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Proppant is porous material such as sand or ceramic beads that are used to prevent newly created fissures and fractures in the shale rock from closing up once it has been hydro-fractured.

A typical hydro-fractured well uses between 300,000 and 500,000 lbs. of proppant.

The objective of hydro-fracturing is to enhance the deliverability of trapped gas by making pathways for the flow of natural gas and other hydrocarbons from the shale reservoir to the wellbore. Two chief factors that influence the flow of gas are permeability and proppant.

Stokes' law can be used to define four variables that affect proppant settling velocity in a column of water:

  1. fluid specific gravity
  2. fluid viscosity
  3. proppant size
  4. proppant specific gravity

The cost of hydro-fracturing can be minimized by by reducing frac fluid viscosity. According to Stokes law, reducing the particle (proppant) size in half cuts the settling rate by a factor of four. However, particle size is also proportional to the conductivity of a proppant pack. Hence, in designing a fracing plan these factors must be weighed against each other in order to optimize the flow of gas from the shale reservoir.

Although naturally occurring sand is frequently utilized as proppant, specially engineered man-made proppants can be used too such as resin-coated sand or high-strength ceramic materials like sintered bauxite. Materials are carefully selected for size and sphericity to provide the most efficient conduit for production of gas and other hydrocarbons from reservoir to wellbore.

There are three main types of proppant that are in use in hydro-fracturing. Listed in order of their unit cost, these include:

  • sand
  • sand coated with resin
  • ceramic proppant

The higher initial cost of ceramic proppant over sand may be justified by higher returns on investment in terms of greater well production rates and total overall recovery of oil and gas from the well. Higher production rates result from the greater strength of ceramic proppant and its more uniform shape and size.

Production engineers use fracture design models as a guide to optimizing fracturing by comparing treatment size versus fracture half-length. The purpose is to design a fracture stimulation plan that optimizes productivity. The lower the permeability of a reservoir the more fracture length determines the effectiveness of the stimulation. However, unless the fractures can be sustained unpropped, that is, unless the fracture length or height created by hydro-fracturing has residual conductivity without propping, it is a waste of fluid. That can reduce the return on investment of hydro-fracturing a well or even turn it into a loss situation.

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