Abstract

Haradh forms the southernmost part of the super giant Ghawar oil field located about 80 km (50 miles) onshore from the Arabian Gulf, in the Eastern Province, Kingdom of Saudi Arabia (Fig. 1).

A major feature of the Ghawar field is its tilted Original Oil Water Contact (OOWC), getting shallower from North to South Ghawar at an average gradient of 0.36 m (1.2 ft) per km. In Haradh, the OOWC displays also a large but localized west-to-east component.

Understanding the OOWC tilt mechanisms is vital since it will improve the effectiveness of the on-going Oil-Water Contact (OWC) delineation program. This will result in better placement of future producers and injectors, which in turn will reduce the cost of future developments.

Statement of Theories and Definitions

To date, two major concepts have been proposed to explain the tilted OOWC in Ghawar and more particularly in Haradh:

• One is that the OOWC tilt is caused by regional changes in reservoir fluid densities¹.

• Another hypothesis is a dynamic flowing aquifer hitting the Ghawar field on Haradh west flank and pushing the OOWC to a shallower level along its south to north journey².

In summary, a static concept of the equilibrium is confronted to a dynamic one. The existence of pre-production pressure gradient in the field has to be verified to support a dynamic cause of the OOWC tilt. Integrating field temperature, salinity and pressure data as arguments, these hypotheses are critically discussed. During this process, a causal link to a major tectonic accident is proposed.

Introduction

Berg completed the surface mapping of the Haradh structure in 1940³. Wildcat HRDH-1 struck oil in the Arab-D reservoir (Jurassic) in 1949. The first OOWC delineation wells were drilled during the seventies.

Although 'Ain Dar and Shedgum (North Ghawar) areas are on stream since 1951, very limited production occurred in Haradh until 1996 when Haradh Increment-I (North Haradh) was put on stream to reach the required production plateau (Fig.2). Prior to that date, the OOWC was roughly defined by several delineation wells with a spacing of 16 miles.

A fresh look at existing field data, recent advances in regional tectonics and simulation results all support a static origin of the OOWC tilt and confirm the conclusions of a previous work¹.

Description of the Haradh OOWC

Haradh delineation wells showed that:

• The OOWC gets shallower from north to south to reach -6,420 ft at the Haradh tip;

• The west flank OOWC gets locally 800 ft shallower than that in the east (Fig. 3).

Note that western and eastern aquifer legs are separated by almost 15 miles of a large anticline structure with a maximum 3°-dip angle.

Drill Stem Test (DST) samples of Well-A (Well location on Fig.2) showed that the formation did not contain recoverable oil to the top, which established the Highest Known Water (HKW) at -5,863 ft in the west and confirmed the Formation Analysis Log (FAL) (Fig.3).

FAL of Well-B (Well location on Fig.2) showed that the formation contained high oil saturation throughout the good quality zones, which established the Lowest Known Oil (LKO) at -6,625 ft in the east (Fig.3).

Aquifer water on the Haradh west flank is anomalously fresh (30,000 ppm TDS at Well-A) and recent (6,000 to 20,000 year-old), compared to the more saline water sampled on the east flank (120,000 to 150,000 ppm TDS). Recent isotopic analysis in the Dhahran Research and Development Labs4 showed the meteoric origin of the aquifer water sampled on Haradh west flank. Note: The Jurassic Arab-D reservoir was deposited some 150 million years ago.