Linking Diagenesis, NMR, and Dynamic Data for Accurate Flow Characterization of Heterogeneous Carbonate Reservoir
Authors: Farooq Umer , Iskandar Reza , Radwan El Sayed Moustafa , Hozayen Magdy Ahmed H 
Published: December 2014

Defining hydraulic properties of carbonate reservoirs has long been a longstanding debate and challenge due to its heterogeneity as a result of deposition and impact of diagenesis. The flow unit identification becomes non-trivial when the reservoirs are affected by extensive diagenesis, which results in a complex pore-size distribution. This condition makes standard logging suits inadequate. Therefore, advanced logs interpretation (NMR) with the integration of geological understanding, core and dynamic data are essential to derive a robust flow characterization.

A comprehensive evaluation on multi wells that link different aspects controlling permeability has been carried out. The study comprised three integrated steps. The first step involved detailed diagenesis study of the reservoir where core analysis: petrographic examination, porosity-permeability measurements, pore throat distributions, and capillary pressure were introduced. The next step focused on the integration of full field NMR logs interpretation. Pore size distribution, porosity partition (macro, meso, micro), and free and bound fluid derived from 10 NMR logs were used across the reservoir section. The final step was to build the link between the pore connectivity obtained from NMR to diagenesis. Dynamic data such as production performance were then used to validate the integrated model.

The result of the study reveals that diagenetic processes particularly dolomitization and grains dissolution have produced pore architecture that can be linked to NMR. This approach has a significant impact on the reservoir flow behavior, and has hence become a strong reference to find the sweet spot for the next drilling targets, which can lead to a proper flow characterization, better understanding of well production performance and dynamic reservoir description.

Introduction

Carbonate reservoirs are strongly heterogeneous in terms of porosity and permeability from the time of their deposition owing to rapid changes in depositional facies, and hence sediment texture. Diagenetic alterations (cementation, compaction, dissolution, micritization, neomorphism, and dolomitization) impose further and profound impact on modification to depositional porosity and permeability, and hence on final quality of carbonate reservoirs (Tucker, 2001; Wierzbicki et al., 2006; Morad et al., 2012). Diagenetic alterations are controlled by a complex array of inter-related parameters, which include depositional facies, mineralogy (aragonite, high-Mg calcite, and low-Mg calcite), changes in relative sea level, and structural deformation (Morad et al., 2000, 2012; Moore, 2005).

The Simsima Formation, which is a major oil producing reservoir in the onshore field (Abu Dhabi, United Arab Emirates), has heterogeneous quality related to depositional and diagenetic processes (Badr and Ayoub, 1989). The Simsima reservoir is subdivided into three units (Upper, Middle, and Lower) on the basis of variations in petrophysical characteristics and depositional environments.