Influence of Property Modeling Practices on Estimated Hydrocarbon Pore Volume
Authors: Bust Vivian K. , Worthington Paul F. 
Published: October 2012

The emergence in geoscience of 3D geocellular modeling has raised questions about the correspondence of hydrocarbon volumetric deliverables with those derived from established 2D zonal modeling. These differences of approach are compounded because the established 2D methods have traditionally used net-pay volumetric algorithms whereas the contemporary 3D methods use net-reservoir volumetric algorithms. Key points of comparison are net-to-gross (reservoir and pay), equivalent hydrocarbon column (EHC) for 1D reference modeling, and initial hydrocarbon pore volume (IHPV) for 2D and 3D modeling. The evaluation of net-to-gross and EHC are key stages in the volumetric determination of IHPV, a parameter that serves as a reference for estimated ultimate recovery.

The objectives of the study were twofold. The first aim has been to generate an internally self-consistent petrophysical model as a basis for volumetric comparisons. For present purposes, a self-consistent petrophysical model is one that can be described by several measureable parameters that show definable interrelationships for each constituent rock type. The reservoir notionally comprises an oil accumulation within a water-wet, lithologically-clean sandstone that is partially saturated with high-salinity brine. The sandstone comprises five rock types, each of which has a defined set of interpretive algorithms. The second objective has been to compare static volumetric estimates of net-to-gross reservoir, net-to-gross pay, EHC and/or IHPV for a range of case models. This objective was approached at four levels. Initially, 1D "along-wellbore" screening studies used different case models representing various stratigraphic sequences. These allowed a preliminary assessment of results arising from the use of net-reservoir and net-pay volumetric algorithms: the screening results indicated measureable differences between the deliverables of net-pay and net-reservoir algorithmic protocols expressed as EHC, with EHC (net reservoir) generally being significantly higher than EHC (net pay). The screening results have been substantiated by field studies. Second, 2D zonal modeling led to values of IHPV based on both net-reservoir and net-pay algorithmic protocols. Third, 3D geocellular modeling led to values of IHPV, also based on both protocols. Finally, these data facilitated equitable comparisons of 2D zonal deliverables with those from 3D geocellular models. A consistent inter-well interpolation methodology was used for each case to maintain consistency and to allow meaningful comparisons.

The study incorporated the influence of stratigraphic sequences of five rock types, petrophysical properties, log upscaling, and other property modeling elements relevant to case modeling. Comparisons of 2D and 3D models showed that net-to-gross and IHPV delivered by established 2D zonal models with net-pay algorithmic protocols are mostly lower than those values delivered by contemporary 3D geocellular models with net-reservoir protocols, but the differences are variable. These outcomes, which have implications for reserves estimation, are primarily governed by the stratigraphic distribution of rock types. They re-emphasize that each case must be investigated separately and thoroughly.