Access: reservoir engineer's dream
- The Njord reservoir. All parts can be reached from the drilling and production floater. [33,778 bytes]
- The subsea wells beneath the Njord platform are configured in a V-formation, each one individually tied back to the platform. [55,854 bytes]
Though not the first to opt for it, Hydro has become a leading exponent of the concept in recent years, according to Thor Tangen, the company's senior vice president with responsibility for the Troll area. It is in the company's back yard, the Norwegian continental shelf, that the concept has been fully developed.
Hydro itself has implemented it on the Njord Field in the Norwegian Sea, and on Visund in the North Sea, where startup is expected in May. It was also to be used on the Fram Field, until the oil price collapse last autumn obliged Hydro to suspend Fram's development.
Saga has now decided to follow suit for the second phase of development of the Snorre field. "We felt vindicated when Saga chose the same concept for Snorre North," says Tangen. "This is the first time another operator has chosen the concept in the Norwegian sector."
AdvantagesAmong the advantages of a semisubmersible with drilling capability are that the wells can be located directly underneath the platform and individually tied back to the platform manifold. "This is the reservoir engineer's dream," says Tangen. "Access is almost as easy as on a wellhead platform, and you have knowledge of the individual well performance."
The minimum number of wells required to make the cost competitive with that of hiring a mobile drilling rig is 12 to 15. On Njord there are plans for 15 and on Visund 21.
In both cases, they are configured in a V-formation, and all are individually tied back. Approximately 20 such wells can be easily accommodated in a V-formation, says Tangen. If more were needed, up to about 30 could be catered for in a W-formation.
In an ideal case, the whole reservoir can be accessed from the platform location. Given the current state of drilling technology, this means that wells can be drilled into parts of the reservoir up to 8,000 meters away - equivalent to a measured wellpath of around 10,000 meters.
Whereas all parts of the Njord reservoir can be reached from the platform, Visund is a little too long for this, and two subsea wells are planned for the northern part.
Life-of-field perspectiveIt is from a life-of-field point of view that the major benefits of having an on-board drilling capability are most evident. Wells can be accessed easily and without major expense to implement improved oil recovery operations. Such operations can already significantly increase recoverable reserves, and given the speed with which downhole technology is advancing, may do so to an even greater extent in coming years.
There is no need to wait until the exploration department decides it can spare a mobile rig, which is especially important if a downhole emergency arises. As the platform rig has been paid for, the cost of using it is restricted to the operational cost, which in recent years has been roughly one tenth of the cost of hiring in a mobile rig.
"There are always surprises, specially when the reservoir is complex," says Tangen. "If you have a rig always available, you can carry out operations such as performing zone isolations, though I know of very few instances when an operator would hire a rig just to do this. Con verting wells from oil to gas production, which is becoming more common, is also easy with your own rig."
"Another advantage is that you have a drilling crew which develops a wealth of experience, both with respect to the individual wells, and in terms of knowing the rig and its capabilities. This in turn makes it easier to get acceptance for drilling difficult wells."
Adding drillingIn this respect, the production ship, which as yet has no drilling capability, is put at a disadvantage. In the case of Visund, the semi plus drilling concept was compared with both a production ship and a subsea tie-back to the Gullfaks Field. "We concluded that the concept which best catered to improved oil recovery was the semi plus drilling, precisely because of the drilling rig," says Tangen.
The production ship has one undeniable advantage over the semi in areas remote from infrastructure - its built-in storage capability. Yet for Hydro, even this is not necessarily a decisive advantage. In the case of Njord, the semi plus drilling concept emerged as more favorable than a production ship, even though the field is remote from infrastructure and a separate storage unit was required.
The semi concept is also relatively straightforward and well-known. Its motion characteristics, for example, are better than those of a ship. It's no accident that the form of drilling rig used in harsh environments is the semi, and not the ship.
But perhaps the forthcoming addition of a drilling capability to the production ship will wipe out many of the semi's advantages? Tangen is skeptical. The turret system is already a complex and critical area on the production ship, and its complexity and criticality will only be intensified by adding in drilling facilities. He waits with interest to see which company will be daring enough to order the first production ship with drilling.
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