHg Capillary oncotic pressure(b) = 25- 30 mmHg Interstitial hydrostatic pressure(c) = 0 mmHg Interstitial oncotic pressure(d) = 5 mmHg net fluid movement = (a-c)-(b+d) about Edema: edema may be a result of 1.high capillary hydrostatic pressure 2.low capillary oncotic pressure 3.high interstitial oncotic pressure etc.
What is water potential , solute potential?
"water potential" (Ψ) is a measure of the free energy of water; pure water (which has a high amount of free energy) is arbitrarily assigned a water potential of zero; units pressure: MPa; can be positive or negative factors that determine plant water potential: 1) amount of solutes- increasing concentrations will lower the free energy (water potential); termed osmotic potential (ψs) 2) turgor pressure (ψp) in plant cell- positive pressure inside plant cells; increases free energy; loss of turgor = wilting 3) surfaces of macromolecules (e.g. cellulose) exerts an attractiveforce on water- matric potential (ψm); lowers the free energy; usually a minor component, and often ignored Total water potential of a plant: Ψplant =ψs + ψp + ψm Soil water potential same components as plants, but with no turgor, and a gravitational componentadded for saturated soils (ψg) Ψplant =ψs + ψg + ψm for most terrestrial ecosystems, the matric potential (ψm) is the most important component of soil water potential; in saline soils ψs is the most significant component. Atmospheric water potential determined by the relative humidity of the air; from plant perspective, it is extremely low below 90% RH Solute potential is also known as osmotic potential. When solute molecules are dissolved into water, the concentration of water molecules is reduced, and therefore so is the water potential. Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules. The more solute molecules are present, the lower (and more negative) is s. Solute potential is always negative. Water potential is affected by both solute potential and pressure potential.
What is uplift pressure?
An uplift pressure is any upward pressure applied to a structure that has the potential to raise it relative to its surroundings. Uplift forces can be a consequence of pressure from the ground below, surface water and so on.When there is an impermeable layer between the bottom of the excavation and a deep aquifer, the ground water head at the acquifer below the subgrade might be sufficiently high to cause soil layers to lose strength.Water pressure can exert an uplift force on a structure due to high rainfall, for example by causing clay soils to expand. This can be problematic if the upward forces are greater than the forces being exerted downwards by the structure. To overcome this risk, the structure must be designed to provide greater resistance against uplift forces, and also a drainage system to relieve the water pressure.
What is the difference between positive and negative pore pressure?
let take a soil sample in it all the pores are filled with the water or soil is saturated.now we applied the pressure on the soil sample,due to the pressure soil will tend to compress and water will come out .if we don't allow the water to come out from pores then what will happen? think!!the soil will tend to compress but there is water in the pores and water is not allowing to come out. so the soil will not compress. its mean pressure is taken by the pore water. and in this condition a pressure in pores will generate .this is called pore water pressure.positive pore water pressure:-the pressure above the atmospheric pressure.work in saturated soils.measured by the inserting piezometer.due to the positive pore wayer pressure the effective pressure is less than the total pressure.Negative pore water pressure:-upper part of the water table where soil is unsaturated.This is primarily due to the surface tension of pore water in voids throughout the vadose zone causing a suction effect on surrounding particles. This capillary action is the "upward movement of water through the vadose zone"soil sample is unsaturated but not drythe pore water pressure below the atmospheric pressure.it is called suction pressure.measured by the Tensiometerdue to the negative pore water pressure the effective stress greater than the total stressIf soil is dry then the pore water pressure will be zero.
Water changes state at 100 degree celcius, how does it evaporate from Sea without Sea reaching 100 degree?
Water boils at 100 deg Centigrade but it is not necessary for it to reach that temperature to evaporate. Try spilling a little water on a plate and see how it eventually dries. What happens is that the groups of free molecules of water (H2O) escape into the atmosphere due to their movement. These are still small pieces of 'water' and can float until they attach to other matter, usually dust, and cause precipitation (rain). Naturally because the collections of molecules are so small they will rise into the atmosphere on thermals as being less dense than the surrounding air and eventually, when they reach optimum height depending on temperature and mass they form clouds. Water vapour as a gas is NOT steam. When water boils and becomes a gas it is colourless and cannot be seen. Look at the spout of a boiling kettle and you will see a small area between the spout and the steam that is true gaseous water. Once the gas hits normal air temperature it forms steam which is a collection of water droplets as the gas turns back into water. Steam, therefore, is not gaseous water but water itself. Evaporation from the sea is exactly the same principle but on a larger scale. Most liquids evaporate eventually, i.e. dry up, under normal conditions of temperature and pressure (See Boyle's Law). When evaporation from the sea takes place the water does not take solubles with it such as salt which is why rain derived from sea evaporation is not salty.