A warning system for exceeding subsidence limits

摘要

In some cases hydrocarbon production is legally restricted to a maximum amount of subsidence that it generates. To support the field management in such circumstances we developed a general framework that enables decision-making when a threshold in a process is about to be exceeded (“an event”). Measurements are combined with prior information to update the probability of such an event. This prior information is derived from the results of an ensemble of model realisations that span the uncertainty present in the model before any measurements are collected; only probability updates need to be calculated, which makes the procedure very fast once the basic ensemble of realisations has been set up. The procedure is demonstrated with an example of a synthetic gas field. Starting with 100 realisations spanning the prior uncertainty of the process, the measurements collected during monitoring bolster some of the realisations while refuting others. In this procedure, more data will mean a sharper determination of the posterior probability. We show the use of two different types of limits: a maximum allowed value of subsidence and a maximum allowed value of subsidence rate for all measurement points at all times. These limits have been applied in real world cases. The framework is general and is able to deal with other types of limits in just the same way. It can also be used to optimise monitoring strategies by assessing the effect of the number, position and timing of the measurement points. Furthermore, in such a synthetic study, the prior realisations do not need to be updated; spanning the range of uncertainty with appropriate prior models is sufficient.