Liquid CO2, fracturing is a patented process that was introduced in 1981. This fracturing process has undergone many technical and operational changes making this technique more effective. Computer simulation models have been developed and correlated to post stimulation results with a high degree of accuracy.
Industry advances in density measurements have been incorporated in the Liquid CO22 blender that ensures quality control. The original Liquid CO2, blender design has been modified to allow higher concentrations of proppant to be added to the Liquid CO2, stream. Improvements have been made to the delivery method of Liquid CO2 that makes it more consistent and safer.
These improvements have optimized operations on the usual range of well candidates, as well as opened new doors to new applications for this technology. Examples included are high permeability gas zones and coal bed methane reservoirs.
Continuous research and development has been performed to improve the quality, cleanliness and performance of fracturing fluids since the first hydraulic fracturing treatment in 19471. The ultimate goal of this research and development was to reduce the negative effects the fluids had on the formation being treated. Previous papers2,3 show that low permeability gas formations can suffer production problems due to treatment fluid invasion. Fluids pumped into formations can change formation saturation properties, especially in dry gas wells. The resulting change in relative permeabilities, capillary pressures and possible aqueous phase trapping can lead to lower production rates. Conventional fracturing fluids include; water, crude and refined oils, solvents, methanol, and combinations of these fluids. The variety of treatment fluids reflects the diversity of formations and the need to choose a treatment fluid that will produce an economical stimulation.
The use of chemically gelled or crosslinked fluids to carry proppant and control leakoff in the fracture is a common practice. This can lead to residual chemicals and gel precipitates left in the fracture proppant pack4,5 that can impair production.
Treatment designs utilizing combinations of fluids with nitrogen or carbon dioxide and even binary foams where nitrogen and carbon dioxide are used, reduce this problem. The addition of nitrogen and/or carbon dioxide provides a non combustible gas that aids in the recovery of the treatment fluids. These gasified fluids also reduce the amount of potentially damaging fluid pumped into the formation.
Gaseous nitrogen is used to create a foamed fluid with excellent leakoff characteristics. The reduced hydrostatic pressure of nitrified fluids limits the depths at which these treatments can be performed. Carbon dioxide is pumped in the liquid phase. When CO2 is commingled with treatment fluids it will prevent the reduction of hydrostatic pressure.
The addition of either carbon dioxide or nitrogen with a fluid is done downstream of the fluid blender. This limits the effective downhole slurry concentration since proppant is only added at the fluid blender. The limits of the effective downhole slurry concentration are dependent on the concentration of the carbon dioxide and/or nitrogen used.
Canadian Fracmaster patented the process and equipment in 1982 that allowed proppant addition to 100% Liquid CO2, (Canadian Patent # 1134258, U.S. Patent # 4374545, British Patent # 8225692).
