Absolute, Effective and Relative Permeability
Permeability is a measure of the rock’s ability to allow fluids to flow. It thus largely controls production rates together with the energy (potential pressure differential) in the reservoir. However, although permeability is mainly related to the pore throat size and distribution for its application in reservoir simulations a second aspect needs to be dealt with. If multiple fluids (with different composition and viscosity) rely on the same pore throat space to move through they affect each other’s capacity to move. The different fluids will hamper each other’s ability to move through the pore space. This effect is captured in the defining relative or effective permeability’s which are related to the saturation of each fluid in the pore space.
To understand this process, consider a busy highway with three types of vehicles: slow moving truck, cars and fast and agile motorcycles. The trucks move steadily with approximately the same speed, as they usually try to anticipate sudden stops and keep a larger distance from the vehicle in front of them. The cars can speed up more easily and can perform sudden stops. They easily overtake trucks and fill the gaps they leave. The nimble motorcycles can move fast in-between the cars and speed through the system, but often restrict movement of the cars by populating the space in between lanes. At the same time the motorcycles can not move at full speed as the cars and trucks restrict their movement. The cars can not move freely as well, as the trucks slow them down and the motorcycles restrict their ability to freely switch lanes. Lastly, the trucks are hampered by all the fast moving vehicles around them.
If the highway would consist only of trucks they would be able to move much faster. Also a highway full of cars would have less of a problem moving along. It goes without saying that a freeway filled with motor cycles would have no problem reaching a steady high speed.
The different vehicle types are much like fluids in a reservoir. They all have different agility and speed (viscosity) and when moving through a pore space simultaneously they hamper each other more than when moving through the same pore space alone. Effective permeability is the measure of a fluids ability to flow through a medium at a certain saturation.
Effective permeability is largely related to a rock property called wettability
. Better said, the preference of a certain fluid to stick to the grains over the other fluid tahts present. If grains are coated in a certain fluid this will allow the other fluid to move more easily as it doesn't stick (wet), but on the other hand the pore throats are reduced by the wetting fluid as is shown in the following image.Determining Effective Permeability
During routine core analysis a single fluid is pushed through a dried (all fluids are removed) core plug and through the pressure differential the permeability is measured. This, however, is the absolute permeability and can either be measured by using air (gas; if gas is used a Klinkenberg correction needs to be applied) or mercury. In this case a single fluid is present.
During a special core analysis multiple fluids are forced through the core plug and for different saturation the permeability is determined. This permeability is the effective peremeability and changes with saturations. Due to the (hampering) interaction between the fluids the effective permeability of all fluids does not add up to the absolute permeability.What is Relative Permeability?
Relative permeability is the ratio of effective permeability over the absolute permeability. It’s a value between 1 and 0, but in practice hardly ever reaches 1. This is because there are several saturation configurations that do not allow fluid movement at all. These saturations are called irreducible saturations. The effect of irreducible saturation of rel. perms will be discussed next.Irreducible saturations and Relative Permeability
Irreducible saturation are those fluid saturation below no fluid movement exists. In a pore system there are many surfaces on, and crooks in, the grains to which fluids can stick. Primarily wettability plays a role in how much a fluid sticks to these surfaces. Fluids that stick can not move and they form the irreducible volume of the pore space, but they still somewhat reduce fluid movement of the other fluid.
For an oil and water system the irreducible water saturation (Swir or Swc for connate Sw) is usually between 5 and 35% and below this saturation you can not displace more water by adding oil and hence you can not measure the relative permeability below these saturation levels. On the other side of the spectrum you will have the irreducible oil saturation (Sor; So=1-Sw) below no more oil can be displaced. This means that only theoretically (or after drying and preparing a core) the absolute permeability can ever be reached. In a oil and/or gas reservoir there is always an irreducible fluid and the absolute permeability can never be reached.Relative Permeability Plots
Relative Permeability is always displayed for two fluids and plotted with two y axis (displaying the relative permeability for each phase) against the saturation of the tested fluids (Sw & 1-So; Sw & 1-Sg or So & 1-Sg). A typical rel. permeability plot is shown below for an oil and water wet system