Geostatic Pressure Gradient
Definition - What does Geostatic Pressure Gradient mean?
Geostatic Pressure Gradient is the rate of change in geostatic pressure with depth. A geostatic pressure gradient of 1 psi per feet results from an average density of 2.3 gram per cubic centimeter. Geostatic pressure is the stress or pressure exerted by overlying rock or sediments above a depth or formation of interest. It is usually considered to be about 1.0 psi per feet although the value can vary depending on the geographical area. Hence this value can be considered for approximations.
Petropedia explains Geostatic Pressure Gradient
When the geostatic pressure changes with depth, the geostatic pressure gradient also changes. This means that with higher depths, the geostatic pressure increases and thus the gradient also increases. Geostatic pressure is commonly referred as overburden pressure. In order to estimate the overburden pressure at any point, it is important to first determine the material density, i.e., the density of rocks and fluids at that point. The equation for estimating the overburden pressure at depth z is given by:
p(z) = density of overlying rocks at depth z
g = acceleration due to gravity
po = datum pressureThere are certain assumptions made while deriving the above equation, such as the acceleration due to gravity is considered to be constant at depth z which is why it is kept outside the integral. However, in actual scenario, acceleration due to gravity will not be constant and changes when moved deeper into the earth. The reason for keeping “g” outside the integral part is to practice most near surface applications which need the determination of lithostatic pressure.
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