Novel Petrophysical Characterization of Complex Microporous Carbonates in the UAE: A Comprehensive Case Study
Authors: Kobenko Igor , Farmer Russell George , Mohamed Yassien , Di Simone Dr. Salvatore , Mohamed Alyazia 
Published: November 2024

This paper addresses challenges in petrophysical analysis encountered when dealing with complex microporous carbonates, evidenced in a case study of the Lower Shuaiba reservoir, UAE.

The Lower Cretaceous carbonates of the Shuaiba Formation represent one of the most prolific carbonate reservoirs in the UAE. These carbonates were deposited on a shallow epeiric carbonate platform during the Aptian period. Within this formation, the lower interval comprises coarsening-upward patterns, ranging from skeletal wacke-packstones to packstones, eventually transitioning into Lithocodium/Bacinella-rich floatstones. In contrast, the middle part of the formation is predominantly characterized by micrite-dominated textures, varying from mudstones to packstones.

Of particular interest is the microporous rock type, frequently encountered in the Lower Shuaiba at the crestal part of our study area, extending upwards to the reservoir's top at the flank area, where diverse dynamic behaviors have been observed in drilled wells. Log evaluation results reveal that this microporous rock type dominates the Lower Shuaiba, accounting for up to 38% of the total formation thickness. The atypical low resistivity behavior, of this microporous rock type, can be attributed to an increased volume of bound water. Although the measured pore volume is considerable, moderate-to-poor core permeability values result from limited pore connectivity.

Our study leverages enhanced integration of petrophysical data with core analysis and dynamic well data, including pressures, sampling, production data and PLT's. This comprehensive analysis was instrumental in defining reservoir properties and constructing a robust rock classification scheme, ultimately refining our characterization of the microporous rock type and enhancing petrophysical evaluation.

Key steps in our workflow included:

Developing a robust rock typing scheme capable of clustering heterogeneous petrophysical properties that are in alignment with geological factors such as lithology, stratigraphy, sedimentology and diagenesis. Generating a petrophysical evaluation that consolidates static rock type characteristics, maintaining coherence with dynamic behaviors, particularly those associated with microporous facies in the Lower Shuaiba reservoir.

The study yielded notable outcomes:

An innovative rock typing scheme, developed through the integrated study of well data and core analysis, furnishes valuable insights into understanding abnormal saturation profiles across the Lower Shuaiba formation and implications for well production performance. A successful implementation of a robust saturation height function tailored to the microporous rock type, offering a comprehensive explanation for the dynamic behavior observed in the field's flank area, where this rock type extends into the upper part of the Lower Shuaiba. The results of the novel application for cementation exponent "m" prediction provided a variable m solution for Archie saturation model. A novel petrophysical workflow, which substantially improved the prediction of static and dynamic reservoir properties and was subsequently incorporated into an updated Field Development Plan (FDP).

A pioneering integrated workflow for petrophysical synthesis of microporous rock types has significantly advanced the characterization of Lower Shuaiba reservoirs. It has led to more reliable predictions of reservoir dynamic behavior, particularly in the context of microporosity driven low resistivity pay effects, as observed in the Shuaiba and Lekhwair formations within the UAE region.

A novel neural network-based approach has been implemented for cementation exponent "m" prediction and the results have been compared with "m" calculation results derived from different techniques including NMR and dielectric logging.