Modeling Asphaltene Precipitation During Primary Depletion

Abstract

This paper describes a thermodynamic model for asphaltene that captures the behavior of asphaltene precipitation during primary depletion. Precipitation occurs above the saturation pressure, reaches a maximum value around the saturation pressure, and decreases as the pressure drops further. The precipitated asphaltene is represented as a pure solid, with the oil and gas phases modeled with an equation of state (EOS). A validation of the model with experimental data is reported. A study of the sensitivity of the model to different parameters is described. Introduction Asphaltene precipitation from reservoir fluids. during oil production is a serious problem because it can result in plugging of the formation, wellbores and production facilities. Asphaltene precipitation may occur during primary depletion or during the displacement of oil by rich gas or C02. de Boer et al. observed that reservoirs that experience asphaltene precipitation during primary depletion usually have the following characteristics:The fluid in place is light to medium oil with small asphaltene content.The initial reservoir pressure is much larger than the saturation pressure.Maximum precipitation occurs around the saturation pressure. Heavier crudes that contain a larger amount of asphaltene have very little asphaltene precipitation problems as they can dissolve more asphaltene. The region where asphaltene precipitation occurs is bound by the asphaltene deposition envelope (ADE). Fig. 1 shows an ADE and PT saturation curve for a typical oil. Precipitation occurs above the saturation pressure (upper ADE pressure), reaches a maximum value around the saturation pressure, and decreases as pressure drops further. Precipitation ceases as pressure drops below the lower ADE pressure. Above the saturation pressure, the precipitation is solely due to pressure, while below the saturation both pressure and composition affect the precipitation behavior.

Prediction of the Gas Injection Effect on the Asphaltene Phase Envelope

“…The asphaltene that this technique gives is physically extracted from oil, and by using some other techniques such as SARA analysis and molecular weight measurement, it can be further studied. The asphaltene onset points at different temperatures were achieved by extrapolation on asphaltene wt% versus the pressure plot by using the Peng-Robinson (PR) EOS (Nghiem et al, 1993;Nghiem and Coombe, 1997), and are summarized in Table 3. Figure 4 is an example of achieving the asphaltene onset pressure at 80°C.…”

Experimental Study and Mathematical Modeling of Asphaltene Deposition Mechanism in Core Samples

“…Solid models may require many empirical parameters and excessive tuning to match experimental data. In this study, the Nghiem model [29] was used; the fugacity of asphaltene in the solid phase is given by:…”

“…In a mixture, when the vapor, liquid and asphaltene phases coexist, the following thermodynamic equilibrium equations should be satisfied [29]:…”

Fully implicit compositional simulator for modeling of asphaltene deposition during natural depletion

“…In this work pure solid model is used to model asphaltene precipitation [20,21] . A first-order chemical reaction was employed to model forward and reverse rates for the conversion of precipitated asphaltene to flocculated asphaltene.…”