Integration of Petrophysical SCAL Measurements for Better Understanding Heterogeneity Effects in Carbonates: Case Study Using Samples from a Super Giant Field in Abu Dhabi
Authors: Serag El Din Samy , Dernaika Moustafa R. , Kalam Zubair 
Published: March 2015

Characterization of carbonate reservoirs is challenging as well as daunting due to the inherent heterogeneities that occur at all scales of observation and measurement. Heterogeneity in carbonates can be attributed to variable lithology, chemistry/mineralogy, pore types, pore connectivity, and sedimentary facies. These complexities can be related to processes controlling original deposition and their subsequent diagenesis.

Reservoir cores from super giant producing carbonate field in Abu Dhabi have been used in this study. X-ray CT scanning, Thin Sections, porosity, permeability, mercury-derived drainage capillary pressure (Pc) and pore throat size distribution (PTSD) have been used to define the petrophysical groups and different sedimentary facies. The effects of carbonate heterogeneity on reservoir behavior have been studied by correlating rock structure/texture and pore-throat size distribution to formation resistivity factor, cementation exponent (m), and to relative permeability. The effects of wettability and rock nature have been discussed based on the relative permeability trends observed on the main rock types covered in the field.

Less heterogeneous reservoir rocks with muddy structure and uniform pore throat size distribution (PTSD) have moderate poroperm characteristics and tend to be less tortuous with average cementation exponent m at 1.85. More heterogeneous rocks with grainy/muddy or grainy structures and bimodal/heterogeneous PTSD have very good poroperm characteristics and tend to be more tortuous with range of cementation exponent m values from 2.20 to 2.50. Large variations have been obtained in imbibition relative permeability curves among the different rock types, which were explained on the basis of rock structure, PTSD curves and poroperm characteristics. The more heterogeneous rock types present more oil-wet behavior than the less heterogeneous rock types.

The full integration of resistivity and phase flow laboratory results with geological heterogeneities showed consistent trends in reservoir characteristics, and offered in-depth understanding of the reservoir rock properties for potential investigations in further field development using IOR/EOR techniques.

Introduction

Accurate knowledge of petrophysical properties is required for efficient management and prediction of future performance of oil and gas reservoirs. This knowledge necessitates the understanding of physical properties of the reservoir rocks and the interactions of various fluids with interstitial surfaces, and the distribution of pores and minerals within the porous medium. The quantification of the effect of heterogeneity on electrical petrophysical properties helps reduce uncertainty in the estimations of water saturation from resistivity logs. Furthermore, the evaluation of the effect of heterogeneity on relative permeability helps in the understanding of the reservoir dynamic behavior and fluid distribution in addition to wettability and residual oil saturation after water and/or gas injection.

Core laboratory data can help establish a sound basis for reservoir modeling and develop effective strategies for reservoir exploitation (secondary and EOR) schemes (Masalmeh and Jing, 2008). However, the full characterization and rock typing of core samples are often overlooked in special core analysis (SCAL) programs. This would lead to unrepresentative selection of the core samples and would raise questions about the effectiveness of the core data in the reservoir model and its calibration.