NPI (log-log)

• The log-log plot of normalized pressure (∆p_p/q) versus material balance time (G_p/q) should have an increasing trend.
• A step change in rate causes a sudden deviation from the inclining trend, but if the data is consistent, it reverts to the original trend.

Reasons for inconsistencies:

• A decreasing trend indicates non-reservoir effects, such as near wellbore cleanup, improving skin, wellbore liquid unloading (if pressures have been calculated from wellhead data).

• Changes in rate or pressure are accounted for in the NPI plot, and should not cause it to develop multiple stems.
• In the case of multiple stems, the stem that represents the correct reservoir model should be used for analysis.

Reasons for inconsistencies:

• The reservoir model has changed (for example, work-over, recompletion, refrac, etc.).
• The wellhead pressures measurement location may have changed (for example, tubing, casing, before / after valve), or there may have been a change of flow path or tubulars.

• Outliers often appear on a unit slope on the NPI plot. This can lead to an incorrect diagnosis of reservoir depletion, or boundary-dominated flow (BDF).

Reasons for inconsistencies:

• Outliers have been included in the analyses. Apply the median filter to remove outliers.