Evaluating Effect of Compositional Gradients on Asphaltene Stability in Reservoirs with CPA and PC-SAFT

Optimal field development requires proper knowledge of the composition and phase behaviour of the fluids that are present in the reservoir. Within the complex matrix of reservoirs, various forces such as gravitational and thermal gradient induced forces as well as mass flux of light hydrocarbons at reservoir boundaries can cause the migration and compositional distribution among the different reservoir units or compartments, affecting optimization and management of hydrocarbon production.  For an asphaltenic reservoir field, the large compositional variations may lead to asphaltene precipitation and deposition within reservoirs, causing formation damage and permeability reduction. Taking into account all the phenomena, to establish a consistent picture of fluid distribution and phase behaviour in the field relies on the fundamental understanding of the effects of compositional variations on fluid phase behaviour.

In this study our well-developed Cubic-Plus Association (MF-CPA EoS) model[1] and the perturbed chain-statistical associating fluid theory equation of state (PC-SAFT EoS)[2,3] are applied to model the effect of compositional gradients as well as changes of pressure and temperature on asphaltene stability in reservoirs. Possible size distribution of the asphaltene monomers based on Gamma distribution is also considered to assess the impact of the size distribution on the phase behaviour of asphaltenes in various reservoir fluids.    

Real field cases with compositional gradients, and severe asphaltene precipitation and deposition problems from a wide range of asphaltenic crudes ranging from heavy oils to light live crudes were used to validate the models.

The validation and comparison demonstrate the capabilities and advantages of both the conventional cubic equation of state plus association and PC-SAFT model.  With its simplicity, capabilities and consistency with the industry standard fluid characterisation methods, MF-CPA EoS has the potential to become a promising and general engineering tool for providing essential knowledge of asphaltene phase behaviour in reservoirs.

[1] Xiaohong Zhang, Nuno Pedrosa, and Tony Moorwood, Energy Fuels, 2012, 26, 2611−2620.

[2] Francisco M. Vargas, Jeff L. Creek and Walter G. Chapman, Energy Fuels, 2010, 24 (4), pp 2294–2299.

[3] Gonzalez, D. L.; Hirasaki, G. J. Creek, J.; Chapman, W. G. Energy & Fuels 2007, 21, 1231-1242.