Organic Deposition During CO2 and Rich-Gas Flooding

Abstract

Summary. This paper provides a 17-stock-tank oil (STO) experimental data base on the effect of oil composition on CO2-induced organic deposition. This paper also compares CO2 to precipitating agents that are paraffinic and presents information on the topography of deposition. The results offer insights on evaluating organic deposition during CO2 and rich-gas flooding. Introduction Asphaltenes and other heavy organic fractions are typically defined by solubility criteria. For example, asphaltenes are soluble in solvents like benzene but insoluble in low-molecular-weight paraffins like pentane. Crude oils can be fractionated with a variety of solvents. One simple scheme separates an oil fraction from three precipitates: first, asphaltenes are separated with pentane; then, resins with propane; and finally, waxes with methy-isobutyl-ketone. This paper provides analytical data that suggest that asphaltenes, resins, and waxes can precipitate in CO2/crude-oil mixtures. Models for predicting asphaltene flocculation by low-molecular-weight paraffins are still evolving because of the complexity of the problem. Besides the prevailing physical conditions, asphaltene precipitation is affected by the actual solvent used. Perhaps of more importance is the chemical type and size distribution of organics in the crude oil, which change for every crude oil examined. By analogy, the phenomenon of CO2-induced organic deposition will be multifaceted, with significant oil-composition effects. Most of the available quantitative data on the nature Of CO2 induced organic deposition were presented in a previous paper, which provided examples of how rock wettability can be altered by CO2-induced organic deposition. Strongly water-wet Berea cores became mixed-wet, and artificially neutral cores became strongly oil-wet. The extent of deposition depended on oil, brine, and rock compositions and whether conditions favored multiple-contact miscibility. Insensitivity to fresh water following deposition and enhanced deposition following clay stabilization implied that clay surfaces are important deposition sites. That study also began to delineate the similarities and differences between asphaltene flocculation by low-molecular-weight paraffins and the precipitation of heavy organics by CO2. This paper provides additional details on how CO2-induced organic deposition is influenced by oil composition. Further information also is provided on the importance of clay surfaces as deposition sites. Additionally, this paper continues the comparative study of precipitating agents. Results are provided for CO2, a rich gas, and the low-molecular-weight paraffins pentane and propane. Materials and Methods Fig. 1 is a schematic of the high-pressure/high-temperature variable-volume circulating cell (VVCC) used in this study. The three salient features of the apparatus were a 2000-CM floating-piston mixing cell that contained the sample, a 1000-CM floating-piston CO2 reservoir, and a flow loop that circulated the sample through a sight glass and precipitate trap. For this study, 0.5-um stainless-steel filters were used as traps. Both the CO2 reservoir and the circulation loop were driven by metering pumps and were manifolded to the sample side of the mixing cell. A dome-loaded backpressure regulator connected to the mixing cell controlled run pressure. The differential pressure across the precipitate trap was displayed continuously. Several platinum resistance probes read and regulated system temperature. The working limits of the apparatus were 5,000 psia and 250 degrees F [34.5 MPa and 394 K]. In a typical experiment, 100 CM of oil plus sufficient CO2 to produce an approximately equimolar mixture was loaded into the mixing cell, and the system was brought to isothermal and isobaric run conditions. Circulation of the sample then began, usually at a constant rate of 50 cm/min, as additional CO2 was metered into the mixing cell at a slower rate of 0.3 CM 3 /min. The overall sample composition could be determined at any time from the known amounts Of CO2 and oil in the mixing cell. Recorded run data included system temperature and pressure, the differential pressure across the precipitate trap, visual observations of the circulated sample, and GOR's of the circulated sample. The latter were compared with overall sample composition to judge mixing efficiency. A maximum differential pressure of 150 psi [1034 kpa]was allowed across a trap before flow was switched to a new trap. Four to eleven stainless-steel fitters were required per run. The run ended when the CO2 content of the sample reached about 96 mol%. The total sample was then positively displaced through the sight glass and trap and flash separated. Finally, the bottom of the mixing cell and circulation loop were flushed with CO2 to mobilize any remaining precipitate. Each stainless-steel filter used during the run was air dried for 48 hours, and the precipitate amounts were determined gravimetrically. Precipitate weight percents were calculated from the grams of precipitate per gram STO initially loaded into the mixing cell. Additional operational details are provided elsewhere. Design of the VVCC was inspired by the continuous multiple-contact (CMC) experiment originated by Off and Silva. Both the VVCC and the CMC setup provide phase-equilibrium data. The primary difference between the two setups is in how the volume increments caused by increasing CO2 content are managed. The CMC setup produces portions of both upper and lower phases to maintain a fixed overall sample volume; thus, the overall sample composition is dynamic, not only because CO2 content increases, but also because phase portions are removed. The VVCC allows overall sample volume to swell with increments of CO2. The overall composition is affected only by the removal of precipitate and the small volume sampling performed to judge mixing efficiency. The reproducibility and accuracy of the VVCC were checked against the pressure/composition phase diagram reported for the CO2/decane binary at 160 degrees F [344 K].7 A 1,570 psia [10.8 MPa] run that traversed the liquid/vapor phase region yielded the following results. Sight-glass observations of the system bubblepoint and dew-point were within 2 mol % of the reported values - Moreover, CO2 levels from flash separations performed to judge mixing efficiency were typically within 2 mol% of the overall sample composition determined from the known amounts Of CO2 and decane in the mixing cell. The weight of precipitate collected was 0.0012 g/g decane charged to the mixing cell. This is judged to be a negligible level of residual oil plus inherent contamination and suggests that the VVCC precipitate determinations are accurate to within 0.1 wt%. The estimated reproducibility of the VVCC results from runs was also within 0.1 wt %. The 17 STO's used in this study came from fields located in seven states and Canada. The oils range in gravity from 19.5 to 46.5 o API [937 to 795 kg/M]. Table 1 lists other properties of the oils. Additional oil compositional data, including asphaltene contents, are presented in the next section. For the rich-gas runs, a live oil was formed by recombining light ends with Sample E oil. SPERE P. 17^