This work concerns with simulation of permeability impairment as a result of asphaltene deposition at the field scale, which is a real situation of oil production from petroleum reservoirs.
The involved processes are the initial precipitation of asphaltene from the reservoir fluid, the flocculation of these precipitates into larger particles and the deposition of these particles in porous media. The alteration of porosity and the impairment of permeability due to asphaltene deposition are also investigated.
The case study reveals that the static surface deposition may occur in certain situations even without the fluid flowing. However, during flow, the dynamic surface deposition is predominant. The plugging deposition may occur under certain conditions, depending on properties of the fluid, porous media, asphaltene precipitates, injection rate and pressure conditions.
In addition, the effect of fracture properties on asphaltene deposition at reservoir condition was investigated. The developed dual-porosity and dual permeability simulation model also has been used for some sensitivity analysis of parameters affecting asphaltene deposition in reservoir. The results of sensitivity analysis showed that dynamic and static parameters of asphaltene play crucial role in the field performance and formation damage. Also, the results confirmed that, the permeability reduction due to asphaltene deposition in the fractures is more considerable than in the matrix media.
Asphaltenes are complex molecules, which are defined as the nonhydrocarbon molecules that are soluble in benzene but insoluble in low-molecular-weight n-alkanes, and can be derived from petroleum oil or shale oil [1]. Asphaltene precipitation from reservoir fluids during oil production is a serious problem, because it can result in plugging of the formation, wellbore and production facilities. Asphaltene precipitation can occur during primary depletion of highly undersaturated reservoirs or during hydrocarbon gas or CO2 injection [2–6].
The region where asphaltene precipitation occurs is bound by the Asphaltene Deposition Envelope (ADE) [5]. Figure 1 shows an ADE and PX saturation pressure curve for typical oil. Many thermodynamic models that describe the phase behavior of asphaltene precipitation have been reported in the literature. These include the use of a liquid solubility model [4], a thermodynamic colloidal model [7], a thermodynamic micellization model [8], a variation of a model for wax [9, 10], or a pure solid model [11, 12]. It must be pointed out that asphaltene precipitation is a result of changing in pressure, temperature and composition which leads to changes in properties such as wettability and permeability in porous media [13, 14]. In this study the effect of asphaltene precipitation and deposition in a naturally fractured Iranian oil reservoir performance, Sarvak formation, has been examined. Warren & Root dual porosity model as well as solid thermodynamic model was used for developing of compositional simulation model, and natural production from Sarvak formation was simulated.
An equation of state (EOS) compositional reservoir simulator has been modified to incorporate asphaltene precipitation, flocculation and deposition. The fluid phase behavior in the simulator is represented with a standard cubic EOS. The following sections describe the solid phase behavior models.
