Step Rate Test (SRT) Guidelines
A Step Rate Test (SRT) is a method used to accurately measure the fracture pressure (or formation parting pressure) of a given geologic formation. The test is conducted by injecting fluid into the formation at a series of increasing rates, allowing each rate to stabilize, and noting the stabilized injection pressure for each rate. A plot of injection pressure versus injection rate is then made to identify the fracture pressure.
The basic equipment required is as follows:
- A water supply truck capable of pumping the water down hole under increasing pressures,
- Surface monitoring equipment capable of measuring injection rate and pressure, and
- A bottom-hole pressure memory gauge (the bottom-hole pressure gauge can be a memory type that does not require live surface communication).
Surface to Bottom-hole Conversion
To convert surface pressure to bottom-hole pressure (BHP), the inside diameter and condition of the tubing must be known to compute frictional losses, and the density of the injected fluid must be known to compute the hydrostatic pressure. A bottom-hole pressure gauge should be used to measure BHP to provide two sources of pressure data for comparison.
The following SRT procedure is recommended to ensure meaningful test results.
- The test well should be shut-in long enough, but not less than 48 hours so that the BHP is near the shut-in formation pressure. The well may need to be backflowed if the shut-in pressure is above the expected fracture pressure.
- The SRT should start at a suitably low rate such that at least two injection rate steps are below the formation fracture pressure and at least three rates are above the formation fracture pressure. A plot of the injection rate versus surface pressure and BHP will facilitate the determination of the formation fracture pressure. The point at which the slope of the lower fracture rates intersects the slope of the higher fracture rates is the formation fracture pressure, which can be expressed as a fracture gradient (psi/ft.).
- Each injection rate step must be allowed to stabilize before proceeding to the next higher rate. Each step should have a duration of 60 minutes for formations with permeability of less than 10 millidarcies, and 30 minutes for formations with permeability of greater than 10 millidarcies. Each step should last exactly as long as the preceding step.
- Once enough data points (rates) have been obtained to provide a clear intersection of rates below versus rates above the fracture pressure the SRT can be terminated. Surface pressure and BHP measurements should be recorded at shut-in (instantaneous shut-in pressure, ISIP), after 5 minutes of shut-in, and after 10 minutes of shut-in.
- A pressure recorder chart and a bottom-hole pressure gauge should be used to document the testing process as recommended.
- All SRT documents must be prepared, signed and sealed by a Professional Engineer registered in Texas, according to The Texas Engineering Practice Act. The following SRT data should be submitted to UIC in Austin for review:
- The formation fracture pressure,
- Fracture gradient analysis and determination,
- A plot of injection rate versus pressure, and
- A plot and table of the SRT data (injection rate and pressure versus time).