Basic Knowledge about Multilateral Well and Completion

A multilateral well is a well with two or more laterals (horizontal, vertical, or deviated) drilled from a main mother well. This allows one well to produce from several reservoirs. Multilateral wells are suitable for complex geology where drilling more new wells to penetrate to those reservoirs is not economical. Lateral sections may be used to produce from a separated section in depleted, faulted, layered and heavy oil reservoirs.

Figure 1 – Multilateral Well Bashkiria drilled in 1953 (Credit –

Figure 2 shows how multilateral wells fit with several geological structures.

Figure 2 – Multilateral well with different geological structure

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Petroleum Engineering Development Timeline from 1848-1959 (1st Part)

Petroleum engineering has been developed since the beginning of the 19th Century. The following images demonstrates petroleum technology milestone from 1848 – 1959. This is the first part of this series and more to come.

1848:World’s First Oil Well – Major Aleveev drills the world’s first oil well at Baku, Azerbaijan using a primitive cable-tool drilling technique which originated in ancient China.

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Floating Offshore Structures – Offshore Structure Series

There are many oil and gas discoveries which are out of reach of fixed structures for one reason or another. They may be in extremely deep water, or the oil or gas deposit might be too small or too widely spread to warrant the high cost of building a fixed structure. In these cases, seabed-completed wells may be connected to a floating platform moored above the field, using a production marine riser. The limiting conditions for fixed installations are not clearly defined, and they have been used in some cases for depths of over 250 meters, although this is in a benign environment. Floating platforms can also be used as the basis for an Early Production System (EPS), in which the appraisal wells drilled from a floating drilling vessel are completed at the seabed and produced to a floating platform carrying the required process plant and other facilities. This allows production to begin and create income whilst a fixed platform is being designed and installed for full field development. In this article, there are some discussions about three main types of floating offshore structures which are Tension Leg Platforms, SPAR and FPSO.

Tension Leg Platforms (Tethered Buoyant Structures)

One more form of offshore platform is what is known as the Tension Leg Platform, or Tethered Buoyant Structure. This method is intended for oil and gas production from water depths of over 500 meters. The platform works in much the same way as a taut moored buoy, which is anchored to the seabed using a vertical wire. The Tethered Buoyant Structure is basically a large, semi-submersible floating vessel, which uses a heavy gravity anchor to moor it to the seabed. Tension force is maintained in these vertical cables by adjusting the buoyancy of the floating platform, to ensure positive tension at all times. This method reduced marine response in the platform to effectively zero in vertical terms, and very little in horizontal terms. Horizontal drift can be further reduced as necessary. By using buoyance against a tension mooring system, this allows the use of a semi-submersible floating platform which can carry an additional load, balancing this out by increasing the buoyancy.

This type of structure is still under development, and there are still many problem points to iron out. How widespread it will become in the future is largely dependent on solving these issues, along with a thorough economic assessment comparing the system to other available ones.

Figure 1 – Tension Leg Platform

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Summary of Comparison between Piled Offshore Platform Structures VS Concrete Gravity Structures

We’ve discussed about two type of fixed platform structures which are piled offshore platforms and concrete gravity structures. In this article, it summarize the comparison between these two fixed offshore structures.

Figure 1 – Piled Structure and Gravity Structure of Brent (Courtesy of Shell UK)

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Concrete Gravity Structures – Offshore Structure Series

As the name suggests, the concrete gravity structure is reliant on its own weight, and the capability of the seabed to maintain that weight, in order to remain stable. They are designed particularly with storm conditions in mind. Like other types of structure, they come in multiple design variations, and may be made out of concrete, steel, or a combination of the two. Concrete gravity structures were first used in the Ekofisk Field off Norway, although the design principle had previously been used in lighthouse construction. The Ekofisk structure, which had originally only been intended for oil storage but was then modified for use as a large gas handling and compression plant, was soon followed up with the construction of multiple additional drilling and production concrete gravity structures made from reinforced concrete. Given the huge demand placed on onshore prefabrication sites, and the significance of the water depths available to constructors for fabricating and towing these structures near to the shore, there has been a wide variety of different gravity designs, despite being constrained by the conditions of the construction site. It has been impossible to create an optimized design which is suitable to be built at all available sites.

The concrete gravity structure is built in a tapered shape, with as much of the mass and bulk concentrated as close as possible to the seabed. Ideally, the platform is constructed close to the shore, and the topside facilities are placed in a sheltered site before the offshore tow begins. Then, the whole thing is moved to its final location through the use of ocean-going tugs. This is done as much as possible using a multi-celled caisson raft, which can measure up to 100 meters high and 60 meters wide. From this raft base, a number of columns will be carried up to the full height of the structure. When the raft reaches the offshore location, the caisson is water ballasted and landed on the sea bed, Offshore installation can therefore take as little as a few days, which is certainly an advantage in harsh areas which have short fair weather periods. Concrete gravity structures can be used in water depths up to 160 meters and with weights of over 300,000 tonnes.

Examples of concrete gravity structures – Ninian Central Platform

Figure 1 demonstrates Ninian Central platform, a large concrete tower with a series of tanks around the base. These concrete fixed platforms are able to store fluids, and can also be attached to export lines, which gives them a significant advantage over steel jacket platforms. Jacket platforms generally lack tanks, although they can be built on deck, which means that their export can be entirely lost if a tanker does not stick to its strict schedule. Concrete platforms also do not need to be secured to the seabed. Thanks to skirts around the concrete, erosion is prevented. Concrete platforms perform exactly the same function as steel jacket platforms, with only the support structure being different.

Figure 1 – Ninian Central platform

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