FMB pressure (Cartesian)

• The Cartesian plot of normalized pressure (∆p_p/q) versus material balance time (G_p/q) should have an inclining trend and tend towards a straight line.

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 FMB-pressure 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 the 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.