Characterization of High-Permeability Streaks Associated with Heterogeneous Stromatoporoid Reefal Facies and Establishment of Modeling Workflow in Carbonate Reservoir, Offshore Abu Dhabi
Authors: Kohda Atsuro , Bellah Sameer , Shibasaki Toshiaki , Farhan Zahra Al , Shibayama Akira , Hamami Mohamed Al , Jasmi Sami Al 
Published: November 2017

The understanding of heterogeneous rock properties especially high-permeability streaks is very important to predict fluid behavior in carbonate reservoirs. An Upper Jurassic reservoir in "Field A" has been producing for 30 years with different production scheme such as crestal water and gas injection at the different stage. The observed water/gas breakthrough and the evolution trend in water cut/GOR indicate reservoir heterogeneity caused by geological complexity. To replicate such complicated fluids behavior in reservoir model, the characterization study for high-permeability streaks was conducted.

Multiple data sources were used to identify and characterize high-permeability streaks.

Interpreted injected gas/water sweep intervals utilizing cased-hole production logging. Identified potential high-permeable lithofacies and its stratigraphic positions by detailed core and thin section descriptions with petrophysical observations. Defined high-permeability streaks based on the integrated interpretation of multiple data sources. Characterized the high-permeability streaks in reservoir model with excess flow capacity estimated from model and well-test permeability.

This study revealed that multiple types of high-permeability streaks present in the reservoir. In particular, it was recognized that a specific thin layer comprises stromatoporoid (epibenthic calcified sponges) patch reef deposits acts as the main contributor for fluids movement. This paper shows how to characterize the high-permeability streaks in reservoir model focusing on stromatoporoid lithofacies.

Thickness of stromatoporoid lithofacies shows heterogeneous variation of 0 to 14 feet. The complex pore system in stromatoporoid lithofacies associated with heterogeneously distributed skeletal fragments with centimeter-scale makes difficulty for capturing accurate permeability from conventional plug measurement. The plug permeability was generally underestimated comparing with actual flow capacity estimated from well-test. Hence the modeled permeability which generated from porosity-permeability correlation coming from plug measurement was required further conditioning based on the pre-established concept for high-permeability streaks.

To fill the gap between modelled and well-test permeability-thickness (KH) i.e. excess KH, the relevance between excess KH and stromatoporoid lithofacies was investigated. As a result, it was found that the zonal well-test KH increases as stromatoporoid lithofacies thickness (STR-H) increases, and there is a good correlation between STR-H and STR-KH estimated as "zonal well-test KH" minus "zonal modeled KH except stromatoporoid lithofacies intervals". Therefore, excess KH was allocated to only into the part of stromatoporoid lithofacies. The prepared STR-H map was directory transformed to STR-KH distributions by the revealed correlation. Through dynamic history matching, permeability distribution was iteratively modified by updating STR-H map in concordance with depositional concept.

Detailed observations and integrated interpretation for multiple data sources allowed identifying high-permeability streaks and establishment of a model workflow for representing its heterogeneity and associated permeability distribution. This workflow enabled geologically reasonable permeability conditioning and iterative model update in conjunction with the depositional concept during dynamic history matching.