Abstract
Probabilistic estimation of well duration has been common practice for over a decade; many papers have been written on the subject, and commercial software is available for the purpose. Is the subject therefore mature? The authors suggest that this is not the case, and show that several essential aspects of both data characterisation and probabilistic analysis have been overlooked in previous contributions.
A database of 104 central North Sea wells was independently re-analysed for non-productive time (NPT) from the original daily drilling reports. Mechanical extreme NPT events (those over 2.5 days) were only 4% by number, but contributed 51% of NPT by duration. It is shown that mechanical parent NPT, mechanical extreme NPT, open water WOW, and riser connected WOW are all statistically distinct, with very different occurrence frequencies and probability density functions (PDFs). These data vary widely by well type, installation type, and time of year.
The paper gives tables of all required occurrence frequencies and PDF parameters, together with a full theoretical basis. Earlier workers did not publish this information, nor did they observe that the four NPT types above are statistically distinct. It is shown that trouble-free time plus mechanical parent NPT is log-normally distributed, whereas the other PDFs are Weibull. These PDF types are not currently available in commercial software, and the authors are working with a software provider to remedy this deficiency.
Example results (PDFs of predicted duration) are given by well type, installation, and time of year. They are in good agreement with the historical database.
