The use of liquid CO2 as a fracturing fluid has proven to be an effective stimulation for the increased production of gas in Canada and the United States. The process involves the injection of 100% liquid CO, as a proppant carrier to create a fracture and form a highly conductive channel from the wellbore into the reservoir.

The process has recently been optimized with the addition of nitrogen gas. Overall treatment costs were reduced by volumetrically replacing the more expensive liquid CO2 with gaseous nitrogen. Additional cost savings were realized with decreased power requirements due to lower liquid pumping rates and reduced friction pressures.

The results of more than seventy treatments utilizing N2/CO2 fracturing operations have been used to evaluate the limitations of this new development. The use of downhole pressure and temperature instrumentations has aided in the development of a computer simulator which evaluates the economic benefits of adding nitrogen to 100% liquid CO2. This paper will describe the computer simulator and discuss the variables in optimizing the new commingled N2/CO2 fracturing stimulation.

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