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Step Rate Test Guidelines
A fracture step rate test is a method used to accurately measure the fracture pressure of a given formation. A fracture step rate test must demonstrate that formation fracturing will not occur at the proposed injection pressure. The test is conducted by injecting into the formation at a series of increasing rates, allowing each to stabilize, and noting the stabilized injection pressure for each rate. A plot of injection pressure vs. 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
To convert surface pressure to bottom hole pressure (BHP), you must know the inside diameter and condition of the tubing to compute frictional losses and the density of the frac fluid to compute the hydrostatic pressure. It is also recommended that a down-hole pressure bomb be used to measure BHP so that you will have two sources of data which can be compared when checking for errors.
The following procedure is recommended to ensure meaningful test results.
The test well should be shut in long enough so that the bottom-hole pressure is near the shut-in formation pressure (No less than 48 hours.). The well may need to be backflowed if the shut-in pressure is above the expected fracture pressure of 0.5 psi/ft. of depth.
Suggested rates for the test are 5, 10, 20, 40, 60, 80, and 100% of the proposed maximum daily injection volume and corresponding pressures. The first rate should be the formation matrix rate (the rate at which the formation begins to accept fluid). In addition, at least two rates must be below the expected fracture pressure of 0.5 psi/ft. of depth.
Each rate must be allowed to stabilize before proceeding to the next higher rate. A step duration of 60 minutes is recommended for formations with permeability less than 10 millidarcies, and 30 minutes for formations with permeability greater than 10 millidarcies.
EACH STEP SHOULD LAST EXACTLY AS LONG AS THE PRECEDING STEP.
Once the fracture pressure is reached, continuing the test by stepping downward will refine the fracture pressure and provide quality control for the test.
The use of a pressure recorder chart to document the testing process is recommended.