Summary
This paper discusses miscible gas injection in reservoir fluid systems exhibiting compositional variation with depth characterized by an undersaturated "critical" transition from gas to oil, where all fluid properties exhibit a continuous variation with depth.
In an undersaturated gas-oil fluid system all fluids throughout the reservoir are initially first-contact miscible with their neighboring fluids. This implies that full-pressure maintenance, gravity-stable updip gas injection will lead to miscible displacement throughout the reservoir, even for down-dip fluids which are not miscible with the injection gas. The pressure maintenance required is to avoid depletion below initial saturation pressure throughout the reservoir, i.e. maintaining a single-phase fluid system. Compositional reservoir simulation studies have been used to verify this behavior for one-dimensional (1D) flow.
Similar results were originally suggested by Hoier and Whitson1. In this paper we study the same mechanism for non-ideal reservoir systems characterized by (1) partial pressure maintenance where reservoir pressures locally drop below initial saturation pressure and two phases form prior to injection gas arrival; and (2) two-dimensional (2D) flow in reservoirs with heterogeneities and strong layer-property contrasts. Both situations may have a negative impact on oil recovery compared with full-pressure maintenance and gravity-stable miscible displacement with expected near-100% recovery.
1D and 2D compositional simulations are used based on the reservoir characteristics of a field with an undersaturated gas-oil fluid system – Smorbukk South Field, located in the Norwegian Sea. This field produces rich gas condensate and volatile oil through an undersaturated gas-oil transition from two main geological layers. We present five years of production history with pressure maintenance by gas injection.
