Abrupt Drawdown Observed in Sandstone-Shale Interbedded Layers Surrounding Underground LPG Cavern

An abrupt drawdown was observed at observation holes located in a geologic formation consisting of an interbedded sandstone-shale bedrock layers surrounding an underground liquefied petroleum gas cavern during operation. The transient hydraulic behavior of a multilayer system, such as interbedded sandstone-shale, is highly dependent on the hydraulic parameters of each individual layer. Therefore, in order to identify the cause of the drawdown, the changes in the hydraulic conductivity of each layer was studied based on compaction theory. It was observed that the compaction in shale occurs due to long-term lowered hydraulic head, and this causes the increase in equivalent hydraulic conductivity of the multilayers. The result of the numerical modeling showed that the altered hydraulic conductivity subsequently induced the additional drawdown.