Systematic evaluation of asphaltene formation damage of black oil reservoir fluid from Lake Maracaibo, Venezuela

Abstract

One of the most concerning areas for asphaltene deposition is the near-wellbore region, where such deposition may cause formation damage. The reservoir fluid of the Lake Maracaibo region is known to cause operational problems such as production losses due to asphaltene deposition. In this paper, we focus on the systematic evaluation of the asphaltene formation damage using stacked composite core samples and live oil samples from Lake Maracaibo area and study the formation damage due to asphaltene deposition inside the composite core stack.A special coreflood system was designed for an asphaltene formation damage study using reservoir core samples and sour–live reservoir fluid samples. The phenomenon of asphaltene precipitation and deposition in porous media and subsequent permeability impairment was evaluated while the fluid pressure decreased below the asphaltene onset pressure (AOP). Live fluid samples were collected and flooded through the stacked composite core plugs under reservoir conditions. Progressive pressure reductions were performed to induce asphaltene precipitation and deposition in the composite core. Changes in the absolute permeability of the composite core assembly due to asphaltene precipitation and deposition were calculated in the context of the pressure depletion process. Significant permeability impairment occurred from the depressurization of the reservoir fluid below the AOP. The upstream inlet pressure of the composite core stack was maintained just above the AOP while the outlet pressure was regulated approximately 50 psig above the saturation pressure throughout the coreflood experiment so that the onset of asphaltene precipitation occurred along the length of the composite core.Following the coreflood experiment, the core stack was depressurized and the remaining fluid was extracted from individual core plugs. Various analytical techniques such as asphaltene content measurement, simulated distillation and optical density were used to perform detailed post-trial analyses of the residual fluids extracted from each individual core plugs. Analyses of the thin sections extracted from various core plugs were also performed to investigate the microstructure of the residual hydrocarbon as well as the distribution of the flow pathways plugged with asphaltene aggregates in the pore structures. The results associated with these post-trial analyses confirmed that the observed permeability impairment was indeed due to the plugging of the flow pathways of the core plugs by asphaltene aggregates, and that the asphaltene-induced permeability loss was somewhat more extensive in the tighter plugs associated with the stacked core assembly. We present the systematic approach to perform asphaltene formation damage study using composite core and live reservoir fluid samples above the saturation pressure.