Reservoir Petrophysics

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Reservoir petrophysics, or the study of the physical properties of certain rock types, concerns itself with the measurement of field data from a well or wells, the processing of that data, its incorporation into physical models that describe the rock in place and field-wide, the calculation and interpretation of certain reservoir properties and finally the integration of these interpreted properties with others from cores (Sidewall and Conventional Whole Cores), tests and actual production to petrologically describe the reservoir in place and field wide. ^[1]

A principal goal of operators is to define a reservoir with the least number of wells as possible. Operators drill need means to obtain as much as data as possible to define the reservoir parameters and extrapolate them spatially across the field to economically evaluate the reservoir and plan the best production method.

"Petrophysics is the study of the physical and chemical properties that describe the occurrence & behavior of rocks, soil and fluids." How well the occurrence & behavior of the physical & chemical properties and processes in rocks, soils & fluids are understood determines how well resources are developed.

In extrapolation and development of petroleum resources, the properties of interest are the porosity, saturation, chemistry and mobility.

These are in pursuit of the questions:

• Is there any place in the rocks for the fluid to exists? (Porosity).
• How much of the porosity is fluid filled? (Saturation).
• What kind of fluids are there? (Chemistry).
• Can the fluids be moved? (Mobility).

None of the above can be measured directly. They are measure in conjunction with some other parameters. For example injection of an electric current and measurement of voltage response. Ohm's law relates the ratio of voltage to current as electrical resistance, which is multiplied by geometric factor become the material property called electrical resistivity. Electrical resistivity is the property that describes the ability if a material to support the process of charge transport. Archie's law describes the relationship b/w electrical resistivity and porosity, fluid saturation, cementation of the rock etc. Thus the injection of current & measurement of voltage can result in determination of porosity, fluid saturation and other parameters. However geometric factors in Archie's law have many built-in assumptions. These include considerations of rugosity of the borehole wall, wettability of the rock, presence or absence of clay minerals & more. Depending upon the choices made about these assumptions, different interpretations result for porosity, saturation & types of fluids. In petroleum reservoir valuation, these have significant impacts & consequences for the extraction of oil. Billions of dollars are wagered on the proper interpretation of these data.

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