The process is typically divided into 2 stages, the pad and the slurry. In the first stage the slurry is injected into the well to break down the formation and thus create the pad. At this stage the fluid injection rate is greater than the rate at which the fluid can escape the formation.
The slurry stage comes after the pad gets to the right size. During this stage the fluid is mixed with proppant topside and the mixture injected into the pad and the subsequent fractures. Eventually the pumps are shut down, the proppant within the fluid is now left behind within the fractures and aids in propping them open. The proppant is able to withstand the pressures within the earth thus keeping the fractures open and allowing the fluid flow.
For their to be fracture propagation we must exceed the breakdown pressure of the formation. We must consider the in-situ stress, in the vertical direction this is the weight of the overburden material. We must also consider the stress between the grains by contact. Now the formation is a 3 dimensional arena, thus stress does not only exist vertically but horizontally as well.
This is where we have the fracture geometry, their are several models, that aim to mathematically describe the way in which the fractures propagate within the formation. There are 3 models agreed upon,
- Radial Fracture Model
- The KGD Model
- The PKN Model
There are also 3 dimensional and pseudo-3D models, that were developed by various third parties, a lot of them oil producers/corporations and service companies. It is important to note that their are many variables and inputs that go into these models, and for the most part they require many assumptions, their is so much about the the various characteristics of the fracture that is not known. This results in some parameters being approximated or assumed.
