Carbonate reservoir complexities are both described and hidden by the use of the common and generic term, "heterogeneous", with usually little serious effort then expended to define and quantify the meaning and dimensions of the term. Saudi Aramco, the custodian of the world’s largest petroleum reserves in carbonate reservoirs, is making long-term efforts to bring light to the carbonate "heterogeneity" darkness. We are constructing the industry’s largest integrated petrophysical and geological databases for our major carbonate reservoirs.
Our large databases contain porosity, permeability, grain density and pore system information from core plugs carefully integrated with a full suite of well log data and core descriptions. These cores have been described in detail using the most current carbonate sequence stratigraphic techniques and all these data have been captured digitally and integrated consistently and carefully. Our database for one field contains 1695 limestone pore systems (a pore system is defined by a single Thomeer hyperbola), obtained from 931 plug samples by Thomeer analysis of core plug mercury capillary pressure data (MICP) with plans to expand the measurements to 1500 plug samples for just this one major reservoir by the year 2020. The current core plugs for the database now include samples from 30 cored wells (up from the 10 cored wells of the previous Rosetta Stone project) to obtain statistically robust reservoir petrophysical data at the facies level. Pore system data are now also being acquired in vertical wells for constrained reservoir layering and vertical reservoir model tie points using nuclear magnetic resonance (NMR) well log data and the CIPHER software for the Thomeer parameter MICP-consistent spectral porosity analysis.
At Saudi Aramco, our extensive facies – petrophysical properties database process delivers the reference reservoir property database for our major carbonate reservoir models. The reference database improves our understanding and modelling of the variation and covariation of the facies and petrophysical rock types (PRTs) for reservoir modeling. It also provides sound statistical support for the population of the reservoir with petrophysical pore system properties within the sequence stratigraphic facies framework. For the reservoir dynamics, the database allows detailed investigations into the statistical linkages of pore system properties which control permeability and relative permeability developed by Clerke and coworkers to the sequence stratigraphic reservoir facies.
We report here selected results from a very large quantity of relationships and statistical attributes that can be derived from this database. At the general carbonates level, these reference databases demonstrate that the reservoir complexity hidden in the shadow of "heterogeneity" is actually the prevalence and statistical distribution of these multimodal pore systems and their attributes.
