Various Worldwide CO2-Enhanced Oil Recovery Operations

It has been well established that only a relatively small percentage of the oil present in an oil field can be recovered using well-established extraction techniques. Injecting CO2 can enhance oil recovery (EOR), a technique well established in the oil industry. Following injection, some of the CO2 remains in the field; hence this technique can be applied for the storage of CO2. In particular, where fields are approaching the end pf their useful working life, the additional income generated from the extra oil produced can offset the costs associated with the CO2 injection technology. When CO2 is injected into an oilfield, it may mix with the crude oil, causing it to swell and thereby reduce its viscosity. This also helps to maintain or increase reservoir pressures. The combination of these processes allows more of the crude oil to flow to the production wells. In other situations, the CO2 is not soluble in the oil and here, injection of the CO2 helps to sweep the oil towards the production well. Up to half of the injected CO2 can be stored in the immobile oil remaining in the reservoir at the cessation of production. The remainder is recovered from the production well and recirculated. It is estimated that globally, some 130 Gtonne of CO2 could be sequestered as a result of CO2-EOR operations.

Commercial injection of CO2 for enhanced oil recovery began in 1972 in the Permian Basin of the USA. CO2 flooding was later implemented at depleted oil fields in the Rocky mountain, Mid-Continent and Gulf Coast, involving different types of reservoir formations (predominantly sandstone rather than carbonate reservoirs prevalent in the Permian Basin). There followed a steady increase throughout the next two decades, followed in turn by a significant acceleration from the late 1980s onwards. Such development took place for three main reasons:

• EOR development and operating costs fell as the technology developed further
• CO2 supplies increased as a result of the construction of new long-distance pipelines
• Major oil companies became organisationally leaner and more efficient

Limited fiscal incentives for EOR projects in the USA also had a favourable, albeit more modest, impact on investment. In such a climate, the result was that CO2-EOR production in the USA increased eightfold from less than 4000 m3/day in 1985, to 31,000 m3/day in 1998. Commercial CO2-EOR operations are currently taking place in four countries. In order of scale of production these comprise the USA, Turkey, Trinidad and Canada. As well as these projects, during the 1980s, there were also several small operations active in Hungary. The latter were operated successfully up to the mid 1990s but are no longer working. By the late 1990s, worldwide CO2-EOR production averaged ~33, 460 m3/day (210,444 BOPD) from 79 individual projects.

The initial interest in CO2-EOR technology arose predominantly in the USA and this region still accounts for the majority of CO2 flooding operations. Total US production in 1998 was estimated to be 31,190m3/day (196,194 BOPD), essentially all from one large field. Turkey was second at 2,146 m3/day.

In the USA, the majority of CO2 flooding activities take place in the south-western region of the country, within the mature Permian basin of western Texas and eastern New Mexico, where numerous depleted oil fields exist and reservoir properties are often suitable for CO2 miscibility. The Permian basin has been the worldwide focus of CO2-EOR technology development and application. Other CO2 floods are located in the Rocky Mountain region (6 projects), Mid-Continent Region (8 projects) and coastal onshore Gulf of Mexico (5 projects). In addition, two large-scale hydrocarbon-miscible floods are located in Alaska, utilising CO2-rich (~22%) hydrocarbon gas re-injected to maintain reservoir pressure and maximise crude oil production.

CO2-EOR projects operating utilise naturally occurring CO2, tapped from high pressure, high purity underground deposits. For example, Shell's McElmo Dome field in south-western Colorado contains >283 million m3 of confirmed CO2 reserves at a pressure of ~2000 psi. However, a small but significant number of EOR projects utilise anthropogenic CO2 sources, such as waste streams from fertiliser or gas processing plants. Major projects are operating in Texas, Colorado, Oklahoma and Louisiana:

State	Plant name	Plant type	CO2 supply(MMcfd)     (106 m3/d)		EOR fields	Operator
Texas	Mitchell, Gray Ranch, Puckett, Terrell	Gas processing	250	1.08	SACROCCrossett	Pennzoil, Altura
Colorado	LaBarge	Gas processing	150	4.25	Rangely	Chevron
Oklahoma	Enid	Fertiliser	35	0.99	Purdy	Occidental
Louisiana	Koch	Gas processing	25	0.71	Paradis	Texaco
	Total		460	13.03

A further project is operating at Joffre, Canada, where Novacor Chemicals, a leading North American producer of ethylene, polyethylene and styrenic polymers, removes CO2 from its ethane feedstock and sends it to Vikor Resources who apply in as part of an enhanced oil recovery scheme.

More recently, a major new project that is substantially extending the use of anthropogenic CO2 has been added, namely the Weyburn CO2-EOR project in Canada. As part of this project, a new pipeline was built from the Dakota Gasification Plant in North Dakota, which transports 2.8 million m3/day of CO2. Spare pipeline capacity is also available for other fields in Montana, the Dakotas and Saskatchewan. It is anticipated that over the next 20-25 years, oil production will be increased and ~19 million tonnes of CO2 will be permanently sequestered in the formation.

CO2-EOR production currently remains concentrated within a small number of extensive, highly productive projects that account for most CO2-EOR production in the USA, along with essentially all of Turkey's EOR production (in the Bati Raman field). Overall, a mere handful of large projects account for about half of the worldwide CO2-EOR production.

Most of the firms involved in this sector are major oil companies that are active in refining and marketing activities as well as production operations. The main production of CO2-EOR resides with the following oil companies:

	Percent of total (%)			CO2-EOR Production
Rank		Company	No. of projects	BOPD	m3/day
1	27	Altura (Shell/Amoco)	13	55,928	8892
2	14	Amerada Hess	3	30,500	4849
3	11	Mobil	7	22,800	3625
4	12	ARCO	3	21,070	3350
5	7	Chevron	3	14,511	2307
6	6	Turkish Petroleum	1	13,500	2146
7	5	Texaco	5	10,270	1633
8	4	Exxon	4	9,250	1471
9	4	Pennzoil	1	9,000	1431
10	3	Amoco	4	7,030	1118
	8	Other US/Worldwide	35	16,585	2637
	100	TOTAL	79	210,444	33,459

In the USA, there is a CO2 pipeline infrastructure >3000 km in length. This feeds CO2 to oilfields, mainly from naturally occurring sources, to EOR operations. Many of these pipelines have been operating since the 1980s, with the CO2 transported as a supercritical or dense phase fluid. This is more economical than transporting it in the gaseous phase. There are also a number of CO2 pipelines in use within Canada, these including a 205 mile long cross-border pipeline from the lignite-fired Dakota Gasification Company synfuels plant site in North Dakota.

The Future:

Enhanced oil recovery from CO2 flooding is expected to continue increasing under most world price scenarios although EOR production will continue to be influenced by oil prices and technological improvements. Higher oil prices enhance revenues and profitability. Technological improvements, such as improved flood monitoring using 4-D seismic surveillance, reduce extraction costs, which in turn enhances profitability, thus stimulating investment and increased production. It is forecast that in the USA, CO2EOR production will remain relatively flat until 2010, after which higher oil prices will stimulate increased activity. Production is forecast to increase to a peak of ~35,000 m3/day (220,000 BODP) in 2015, before declining as the resource base currently identified becomes substantially depleted.