Choosing the right method

The process of deciding which formation strength testing method to choose is characterised by two, usually conflicting considerations:

• the required accuracy and significance of the results;

• the requirement to avoid the risk of reduced formation strength, caused by formation breakdown.

In the design stage a trade-off has to be made between the risk of formation strength reduction and the need for realistic formation strength data. These two aspects are discussed below.

Accuracy of formation strength testing method

The different methods for formation strength testing are summarised in the table below. They are given in order of increasing accuracy and significance of the results. Laboratory tests and wireline testing methods are not mentioned here, because they only offer additional information or are not yet operationally available.

A successful Limit test only confirms that the Formation Breakdown Pressure of the formation below the casing shoe is higher than the limit pressure (LP). If the required capacity is confirmed, drilling can continue. However, little is actually learned about the formation itself, and therefore, this information cannot be transferred to other wells.

The occurrence of Leak-off is the first indication related to the strength of the borehole and the mudcake. It is traditionally used as an indication of impending formation breakdown, and the LOP has been often used as an estimate of FBP or minimum in-situ stress. However, the use of the Leak-off test as a method to characterise formation strength has the following drawbacks:

- Sometimes breakdown occurs without indications of Leak-off.

- Leak-off is dependent on parameters that are not related to formation strength (e.g. mud type, length (if the open hole section, whether any natural or drilling induced fractures are exposed, borehole condition).

- Leak-off testing relies on subjective interpretation of the engineer. Interpretation is difficult, especially in unconsolidated formations.

- Leak-off tests do not repeat well. Leak-off pressures tend to increase with time, especially in sandstone.

- Leak-off pressures have been shown [5] not to correlate closely with other, more significant formation strength parameters (for example FBP and FCP or minimum in-situ stress).

During a Formation Breakdown test, the FBP is determined, and the FCP can be estimated.

The FCP is equal to the minimum in-situ stress, which is to be preferred as a measure of formation strength. Its value is not dependent on the mud or the borehole orientation or geometry, and can be correlated regionally from well to well. Knowledge of the minimum in-situ stress also offers the possibility to predict FBP for nearby wells at different deviations (see Eq.

App. 3-6 to Eq. App. 3-9).

A micro-frac or mini-frac test allows the minimum in-situ stress to be derived with a higher degree of accuracy. Data from these tests can be used to derive regional models of in-situ stress and formation strength. Additional information about fracture propagation is obtained from these tests. This can be used to design well stimulation treatments.

TABLE OF TEST TYPES AND USES

Operational considerations

The main consideration that constrains the selection of the formation strength testing method is the risk that the reduction in formation strength of a fractured wellbore, jeopardises the success of the well.

The magnitude of the reduction in strength after formation breakdown is unknown, and it is not certain that "clay healing" will restore the strength of the wellbore.

For an optimum well design, the predictions of the formation strength at the scheduled casing shoes must be as accurate as possible. The accuracy of the prediction depends on the validity of the formation strength model and the accuracy, significance and amount of available formation strength data.

If no data are available, assumptions have to be made about the state of stress and only a rough estimate can be made of formation strength. This will usually result in a sub-optimal well design, (either conservative or over-optimistic).

If data from one or more offset wells are available, the basic assumptions on the state of stress can be confirmed, and the accuracy of the prediction increases. If enough high quality data (e.g.

micro frac, mini frac or formation breakdown data) are available, a regional strength model can be derived, which will allow a more optimal well design. For some areas in the world formation strength data have been used to determine the relationship between minimum horizontal stress, depth and pore pressure (see Figure C-8). Correlation formulae exist for the Gulf Coast, Venezuela, Brunei, the North Sea and The Netherlands [6,7]. The application of numerical programs, like STABOR [8], will assist in the model formulation, since wellbore collapse case analysis can be performed and add to the understanding.

FIGURE C-8 : MINIMUM HORIZONTAL STRESS, CORRECTED FO PORE PRESSURE AS A FUNCTION OF DEPTH, US Gulf Coast

In area where formation strength determines well design, it is recommended that Opcos develop similar correlations. To enable this, it is recommended that formation breakdown tests or microfrac tests are carried out, to determine FBP and FCP, (and the state of stress). If operational considerations do not allow these tests to be performed during drilling, it should be considered to do these tests on abandonment of wells, or in existing wells.

In view of the importance of stress and strength data, not only for subsequent wells, but also for the production phase (e.g. sand failure, compaction, stimulation, etc.) no opportunities should be missed to perform these tests which are relatively cheap in the drilling phase.