Asphalt flocculation and deposition. V. Phase behavior in miscible and immiscible injections

Asphalt formation and precipitation under primary oil recovery conditions a s well as secondary recovery by immiscible gas injection were studied. The crude oils were from a carbonate and highly fractured reservoir. No asphalt precipitated under these conditions. Since the oil reservoir had experienc ed significant formation damage and plugging of its well's tubing due to as phalt formation and precipitation, the absence of asphalt precipitation in our experiments points to the important influence on asphalt formation of f low of the oil, the streaming potentials arising from the flow, and their i nteraction with the electrical properties of the asphalt aggregates. Also i nvestigated was asphalt precipitation when an enriched gas was injected int o the oil under dynamic (multiple contact) miscibility conditions. Only tra ce amounts of asphalt precipitated when the dilution ratio R was high enoug h. However, significant asphalt precipitation occurred in completely miscib le injections. For this case, we report extensive new experimental data for the amount of the precipitated asphalt formed with various precipitation a gents over wide ranges of pressure, temperature, and crude oil composition. Increasing the pressure decreases the amount of asphalt precipitation. How ever, depending on the crude oil, two opposite trends in the amount of the precipitated asphalt were observed when the temperature of the system was r aised. A scaling equation of state is shown to provide accurate predictions for the data. The scaling equation also yields a novel analytical equation for R-c, the critical dilution ratio (measured in cm(3) of the diluent or the precipitation agent per gram of crude oil) at the onset of the precipit ation given by R-c = c(MT)(1/4), where M is the (average) molecular weight of the precipitation agent, T is the temperature in degrees C, and c is a c onstant on the order of 10(-2). The predictions of this equation are in exc ellent agreement with the experimental data. Thus, this equation may be use d in the design of gas injection operations for enhanced oil recovery, such that precipitation of the asphalt aggregates in the reservoir can be preve nted.