Porosity Heterogeneity Analysis Using Geostatistics

Abstract

Abstract High levels of heterogeneity in many carbonate fields have raised concerns about the validity and relevance of small-scale measurements from core plugs and high-resolution logs. Although the measurements may be precise, they may not accurately describe average formation properties. A related question is one of reconciling the variability observed in small volume-of-investigation data (e.g., core plug), with the variability in relatively large volume-of-investigation measurements (e.g., wireline logs). In a recent study of early Cretaceous carbonate reservoirs in Abu Dhabi, we developed a technique to address this issue using high-resolution borehole imaging logs. The resistivity image is first transformed into a porosity image using Archie's equation. The resulting porosity variability is then analyzed using geostatistics. This analysis allows us to extract geostatistical parameters (range and sill) at the scale of the borehole image measurements (0.1 in.) that provide a heterogeneity index for the rock. We found that the scale of the heterogeneity is sufficiently small (typically less than 1 in.), such that significant heterogeneity will be observed at the core plug and smaller scales of measurement, while the impact on larger-volume logging tool response is minimal. The fact that standard wireline logs are insensitive to most of the "heterogeneity" in these formations explains the difficulties in determining reservoir rock types (RRTs) from standard logs, because the properties distinguishing the RRTs from one another (macrofossils and molds) are present at a scale below the sensitivity of standard logs. In contrast, properties measured by the logs can be expected to reflect the larger-scale reservoir properties around the well. These findings have significant impact on how we calibrate logs using core data and how we should upscale reservoir properties derived from core and log data in our 3D earth models. The findings support our recommendation to use core plugs larger than the conventional 1-in. plugs in carbonate formations to ensure that the small-scale heterogeneities are captured within the core plug itself.