A Systematic Approach for Experimental Study of Asphaltene Deposition for an Abu Dhabi Reservoir Under WAG Development Plan

This paper discusses experimental work associated with the evaluation of asphaltene deposition for a field in Abu Dhabi, UAE. The subject reservoir is in the early stages of development and will be put on production using a combination of gas, water, and water alternating gas (WAG) injection schemes in early 2006. The field has not shown operational problems resulting from asphaltene precipitation during primary production. Laboratory experiments using the transmittance of an optimized laser light in the near infrared (NIR) wavelength (∼1,600 nm) were used to first confirm the stability of asphaltene in the reservoir fluid. Two cases, covering the expected extremes in terms of the field gas/oil-ratio (GOR) were evaluated. Isothermal depressurization tests were also conducted at reservoir, wellhead, and separator temperatures (250°, 190° and 130°C). Several additional light transmittance experiments were conducted to evaluate the asphaltene instability regions resulting from reservoir fluid contact with various concentrations of a rich gas and carbon dioxide (CO₂). Measurements using high-pressure filtration were also conducted to quantify the bulk precipitation of asphaltene with various molar concentrations of gas. Finally, tests were conducted using state-of-the-art technologies to evaluate the consistency of the initial NIR runs. These technologies involved the use of a spectral analysis system (SAS) to evaluate the asphaltene particles size and growth rate and high-pressure microscopic images to visually confirm the measurements. Results indicated that rich hydrocarbon gas, in contact with reservoir fluid, destabilizes asphaltene. The amount of the bulk precipitation increased with higher concentrations of rich gas in the reservoir fluid. Particle sizes were in the range of 0.5 to 1 micron and did not pose any threat to plug pore throats, and consequently did not affect the recovery potential. The effect of CO₂ was found to be less severe with regards to asphaltene instability.