Extended Leakoff Testing

Extended leakoff testing ideally differs from conventional leakoff testing in the following ways:

• At least two and preferably three injection-shut-in-leakoff cycles are performed.
• It is desirable to use electronic pressure gauges to measure surface and ideally bottomhole pressure. Five second data collection (or more frequent) has been advocated.
• Shut-in is monitored for a period of time longer than 10 minutes. Some people say 30 minutes. There is a degree of subjectivity here. It may be possible to reduce this, depending on what you observe during the first injection cycle.
• If bottomhole pressure is inferred, fluid densities are measured accurately. Corrections are applied for oil-based fluids. The operations can include circulating until the density is constant and measuring the drilling fluid density with a pressurized mud balance (API RP 13B-2).
• Recorded data are processed using fundamental reservoir engineering concepts to determine when the created fracture has closed and to consequently infer the minimum stress.

Procedures

Kunze and Steiger recommended the following.

1. Drill out cement and 10 feet of new formation.
2. Rig up surface transducer to choke manifold for annular measurements.
3. Circulate drilling fluid at least long enough to receive bottoms up and check properties in and out. Final density measurements are made in triplicate with a pressurized mud balance.
4. Pull drill bit ten feet into casing and hang off drill string.
5. If a wireline downhole gauge is used, rig up a pump-in sub, wireline BOP, and pressure lubricator with the downhole gage assembly. The required well control equipment will vary and appropriate regulations and safety precautions should be followed. Connect surface transducer to pump-in sub for drillpipe pressure measurement.
6. Rig up cement pumper.
7. Run downhole gauge to top of bit or baffle plate on wireline. Pull up 25 feet. Hang off.
8. Shut-in annular BOP.
9. Pump at 0.25 bpm (0.04 m³/minute) or lower - keep the rate constant. Pump until the pressure rise shows a definite change in the rate of increase. Pump an additional 0.25 bbl.
10. Shut-in pumps and monitor the pressure decrease for 30 minutes.
11. Bleed off pressure. Record the volume of fluid returned.
12. Repeat steps 9 through 11 for cycles 2 and 3.

What Can be Learned from Extended Leakoff Testing?

If the quality of the test is acceptable, inferences can be made of the maximum and minimum principal stresses in the drilled out zone below the shoe. There may also be some possibilities for determining modulus and permeability but these are not routine procedures.

Interpretation

Quantitative stress measurement is quite feasible with extended leakoff testing. However, it is commonly done incorrectly or important information is missed. Interpretation of all small volume hydraulic fracturing stress measurement data is covered in the next section on Microhydraulic Fracturing.

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