The largest gas discovery in Taiwan is the Tiehchenshan (TCS) gas field, which is a lean gas condensate reservoir with strong underlying aquifer. The gas reservoir has been produced since 1965, and in 1990, in light of considerations regarding domestic gas demand and available infrastructure, it was converted to underground gas storage (UGS) by injecting re-gasified LNG.

It was recently found that one of the blocks serving for gas storage is facing gradual water encroachment. In addition, between 2011 and 2012 it was discovered that average reservoir pressure suddenly increased by about 170 psi. It was surmised that this could be a result of the influence of strong water-drive due to the imbalance of injection and production over a very long period.

The aims of this study are to effectively assess the extent of gas bubble in the reservoir through the integration of a geological model, reservoir simulation, and history matching process associated with a 54-year production history; and, on this basis, to re-evaluate working and cushion gas requirements for feasible operating conditions. However, owing to the poor quality of oil production data, the black oil model is utilized for reservoir simulation rather than the compositional model. Tracer tracking of reservoir simulation is also applied to identify the movement of injected gas due to the limitations imposed by the lack of the composition model.

The reservoir simulation model for describing the dynamic behavior of the UGS was successfully developed and verified, and it was also applied to evaluate the possibility of the UGS expansibility. Results of the study suggested that one of the gas storage blocks in the UGS is no longer appropriate for its function. Instead of using it for storage, it is recommended that the gas from it be produced as quickly as possible accompanied by water encroachment.

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