With proper engineering, liquid nitrogen can be used safely as a hydraulic fracturing fluid. The fluid's extremely cold temperature (-300 to -320°F) will induce thermal tensile stresses in the fracture face. These stresses exceed the tensile strength of the Devonian shale, causing the fracture face to fragment. Because of the extreme temperatures, treated water can be used as a diverter between fracturing stages. This water freezes instantly when it contacts the first treatment zone. This paper discusses the successful application of this technology to stimulate the Devonian shale in Eastern Kentucky.
Liquid nitrogen has been applied in coal seams and sandstone reservoirs for the stimulation of gas production in the San Juan Basin of New Mexico. For nitrogen to be pumped safely into a well, the entire surface manifold and wellhead must be made of stainless steel. In some cases, operators may use free-hanging fiberglass tubing to protect the casing from extremely cold temperatures.
Using liquid nitrogen as a fracturing fluid is a process that is in an early stage of development. Theoretically, self-propping fractures can be created by the thermal shock of an extremely cold liquid contacting a warm formation. As the fluid warms to reservoir temperature, its expansion from a gas to a liquid results in an approximate eightfold flow-rate increase. For example, a12-bbl/min liquid nitrogen flow can result in a flow rate as great as 96bbl/min some distance from the fracture.
The Devonian shale, a thick formation beneath a large portion of Eastern Kentucky and West Virginia, has been described previously. This Devonian shale has a low permeability, is somewhat water-sensitive, and typically produces through natural fractures. Operators have used a number of different techniques to stimulate wells in the Devonian shale. However, stimulating these wells with a nitrogen-gas fracturing treatment is currently a common method. Fracturing this type of formation with cold liquid nitrogen rather than warm gas could prop fractures open more efficiently.