Procedures for leak-off and limit tests. Leak-off and Limit tests are carried out during the drilling phase of the well. The BOP is closed around the drillpipe, and the well is slowly pressured up, using mud. At the first sign of fluid leak-off into the formation the pumping is stopped. Leak-off tests are carried out until leak-off is observed; limit tests are carried out until a pre-determined test pressure is reached.

1 Introduction to leak-off test procedure

Leak-off and Limit tests are carried out to:

• confirm the strength of the cement bond around the casing shoe and to ensure that no flow path is established to formations above the casing shoe or to the previous annulus;
• investigate the capability of the wellbore to withstand additional pressure below the casing shoe (to handle an influx, and to allow safe drilling depth of the next hole section;
• collect regional data on formation strength for the optimisation of future well design.

These tests are sometimes called: casing seat, formation intake, formation strength or formation integrity tests.

Proper planning, execution, interpretation and reporting of these tests is essential for well safety and in order to gain maximum benefit from the experiment.

2 Leak-off test procedure

2.1 Planning the test

• Estimate the surface leak-off pressure :
• Calculate surface limit pressure. For a Limit test the Limit Gradient (LG) may be given in the well program. For a Leak-off test it is recommended to limit the test pressures to a maximum of the over burden gradient or to another realistic limit. This is done to reduce the chance of untoward formation breakdown.
• Confirm the accuracy of the pressure gauges that will be used for the experiment. The absolute accuracy of the gauges should be 0.5% of the expected downhole test pressure. The resolution (relative accuracy) of the gauges should be 2% of the expected surface test pressure. Calibrate the mud balance to confirm its accuracy (±0.5% for a pressurised mudbalance). Usually the pressure is measured and recorded at surface, but for high mud weights the application of downhole gauges with surface read-out should be considered.
• The pressure exerted during a Limit or Leak-off test should never exceed the maximum burst pressure of the casing (using the recommended design factor (DFburst) for casing burst) and the associated surface equipment. To calculate the pressure at the outside of the casing, assume a fluid gradient equal to the mud gradient that the casing was run into. Add the test pressure to the static mud column on the inside, to calculate the pressure at the inside of the casing.
• Estimate the volume of mud to be pumped, and determine the increment volume. To be able to establish a clear trend in the test results, a minimum of about 8 equal increments should be pumped before the (expected) test pressure is reached.
• Complete the pre-test part of the test report. Prepare a large scale graph (e.g. A3) to plot the results during the test. Draw the expected volume´pressure line and the surface limit pressure and the casing burst pressure in the same plot.

Typical values for mud compressibility are given below. Observed values may be higher due to the additional expansion of the casing and lines. The combined compressibility of well and mud can be calculated with the results of a previous leak-off test or casing pressure test after the cementation. If the actual volume´pressure relationship during the test is radically different from the plan, this might indicate that the pump unit is not lined up properly, the BOP stack not properly closed, a leak in the surface lines or a very porous formation.

Fluid Compressibility (1/kPa)         /       (1/psi)

water, and WBM      0.45x10-60    /       31x10-5

base oil                   0.69x10-60    /       47x10-5

OBM                      0.80x10-60     /      55x10-5

2.2 Execution leak-off test procedure

The following Leak-off test procedure should be applied:

1: Drill out cement plus ca. 20 ft (6 m) of new formation. (Consider to repeat the experiment when the first sand is encountered, or when a weak formation is penetrated.)

2: Circulate and condition mud.

• while circulating the hole clean of cuttings, circulate/ treat the mud to achieve as low as possible gel strength; (excessive gels may reduce the pressure transmitted down the well to the formation). Consider pulling the bit into the casing shoe to prevent wash out.
• accurately measure the mud weight with a recently calibrated pressurised mud balance;
• confirm that mud weight-in is equal to mud weight-out;
• do not change the mud weight until after the test.

3: Pull the bit back into the casing shoe. If high leak-off pressures are expected consider the use of a downhole packer to isolate the cement sheath to prevent micro-annuli development during the test.

4: Make sure the hole is filled up and close the BOP around the drillpipe. Where practicable, open and top up the annulus between the last and previous casing string, and check for returns during the test.

5: Use a high pressure, low volume pump (usually the cement unit; rig pumps are unsuitable). Line up to establish a clear flow path from the pump to the open hole annulus. Consider the risk associated with testing through a mudmotor or a non return valve.

6: Line up calibrated pressure gauges, covering various pressure ranges and preferably mounted on a special manifold. The standard gauges on the drilling console or the cement unit are not accurate enough for these measurements. Usually the pressure is measured and recorded at surface, but for high mud weights the application of downhole gauges with surface read-out should be considered.

7: Pump mud slowly (less than  ½ BPM, 80 l/min.) until the pressure builds up. Record and plot the volume pumped against pressure.

8: Pump a small increment of mud, and wait for 2 minutes or the time required for the pressure to stabilise in case this takes longer.

