Characterization of Complex Carbonate Heavy Oil Reservoir—A Case Study

Abstract

The Paleocene[Eocene age 1st Eocene Reservoir is the shallowest producing interval of Wafra Field in the Partitioned Neutral Zone (PNZ), Saudi Arabia and Kuwait. Characterization of this heavy oil reservoir is challenging due to observed variations in oil viscosity, heterogeneity related to complex mineralogy, a possible dual porosity system, and the presence of fractures at varying scales. This case study of the 1st Eocene reservoir characterization in the steam flood pilot area will improve our understanding of the range and distribution of formation properties which is critical for management of the current pilot project. This study presents several aspects of an integrated approach to characterize the 1st Eocene reservoir. The approach includes the quantification and distribution of the evaporite minerals and porosity, analysis of a possible dual porosity system, and evaluation of permeability using a new porosity partitioning technique. Data used in this study includes conventional open-hole well logs, borehole images, nuclear magnetic resonance (NMR) logs, Elemental Capture Spectroscopy (ECS) logs, formation pressure measurements, and routine core analysis. The porosity partitioning model allows the core porosity-permeability relationship to be reproduced from logs by incorporating dual porosity information. The integration of NMR logs and image logs used in the porosity partitioning model provides a well log based permeability that has a good correlation with core plug data. Introduction The 1st Eocene reservoir of wafra filed is located in the Divided Zone (DZ), Saudi Arabia and Kuwait (Fig. 1). Currently, a large scale Steamflood pilot (LSP) consisting of sixteen, 2.5 acre inverted 5-spot patterns is being installed. This will be the first multi-pattern steam flood of a carbonate reservoir in the Middle East. Fig. 2 shows the locations of the LSP Steamflood project. Oil was first discovered in the 1st Eocene in 1954 and full scale development began in 1956. The 1st Eocene has produced only 4% of OOIP and primary recovery is not expected to exceed 10%. The oil API in this heavy oil reservoir varies from 14–20ºAPI. The 1st Eocene is a depletion drive reservoir with partial solution gas drive and limited aquifer support. The estimated gross OOIP within the currently delineated field limit is more than 10 billion reservoir barrels. A generalized Stratigraphic column for PNZ is given in Fig. 3. The reservoir mainly consists of dolomitized packstones and grainstones deposited under arid to semi-arid conditions on a shallow, very gently dipping, low to moderate energy inner shelf or ramp setting in a gently dipping, restricted ramp environment. The presence of minor interbedded evaporites suggests restriction was occasionally sufficient for the development of hyper-saline lagoons and sabkhas. The shallowing-upward cycles are capped by mud-dominated rocks, hardgrounds and exposure surfaces. The average porosity is 35% and the average permeability is 250 md over the gross interval. Well log and core plug porosity values over 50% are common and measured permeability values range up to 5000 md. The 1st Eocene reservoir has an average depth of about 1000 feet and a gross thickness of about 750 feet.1 A structure map is given in Fig. 2