Impact of a Secondary Condensate Charge into an Oil Reservoir Evaluated by Downhole Fluid Analysis, Core Analysis, and Production

Abstract

Abstract In deepwater, evaluation of reservoir connectivity is critical to reduce uncertainties in field development planning. Downhole fluid analysis (DFA) using wireline formation testers (WFTs) has made it routine to measure compositional gradients of reservoir fluids, both vertically and laterally. Thermodynamic modeling of these gradients proceeds with the cubic equation of state (EoS) for GOR gradients and the Flory-Huggins-Zuo (FHZ) EoS with the Yen-Mullins model for asphaltene gradients. Thermodynamic equilibrium implies reservoir connectivity while stair step discontinuities imply compartmentalization. Fluid gradients that are in thermodynamic dis equilibrium are often associated with connected reservoirs with current or recent charging. These methods have been applied, for this case study, in a field in development, deepwater Gulf of Mexico. In one of the primary multi-lobe sand intervals being developed, higher GOR fluids exist on a structural flank of the field. The GOR varies between sand lobes of variable permeability, with higher GOR in the more permeable lobes. Laterally away from the high GOR fluids, and in a similar structural position, there is an abrupt change in GOR. DFA gave the first indication of GOR variability within the different lobes, and subsequent static and dynamic data pointed to a recent gas condensate charge into the oil reservoir in this flank. Away from the flank, the fluids are equilibrated indicating connectivity. This recent charge, not potential compartmentalization, accounts for the fluid variability. Core analysis and WFT permeability measurements indicated the expected asphaltene instability in sections of the field associated with such a recent charge. Pressure measurements during production confirmed reservoir connectivity. Moreover, temporal variations of the produced fluids confirmed the recent gas condensate charge into the oil reservoir. The mixing of fluids with different GORs could have had a very negative impact on the stability of the asphaltenes during production. Fortunately, the higher GOR fluid was produced off in a short time frame and has not had any negative impact on production to date. This case study represents the end member of ‘no deleterious impact’ for a gas condensate charge into an oil reservoir. This conclusion is in stark contrast to other published case studies where such similar recent charges gave rise to tar mats and large changes in fluid properties. Moreover, this case study extends the understanding of reservoir filling. A common observation in trap filling is increasingly denser fluids at increasing depth independent of the relative timing of charge of these different fluids; this we refer to as density stacking of reservoir fluids. Naturally, within migration pathways at or near the reservoir, fluid complexities can occur that do not necessarily correspond to density stacking, for example, with a late gas or gas condensate charge. In this case study, areas near charge points of a reservoir are shown not to conform to density stacking during trap filling (in a connected reservoir) but instead exhibit a lateral fluid front with different density fluids at the same subsurface structural elevation across the reservoir.