Impact of Perched Water, Compartmentalization, and Tilted Contact, on an Offshore Gas Development and its Implications for Project Execution

Abstract Full field simulation considering a regional aquifer view is used to quantify the impact of alternative gas-water contact scenarios on a green field development of a large offshore gas field. Results are used in recoverable volumes quantification, completion option decisions, and concept evaluation of potential next development phases. Two gas-water contacts have been measured in the field's main reservoir with a large difference in contact depth. This has led to the reservoir being described as compartmentalized and modelled with segments with zero transmissibility multipliers along seismically and geologically defined boundaries. Further review of potential explanations for the difference in gas-water contact depths and of regional pressure data resulted in two alternative scenarios being added, namely a perched water scenario and a hydrodynamically tilted contact scenario. A combination of water being trapped (during charge) stratigraphically or structurally by the base of the reservoir enables the concept of perched water, with communication in the overlying gas zone across the full reservoir. The other alternative scenario, namely the tilted contact, is enabled by a regional aquifer pressure gradient resulting in continuous flow of ground water, which makes the gas water contact tilted in the direction of aquifer flow. Model changes for these scenarios are limited to the removal of zero transmissibility multipliers in between, and to redefinition of, initialization regions. The pragmatically modeled tilted contact scenario also uses threshold pressures. Both additional scenarios were investigated with multiple model realizations. Simulation results of the various contact scenarios confirm the optimum placement of the development wells, mitigating the risk of early water breakthrough, and enable better-informed decisions on drilling and completion options. The described pragmatic modeling of alternative realistic gas water contact scenarios can be an important element in the quantification of recoverable volume uncertainty ranges, which may have commercial and further development implications, and in taking well-informed decisions on completion options. It also supports the risk mitigation of over- or under-designed surface facilities with respect to produced water management.