Reservoir Simulation Box Model to Provide Quick-Look Forecast of Remobilizing Trapped Gas in a Gas-Water Jurassic Reservoir with Large Aquifer
Authors: Daae Helene V. , Susanto M. Indriaty , Morgan Benjamin 
Published: October 2023

Abstract
Log data shows existence of residual trapped gas below the Free-Water-Level (FWL) in a high-quality (multi-Darcy) Jurassic reservoir. During reservoir depletion, it is possible for palaeo residual gas phase to remobilize. A box model was utilized, with a relatively short simulation run-time, to understand potential impacts of palaeo gas on incremental gas and condensate recovery, additional pressure support and effect on water production. Note that limited literature/field experience is currently available on this topic.
This paper outlines a workflow to build a fit-for-purpose box model that represents the dynamic performance of gas, palaeo gas and aquifer zones, that contains similar volumes to the existing full field model. The box model also adopts similar fluid composition, equation of state and capillary pressure/relative permeability in the gas zone. The palaeo trapped gas and aquifer are modelled below the current FWL recognising the key uncertainties such as palaeo FWL depth, residual gas saturation, and gas remobilization saturation threshold. The box model initialization is explained to include relative permeability hysteresis, critical gas saturation and the equilibrium test parameters.
The box model is a simplified representation of the full-field reservoir grid in which in-place volumes are matched. A vertical producer drains the reservoir. Homogeneous properties of net-to-gross, porosity and permeability are set using average values from the full field model, representing tank-like behaviour. The box model was first initialised and simulated without palaeo gas, to calibrate its volume to the full-field model. Three saturation regions are assumed; a live gas column, a trapped palaeo gas column and a 100% water column honouring the current and interpreted palaeo FWLs. Initialisation with palaeo gas uses a primary imbibition relative permeability model for all regions and modifies the critical residual gas saturation to match the gas remobilization saturation threshold to create secondary drainage, applicable for the trapped gas region. A residual gas saturation (Sgr) and gas remobilization saturation threshold (Sgrc) were based on analogues and literature reviews. The result shows ~50% total incremental resources and ~10 years longer production (without economic cut-off), and improved pressure support, but with additional water production. The box model was used to explore sensitivity to three critical uncertainty parameters: depth of the palaeo FWL, Sgr and Sgrc. Sensitivity shows that recovery increases with deeper palaeo FWL, higher Sgr and lower Sgrc.
Trapped gas remobilization is currently treated as upside potential, rather than as inclusion to the reserves range. However, modelling trapped palaeo gas is important to give insight to how gas remobilization could increase both hydrocarbon and water production as well as pressure support, which could potentially affect fluid handling capacity, commercial negotiations, and development project economics. The ability to produce quick-look models and visualize the impact can influence the development project, especially those with marginal return.