This article gives a preliminary analysis of the impact of application of horizontal drilling and production techniques on the economy of offshore reservoir developments.
If we consider wells with a medium and long radius of curvature ensuring horizontal drilling in reservoirs with horizontal sections of greater than 300 m (1000 ft), theoretical research shows that flow ratios of four or more can be obtained in practice in practically all reservoirs, provided they are vertically permeable ana no more than 60 m (200 ft) thick. These theoretical results agree well with what is observed in the field.
Analysis of the data available on horizontal drillings performed so far shows that offshore, the cost per metre drilled in a horizontal well tends with experience towards the cost per metre drilled in a conventional well. The excess cost of a horizontal well is essentially due to the greater length drilled.
Offshore, the capital and operating costs bear an increasing relationship to the number of wells and production level. From a model of these costs based on 28 projects in the North Sea, the impact of replacing conventional wells with horizontal wells was studied. In the case of complete development, the reduction in the number of wells for the same production flow enables the technical cost of the oil to be decreased by 2 to 8 dollars per barrel, depending on the case. The parameters of importance are firstly, the ratio of the flowrate of the horizontal well to the flowrate of the conventional well, secondly the ratio of the cost of drilling to the total cost and thirdly the flowrate of the conventional reference well.
Cases of application yielding the greatest savings are those where the share of drilling costs out of the total cost is high, over 25 an a cases where the flowrate of conventional wells is low, below 2000 Bopd.
Drilling and bringing into production horizontal wells has been growing considerably since the beginning of the 1980s. Originally restricted to technological operations and subsequently to pilot projects, horizontal drilling is now being applied to complete developments of offshore fields. Development of the Rospo Mare reservoir in Italy by Elf, Unocal operations in Dutch waters in the North Sea, Arco in Indonesia, Standard Oil (ex Sohio) in Alaska, where the situation can be assimilated to an offshore situation, all bear witness to the fact that horizontal drilling has now come of age.
If one excludes a few operations for essentially technological purposes with a view to gathering know-how, trying out new techniques or involving applied research, the great majority of horizontal wells have been drilled offshore (see figure 1). Why ? for mainly economic reasons, since there is no production-related reason that dictates that an offshore reservoir is a better case of application than an onshore reservoir. From the drilling standpoint, it can be said that offshore, the habit of drilling deviated wells has provided a foundation on which horizontal drilling can rest, though cost factors are the main motive.