UNDERSTANDING EFFECTIVE AND TOTAL POROSITY: THE KEY TO OPTIMISING MINERALOGICAL PETROPHYSICAL EVALUATIONS

Abstract

Use of petrophysical evaluation models that rely on predicting mineral composition of reservoir rocks is becoming more prevalent in the oil industry. Evaluation tools such as Schlumberger's Elan or Mincom's Geolog/Multimin allow prediction of variability in reservoir composition and petrophysical parameters. These are particularly useful in cases where assuming constant lithology and petrophysical parameters can result in significant variation in well evaluation, hydrocarbons in place estimates and volumetric reserves.

Mineralogical evaluations have historically been questioned in cases where evaluated effective porosity differs significantly from core total porosity. Effort has been made over the last year to obtain maximum value out of these tools by providing all data as input to an integrated evaluation using total porosity. The key feature is incorporating all possible core data for calibration purposes, including:

using all core total porosity, permeability and capillary pressure data, where available, to calibrate modelsusing petrological data to obtain mineral-by-mineral calibration of rock compositionoverlaying a regional total porosity curve in the absence of core data to provide a quality control on evaluated total porosity.

The success of such approaches is demonstrated in an offshore WA greensand and an Eromanga shaly sand. These methods are now used on a routine basis for detailed field studies and hold the promise of both permeability evaluation and improved evaluation of wells with restricted logging suites.