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On this occasion I would like to share with you an audiovisual resource that explains the feasibility of continuing to drill an oil well under certain conditions:
Reservoir pressure = 4300 Psi
Drilling Fluid Density = 10 Pounds/Gallon
True vertical depth = 9500 feet
The approach in the solution of this example, which is summarized below, is to exemplify a possible scenario that may occur in the field, according to PDVSA's recommendations, an oil well should be drilled with a pressure in favor of the drilling fluid of 500 Psi, it is under this point that the following question occurs:
What happens if you have a delta pressure in favor of the drilling fluid lower than 500 Psi?
If we drill an oil well with a pressure margin in favor of the drilling fluid (over balance condition) there is a risk that fluids such as gas and/or oil may invade the well causing a fluid burst at the surface and causing an explosion where equipment of significant monetary value is lost, but above all, human lives are at risk.
What happens if you have a delta pressure in favor of the drilling fluid greater than 500 Psi?
If we drill a well with a delta pressure in favor of the drilling fluid greater than 500 psi we run the risk of fracturing the formation at its weakest point, i.e. in the section of the hole where there is no casing, if the formation fractures all the drilling fluid will be lost to the formation, and if this occurs the drilling fluid column drops and thus the static drilling fluid pressure may drop to values that are below the reservoir pressure.
What happens if the hydrostatic pressure is below reservoir pressure?
If the hydrostatic pressure is below the reservoir pressure, the condition already explained in previous paragraphs may occur, that is, a fluid burst such as gas and/or oil on the surface, resulting in a tragedy as shown in the following image:
Although there may be many causes for this type of accidents, in this post I have dedicated myself to explain only one of the many cases that may cause it. For the technical case explained in the video presented in this post, the calculations of hydrostatic pressure of the drilling fluid are made, In order to make a decision on whether to continue drilling, it is necessary to subtract the hydrostatic pressure of the drilling fluid from the reservoir pressure. In the case that is placed as an example in this post, the pressure delta in favor of the drilling fluid gives us a differential of 640 psi.
With that pressure margin we would be exceeding 140 psi above what is recommended by PDVSA and with which there is a risk of fracturing the formation.
The practical solution in the field would be to lower the density of the fluid until we obtain a differential pressure of 500 psi in favor of the drilling fluid, it does not have to be an exact calculation of 500 psi you can go through a few pounds or be below a few pounds, but never exceed so much above or below 500 psi.
To learn the technical details of the mathematical calculations from which the decision to lower the density of the drilling fluid is made and to adjust the pressure differential values in favor of the drilling fluid to about 500 psi I invite you to view the following video:
Watch on Youtube
To have a panoramic view of the entire solution of the mathematical calculations to address the engineering and field problem of the feasibility of continuing to drill the oil well at 9500 feet I present the following photographic image: