Landing and Navigating Thin Reservoirs for UGS by Means of Integrated LWD Measurements and Reservoir Modelling

Underground Gas Storage (UGS) reservoirs are often a late development of hydrocarbon reservoirs converted to storage after reservoir depletion. This conversion commonly requires the drilling of new wells tailored to the needs of UGS and the deployment of services like geosteering to optimize the well placement in the target UGS reservoir. However, UGS is an application with specific technical challenges. Modelling of underground conditions can be difficult due to lack of suitable data, or due to the changing fluid properties connected to the cyclicity of charging and depletion of the reservoir, adding additional uncertainty to the challenges posed by geological variability. This case history details the integration of Logging While Drilling (LWD) data to land and geosteer two infilling wells for UGS activity optimization.The target for both wells was a sand layer, a few meters-thick, with optimal petrophysical properties at the top of the reservoir sequence. The results of the geosteering operations confirmed the uncertainties regarding the structural trend and formation properties in a peripheral area of the field. Unexpected variations in the reservoir architecture required rapid changes of wellbore trajectory to achieve the landing objectives. After landing, both wellbores were geosteered within the few meters-thick target interval, although lateral formation changes generated anomalous tool responses compared to the data measured in the offset wells. After achieving the operational target length, Well A exited the reservoir through the roof due to a rapid steepening of the formation dip, exceeding the project’s dogleg limitations. TD was called without further extension of the hole section. Well B, instead, was successfully geosteered beyond the planned TD.