Surface Pressure
Surface pressure is a key parameter in hydraulic fracturing because it directly determines the required pumping power at the surface. It represents the pressure that must be applied at the wellhead to overcome subsurface stresses and all pressure losses along the wellbore.
Surface pressure can be estimated as:
The minimum horizontal stress controls fracture propagation, while the net pressure represents the additional pressure required to open and maintain the fracture. Net pressure depends on fracture geometry, rock mechanical properties (such as Young’s modulus), injection rate, and fluid viscosity, and is typically on the order of ~1000 psi.
The hydrostatic pressure reduces the required surface pressure and depends on fluid density and true vertical depth. In contrast, frictional losses increase surface pressure. Pipe friction pressure depends on tubing diameter, fluid viscosity, flow rate, and whether the flow is laminar or turbulent. Perforation friction depends on perforation diameter, number of perforations, and fluid properties.
In addition to propagation pressure, the breakdown pressure must be considered at the start of the treatment. This pressure is often higher than the minimum horizontal stress due to fracture initiation effects and near-wellbore stress concentrations.
Pump power is directly related to both pressure and rate:
This means that higher injection rates or higher surface pressures significantly increase horsepower requirements.
From an operational standpoint, pressure can be reduced by increasing the tubing diameter or optimizing the perforation design to minimize friction losses. However, pumping down the casing rather than the tubing, while reducing friction, can introduce risks such as cement damage, which must be carefully considered in the design.
Example:
Let’s assume:
- Fracture gradient = 0.85 psi/ft
- Perforations at 8,500 ft TVD (True Vertical Depth)
- Fluid density = 10 ppg
- Rate = 50 bpm
Step 1: Minimum Horizontal Stress
Step 2: Hydrostatic Pressure
Step 3: Surface Pressure
Step 4: Horsepower
