Economic Evaluation of Subsea Development Options in the North Sea

Abstract

Summary New fields in the northern and central North Sea are decreasing in size as this major oil- and gas-producing region matures. This paper analyzes technical options and costs to develop these smaller fields. Introduction The northern and central North Sea are major oil and gas producing areas and will continue to be for some time, with undiscovered reserves estimated to be between 4 and 13 billion bbl. As the region matures, however, the sizes of the fields are decreasing, and it seems that it will be necessary to use subsea development techniques to develop the fields economically. This paper presents technoeconomic evaluation of the application of such techniques to develop small fields. The number of undiscovered fields and their fundamental characteristics (reserves, distance to existing facilities, and water depth) are identified from historical and statistical data. These characteristics are used to model typical fields, and then a range of potential subsea development schemes are evaluated for cost with typical (1989) cost data for the North Sea. Potential development schemes include fixed platforms, subsea tiebacks, semisubmersibles with a catenary anchor-leg moored buoy (CALM) and a floating storage and offloading (FSO) unit, semisubmersibles with pipeline export, and floating production, storage, pipeline export, and floating production, storage, and offloading (FPSO) units. After base costs are established for each development, sensitivity analyses are performed t show the importance of the various technical and economic parameters involved in the cost estimation. Analysis of Field Characteristics In this section, we analyze published data for undeveloped fields and discoveries in the northern and central North Sea. From these data and the estimated undiscovered reserves, we develop an approximation for the numbe of undiscovered fields and their fundamental characteristics. Reserves. An estimated 4 to 13 billion bbl oil still remain undiscovered in the sedimentary basins of the northern and central North Sea. This estimate, of course, is updated continually and is based on analysis of exploration data and an ever-increasing understanding of the basins' geology. Analysis of discovered but undeveloped reserves data for the North Sea gives the distribution of small field sizes shown i Table 1. For the purposes of this paper, "small reservoirs" are purposes of this paper, "small reservoirs" are reservoirs containing less than 100 million bbl recoverable reserves. Table 1 shows that 75% of the undeveloped fields have less than 100 million recoverable oil, which corresponds with predictions 6 that 80% of future discoveries predictions 6 that 80% of future discoveries will have reserves of less than 100 million bbl. To obtain an approximation of the frequency distribution of future field sizes, and therefore a prediction of the total number of fields, the current field-size distribution was projected onto the estimated amount of undiscovered reserves. From the current statistics, 75% of the estimated reserves (3 to 9.75 billion bbl) will be found in reservoirs with less than 100 million bbl. Table 2 shows the projected reserves distributions for the minimum (3 billion bbl) and maximum (9.75 billion bbl) cases. These simple projections indicate that 69 to 226 small undiscovered fields exist in the northern and central North Sea, with 55% of these fields having recoverable reserves less than 60 million bbl. Depending on the type of analysis used, these estimates may vary widely, but on the basis of statistical analysis of the frequency distribution of reservoir size, the size of the fields predicted is, if anything, optimistic. Distance to Nearest Platform. One of the most popular methods of developing small reservoirs popular methods of developing small reservoirs in the British sector of th North Sea is to use satellite wells tied back to existing facilities with excess production capacity. Currently, 11 miles (between Don and Thistle oil fields) is the longest distance (in the British sector) over which an oil-producing satellite has been tied back with no primary processing of the well fluids. Thus, the proximity of a new field to existing facilities can hava large effect on its potential development. Table 3 shows that >50% of undeveloped fields (and discoveries) are >9 miles from a producing platform.