Detecting Free Water Above Jonah Pay Using Pickett Plots and Core Analysis

Aaron Fisher
EnCana Oil and Gas U.S.A, Denver, CO

Defining the top of reservoir in the Jonah field has been an ongoing project since the beginning of development. In past years a considerable amount of money has been spent on fracing, testing, and squeezing perforations due to high production of free water. In addition to money being spent, many possible gas producing zones have been bypassed due to the fear of completing a zone which will produce high amounts of free water. As a result, recent studies have resulted in a top of reservoir correlation that has all but eliminated completing free water bearing zones, as well as targeting pay that would have been previously bypassed.

The Pickett Plot is a technique commonly used to quantify water saturation and identify water-bearing intervals. The crossplot is essentially a graphical representation of the ‘Archie’ equation. In unconventional reservoirs such as Jonah, the technique is not as straightforward due to a variety of factors including, but not limited to pore structure, grain sorting, and the presence of detrital and authigenic clays. However, strong graphical trends have been identified and utilized to differentiate zones bearing free water from zones at irreducible water saturations. Core analysis, specifically tritiated filtrate-corrected water saturations and previous completion results are used to verify trends seen during Pickett Plot analysis. Field-wide mapping of these trends has produced a stratigraphic top that serves as the upper boundary of the Jonah reservoir.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado