USING HIGH FIDELITY CONTINUOUS CORE DATA FOR A FAST AND OBJECTIVE ESTIMATION OF RESERVOIR QUALITY

Abstract

With this paper, we demonstrate how Core DNA, a trans-disciplinary suite of high-resolution, non-destructive measurements performed on whole cores at the onset of core analysis programs, helps operation geologists and petrophysicists with an innovative, cost effective and objective way to characterize the reservoir quality of highly laminated hydrocarbon-bearing formations where the standard practice (systematic plugging every foot) fails to provide a correct estimate. The case study focuses on core data from three wells intersecting formations characterized by very thin (millimetre-scale) sand and clay/silt laminations where the resolution of conventional wireline and lab gamma ray logs were not sufficiently sharp for an effective evaluation of reservoir quality. Although a high volume of routine core analysis data was already available for these wells, the remaining uncertainty on reservoir evaluation was deemed high enough by the study team to motivate the acquisition of additional data comprising ultra-high resolution pictures (1.8μm/px) and topographic maps created from micron-accurate laser scans. We explain how continuous profiles of grain size indicators could be used for the prediction of permeability variations across these laminated formations and for the definition of a permeability cut-off for the identification of poor vs good reservoir ratios compatible with the reservoir characteristics. Core DNA test procedures are specifically designed to greatly accelerate the deliverables of core analysis, so that petrophysical evaluation may start right from the moment cores arrive from the well site, which is usually month before routine core analysis results are known. In the context of this paper, Core DNA results were confirmed a-posteriori by the permeability measured on plugs samples from the two first wells. In the third well however, some marked differences were observed: although permeability ranges were found similar by the two methods, the distribution of permeability values obtained from routine core analysis conducted according to standard guidelines (one sample per foot) gave a more optimistic picture of permeability (90% rock above the 1mD cut-off) than the alternative approach based on high resolution continuous grain size data (70% rock above the 1mD cut-off). From the above findings, we conclude that a standard 1-ft interval for plug acquisition is not enough to fully characterise the distribution of permeability in highlyl aminated formations. Alternatively, a continuous profile of permeability index based on high resolution grain size measurements offers a fast and cost-efficient solution to obtain representative reservoir quality data, which enable objective well and reservoir management decisions few days after barrel opening without compromising core integrity for further studies.