It is note quite sufficient to say that to be a reservoir , a rock requires porosity and permeability . Reservoir behavior relative to oil and gas accumulation and production certainly involves porosity and permeability , but its performance is based upon several important engineering factors .
Porosity :
Porosity represents the amount of void space in a rock and is measured as a percentage of the rock volume. Connected porosity where void space has flow-through potential is called effective porosity .
Noneffective porosity is isolated .Summation of effective and noneffective porosity produces total porosity , which represents all of the void space in a rock .
Pore space in rocks at the time of deposition is original , or primary porosity . It is usually a function of the amount of space between rock -forming grains . Original porosity is reduced by compaction and groundwater -related diagenetic processes .
Grownd water solution , recrystallization , and fracturing cause secondary porosity , which develops after sediments are deposited .
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Effective , noneffective , and total porosity |
Permeability :
A rock that contains connected porosity and allows the passage of fluids through it is permeable .Some rocks are more permeable than others because their intergranular porosity , or fracture porosity , allows fluids to pass through them easily .
Permeability is measured in darcies . A rock that has a permeability of 1 Darcy permits 1 cc of fluid with a viscosity of 1 centipoise (viscosity of water at 680 F) to flow through one square centimeter of its surface for a distance of 1 centimeter in 1 second with a pressure drop of 14.7 pounds per sequare inch .
Permeability is usually expressed in millidarcies since few rocks have a permeability of 1 Darcy . Intergranular material in a rock , such as clay minerals or cement , can reduce permeability and diminish its reservoir potential . It is evident , however that mineral grains must be cemented to some degree to form coherent rock and that permeability will reduce to some extent in the process .
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Fluid flow through Permeable Sand |
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Clay cement and porosity and permeability |
Relative Permeability :
When water , oil , and gas are flowing through permeable reservoirs , their rates of flow will be altered by the presence of the other fluids . One of the fluids will flow through a rock at a certain rate by itself . However , in the presence of one or both of the other fluids , its rate of flow can be changed .
The flow rate of each fluid is affected by the amounts of the other fluids , how they reduce pore space , and to what extent they saturate the rock. Comparison of the flow rate of a single fluid through a rock its that same flow rate , at the same pressure drop , in the presence of another fluid determines the relative permeability of the system .
When rock pores decrease in size , the surface tension of fluids in the rock increases .
If there are several fluids in the rock , each has a different surface tension , which exercises a pressure variation between them .This pressure is called capillary pressure and is often sufficient to prevent the flow of one fluid in the pressure of another .
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Relative permeability From Clark , 1969. Copyright 1969, SPE-AIME |