Permeability is the ability of the formation to conduct fluids. From usage the name for fluid conductance capacity of a formation is permeability. It is a property of the porous medium and is measure of capacity of the medium transmitting fluids. The measurement of permeability, then, is a measure of the fluid conductivity of the particular material. It can be determined from samples extracted from the formation or by in-place testing. Two methods are used to evaluate the permeability of cores. The method most used on clean, fairly uniform formations utilizes small cylindrical samples, perm plugs. The second method uses full-diameter core samples in length of 1-1.5ft. The fluid used with either method may be gas or any nonreactive liquid.
So far permeability is referred to rock conditions where a single-phase fluid saturation was considered. In petroleum reserviors, however, the rocks are usually saturated with two or more fluids, such as interstitial water, oil and gas. Effective permeability is introduced here to describe the simultaneous flow of more than one fluid. In the definition of effective permeability each fluid phase is considered to be completely independent of the other fluids in the flow network.
The effectiive permeability is a relative measure of the conductance of the porous medium for one fluid phase when the medium is saturated with more than one fluid. This definition of effective permeability implies that the medium can have a distinct and measurable conductance to each phase present in the medium.
Experiments have established that effective permeability is a function of the prevailing fluid saturation, the rock-wetting characteristics, and the geometry of the pores of the rock. It becomes necessary, therefore, to specify the fluid saturation when stating the effective permeability of any particular fluid in a given medium. The effective permeability is stated as some numerical value at some given saturate conditions. Just as “K”is the accepted symbol for permeability, “K0””Kw”and “Kg”are the symbols for the effective permeability to oil, water, and gas respectively. The saturation, if known, should be specified to define completely the conditions at which a given effective permeability exists. Unlike the previously defined permeability, many values of effective permeability now exist, one for each particular condition of fluid saturation.
Effective permeabilities are normally measured directly in the laboratory on small core samples. However, owing to the many possible combinations of saturation for a single medium, laboratory’s data are usually summarized and reported as relative permeabilities. Relative permeability is defined as the effective ratio of the effective permeability of a fluid at a given value of saturation to the effective permeability of that fluid at 100 per cent saturation. It is normally assumed that the effective permeability is the same for all fluids at 100 per cent saturation, this permeability being denoted as the permeability of this porous medium.
In addition, there are many instances in which not two fluids, but three fluids exist in the rock simultaneously. Thus two-phase relative permeability data had to be amplified and extended into three-phase ones for three-phase systems.