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Injection Mechanism - High permeability and low permeability geology

BACKGROUND
When injecting fluids (gas, liquids or solids) into different geologic formations, the flow of the injected material within the geologic media varies greatly.

In Low Permeability materials such as silt, clay and fractured rock, discrete fracture flow usually results. During the injection event itself, examination of the pressure-time history curves can provide evidence of how the fluid propagation is occurring within the geologic formation. The pressure-time history curves provide both the fracture initiation pressure (the initial spike in pressure) and the maintenance pressure (subsequent pressure required to propagate the fractures).

 

Fluid propagation distances in low permeability geology are in the range of 20 to 40 feet for unconsolidated geologic materials and 35 to 60 feet in consolidated geologic materials. The resulting effect of fluid injection within this type of material results in the creation of a dense fracture network. When injecting a liquid or solid material, the created fracture network becomes the preferred pathway for placement of the injected material.

High Permeability materials or coarser grained geology such as sands and gravels respond to injections differently than low permeability materials. A mixing action often takes place instead of true fracture propagation. The large volume of gas injected into the formation results in fluidization of the geologic matrix. As the applied pressure equals or exceeds the overburden and hydrostatic pressures, soil particles are mobilized and intermixed much akin to the “numbered ping-pong balls in a lottery machine”. This phenomena can lead to a homogenizing effect within the vicinity of injection points. When injecting into this type of media, the characteristics of the pressure-time history curve is quite different from that of the low permeability material.

 

Usually, no distinct fracture initiation pressure “peak” is identifiable. Fluid propagation distances for this type of formation typically range from 15 to 25 feet. As a result, an injected liquid or solid becomes intermixed with the soil matrix.

 

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