Proppant transport is a key function of the fracturing fluid. Its purpose is to carry proppant into the fracture and place it deep within the fracture network to maintain conductivity over the life of the well.
In unconventional reservoirs, it is not sufficient to simply create fractures. Proppant must be effectively transported and properly distributed to ensure that fractures remain conductive.
Proppant Transport Mechanisms
- Settling and Convection
Proppant settling is a gravity-driven process in which particles move downward in the fluid. The settling behavior depends on:
- Density difference between proppant and fluid
- Particle size
- Fluid viscosity
Larger particles settle faster, while higher fluid viscosity reduces settling. In addition to settling, proppant convection occurs when higher-concentration slurry displaces lower-concentration slurry.
- Transport in High-Viscosity Fluids
In high-viscosity fluids:
- Proppant remains suspended
- Settling is minimized
- Proppant moves with the fluid
These fluids are often assumed to provide near-perfect proppant transport.
- Transport in Low-Viscosity Fluids
In low-viscosity fluids:
- Proppant settles quickly
- A proppant dune forms at the bottom of the fracture
- Flow velocity increases above the dune
The dune moves away from the wellbore during pumping, allowing later proppant to travel further into the fracture.
- Sheet Flow and Channeling
Proppant may accumulate in the center of the fracture, forming a sheet due to higher velocity in that area. Flow might also follow paths of least resistance, enabling deeper proppant penetration.
Proppant Concentration
Proppant concentration is gradually increased during the treatment to achieve effective placement and desired fracture conductivity. At the beginning of the slurry stage, the concentration is kept low to facilitate transport and avoid early settling. As pumping continues, the concentration increases progressively until it reaches a maximum value near the end of the treatment. The distribution of proppant within the fracture depends on fluid efficiency and fluid loss. Only the proppant that remains suspended in the fluid contributes to transport, while fluid leakoff reduces the effective concentration inside the fracture. Proppant concentration is often related to fluid efficiency, and this relationship is used to design the concentration schedule during the treatment.
