Paper: Recent Developments In Gas Frac

Use of liquefied gases as fracture fluids has been a commercially available service for more than a year and both results and experience have helped dictate new developments and define candidate reservoirs. Designed originally for gas well stimulation, the new service uses a fluid made up of liquefied CO2, LPG and methyl alcohol. The liquefied gases are mixed in such a proportion that they remain a liquid and behave as a liquid as long as they are under adequate pressure and below the critical temperature of the mixture. When they are heated above their critical temperature in the reservoir and pressure released, the liquid reverts to a gas. This results in extremely rapid clean-up and minimizes problems of water sensitivity and liquid retention damage. In practice, the liquefied gases are mixed in varying proportions depending on reservoir temperature. For each well, the proportion is designed to provide a mixture that will have a critical temperature about 15% below the reservoir temperature. Since the mixture is not an effective frac fluid in itself, a gelling agent in methyl alcohol is added as a third component to provide the desired properties of viscosity, low leak-off or fluid loss control, low friction pressures, and good prop-carrying ability. In a stimulation treatment, the liquefied gases are mixed in proper proportion on location and pumped to the wellhead. The gelled alcohol and prop are mixed using a special pressurized blender and pumped to the wellhead where they mix with the liquefied gas mixture to produce a uniform prop-laden frac fluid. Not only is it necessary to design the liquefied gas mixture to have the proper critical temperature for each well, but it is also necessary to design the methyl alcohol gel to provide the desired frac fluid properties to the final fluid for each well. Properties of the final fluid are determined from conventional fracture design calculations. (Although addition of the gelled alcohol to the liquefied gases increases the critical temperature of the mixture, laboratory data indicate that this increase is more than offset by designing the liquefied gas mixture to have a critical temperature 15% below reservoir temperature. In low-temperature wells, design is based on vapor pressure of the liquefied gas mixture as compared to reservoir pressure.)