Resistive imaging logs (FMI and EMI) have been embraced by those challenged to characterize flow properties of massive carbonate reservoirs. These logs provide valuable insights for assessment of in situ heterogeneities but are not intended to directly indicate those features that provide the best connections between the wellbore and the formation. High resolution flow surveys of openhole completions in massive carbonates can offer a glimpse of flow capacity that has not suffered cement and perforation damage related to cased completions. Accurate reservoir description can only be accomplished by distinguishing between those features that are effective flow conduits and those that simply appear as large anomalies at the wellbore.
Many reservoirs throughout the world conform poorly to the single porosity sandstone matrix model that is so commonly used as the foundation for attempting to predict reservoir behavior. In these reservoirs, recognition and accurate assessment of reservoir flow features is critical for reservoir characterization, completion interval selection, well operation, and improved recovery process design and application. Massive carbonates provide an exceptionally challenging evaluation because;
they are thick and variable-often exhibiting pore-type variation and occasional inter-layers rich in clay or sulfates,
carbonate pores exhibit extreme variation in flow capacity typically without the constraints of pore throat size as a function of grainsize, and
flow heterogeneities can be extremely large scale (i.e. fracturing with solution enhancement or cement precipitation impacting either the matrix or fractures).
Core data and log analysis may adequately characterize the small-scale storage and flow heterogeneities of a carbonate formation. The greatest opportunity for improving current methods of reservoir characterization is in quantifying the large-scale features that extend beyond wellbore resolution(from bed thickness to greater than one well spacing). Assessment of the magnitude, geometry, and connectivity of the highest flow capacity features in a wellbore provides a basis for developing an interwell-scale flow model. The optimum well objective, completion design, and operation should be assessed after applying individual wellbore insights to establish an interwell flow model. Flow feature quantification in openholes can offer valuable insights to engineers and geologists who are not blessed with undamaged openhole completions.
The objectives of this paper are;
Review the several of the various types of flow features in massive carbonates
summarize methods of flow feature identification
offer a simple approach for quantifying flow features
review the value that flow feature description can offer.
Two types of large-scale flow features are discussed in reviewing field data of individual wells. The first is bedding-parallel zones of large pores," touching" vugs, or carbonate breccias. These zones as discussed by Palmer and Palmer (1995) may result from dissolution of evaporates which had been deposited with the carbonate (or may result from other fabric selective solution). A combined result of periodic evaporite deposition and low concentration in the deposit provides improved opportunity for post-depositional dissolution and generation of heterogeneity that is significant at the interwell scale. Thick zones of this type that extend across a multiple well area with good continuity are adequately characterized using typical geologic methods. Thin discontinuous zones require recognition and analysis using engineering (flow) and geology (logs) as discussed.
The second type of large-scale flow features are vertical and sub-vertical fracture flow conduits that are infrequently observed in vertical wellbores relative to the "planes" of bedding- parallel touching vugs discussed above. P. 721^