Integration of Core Analysis, Wireline Logs, and Geologic Descriptions

F. Jerry Lucia, Senior Research Fellow
E-mail: jerry.lucia@beg.utexas.edu
Telephone: (512) 471-7367

ABSTRACT
 Reservoir characterization is primarily a problem of distributing petrophysical properties in 3D space. Petrophysical measurements and wireline logs contain no spatial information and must be linked to spatial information to be used in building a reservoir model. Properly done geologic descriptions contain vital spatial information that can be linked to petrophysical measurements and wireline log responses. This link is accomplished through the process of integrating rock-fabric descriptions with petrophysical and wireline log measurements. Basic rock-fabric descriptions include grain/crystal size and sorting, interparticle and separate-vug pore space, and touching-vug pore systems. The descriptions of touching-vug pores systems, which include large vugs and fractures, can best be described from core material. These pore systems are important because they are difficult to characterize petrophysically and often comprise super-k intervals. Capillary properties and permeability of interparticle and separate-vug pore systems are related to grain/crystal size, grain/crystal sorting, and separate-vug porosity. In many reservoirs, grain/crystal size and sorting can be determined from cross plots of water saturation and porosity. Interparticle and separate-vug pore space can often be estimated from cross plots of acoustic travel time and total porosity, although there are many exceptions to this method. These rock-fabric elements provide a vital link between geologic descriptions used to build 3D reservoir models and petrophysical measurements of permeability and water saturation.

 The Spatial Statistics of Permeability in Carbonates and
the Effects of Scale in Data Analysis and Modeling

Dr. Jim W. Jennings, Research Scientist
E-mail: jim.jennings@beg.utexas.edu
Telephone: (512) 471-4364

ABSTRACT
Permeability data from Permian and Cretaceous carbonate outcrops and subsurface reservoirs exhibit many orders of magnitude variability, much of it occurring within distances of a few feet or less in single rock-fabric units. This short-range variability has weak spatial correlation and composes most of the overall variance in each case. A variety of longer-range features are also observed, but they compose a smaller fraction of the overall variability and require careful analysis to detect. Stochastic models explore the fluid-flow effects of these heterogeneities. Tracer and waterflood simulations demonstrate that the long-range features can control interwell-scale displacement, even though they compose much less than half of the total variance.< The high degree of variability concentrated at small scales, together with the more subtle but nevertheless important larger scale spatial patterns, plays an important role in the analysis, modeling, and scaleup of petrophysical data in carbonate reservoirs. A few of the issues that can arise are illustrated with outcrop and subsurface data and models at the scale of minipermeameter measurements (millimeters to centimeters), plug and whole core data (inches), well log data (feet), rock-fabric units (several feet to a few tens of feet thick), and sequences of rock-fabric units (hundreds of feet thick). Adequate characterization and modeling of carbonate reservoirs require consideration of all these scales. Quantification of the small-scale variance is required for unbiased estimates of effective permeability and rate. Recognition of the larger scale features is required for accurate modeling of interwell displacement patterns.