Many oil-bearing formations in the earth crust have a permeability (degree of pores connectivity) that is too low to justify productions. Many also have been produced but the rate of production have reached a level that is not satisfactory anymore. These kinds of formations require direct and drastic changes made to its permeability. One of the action that can be done is brutally open up channels inside the formations. The act of opening up channels forcefully inside a formation is called fracturing.
If the fracturing uses pumped liquid to forcefully open the formation, it is called Hydraulic Fracturing.
To understand fracturing, one must understand that every oil-bearing matrix has its own overburden pressure. This overburden pressure is there simply because of the existence of sheer weight of the earth layers on top of the formations. This overburden pressure also has different pressure direction preferrence caused by the shape of oil bearing matrix (whether smoothly horizontal or have faults or leaps in it. Hydrauling Fracturign designers must know this pressure direction preferrence because they need to ensure that the fracture they will create goes into favorable direction.
The existence of gas and water layer inside the formations must also be known and its borders estimated. This will ensure that the fracture channels that will be created during hydraulic fracturing don't reach the gas or water layers . If they reach the gas layers or/and water layers, fluid separation systems need to be built on the surface to separate oil, gas, and water.
When designing a hydraulic fracturing job, service companies such as BJ or Halliburton will ask for the formations properties to calculate fluid usage and formulate steps to be used. The properties can be gained from rock cores obtained during the wells drillings. If there are no rock cores available for the well to be fractured, the adjacent wells cores will be used and the data for the object wells will be extrapolated from the available cores.
Once the data are studied and the calculations and formulas are completed, the job will be performed. Well completion string will be pulled out, and stimulation string (complete with isolating packers) will be installed downhole. This will be the conduit in where the fracturing liquid will flow.
The fracturing company will then perform a mini-frac, by pumping small amount of liquid into the formation. Data gathered from this mini-frac will be used to confirm the formulated steps and to make some adjustments based on real-time data.
Once adjustments are based on the real-time data drom mini-frac is finished, then the real hidraulic fracturing pumping will commence. Hydraulic liquid water rich in gels and polymers will pumped to the formation with rate reachin 20 barrel per minutes and pressure of 6000 PSIG. The rate will be increased from zero to highest necessary pressure to prevent unnecessary shock. This pressurized liquid will then create cracks, channels, and fractures inside the formation.
Once the hydraulic liquid water is done pumped downhole, another liquid is pumped down. The later liquid is rich in sand-like proppants that will fill up the fractures so that they won't collapse due to overburden pressure mentioned above and the integrity of the fractures can be maintained.
The final step in the fracturing would be to pump out the unused liquids out to clean out the well and then install the well completion production strings and then put the well online.
