What is the conversion factor to obtain field units for Darcy's Law?

• A. 1.127 x 10^-3
• B. 1.127 x 10^3
• C. 1.12 x 10^-6
• D. 1.127 x 10^-5

Answer: (B)

In Darcy’s law you must keep track of units across every part of the equation. When you apply it in field-scale oil/gas contexts, you typically use field units: permeability in millidarcies, area in acres, pressure in psi, thickness in feet, viscosity in centipoise, and flow rate in barrels per day. To make the equation work with those units, a constant is introduced that bundles all the unit conversions (acres to feet, barrels to cubic feet, psi to psf, days to seconds, etc.). That conversion factor, in the common oil-field form of the equation, is 1.127 × 10^3. It converts the right-hand side, written with lab-style inputs, into a flow rate in STB/day when K is in md, A in acres, Δp in psi, and Δz in ft (with μ in cp). If you switch to different unit sets, you’d use a different factor, but with these standard field units this is the familiar constant that makes Darcy’s law give a practical flow rate.