Technical Discussion of Injection Pressure Issues

Statewide Rules 9 and 46 both require that injected fluids remain confined to the authorized injection/disposal interval. The technical review of the proposed injection pressure verifies that the injection pressure does not exceed the formation fracture gradient and that the existing casing cement is adequate to ensure that injected fluids are confined to the proposed injection zone.

If you are requesting an injection pressure higher than the 1/2 psi per foot of depth to the top of the injection/disposal interval, you will need to run a Fracture Step Rate Test (FSRT) to demonstrate that he requested injection pressure is below the fracture gradient for the injection zone.

The FRST involves shutting in the well to allow the reservoir pressure to to stabilize, then injection at a minimal rate to establish the minimum rate and pressure at which the reservoir will take fluid. After a set time interval, the injection rate is increased and maintaind for the set time inteval. This process is repeated to provide several pairs of injection pressure/rate data points. These data points are then plotted on graph paper to plot the linear relationship between injection pressure and rate. The linear relationship will end when the fracture pressure is reached.

The highest injection pressure justified by the FSRT will be the last data point set just below the fracture pressure. Note that this may be significantly lower than the fracture pressure depending on how the "steps" were chosen. One important option is to "step down" after reaching the fracture pressure to place data points between the fracture pressure and the data point just below it. For example, if the FSRT shows no fracture up to an injection pressure of 800 psi, but does show a fracture occuring at 1,000 psi, then 800 psi. is the highest pressure that was shown to be below the fracture pressure. Stepping back down to a pressure/rate data point around 900 psi. will further refine the fracture gradient, as well as serve as quality control for the FSRT.

Note that if the formation is overpressured, the well will have to be backflowed for a significant length of time in order to establish the linear relationship with at least two pressure/rate data points below the expected fracture gradient of 0.5 psi/ft. If this is not possible or feasible, then the injection permit will be limited to the 0.5 psi/ft. pressure limit.

Last Updated: 3/23/2016 2:20:09 PM