Cyclic steam stimulation (CSS) was tested in some of the wells of a Kuwaiti heavy oil field. It was observed that the produced water-oil ratio of the CSS wells was much less than the expected range of 0.5 to 1 in a first cycle and up to 2 in a second cycle. High water retention was diagnosed in the formation which led to further investigation to confirm it and to find out its causes.

The analysis presented is based on several approaches. Field injection and production data of each well were analyzed to estimate water-oil ratio. Temperature survey was carried out during soaking period to observe vertical movement of steam, if any. Laboratory steamflood experiments were performed on composite plugs. It was followed by oil injection (drainage) to mimic the CSS process and to quantify the water retention and to compare it with the field data. Moreover, detailed geological analysis is discussed to understand the nature of the formation.

The analysis reveals that the baffle located just above the oil formation becomes active at high pressure and allows steam to pass through it. Heat is transferred upwards not only by conduction but also by convection. Migration of steam into baffle and an upper horizon was confirmed by temperature surveys taken in the CSS wells during soaking period. Baffle acts as a thief zone and retains a great deal of water and not allowing water to come back during production phase even when pressure goes down. In summary presence of baffle is the main reason behind high water retention in the formation.

The above methodology has been adopted for the first time in the field under consideration. This will help in optimizing the steam injection strategy in future CSS wells.

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