Influence of Temperature and Pressure on Asphaltene Flocculation

A thermodynamic liquid model has been developed to describe the behavior of asphalt and asphaltenes in reservoir crudes upon changes in pressure, temperature, or composition. Asphaltene solubility properties used as input to the model may be obtained from titration experiments on tank oil. High-pressure flocculation experiments confirm the potential of the model. The model appears to be well applicable to conditions at which asphaltenes are associated with resins. The model may be used to identify field conditions where asphalt or asphaltene precipitation will occur. Introduction Scope of study. Miscible flooding with enriched gas or CO has the potential of recovering a significantly larger volume of oil more economically than conventional water flooding. One of the problems in gas drives is asphaltene instability, which might result in plugging or wettability reversal. Asphalt or asphaltenes precipitation may also affect production in the course of precipitation may also affect production in the course of reservoir development by natural depletion. The parameters that govern precipitation appear to be composition of the crude, pressure, temperature, and properties of asphaltenes. For a specific project one can properties of asphaltenes. For a specific project one can investigate the flocculation process experimentally. This proposition is usually impractical because it requires a proposition is usually impractical because it requires a large number of experiments at reservoir conditions of pressure and temperature. Hence, there is a need for a pressure and temperature. Hence, there is a need for a theoretical description using only a limited amount of experimental data to predict precipitation. The search for such a model has been hampered by the widely held notion that asphaltene precipitation is not a (fully) reversible process. Re-examination of experimental information indicates that reversibility of asphaltene precipitation should be considered an open question. If reversible, the process can be described with a thermodynamic model. The aim of the present paper is to demonstrate that flocculation of asphalt and asphaltenes in light crudes (formation of a bituminous phase) can be described with a simple molecular thermodynamic model. The key concepts of asphalt, asphaltenes, and resins are defined in the next section. The model proposed is described in the following section, in which we also review previous studies. We then discuss field experiences. Experimental data are presented on the phase behavior of two light crudes: an Iranian crude oil with an n-heptane asphaltene content of 1.9 wt% (of tank oil) and a North Sea crude with a low (0.3 wt%) asphaltene content (see PVT properties in Tables 1 and 2). We first use the proposed model (Appendix A) to determine the solubility properties of asphaltenes in Crude No. 1, from a series of titration experiments on tank oil. Using, these results, we compare the measured and predicted amounts of asphaltenes precipitated on mixing recombined Crude No. 1 with three potential injection gases (Table 3). We discuss the pressure dependence of asphalt precipitation and compare measured and predicted pressure dependence of the amount of asphalt precipitated from a mixture of crude No. 2 and propane. Possible improvements of the model are also discussed. Finally, the model is used to predict field conditions favorable to asphalt and asphaltene precipitation. Asphaltenes, Resins, and Asphalt. Asphaltenes are defined as the n-heptane insoluble fraction of crude oil obtained following the Inst. of Petroleum (IP) Method Test 143. Resins can be defined as the fraction of crude oil not soluble in ethylacetate but soluble in n-heptane, toluene, and benzene at room temperature. Asphalt is used here as a general term to designate the combination of asphaltenes and resins. Asphalt precipitated by propane can be molten. n-heptane asphaltenes are solid and decompose upon heating. Asphaltenes and resins are heterocompounds and form the most polar fraction of crude oil. Recent studies on asphaltene structure show that the basic asphaltene "molecule" (asphaltene sheet ) has a molecular weight of the same order of magnitude as that of resins (5 × 10 to 10 3 ). Depending on "purity" and concentration asphaltenes form aggregates with a molecular weight of the order of magnitude of 10 to 10 (asphaltene particles ). Resins have a strong tendency to associate with particles ). Resins have a strong tendency to associate with asphaltenes. This reduces the aggregation of asphaltenes, which determines to a large extent their solubility in crude oil. The most common model for asphaltene/resin interaction is the colloidal model. Asphaltene micelles (aggregates) are assumed to be kept in solution (stabilized or peptized) by a layer of resins ("onion-skin model"). peptized) by a layer of resins ("onion-skin model"). However, the studies of Yen, Speight, and Briant provide a basis for developing a molecular model for provide a basis for developing a molecular model for asphaltene/resin interaction. SPEJ P. 283