Formation evaluation is a log analysis technique used for determination of both physical and chemical properties of rocks and fluid that they contain. The objective of formation evaluation is to locate, define and estimate a given reservoir by drilling as few wells as possible.Ultimate objective of formation evaluation is to find the water saturation and hence estimate hydrocarbon saturation.
Consider a formation with given amount of porosity is completely filled with saline formation water of given resistivity. Resistivity of formation water depends on salinity of formation water and other factor called formation factor (F).
F α 1/Ø
Where Ø is porosity of the formation.
F= a/ Ø m
a is tortuosity factor
m is cementation exponent.
If Ro is overall formation resistivity of water bearing zone than
Ro= F Rt
Rt is true resistivity of formation
Water saturation is the percentage of pore space with in a formation that is occupied by formation water. Water saturation can be physically related to the physical properties of formation and conductive properties of fluid it contains.
Swn = Ro/Rt = F x Rw /Rt = a Rw/ Ø m Rt
Sw = [(a x Rw)/ (Ø m Rt)] 1/n
Sw is water saturation
n is saturation exponent.
a is tortuosity factor
Ø is porosity
m is cementation exponent
Rw is formation water resistivity
Rt is true formation resistivity.
Above equation for water saturation estimation is called Archie equation. It is important to recognize that water saturation in the equation represents percent of water present in the pore space. It does not represent ratio of water to hydrocarbon that will be produced from the reservoir.
Archie equation considers only clean sand is present in reservoir, it does not consider presence of shale in reservoir, what if shale impurities are present?
Presence of shale in the reservoir may either be good or bad in terms of reservoir quality. Small amount of shale within the pore space of reservoir because of increased surface adhesion and capillarity may trap interstitial water. The result could be a virtually water free Hydrocarbon production. On the other hand shale can reduce the porosity and permeability of the formation to such an extent that the result could be non-productive reservoir.