9: Record the cumulative volume pumped, the initial static pressure and the final static pressure after the waiting period. The initial static pressure is the pressure immediately after pumping has stopped and the transients have died out.

10: Repeat items 8 and 9 and plot both pressure values against cumulative mud volume for each increment until leak-off is observed, or until the predetermined limit pressure has been reached.

• Keep the waiting period and the volume increments constant.
• Leak-off is defined as the first point on the volume/pressure plot where either the initial static pressure or the final static pressure deviates from the trend observed in the previous observations.
• If the pump pressure suddenly drops, stop pumping but keep the well closed in. This indicates a leak in the system, cement failure or formation breakdown. Record the pressures every minute until they stabilise. If the drop in pressure is related to formation breakdown, this data can be used to derive the minimum in-situ stress.

11: Keep the well closed in to verify that a constant pressure has been obtained. Record and plot the closed-in pressure every minute. If the pressure does not stabilise, this may be an indication of a system leak or a bad cement bond.

12: Bleed off the pressure and establish the volume of mud lost to the formation.

13: Top up and close the annulus between the casing and the previous casing string.

2.3 Interpretation of the leak-off graph (leak-off test procedure)

Results should be plotted and interpreted on the large scale volume versus pressure plot during the test. "Leak-off" is a downhole event, indicated in the leak-off graph by the first deviation from the trend of either the initial static pressure or the final static pressure. In many cases it can only be identified positively if two points on the curve deviate from the trend. The " surface leak-off pressure" is the (interpolated) value of the initial static pressure at the first indication of leak-off.

In a successful limit test no leak-off is observed when the initial static pressure reaches the "surface limit pressure". It is confirmed that the borehole is strong enough to hold this additional pressure without formation breakdown.

For a hard impermeable formation (e.g. a shale) the pressure increases linearly with volume. Leak-off is simple to identify.

For a permeable formation with an ineffective mudcake, the mud leaks away slowly, the graph is slightly curved, and the final static pressure curves away from the initial static pressure. Leak-off should now be identified either by the change in curvature, or from the increasing difference between initial static and final static pressures.

For non-consolidated, plastic, loose or highly permeable formations where even low test pressures cause loss of mud, the exact determination of the leak-off point is difficult. The initial static pressure will always be considerably higher than the final static pressure, and the graph will be curved considerably. Leak-off can only be established approximately from the leak-off graph. (In many cases the information that no breakdown is observed will suffice, since it is obvious that the formation is weak and the main purpose of the test is to establish the absence of communication around the casing.)

2.4 Formation breakdown, fracture re-opening (leak-off test procedure)

Formation breakdown during a Limit or Leak-off test should be prevented, because a fracture may permanently impair the capability of the wellbore to withstand pressure. However, if breakdown occurs, it should be treated as an opportunity to derive real formation strength parameters. "Formation breakdown" is indicated by a sharp pressure drop on surface. The highest pressure recorded immediately before the pressure drop, is the "surface breakdown pressure".

If formation breakdown occurs, pumping should be stopped, but the well should be kept closed-in, and the pressure decay curve should be recorded. "Fracture closure" is indicated by the stabilisation of the pressure decay curve to a constant pressure value. The FCP can be determined from the "surface fracture closure pressure". The results may be used to determine the in-situ stress, which may be very useful for future operations.

To confirm these observations, the test may be continued with a fracture re-opening cycle. After the pressure is released, and the well is flowed back, the well is pressured up in steps. When the fracture re-opens, the pressure volume graph deviates from the trend (similar to leak-off), and the "surface fracture re-opening pressure" can be determined. After re-opening, the well is shut-in and the FCP is again determined from the pressure decline. Theoretically the FRP and the FCP are equal, but differences may occur.

If the first and second FCP and the FRP are not consistent enough another cycle should be considered.

2.5 Reporting (leak-off test procedure)

Formation strength tests should be reported in a consistent manner.

Actual measurements of pressures and volumes and an interpretation of the results should be reported. An accurate graph on a large scale of volume pumped versus surface pressure should be included in this report. It should be indicated whether leak-off or formation breakdown was observed.

If the measurement relates to a formation some distance below the casing shoe, the conversion may be slightly inaccurate. In practice this difference is not taken into account, except when a leak-off test is repeated at a different depth.

2.6 Repeating a test (leak-off test procedure)

A leak-off or limit test may be repeated some distance below the previous measurement. This may be done to confirm that the strength of a new formation still satisfies the requirements for safe drilling, or to gain some additional formation strength data.

It is recommended not to exceed the previous downhole test pressures unless there are reasons to assume that the formation strength has increased (for example after a change in mud system).

If leak-off or formation breakdown is observed during a subsequent test, it is difficult to identify the formation and the exact depth that the measurement relates to. The test can be used to define a "safe" area on a depth pressure plot. There is no clear cut method to generate such a chart. Common sense should be used to interpret the measurements to determine the safe drilling envelop in which no formation breakdown will occur.