Whole Core vs. Plugs: Integrating Log and Core Data to Decrease Uncertainty in Petrophysical Interpretation and Oil-In-Place Calculations

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 137679, “Whole Core vs. Plugs: Integrating Log and Core Data To Decrease Uncertainty in Petrophysical Interpretation and STOIP Calculations,” by S. Serag El Din, SPE, ADCO; M.R. Dernaika, SPE, Weatherford Laboratories; I. Al Hosani, ADCO; L. Hannon, Weatherford Laboratories; S.M. Skjeveland, SPE, University of Stavanger; and M.Z. Kalam, ADCO, prepared for the 2010 Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, UAE, 1–4 November. The paper has not been peer reviewed. Most carbonate reservoirs are characterized by multiple-porosity systems that impart petrophysical heterogeneity to the gross reservoir interval. The specific types and relative percentages of pores present, and their distribution within the rocks, exert strong control on production and stimulation characteristics of carbonate reservoirs. The effects of heterogeneity on core and log measurements are presented. The challenge is to determine the reliability of applying relatively small-scale properties measured by a log or core to the large-scale reservoir property. Introduction Most sandstone reservoirs are single-porosity systems (i.e., interparticle pores) of relatively uniform (homogeneous) nature. However, most carbonate reservoirs are characterized by multiple-porosity systems. Reservoir heterogeneity results from physical and chemical reorganization processes over time (e.g., compaction, solution, dolomitization, and cementation). Heterogeneity complicates the reservoir-description task, in which reservoir properties (e.g., permeability, porosity, saturation, and rock types) tend to vary as a function of spatial locations in vertical and areal directions. Vertical heterogeneity has shown large effects on sweep efficiencies of waterfloods and conventional gas injection into mixed- to oil-wet carbonate reservoirs. These effects are a result of large variations in permeability between different strata. Permeability and other rock properties can vary along the areal (lateral) direction of the reservoir, which necessitates establishing an accurate and detailed understanding of geological heterogeneities and their effect on petrophysics and reservoir engineering. The effect of rock heterogeneity becomes greater when petrophysical properties are measured at scales ranging from a small trim sample in the laboratory to log measurements in the field. Permeability variation of three orders of magnitude has been reported over a distance of a few centimeters in a carbonate-core plug. In heterogeneous reservoirs, the real challenge is predicting reservoir performance through the acquisition of representative measurements. The challenge would be to reconcile the variability seen in high-resolution (e.g., trim or plug samples) and low-resolution (e.g., wireline logs) measurements and to quantify the technical and physical considerations at the various scales.