Mohammadzadeh, Omid (Schlumberger-Doll Research Center) | Taylor, Shawn David (Schlumberger-Doll Research Center) | Eskin, Dmitry (Schlumberger-Doll Research Center) | Ratulowski, John (Schlumberger-Doll Research Center)
One of the complex processes of permeability impairment in porous media, especially in the near wellbore region, is asphaltene-induced formation damage. During production, asphaltene particles precipitate out of the bulk fluid phase due to pressure drop, which may result in permeability reduction due to both deposition of asphaltene nanoparticles on porous medium surfaces and clogging of pore throats by larger asphaltene agglomerates. Experimental data will be used for identification of parameters of an impairment model being developed. As part of a larger effort to identify key mechanisms of asphaltene deposition in porous media and develop an asphaltene impairment model, this paper focuses on a systematic experimental study of asphaltene-related permeability damage due to live oil depressurization along the length of a flow system.
An experiment was performed using a custom-designed 60-ft slimtube coil assembly packed with silica sands to a permeability of 55 mD. The custom design included a number of pressure gauges at regular intervals along the coil length which enabled real-time measurement of the fluid pressure profile across the full length of the slimtube coil. Test was performed on a well-characterized recombined live oil from the Gulf of Mexico that is a known problematic asphaltenic oil. After saturating the slimtube coil with stock tank oil (STO) to restore wettability and attain the initial state of the test, the STO was then gradually displaced by flooding at least 3 pore volumes of live oil above the asphaltene onset precipitation (AOP) pressure. The impairment portion of the experiment was then initiated by maintaining initial pressure at the inlet while the outlet pressure was regulated slightly above the saturation pressure. Under this constant differential pressure, the injection flow rate through the slimtube decreased over time as the porous medium became impaired. During the impairment stage, samples of the produced oil were collected on a regular basis for asphaltene content measurement. After more than a month, the impairment test was terminated, and the live oil was purged from the slimtube coil with helium at a pressure above AOP pressure, and then the whole system was gently depressurized to bring the coil to atmospheric conditions while preserving the asphaltene damaged zones of the coil. Changes in permeability and porosity of the porous medium were obtained due to asphaltene impairment caused by pressure depletion.
Results indicated that the coil permeability was impaired by about 32% due to pressure depletion below AOP pressure, with most of the damage occurring in the latter section of the tube which operated entirely below the AOP pressure. Post analytical studies indicated lower asphaltene content of the produced oil samples compared to the injecting fluid. Asphaltene deposition distribution along the length of the coil was determined by cutting the slimtube coil into 2 to 3 ft long sections and using solvent extraction to collect the asphaltenes in each section. The extraction results confirmed that the observed permeability impairment was indeed due to asphaltene deposition in the middle and latter sections of the coil, where the pressure was below the AOP pressure.