FORMATION INTEGRITY TESTS

To determine the fracture pressure of a formation, a leak-off test (LOT) or a formation integrity test (FIT) may be performed with a solids carrying fluid or mud. Where solids free workover fluids are used, a formation integrity test cannot be conducted and in these cases the formation is protected solely by a MAASP which is set at a safe percentage of the original casing pressure rating; Refer to Section 6.5.

LOTs and FITs determine if the cement seal between the casing and the formation is adequate and the maximum pressure or fluid weight that the formation(s) can withstand without fracturing. As the leak-off test actually causes a fracture to determine the fracture gradient, it is rarely used in well servicing operations and the FIT is adopted.

Whichever is to be performed, it must be ensured that the well is fully circulated to the correct weight workover fluid and the pump deliverability is sufficient.

Leak-Off Test

The test is performed by applying an incremental pressure from the surface to the closed wellbore/casing system until it can be seen that fluid is being injected into the formation.

Leak-off tests should normally be taken to this leak-off pressure unless it exceeds the pressure to which the casing was tested.

A typical procedure is as follows:

• Before starting, gauges should be checked for accuracy. The upper pressure limit should be determined.

• The casing should be pressure tested before well operations commence.

• Circulate and condition the mud, check mud density in and out.

• Close BOPs.

• With the well closed in, the pump is used to pump a small volume at a time into the well typically a 1/4 or 1/2 bbl per min. Monitor the pressure build up and accurately record the volume of mud pumped. Plot pressure versus volume of mud pumped.

• Stop the pump when any deviation from linearity is noticed between pump pressure and volume pumped.

• Bleed off the pressure and establish the amounts of mud, if any, lost to the formation.

Examples of leak-off test plot interpretation:

In non-consolidated or highly permeable formations fluid can be lost at very low pressures. In this case the pressure will fall once the pump has been stopped and a plot such as that shown in Figure 6.6a will be obtained. Figure 6.6b and Figure 6.6c show typical plots for consolidated permeable and consolidated impermeable formations respectively.

Figure 6.6 - Idealised Leak-Off Test Curves

Pressure

Cumulative Volume a) Unconsolidated Formations

Pressure

Cumulative Volume

b) Consolidated Permeable Formations

Pressure

Cumulative Volume

c) Consolidated Impermeable Formations

Final Pumping Pressure After Volume Increment

Final Static PressureAfter Each Volume Increment Leak-Off Point

Formation Integrity Test

An FIT can be performed when it is not acceptable to fracture a formation. In a FIT, fluid is pumped into the shut in well until a predetermined pressure is reached that is determined to be below the pressure to break down the formation. This value used is usually obtained by assessing information from well’s completion report and nearby well data.

The procedure is:

1. Before starting, gauges should be checked for accuracy.

2. The casing should be pressure tested before well operations commence.

3. Circulate and condition the mud, check mud density in and out.

4. Close BOPs.

5. With the well closed in, the pump is used to incrementally raise the pressure in the well to the test pressure and monitor the pressure to ensure that there is no leak off.

6.5 MAXIMUM ALLOWABLE ANNULUS SURFACE PRESSURE - MAASP

With data from the formation integrity test, the maximum pressure which can be applied without fracturing the formation and the maximum fluid weight can be determined.

The formation breakdown pressure

= Applied surface pressure + hydrostatic pressure of fluid in the casing The applied surface pressure at which leak-off occurred or at FIT pressure, is the maximum allowable annulus surface pressure with the fluid weight in use at that time. MAASP is the maximum surface pressure that can be tolerated before reaching the formation fractures.

MAASP = Formation breakdown pressure - HP of fluid in use at the formation or re-written as:

MAASP = (Fracture gradient - Fluid gradient) x TVD of formation or as:

MAASP = (Max. equivalent fluid weight - Fluid weight in well) x

(0.052 x TVD of formation).

MAASP is only valid if the well is full of the original fluid during the LOT or FIT; if the fluid weight in the well is changed, MAASP must be recalculated.

The calculated MAASP is no longer valid if influx fluids enter into the well.

However, in practise MAASP is calculated as a percentage of the original casing burst pressure rating. This percentage is derived from experience and the age of the well casings, i.e. if the well is old and it is suspected there is casing corrosion or wear, the percentage will be lower than that of a more recently developed well. In general, the pressure rating is 80% of original burst. This pressure is used in the equation in place of the formation breakdown pressure.

6.6 CIRCULATING PRESSURE LOSSES

Friction is resistance to movement. A force is required to overcome friction of a body or substance from a position of rest to movement. The amount of friction to overcome this resistance is dependent upon a number of factors:

• Density of the body or substance.

• Type of substance.

• Roughness of the surfaces making contact.

• Surface area in contact.

• Thermal and electrical properties.

• Direction of movement.

• Velocity.

• The force required to overcome friction is termed frictional loss.

SECTION 7

7. PRODUCTION WELL KILL PROCEDURES