Petroleum Engineering: Hydraulic Fracking Overview
Hydraulic fracturing is a technique that is useful for oil production, because when the reservoir pressure is depleted there is still oil in the rock, so the rock can be fractured and that oil can be extracted. How is the rock fractured in the subsurface? Very simply, by injecting a special pressurized fluid at the bottom.
The purpose of hydraulic fracturing is to stimulate the well or the set of wells that are producing oil from the same reservoir. To achieve the effectiveness of this production and stimulation technique, a special high-pressure fluid must be injected at high pressure, a fluid that can be prepared with water, sand and other chemical additives with the main objective of being injected into the reservoir in the subsoil so that it can generate cracks and through these cracks the natural gas and oil can flow and improve the production of the well.
What happens when the hydraulic pressure decreases?
When the hydraulic pressure exerted by the hydraulic fluid injected to the bottom of the well begins to decrease, there is no problem for the generated cracks to close, since the hydraulic fracturing fluid is prepared with small grains of proppants which manage to keep the generated cracks open.
Hydraulic fracturing as a hydrocarbon production technique is not allowed in all countries, for example in my country Venezuela this activity is not contemplated, since there is a very high probability of generating a high negative impact on the environment, since in the process can occur leakage of methane gas from the subsoil to the surface, and as we all know the release of this gas has a negative impact on the greenhouse effect of the planet. It can also contaminate fresh water aquifers in the subsoil, and finally it can compromise the natural geology of the subsoil, which could contribute to tetanic movements in the future, of course it is a probability since there are no solid studies to confirm it.
Engineering involved in the process
The pressure necessary for the hydraulic fracturing fluid to generate cracks in the rocks will depend on the speed with which the pumping and circulation of the hydraulic fracturing fluid from the surface to the bottom of the well is designed, it is necessary that the engineer in charge of the design of the pumping of the hydraulic fluid considers a certain speed for the diameter of the perforations made to the casing at the bottom.
The fracture gradient of the rock must be a value to be known by the engineer designing the hydraulic fracturing fluid properties, since the pressure exerted by the fluid must exceed the fracture pressure of the rock in the subsurface.
It is important to maintain control of the pressure and circulation of the hydraulic fracturing fluid, since during the first minutes of pumping there is a leakage of fluid due to the fracture of the rock, which is why once a decrease in pumping pressure is noticed, the pumping rate must be reduced, otherwise up to 70% of the fluid in circulation may be lost.
To get an idea of what the wellhead looks like at the surface once all the necessary accessories for pumping the hydraulic fracturing fluid are connected, I show you the following image:
Image source
It is important to maintain safety protocols at the surface due to the high pressures that are handled in the pumping of hydraulic fracturing fluid, which is why the logistics in the search for accessories such as connections, hoses, valves, among others must be reviewed and tested, all with the intention of preserving the safety of operators.
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