Hello friends and Stem content lovers, in this opportunity once again sharing with you a petroleum engineering topic related to the pressure that we can obtain at surface when we perform a formation test.
Performing a formation test involves performing a test where while we are drilling an oil well we are looking for a way to generate a pressure imbalance, where the hydrostatic pressure of the drilling fluid will be lower than the formation pressure, where the thrust of the gas and oil into the well will exceed the pressure exerted by the column of drilling fluid.
The pressure that we are going to obtain at surface we estimate it with the following equation:
Example in the calculation of the static surface pressure during a formation test
You have an oil-bearing sand layer at a depth of 12500 ft with a formation pressure equivalent to 13.5 lb/gallon, considering that the specific gravity of the oil is 0.5. calculate the surface pressure?
As shown in the previous equation, to calculate the surface pressure, we simply calculate the formation pressure and subtract the hydrostatic pressure of the oil.
To do this we first calculate the formation pressure as follows:
As it is well known to calculate the pressure gradient of a fluid in Psi/foot it is necessary to multiply the density of said fluid in pounds/gallon by 0.052 which is the conversion factor to convert density to pressure gradient, and if we multiply this pressure gradient by the depth of the oil-containing sand, then we will end up calculating the formation pressure in Psi units.
We now proceed to calculate the hydrostatic pressure of the petroleum:
We know that Sg is the specific gravity of the oil, which for the case of this example is 0.5 and the value of 8.33 is the density of water in units of pounds/gallon, as is well known to calculate the density of any fluid must multiply its specific gravity by the density of water, 0.052 is the conversion factor to convert density to pressure gradient (psi/ft) and that this value is multiplied by the depth of the sand containing oil, and the result will give us the hydrostatic pressure of the petroleum:
Para calcular la presión en superficie de la prueba de formación Se restan los valores de presión de la siguiente forma:
Conclusion
In the interaction between the formation pressure and the hydrostatic pressure of the oil, a resulting surface pressure of 6068 Psi is obtained, which represents a surface pressure derived from a formation test in which the productivity and the contribution of the formation is estimated while still in the drilling activities of an oil well.
It is important to mention the previous knowledge that must be had for the calculations made in this example, since it is necessary to know how to calculate density of a fluid from its specific gravity, to know how to calculate hydrostatic pressures and in general to analyze the behavior of fluids.
Bibliographic references consulted and recommended
- Well Control Manual Well Control School (WCS). Harvey Louisiana. Year 2003.
Note: The equations used for the calculations and examples shown in this post were elaborated by myself using Microsoft Word's equation insertion tools.
