In enhanced oil recovery (EOR) and carbon capture and storage (CCS), the traditional approach to COcompression has been to use high-speed reciprocating compressors, but recent investigations have revealed this technology has several limitations, among them the significant restriction of volume flows due to their mechanical design.

For this reason, centrifugal systems have become a promising solution for future COprojects. Centrifugal compressors fall into one of two technology groups, namely single-shaft centrifugals and multi-shaft integral-gear centrifugals.

In EOR, when water injection is used to push the remaining oil out of a reservoir, it leaves a significant residue behind. COflooding is a significantly more effective method. Oil and COmix above a pressure known as the minimum miscibility pressure (MMP). At or above the MMP, COacts as a solvent, sweeping the reservoir and leaving very little residue behind. At pressures below the MMP, COalso assists oil production by swelling the oil and reducing its viscosity.

This technique also can be used in natural gas fields to recover additional gas from exploited fields. The elimination of COfrom the atmosphere is a welcome side effect.

Compression benchmark

In 2006, MAN Diesel & Turbo delivered the third unit of an eight-stage integrally geared compressor for compressing COto 187 bar, a record achievement for this technology in the gasification industry. Two prototypes of this RG type compressor had been in continuous operation for the same customer in North Dakota since 2001.

The delivery consisted of a full package, including compressor core unit, driver, process gas coolers, lube oil system, process piping and auxiliaries, and the Turbolog control system.

In this project, COis compressed to 187 bar and fed into a 330-km (205-mile) pipeline that runs north into Canada and ends at EnCana’s Weyburn oil field, where it is injected into the field. The results at Weyburn have been spectacular. The field currently is producing 28,000 b/d of oil. This represents an 18,000 b/d increase over the 10,000 b/d that would be produced without the CO2 flood.

Traditional approach

The traditional approach to COcompression has been to use high-speed reciprocating compressors, with the main reasons being:

Flexibility with regard to pressure ratio and capacity (if equipped with variable speed drive or valve unloaders);

Short delivery times;

Lightweight skid-mounted units that can be relocated at will; and

Familiarity of the field operators with these machines and their suppliers. By comparison, centrifugal compressors offer:

Higher capacity volume flows;

Superior efficiency;

Oil-free compression;

Higher speed that is better matched to the high-speed driver commonly used in the 10-40 MW range; and

Extended intervals between overhauls.

Centrifugal options

Within the centrifugal compressor markets, there are two technologies, namely single-shaft (in line, between bearings) centrifugals and multi-shaft integral-gear centrifugals, both of which have been manufactured by MAN and have been used in CO2 service. The company has come to the conclusion that, for most high-flow onshore COapplications, the multi-shaft integral-gear design offers higher efficiency. The reasons include:

Optimum impeller flow coefficient because optimum speed can be selected for each pair of impellers;

Axial in-flow to each stage;

The ability to use shrouded or unshrouded impellers;

Small hub/tip ratio;

Intercooling after each stage (impeller);

External connection after each stage, which allows more flexibility in selecting the pressure level for the dehydration system if applicable;

Practically no limit to the possible number of stages in one machine; and

The ability to direct-drive by a four-pole electric motor on the bull-gear or a steam turbine on one of the pinions.

Challenges for offshore COcompression

Overall, for volume flows >12 kg/s and pressures up to 250 bar, integral-gear compressors have a definite advantages over reciprocating or supersonic technologies and inline centrifugals in most COservice. Inline compressors require approximately twice the number of stages that integral-gear compressors require, leading to one or two additional casings. Another advantage of integral-gear compressors is their higher efficiency, which they deliver without additional maintenance requirements.

For future offshore COcompression systems to be installed more regularly on platforms, FPSOs, and subsea installations, several challenges will have to be overcome. These include space restrictions, yaw and roll on FPSOs, and a need for high reliability and very high pressure levels.

To date, most compressor manufacturers have preferred to use inline barrel compression systems for offshore high-pressure COapplications and in situations where the pressure is >250 bar. One example of the offshore installation of this type of compression system at these pressure levels is on an FPSO that will be producing offshore Brazil in a 550-bar COcompression train. While there has been some application of this technology offshore, future research work will have to be done to effectively transfer experience from onshore COsystems to the offshore environment and improve the existing compressor design.

Includes excerpts from OTC Brasil paper 22659.

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