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 CO2 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.
Over 1,100 liquid CO2 fracturing treatments have been performed in Canada, United States and Europe since the introduction of this process in 1981. The advantages of this fracturing process include:
the fracturing fluid is nondamaging to the formations, fracture faces and proppant packs;
rapid, complete cleanup and quick return to production even in low permeability, water-sensitive and under-pressured zones;
environmentally friendly with no fluid disposal problems.
Recently, the addition of gaseous nitrogen to 100% liquid CO2 has been proven to be an effective method in reducing the overall treatment expenses. An equipment layout for a N2/CO2 treatment is presented in Figure 1. Based on field operations, the fluid characteristics of the N2/CO2 mixtures are similar to 100% liquid CO2. The postfracture production of the two stimulation methods does not vary significantly.
Factors that contribute to the lower costs of the N2/CO2 treatments are lower friction pressure of the fluid; lower costs on equipment and the fact that nitrogen gas is less expensive than carbon dioxide on a volumetric basis. The nitrogen pumpers carry their own nitrogen gas, but CO2 pumpers are conventional frac pumpers and do not carry liquid CO2, the liquid CO2 must be loaded into a CO2 storage unit prior to the treatment. When liquid CO2 is replaced by nitrogen, less liquid CO2 products are needed for the treatment and the time required to transport the liquid CO2 is reduced. The substitution of N2 for liquid CO2 in the system makes it logistically easier to perform multiple treatments per day and increase potential cost efficiencies.
In addition to cost savings, other advantages of the N2/CO2 process include faster clean up due to the immediate flow back of nitrogen gas without accounting for the vaporization stage and possibly lower fluid leakoff rate to formation.
Nitrogen gas is highly compressible, therefore its volume and density will vary depending on pressure and temperature. The volume, temperature and pressure relationships can be calculated. However, the calculations are very tedious.