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formula and area edit
Darcy's law is a macroscopic law that relates fluid flow through a section of porous medium such as sedimentary rock or sedimentary sand or soil. The sample of sand may be filled into a pipe which is usually circular in cross-section. Likewise as sample of rock may be put into a pice of pipe (core plug container). This is the experimental setup in the laboratory. When the fluid is flowing in the soil or in the rock (the oil reservoir), the area is a representative macroscopic area of the soil or rock that the fluid is flowing through. The fluid is not flow through the sand grain which is solid rock. In a modern computer model of an oil reservoir the representative area is the contact area between two neighboring grid cells. The formula should be
Da = K/A = 4*K/(PI*d^2) = K/(X*Y)
where K = permeability of the sediment sample (m^2 in base SI units or D (darcy) or mD (millidarcy) units in (petroleum) Applied SI units or Field units) A = representative macroscopic (geometric) area that the fluid is flowing through (m^2) d = inner diameter of the pipe (core plug container) containing the sediment sample (m) X, Y = inner widths of a rectangular pipe containing the sediment sample ([X]=m, [Y]=m) ...