Friday, April 8, 2011

Ghawar Oil Field

The Ghawar Oil Field is by far the largest conventional oil field in the world and accounts for more than half of the cumulative oil production of Saudi Arabia. Although it is a single field, it is divided into six areas. From north to south, they are Fazran, Ain Dar, Shedgum, Uthmaniyah, Haradh and Hawiyah. Although Arab-C, Hanifa and Fadhili reservoirs are also present in parts of the field, the Arab-D reservoir accounts for nearly all of the reserves and production.

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The Ghawar Field was discovered in 1948. Production began in 1951 and reached a peak of 5.7 million barrels per day in 1981. This is the highest sustained oil production rate achieved by any single oil field in world history. At the time that this record was achieved, the southern areas of Hawiyah and Haradh had not yet been fully developed. Production was restrained after 1981 for market reasons, but Ghawar remained the most important oil field in the world. The production of the Samotlor Field in Russia was greater during the mid-eighties, but this was because production at Ghawar was restrained. Development of the southern Hawiyah and Haradh areas during 1994 to 1996 allowed production from the Ghawar Field to exceed 5 million barrels per day once again, more than Samotlor ever produced.

This remarkable production history is because of the enormous size of the Arab-D reservoir in the Ghawar Field. Alsharhan and Kendall (1986, Table 1) provide a figure of 693,000 acres for the productive area of the Ghawar Field. This represents a single, pressure-continuous reservoir. Cumulative production by yearend 2000 was about 51 billion barrels of oil.

The anhydrite in the Upper Arab-D forms the seal for the 1,300-foot oil column in Ghawar. It is composed of sabkha evaporites and subaqueous evaporites with thin carbonate interbeds that can be traced for hundreds of kilometers. The anhydrite thickens to the south at the expense of the reservoir zones; the combined thickness remains relatively constant.

The Arab-D reservoir at Ghawar comprises two major shoaling-upward cycles deposited during a relative highstand in sea level (Mitchell et al, 1988). It is composed of skeletal grainstones and packstones with ooid grainstones locally common in the upper Arab-D. The diagenetic processes that have affected the Arab-D reservoir include dolomitization, leaching and recrystallization, cementation, compaction and fracturing.

Interparticle porosity is abundant in the Arab-D reservoir in the Ghawar Field and moldic porosity is common as well. Intercrystal pores are common in dolomites and microporosity is abundant in both limestone and dolomite lithologies.

In the uppermost part of the Arab-D are occasional zones that contain more than 10% of a stromatoporoid sponge known as cladocoropsis. Where this facies is dolomitized, the relatively fine-grained matrix is replaced by dolomite and the cladocoropsis is leached, causing a phenomenon described by reservoir engineers as super-permeability. These super-permeable zones, where present, offer so little resistance to fluid flow as to be difficult to model for reservoir engineering purposes.

The source rock for the Ghawar oil is believed to be the Tuwaiq Mountain Formation, which underlies the Hanifa. It is Callovian and Oxfordian in age and reaches a thickness of more than 300 feet in the basinal area between the Ghawar and Khurais Fields. That this moderate volume of source rock should produce the largest accumulation of light oil in the world indicates very efficient migration and entrapment. The fact that the Ghawar oil-water contact is substantially higher on the west flank than on the east indicates a hydrodynamic gradient to the east, which may explain the much larger volume of oil in Ghawar than in Khurais.

This peripheral waterflood project began in the early sixties in the northern parts of the field. By the time that the southern Hawiyah and Haradh areas were developed during 1994 to 1996, horizontal-drilling technology was available. Horizontal injectors were completed above the tar mat to provide line source distribution of water along the periphery of the field.

How much oil in Ghawar?

No one exactly knows how much oil lies beneath Ghawar. Some estimates (World Energy Outlook, 2005 and 2008) put the oil in place as high as 250-300 billions! However, simple calculations using the reservoir volume, porosity, and water saturation for all the five areas of Ghawar (Table 2) would give the maximum amount of oil in place at about 190 Bbo. How much of this oil (if true) is recoverable is another story. Total recoverable oil reserves from Ghawar have been reported variously as 75 Bbo (Halbouty et al., 1970, AAPG Memoir 14), 68-46 Bbo (Oil & Gas Journal issues 1973-77), 80.3 Bbo (Beydoun, The Middle East, 1988), 66 Bbo (Mann, 2003, AAPG Memoir 78), and 140 Bbo (World Energy Outlook, 2008). The latter source places the natural gas reserves of Ghawar at 186 Tcf, thus giving combined reserves of 97 Bboe.

Beydoun in his book (The Middle East, 1988) reports that Ghawar had produced 19 Bbo by 1979. According to an article on Ghawar in the AAPG Explorer(January 2005), the cumulative production from the field was 55 Bbo. The International Energy Agency in its 2008 World Energy Outlook states that the oil production from Ghawar reached 66 Bbo in 2007 and that the remaining reserves are 74 Bbo.

Data on Ghawar reported in the past issues of Oil & Gas Journal indicate that when Ghawar came on stream in 1951 it produced 126,000 bopd but production steadily rose with a major boost soon after the 1973 oil shock so that the field’s 1975 output was 4.2 MMbopd; this reached a maximum production of 5.7 MMbopd in 1981. From 1982-1990, the Saudis lowered their oil production for market considerations (most notably the oil crash of 1985) and thus Ghawar’s production was 2.5 to 3 MMbopd during that decade. A senior geologist with Saudi Aramco, A. M. Afifi, in his 2004 AAPG Distinguished Lecture, reported production values of 4.6-5.2 MMbopd for Ghawar from 1993 through 2003. These data indicate that 50-65% of Saudi Aramco’s oil production has traditionally come from Ghawar. Apparently, one half of Ghawar’s production (2.0 to 2.7 MMbopd) comes from the Ain Dar and Shedgum areas, while Uthmaniyah provides 1 MMbopd, and another million barrels or so comes from Hawiyah and Haradh combined.

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1 comment:

  1. Chemical Injection Pumps are so important to maintain smooth oilfield production. Nice reading. Thanks