The Utility of Pressure Core and its Evaluation

Chad Hartman
Weatherford Laboratories

Understanding of the subsurface enables operators to minimize risk and maximize return on capital investment. By collecting core and open-hole geophysical logs from a number of select wells, drilled in strategic locations across a given basin or field, an operator can accurately characterize a given target reservoir and evaluate the prospect within a solid stratigraphic and geological framework. By doing this work upfront risk is mitigated, intelligent production wells can be drilled, and reliable economics can be predicted for the play.

Core acquisition not only facilitates determination of the petrophysical, petrological, and geochemical attributes of a target reservoir, it can also provide a direct empirical measurement of the volumes and types of hydrocarbons inherent in the reservoir. However conventional, non-pressurized, coring techniques are subject to alterations of insitu fluid saturations during core recovery due to pore pressure reduction and in-situ fluid expansion and expulsion. Pressurized coring, on the other hand, inhibits loss of in-situ fluids to the drilling mud in the well bore. Because pressure cores contain all in-situ fluids associated with the cut rock volume they can provide reliable information from any one or all of the following:

1. Stock Tank Fluid Saturations
   • Oil Saturation
     • From pressure depletion
     • Total oil saturation
   • Water Saturation
     • From pressure depletion
     • Total water saturation
2. Gas Analysis
   • Composition, mol%
   • Gas Volume, cc@STP
   • Gas Gravity
   • Isotopic analyses
3. Oil Analysis
   • Composition
   • Oil volume, cc@STP

Intimately coupled with the acquisition of pressure core is the instrumentation and analytical protocols necessary to evaluate and depressurize the core at surface followed by extrusion of the core from the tool assembly and subsequent laboratory analyses. Extreme care must be taken to ensure total material balance of fluids and fluid characterization. When done correctly the resulting data provides a “ground truth” assessment of oil and gas quality and hydrocarbons-in -place. This in turn provides extremely reliable information for use in reservoir modeling, estimates of ultimate recovery, and booking of reserves.

AAPG Search and Discovery Article #90178©2013 AAPG Geosciences Technology Workshop, Baltimore, Maryland, July 16-17, 2013