Tapping Difficult Oil in a Giant Carbonate Field in Qatar
Authors: Carpenter Chris 
Published: February 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 18296, “Tapping the Difficult Oil and Enhancing Reservoir-Development Strategy To Maximize Recovery From a Mature Waterflood Giant Carbonate Field in the Middle East: Arab C Reservoir, Dukhan Field, State of Qatar,” by Mohamed Naguib Bin Ab Majid, Carlos Troconiz, Mohammed Nedham Al-Shafei, Gheorghe Luca, and Ariel Cachi, Qatar Petroleum, prepared for the 2015 International Petroleum Technology Conference, Doha, Qatar, 7–9 December. The paper has not been peer reviewed. Copyright 2015 International Petroleum Technology Conference. Reproduced by permission.

After 70 years of production, more than 30% of the Arab C reservoir stock-tank original oil in place has been recovered through various mechanisms including natural depletion, waterflooding, gas lift implementation, and horizontal-well development. Extending production into future years requires a strategic approach focusing on innovative development to target the remaining oil saturation. Integration of a recently acquired, high-resolution 3D-seismic survey complements the data available for subsurface description and characterization, positively affecting reservoir-model history-matching metrics.

Introduction

The large, mature Dukhan field is located onshore Qatar, approximately 80 km west of Doha. The Arab C reservoir interval is a carbonate anticlinal structure lying 5,500 to 7,000 ft below the surface. Areally, the Arab C reservoir has been divided into four structural elements from north to south (i.e., Khatiyah, Fahahil, Jaleha, and Diyab). The first three sectors comprise the continuous oil-bearing extent of the reservoir, while Diyab is  water-bearing on the basis of results from wells drilled to date (Fig. 1).

Arab C is an undersaturated-oil reservoir. The original oil column ranged from 1,400-ft thickness in the Khatiyah sector to 400 ft in the Jaleha sector. It has a weak to moderate connected aquifer lying below the oil column. Arab C development started with vertical wells, initially completed openhole. Increase in water production led to a well-completion-scheme change; vertical wells were then completed cased-hole and perforated selectively. Horizontal drilling commenced in 1992 to improve recovery and enhance production. As the water front from injection progressed, gas lifting in Arab C was initiated in 2003 to continue producing the high-water-cut wells. Currently, 60% of the Arab C producers are flowing under gas lift assistance.

Reservoir Description

Arab C is a heterogeneous organization of limestone and dolomite lithologies deposited on a shallow-water Jurassic ramp system. Hydraulically, the 80-ft-thick interval represents a net-work of grainstone conductors compartmentalized by muddy carbonate baffles resulting in  layer-constrained dynamic behavior. Lateral ranges of 1 to 4 km for baffling thin beds support a localized layer-constrained dynamic behavior and a high degree of vertical heterogeneity, though communication pathways are impacted by sporadic occurrences of cross-cutting conductive and resistive faults. Reservoir porosity is 15 to 20%, and the average permeability is approximately 150 md, but this varies widely on the basis of reservoir zone.