| dc.description.abstract | The objective of this master thesis was to evaluate and identify the potential and limitations when slim well technologies are implemented in the basis of design (BOD). The obvious potential from reducing the conventional well construction used at present will have a positive effect on the economical and environmental impact.
Several technologies are evaluated separately and combined in order to present an optimal well construction and layout for a successful drilling operation. Both production and exploration wells were undertaken in the thesis, resulting in an innovative exploration concept and a production well for the re-development of the Frøy field where solid expandables were implemented in the BOD. This implies that the thesis is somewhat bipartite, between futuristic thinking for the exploration concept and a feasibility study for the production well.
The incorporation of solid expandables in the production wells planned for the re-developmentof the Frøy field mitigates, or at best eliminates the expected hole stability issues. Another favourable aspect with the use of expandables is that it allows the operator to decide whether they prefer to maximize the production tubing size or slim the entire well construction while still maintaining the basis of design of the well. It is recommended that the operator maximize the production tubing size, since the value of increased flow rate surpass the savings from slimmingthe well construction. However, this conclusion should be challenged and verified by aproduction optimization analysis. In addition, the well construction cost is significantly higher when solid expandables are implemented, and this aspect has to be accounted for when evaluating the economics of the Frøy field development.
The ultra slim exploration concept is based on the objective of slimming the conventional design of exploration wells to the maximum. The approach is a bottom-up design where the lower limit of 3” open hole is the starting point. This is the minimum open hole size which allows the formation evaluation tools on the market at present to pass through. The use of solid expandables in well construction results in an ultra slim well design, and the high cost semisubmersible may be replaced with a light specification, low cost drilling vessel.
The integrity of the ultra slim design has been challenged through extensive and careful load andhydraulic analysis. The results show that the use of expandables should be evaluated on case-by-case basis due to their lower performance rating compared to conventional API casings. In addition, the need for accurate and comprehensive drilling fluid engineering becomes vital when drilling slim wells due to the high ECD values and excessive pressure losses encountered when circulating cement and drilling fluid. Taking all these aspects in to consideration, one can conclude that the concept is feasible.
The application window of the presented ultra slim exploration concept covers the conditions Det norske oljeselskap ASA are normally operating within. The maximum water depth when drilling slim wells from a vessel is 600 m, given that it is equipped with an 11” ID riser and 11-3/4” subsea BOP. In addition, this riser/ BOP arrangement allows for a well design consisting of a conductor, surface casing, and 3 additional casings prior to the open hole section through the reservoir.
The economical assessment shows that the total savings when the ultra slim exploration conceptis preferred is approximately 40%. This reduction of total cost is mainly due to the significantly lower day-rate of a vessel compared to a semi-submersible.
Another benefit from drilling slim wells from a vessel is the reduced environmental impact interms of reduced emission of pollutive gases such as CO2, NOx and SOx. It is expected that the total emission of these pollutive gases is approximately 30%. In addition, the effect from reducing the amount of required consumables will have a positive environmental impact on the entire chain from manufacturer to operation.
In conclusion, the ultra slim exploration concept is a feasible and encouraging alternative to the conventional option preferred by Det norske oljeselskap ASA and other operators to day. | nb_NO |
