Core-Analysis Elephant in the Formation-Evaluation Room
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 158087, ’The Core-Analysis Elephant in the Formation-Evaluation Room,’ by Colin McPhee, SPE, Senergy Limited, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8-10 October. The paper has not been peer reviewed.

The variable data quality of core analysis, the sensitivity of results to different test methods, poor reporting standards, and the reluctance of some vendors to share experience and expertise have contributed to basic mistakes and poor data quality. In many cases, an inconsistent or inappropriate approach to the design, management, and interpretation of the core-analysis program has been adopted and exacerbated by the conflicting requests from the end users. In combination, approximately 70% of legacy special-core-analysis-laboratory (SCAL) data are not fit for purpose. A core-analysis-management road map was designed to increase the value from core-analysis investments by enabling a more-proactive, more-coherent, and more-consistent approach to program design and data acquisition.

Introduction

In hydrocarbons-in-place and core analysis, the volume of stock-tank oil initially in place in a reservoir can be calculated. The gross rock volume and gross factor in the net-/gross-pay ratio are the primary responsibilities of geophysicists and geologists. The reservoir engineer is responsible for oil-formation-volume factor from pressure/volume/temperature experiments. The petrophysicist is responsible for net reservoir thickness, porosity ø, and water saturation. Data input relies principally on logs, but log interpretation must be calibrated or verified by measurements on cores. For example, net reservoir thickness normally is defined by a permeability cutoff, and high-resolution permeability data are available only from core analysis. Porosity interpretation (e.g., from density logs) should be verified by, or calibrated to, stressed-core porosity data. Resistivity logs require Archie’s cementation and saturation exponents m and n, respectively, to determine water saturation quantitatively in clean formations. These exponents are measured on cores. Water saturation can be determined directly by extracting water from cores with Dean-Stark methods, or indirectly from core-derived capillary pressure measurements.

Importance of Core Data and Data Issues. Core analysis is the only direct and quantitative measurement of the intact-reservoir properties, and it should provide the foundation for formation evaluation. However, the author found an unfortunate negativity about the value of core data. In a multiple-laboratory comparative SCAL study, it was found that some core-service contractors provided very-poor-quality data on some of the tests, and it was concluded that some of the laboratories do not have in-house quality-control protocols and report data just as acquired. Other studies have found inexplicable relative- permeability-data discrepancies between commercial laboratories testing exactly the same core material and fluids. Too often, core-analysis programs are ill considered, badly designed, and poorly supervised and the results are only crudely integrated with other well and reservoir data. The results, in terms of data acquired, often are unrepresentative or even contradictory. A conservative estimate from review and audit of more than 30,000 SCAL measurements of different vintages indicates that approximately 70% of the data are unfit for purpose because of their unreliability, inapplicability, or inappropriateness.