Flowlines for oil gathering systems in water-flooded oilfields
Secondary oil recovery in full flow
Although secondary oil recovery by means of water flooding or re-injection can almost double the recovery rates from oilfields, this technique entails more exacting demands on the steel pipe used in the oil gathering lines. Salzgitter Mannesmann Line Pipe has now developed a particularly efficient system solution to address these challenges.
Primary oil recovery – i.e. by means of the reservoir pressure alone – ranks as a fully developed technology. However, as the reservoir pressure decreases over time, so do the recovery rates. In secondary oil recovery, water is injected into the reservoir to maintain the reservoir pressure. This raises both recovery rates and the overall recovery from an oilfield quite considerably. The mode of recovery is based on the use of injection lines for the water and gathering lines – or flowlines – for the recovered medium, an oil-water mix.
Conveyance of aggressive media
While polyethylene and polypropylene coatings provide for efficient external corrosion protection in all kinds of soil, up to and including severely aggressive, pipes used for conveying hostile media require equally efficient internal corrosion protection. Compared to primary recovery, secondary recovery features significant differences regarding the composition and properties of the media conveyed. Chemical analysis of these oil-water mixes reveals high mineral contents, the presence of solids and possibly dissolved gases such as H₂S or CO₂. Re-injection of the water also places high requirements on the corrosion protection of the line pipe used.
In an initial field test, the pipes were exposed to real service conditions for a period of five years. During this time, the pipe string was operated parallel to another pipe system in order to check the resistance of the lining.
Flowlines
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System solution with modified cement mortar lining
For this application, Salzgitter Mannesmann Line Pipe has developed a system of steel pipes and fittings with a modified cement mortar lining (mCML) and slip welding joints. Precisely matched to the composition of oil-water mixes, the modified cement mortar is applied using a special technique that prevents demixing while ensuring maximum compaction and a smooth surface. The use of slip-welding joints seals the pipes and ensures seamless corrosion protection along the pipe string. Besides passing its laboratory tests, the system's reliability has proven itself over several years of successful field testing.
Simple pipe-laying
Pipe-laying is facilitated through the use of a caulking tool perfectly adapted to the design of the slip-welding joint. Before lining, an oil-resistant rubber stop ring is placed into the socket base before inserting the spigot end. Firmly anchored in the cement mortar, it prevents lining damage and spalling during the installation of the pipe joint. The joint area is protected with a special sealant, an elastic thermosetting material that is applied to the socket base before inserting the spigot end. After tack-welding the spigot end, any excess sealant can be smoothed out with the aid of a scraper before welding the pipe joint.
Operating pressures up to 200 bar
Depending on the size, the pipes and fittings can be used at operating pressures of up to 200 bar and service temperatures of up to 130 °C. The lining is suitable for liquid media containing dissolved salts and gases or solids particles and is resistant to alkaline, neutral, and weakly acidic environments (pH >6). The application range may be widened should additional field tests prove successful.