The paper presents a field case based upon a reservoir operated by Saga Petroleum in the North Sea. The reservoir stress path was determined from a series of minifrac and fall off analyses and step rate tests on various appraisal tests and injection wells. The results show a strong heterogeneity of the stress path with a large difference between the heavily faulted Southern part of the field and the more quiet Central part of it. The stress path was further confirmed by a series of mud loss incidents during drilling that are briefly summarised.
Furthermore, in the case of two injectors, the step rate tests were performed after a significant re-pressurisation of the reservoir -i.e. 30 and 70 bars respectively. Such tests gave extremely low fracturation pressures compared to what was the reservoir stress path upon depressurisation. In fact, the fracturation pressures measured are consistent with the maximum depletion but not the reservoir pressure at the time of the test. In other words, the total minimum stress in the reservoir seems to be governed by the maximum depletion of the zone and not by the present level of pore pressure. Two wells from another North Sea reservoir will also be presented showing a similar behaviour upon repressurisation. Note that such an observation is consistent with one measurement performed on Ekofisk and published earlier on.
These observations of heterogeneous and irreversible stress paths will be analysed and compared to other observations such as the behaviour of surface subsidence during aquifer depletion and repressurisation.
In particular, it is shown that the reservoir stress path is compatible with a rigid plastic behaviour of the rock mass that has already been more or less observed in the case of the Venice subsidence. However, core measurements do not seem to obey such behaviour. The difference can then be attributed to scale effects as the stress measurements via fracturing tests affect large rock volumes.
As a consequence, the results reported here give an insight about what could be the rheological behaviour of a reservoir rock mass at a very large scale. Possible applications to large scale rock mechanics modeling such as compaction, subsidence, hydraulic fracturing, are then highlighted. Finally, the practical consequences of such a state of affairs in terms of reservoir management are drawn.