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In a few years there will be a need for performing a considerable number of subsea plug and abandonment (P&A) on the Norwegian Continental Shelf (NCS). There are certain challenges associated with that such as more difficult access to subsea wells compared to ordinary platform wells, high daily rates of semi-submersible rigs and the fact that these rigs will be allocated for drilling new exploration and production wells to sustain the hydrocarbon production. For the moment there is large focus on looking for rigless technologies to decrease P&A cost and use of rig time. A probabilistic approach should be used to assess the cost and duration saving potential of such technologies relative to semi-submersible rigs.In this paper, we will consider rigless technologies for performing parts of the P&A operation. For the first time, a risk based probabilistic approach including learning curves, correlations and possible risk events is used to evaluate subsea batch operated P&A.A realistic example case which includes the use of a semi-submersible rig for batch operated P&A of subsea production wells is used to show how a probabilistic methodology can be implemented for obtaining probabilistic estimates of P&A cost and duration. Inclusion of learning curves and correlations makes the application of a probabilistic approach for multi-well campaigns challenging. It is demonstrated how to incorporate learning curves, correlations and possible risk events to capture a realistic range of cost and duration for multi-well P&A. In a second operational example, a light well intervention vessel is employed to accomplish preparatory work as well as wellhead cutting and removal for P&A of the subsea production wells in batch campaigns. These two case studies will be compared from a cost and duration point of view and it is shown that there are significant cost and duration savings related to transferring P&A activities from rigs to alternative intervention and vessel technologies.
In a few years there will be a need for performing a considerable number of subsea plug and abandonment (P&A) on the Norwegian Continental Shelf (NCS). There are certain challenges associated with that such as more difficult access to subsea wells compared to ordinary platform wells, high daily rates of semi-submersible rigs and the fact that these rigs will be allocated for drilling new exploration and production wells to sustain the hydrocarbon production. For the moment there is large focus on looking for rigless technologies to decrease P&A cost and use of rig time. A probabilistic approach should be used to assess the cost and duration saving potential of such technologies relative to semi-submersible rigs.In this paper, we will consider rigless technologies for performing parts of the P&A operation. For the first time, a risk based probabilistic approach including learning curves, correlations and possible risk events is used to evaluate subsea batch operated P&A.A realistic example case which includes the use of a semi-submersible rig for batch operated P&A of subsea production wells is used to show how a probabilistic methodology can be implemented for obtaining probabilistic estimates of P&A cost and duration. Inclusion of learning curves and correlations makes the application of a probabilistic approach for multi-well campaigns challenging. It is demonstrated how to incorporate learning curves, correlations and possible risk events to capture a realistic range of cost and duration for multi-well P&A. In a second operational example, a light well intervention vessel is employed to accomplish preparatory work as well as wellhead cutting and removal for P&A of the subsea production wells in batch campaigns. These two case studies will be compared from a cost and duration point of view and it is shown that there are significant cost and duration savings related to transferring P&A activities from rigs to alternative intervention and vessel technologies.
High demand for plug and abandonment (P&A) of subsea wells in the forthcoming years on Norwegian Continental Shelf, low availability and high daily rate of semisubmersible rigs as well as high proportion of P&A cost to total cost of drilling exploration wells challenge industry and intensify looking for alternatives and rigless technologies which can make P&A operation more cost effective and release time of semisubmersible rig earlier. Light well intervention vessels currently can take over some parts of P&A operation and release time of semisubmersible rigs in order to function in drilling and completing of new wells. In addition these vessels have a low daily rate compared to semisubmersible rigs and decrease cost associated with P&A. A systematic yet simple approach is required to acquire a correct picture for expenditure and time of these alternative technologies. In this paper, potentials and experiences of light well intervention vessels for P&A and benefits associated with them will be discussed. An example case is considered wherein a new technology is deployed from a light well intervention vessel to set surface plugs in a multi-well campaign. Thereafter, benefits associated with application of this rigless technology are discussed and highlighted. The paper will discuss different approaches for structuring probabilistic time and cost estimation of multi-well P&A campaigns discussing at which level these should be performed and what is the most appropriate approach with respect to modularity and efficiency. This will also be seen in relation to the Oil and Gas UK cost estimation guideline for P&A and the NORSOK technical specification. The objective is to keep the simulations simple while still acquiring the correct range of time and cost for multi-well P&A operation in order to make the proposed approach applicable for the industry.
Well abandonment operations can be very time-consuming and costly, and thousands of wells need to be permanently plugged and abandoned offshore Norway during the upcoming years. One possible way to reduce costs during P&A operations is to leave most of the production tubing in the well, as this would save significant rig time. A major concern with such an approach is, however, whether the cement will properly displace the original fluid, due to lack of tubing centralization and possible unfavorable flow dynamics in the annulus. In this paper, we demonstrate by full-scale experimental tests that it is possible to obtain good cement placement when the tubing is left in the hole, with and without control lines. Full-scale tests have been performed with both conventional and expandable cement to determine the sealing ability of annulus cement when tubing is left in hole. The quality of the cement placement was evaluated by pressure tests with water; where leakage rates and pressure drops over the test sections were recorded, and by visual inspection after cutting the test assemblies at different places. It is seen from the experiments that cement is well placed in the annulus when tubing is left in hole, but some microannuli are detected.
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