Comprehensive Study of Asphaltene Precipitation due to Gas Injection: Experimental Investigation and Modeling

Roshanaei Zadeh, Gholam Abbas (National Iranian South Oil Co) | Moradi, Siyamak (Islamic Azad University-Mahshahr Branch) | Dabir, Bahram (Amirkabir University of Technology) | Ali Emadi, Mohammad (NIOCRTD) | Rashtchian, Davood (Sharif University of Technology)


Asphaltene precipitation during natural depletion and miscible gas injection is a common problem in oilfields throughout the world. In this work, static precipitation tests are conducted to investigate effect of pressure, temperature and gas type and concentration on asphaltene instability. Three different oil samples are studied under reservoir conditions with/without nitrogen and methane injection. Besides applying common thermodynamic models, a new scaling equation is presented to
predict asphaltene precipitation under HPHT gas injection. Published data from literature are also used in model development. The scaling approach is attractive because it is simple and complex asphaltene properties are not involved in the calculations. Moreover, the proposed model provides universal parameters for different fluid samples in a wide range of pressure and temperature that makes it novel for evaluation of future gas injection projects when simple PVT data are available.

Keywords: Nitrogen injection/Asphaltene precipitation/Pure solid modeling/Scaling equation.

1- Introduction
Miscible/partial miscible gas injection is a promising enhanced oil recovery technique for many reservoirs (Huang et al, 1993). It is well known that the injection of CO2, N2 and hydrocarbon gases change the solubility of heavy components in the reservoir oil and causes asphaltene instability (Yang et al., 1999; Srivastava et al., 1999; Jamaluddin et al., 2002; Takahashi et al., 2003; Hu et al., 2004; Negahban et al., 2005; Verdier, 2006; Dehghani et al., 2008).

Hirschberg et al. (1988) applied polymer solution theory to present a solubility model for asphaltene precipitation. In this approach, asphaltene was expected as a heavy liquid and phase behavior calculations were performed for gas-oil and oilasphaltene systems. The model was later modified to account for asphaltene polydispersity (Monteagudo et al., 2001). Yang et al. (1999) proposed a modified Hirschberg model and defined asphaltene solubility parameter as a function of specific gravity, molecular weight and boiling point. Pazuki et al. (2006) defined an interaction parameter between polymer and solvent and correlated it to molecular weight of solution and asphaltene. Later, they applied the perturbation theory and proposed a new equation of state to study phase behavior of crude oil and asphaltene (2007). Vafaie-Sefti et al. (2003, 2006) incorporated association term in Peng-Robinson equation of state. Simple association factors were obtained from molecular weight distribution (fractional molecular weight) and average asphaltene molecular weight. However, this model includes more uncertain matching parameters compared to previous attempts.

Leontaritis and Mansoori (1987) presented a colloidal model by applying statistical thermodynamics. They defined a critical resin concentration (CRC) to estimate critical chemical potential of resins. According to this approach, asphaltene precipitation will occur if chemical potential of resins in the mixture is less than the critical value. Escobedo and Mansoori (1994) concluded that this model is accurate for determination of onset of asphaltene precipitation and does not quantify the precipitates. Later, Wang et al. (2001) mentioned that some experimental evidences were against this model.

Pan and Firoozabadi (1998, 2000) proposed thermodynamic micellization model that describes asphaltene precipitation by minimizing total free Gibbs energy of system (including asphaltene monomers and micelles). Although their model considers colloidal nature of oil, it must be optimized to comprise effect of resins and precipitants successfully.