Fracturing treatment designs are implemented through a pump schedule, which defines how fluids and proppants are injected during the treatment. The design is guided by treatment objectives and constrained by reservoir conditions, operational limits, and equipment capabilities.
Design of the Pad Stage
The pump schedule begins with the pad stage, which consists of injecting fluid without proppant.
The purpose of the pad is to:
- Initiate the fracture
- Generate the desired fracture geometry
- Create sufficient fracture width for proppant placement
During injection, part of the pad fluid is lost to the formation due to fluid leakoff. This lost fluid does not contribute to fracture growth. Leakoff is typically highest at the fracture tip, where new rock is continuously exposed.
Depending on the magnitude of fluid loss, the fracture may grow differently than initially predicted.
Fluid Efficiency
Fluid efficiency represents how much of the injected fluid remains inside the fracture compared to the amount lost to the formation.
It depends on:
- Volume of fluid in the fracture
- Volume of fluid lost to the formation
- Injection time
- Fracture closure behavior
Fluid efficiency is commonly evaluated using dimensionless relationships that relate fluid loss to fracture closure time.
Pad Volume Design
The required pad volume is determined using the relationship between:
- Injection time
- Fluid efficiency
A higher fluid efficiency means more fluid remains in the fracture, reducing the required pad volume.
The pad volume fraction is defined as the ratio between:
- Pad fluid volume
- Total injected volume (pad + slurry)
This relationship is used to estimate the required pad volume for the treatment.
Important Considerations
- Spurt loss is not included in this analysis and can be significant in high-permeability formations.
- Fluid efficiency calculations may overestimate performance because they combine fracture growth and pressure decline behavior.
- Pressure-dependent leakoff and fracture closure effects can impact accuracy.
