Formation water resistivity represents the resistivity value of the water (uncontaminated by drilling mud) that saturates the porous formation. It is also referred to as connate water or interstitial water. Its resistivity can be determined by a number of methods, one of which is by the SP curve discussed in this work. Analysis of wire-line log data depends on the assumption that the only conductive medium in a formation is the pure water which supplies the energy and drive in reservoirs. So, physical properties of this formation water can be determined, one of which is its electrical resistivity and this eventually leads to water saturation determination -an important aspect of reservoir evaluation. This paper presents a review and comparative assessment of the graphical, vis-à-vis the calculative means of R w determination by the SP method. @ JASEM Many of today's oil reservoirs are composed of sediments, which were once deposited in Marine, deltaic and other aquatic environments. Consequently, these sedimentary beds were originally saturated by salt water. Part of this water was displaced in the process of diagenesis and oil accumulations, the other remains, suspending the hydrocarbons because of their density contrast. That which remains generally is known as "Connate" or "Interstitial" water because the water was "born with" and is stored in the interstices of the sediments.Schlumberger (1989) defined formation water as the water uncontaminated by drilling mud that saturates the formation rock. Analysis of wire line log data depends on the assumption that the only conductive medium present in the formation is the pore water; the matrix and hydrocarbons are nonconductive. Physical properties of this formation water can be determined, one of which is electrical resistivity. Formation water is the free water which supplies the energy for the water drive in reservoirs; and its resistivity is variable depending on the salinity, temperature and whether or not the formation contains hydrocarbons. At a given salinity, the higher the temperature the lower the resistivity, and the water resistivity at any formation temperature, can be calculated from the water resistivity at another formation temperature, knowing both the temperature and temperature offsets using this formula: R w at FT 2 = R w FT 1 , (FT 1 + C)(FT 2 + C). Where FT 1 = Initial formation temperature FT 2 = Formation Temperature for which R w is being determined. C = 21.5 for Temperature in °C (Smolen, 1977).It has also been established (Schlumberger, 1989) that the water resistivity determined from a hydrocarbon-bearing zone is usually greater than that from the zone bearing only formation water. Determination of formation water resistivity is very important in calculating water and/or hydrocarbon saturation, in the determination of salinity if temperature is known and in understanding the variations of resistivity from the well wall into the formation by comparing it with the resistivity of the mud filtrate. In both SP and R wa comparison m...
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