Summary

Conventional drilling leakoff testing are performed below the shoe. Commonly, this is in shale. Generic procedures can be as follows:

• After the cement sets, drill ahead for 10 to 20 feet of new interval.
• Pump drilling fluid at between 0.25 and 1 BPM (0.04 to 0.16 m³/minute.
• Continue pumping until the rate of pressure increase slows, possibly indicating formation breakdown. Figure 1 is an idealized schematic.



Figure 1.   An idealized schematic of the variation of pressure with volume/time during a conventional leakoff test.

• After breakdown, the well is shut-in and the pressure is monitored for ten minutes or so. Some people think that this may correlate to a fracture with a dimension of about 25 feet in diameter.

Drawbacks

1. Most of these tests incorporate only one cycle of injection and shut-in. To evaluate stresses from elastic theory it is necessary to know if there is any significant tensile strength. If there is more than one injection cycle, the difference between the peak pressures has been used to estimate the tensile strength.
2. Data recording is usually sparse. Commonly pressures are recorded every one-half barrel or every minute. This is commonly insufficient for identifying subtle changes in the pressure that can be diagnostic of breakdown or fracture closure.
3. Ten minute monitoring of pressure decline after shut-in is usually insufficient to allow the created fracture to close and to infer stress conditions.
4. If surface pressure alone is measured, accurate drilling fluid density is required. There can be difficulties with entrained air and with compressibility (particularly for oil-based muds.

What Can be Learned from 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.

What are Improvements That Can be Made?