The Value of Integrated Downhole Passive Acoustic Monitoring During an Extended Leak Off Test to Prove Formation Integrity. A Case Study

The Extended Leak Off Test (XLOT) is a sophisticated formation integrity test that can be performed during drilling, recompletion, or at the well abandonment stage. The test is usually characterized by multiple cycles, creating and manipulating a fracture that can extend several meters away from the wellbore. The test can provide more data (both formation stress and fracture mechanics) compared to traditional leak-off tests. This data is used extensively both for determination of the in-situ formation stress for well barrier integrity assessment and for more general rock mechanical work such as quantifying fracture gradient for use in wellbore stability programs for drilling and completion operations.

The interpretation is performed by analysis of the surface pressure and, often with downhole data from memory gauges (or, increasingly, with real-time data from wired pipe) at different stages of the XLOT test. The typical XLOT pressure analysis chart is shown below (see Fig.1). The key determined parameters are:

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  Leak Off Pressure (LOP)

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  Fracture Initiation Pressure (FIP)

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  Formation Break Down Pressure (FBR)

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  Formation Propagation Pressure (FPP)

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  Instantaneous Shut-In Pressure (ISIP)

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  Formation Closure Pressure (FCP)

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  Fracture Reopening Pressure (FRP)

Figure 1

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The traditional XLOT interpretation plot.

Figure 1

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The traditional XLOT interpretation plot.

A key requirement of the test is to ensure hydraulic connectivity to the targeted formation only. This can be achieved in the case where annulus barriers are in place and perform well. Unintentional communication to non-targeted zones may result in abnormal behavior, more complex interpretation of obtained data, larger uncertainty in the meaning of the results and ultimately failure of the XLOT test.

To verify the well barriers integrity prior to the XLOT different techniques can be utilized. The main one is cement bond logging across the cemented barriers. This indicates the condition of the cement behind the first casing and increases the level of confidence the test will be conducted successfully. "However, recent case studies have shown that an indication of good bond above and/or below the target formation from a cement bond log cannot guarantee the isolation required to sufficiently hold the applied pressure [Maxim Volkov]."

The paper demonstrates an approach taken by Equinor in a special application where XLOT testing was advanced by adding downhole monitoring during the test. This targeted the following parameters to evaluate the new essential components of XLOT interpretation:

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  depth and capacity of opened and re-opened fractures,

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  actual sealing of the cement barriers above and below the targeted zone,

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  failure investigation in case the FBP cannot be achieved.