The fluid level, however, is below the surface (zero) at 500 ft. Our total depth (2000 ft.) and gradient (0.425 psi/ft.) are the same as before. If the fluid doesn’t reach the surface, then there is some ‘fluid level’, or depth, where the pressure is zero and then the pressure increases according to the gradient. Pressure depth = 850 psi + 150 psi = 1000 psi. If my surface pressure was 150 psi, then the pressure at depth would increase by the same amount: If the pressure at surface isn’t zero, then the whole line shifts over according to the surface pressure.įrom the previous example, a gradient of 0.425 psi/ft at 2000 ft. and my gradient is 0.425 psi/ft, then my pressure is 2000 ft. Obviously, denser fluids, and therefore higher fluid gradients result in higher pressure.įor example, if my depth is 2000 ft. We can draw this on a graph that we call the pressure-depth plot. To find a pressure at a given depth, simply multiply the VERTICAL depth by the given fluid gradient.Īssuming that the fluid is incompressible, this is a linear relationship. If you are given the API gravity of oil instead of the SG, then use this formula: Most of the time you will not be given a fluid gradient or an average specific gravity, you will need to calculate it.įirst calculate your average specific gravity:Īverage SG = SG of water x Water Cut + SG of oil x (1-Water Cut) NB: Specific Gravity is always relative to pure water. To convert to gradient:Ġ.433 is the gradient for pure water (SG = 1) in Imperial units, remember it. Pressure gradient seems difficult, but it is simply using the density of the fluid and converting units: Pressure gradients for incompressible fluids have units of pressure/depth. The easiest way to calculate pressure from depth is to use the pressure gradient of the given fluid. Your understanding of the concept is crucial. “Pressure and Depth” is the FUNDAMENTAL relationship in the oil industry.
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