Comparison of First and Multiple Contact Carbon Dioxide Induced Asphaltene Precipitation

Abstract

Precipitation of solids, believed to be asphaltenes is common in most carbon dioxide floods. The oils in most carbon dioxide flood applications, however, are light crudes with low initial asphaltene content. Precipitation is common after carbon dioxide breakthrough in production equipment. It is believed that precipitation does not occur in the reservoir. The objective of this paper was to identify the reasons for this phenomenon. The mature carbon dioxide flood at Rangely field in Colorado was selected as an example. A high-pressure, high-temperature pressure-volume-temperature apparatus in conjunction with a high-pressure core flooding system was used to study carbon dioxide induced precipitation from the Rangely crude oil. Rangely crude contains about 1% heptane insolubles. It was determined that first-contact precipitation, at different carbon dioxide concentrations was negligible. Core flooding experiments at field conditions revealed that precipitation did occur at the core exit. This essentially showed that the precipitation phenomenon in crudes that are likely CO2-flood candidates is due to the multiple contact process and the associated compositional changes. Multiple contact experiments conducted to prove this hypothesis showed that multiple contact precipitation amounts were 3–5 times the first-contact precipitation values. The compositional changes (in the original crude) to cause widespread precipitation were established. The precipitates from the field and various laboratory experiments were analyzed using GC and SARA, which showed that field precipitates contained compounds other than original heptane insolubles and also exhibited some unique features. Introduction Asphaltenes by definition are a compound class. Even though precipitation has been observed in most CO2 flooding applications, there is uncertainty with respect to the thermodynamic conditions that lead to precipitation. The differences between asphaltenes or solids precipitated by different means are also not well established. Asphaltene content in the crude as determined by conventional methods is not a good indicator of asphaltene precipitation problems. It has been reported that a crude with more than 17 wt % asphaltenes did not have asphaltene problems, while a crude with only 0.1 wt % had serious asphaltene deposition problems1. Asphaltenes precipitate because of changes of pressure, temperature and composition and the thermodynamic instability that these changes entail. The use of carbon dioxide to increase oil recovery has received considerable attention. In United States, there were 60 active miscible CO2 projects in operation in 1996, whereas in Canada there were 40 hydrocarbon miscible active projects2. It has been projected that CO2 miscible flooding potentially could recover as much as 40% of the total oil to be recovered by all enhanced recovery methods3. Carbon Dioxide is highly soluble in oil and to a lesser extent in water. Carbon dioxide improves oil recovery by the following mechanisms: reduction in crude oil viscosity and increase in water viscosity; swelling of crude oil; reduction in oil density; and miscibility effects. The miscibility between the oil and CO2 or hydrocarbon solvents eliminates interfacial tension and capillary forces and could help recover, in theory, all of the residual oil.

Ready for Gas Injection: Asphaltene Risk Evaluation by Mathematical Modeling of Asphaltene-Precipitation Envelope (APE) With Integration of All Laboratory Deliverables

“…The increase of asphaltene risks through gas injection has been recognized as a general rule (Kokal et al 2003;Srivastava et al 1999;Parra-Ramirez et al 2001;Sarma 2003;Jamaluddin et al 2000;Kokal et al 2004;Negahban et al 2005;Okwen 2006; Moghadasi et al 2006). A preparatory evaluation of asphaltene behavior under gas-mixing conditions is important to minimize risks and to optimize the gas-injection project.…”

Lab and Reservoir Study of Produced Hydrocarbon Molecular Weight Selectivity during CO2 Enhanced Oil Recovery

“…For example, a CO 2 flood in the Midale reservoir in southeastern Saskatchewan showed that a considerable amount of solid deposit (consisting of asphaltenes, wax, and trapped oil) formed in the wellbore equipment and downhole facilities shortly after CO 2 breakthrough in the production wells . Similar problems were reported in many other oil fields with CO 2 EOR operations across the world. − Over longer time scales, the phenomenon of selective dissolution is a potential problem because this can negatively impact CO 2 injectivity, sweep efficiency, and incremental oil production.…”

Comparison of Experimental Techniques for Evaluation of Chemistries against Asphaltene Aggregation and Deposition: New Application of High-Pressure and High-Temperature Quartz Crystal Microbalance

“…Some of the significant parameters involved in the asphaltene deposition phenomenon are asphaltene onset pressure (AOP), lower and upper asphaltene envelope, reservoir pressure, and bubble point pressure. − Gas injection is one of the most common EOR approaches that increases ultimate oil recovery in many cases because of induced oil viscosity reduction or pushing the oil toward the wellbore . One of the most effective agents in this type of EOR method is natural gas, which leads to increased oil production, although the natural gas injection into an oil reservoir could change the flow behavior and the fluid equilibrium properties, which cause asphaltene precipitation and deposition problems. , Despite several decades of research and development projects, gas-induced asphaltene deposition is still a major flow assurance challenge. , There are different approaches for asphaltene remediation and inhibition, including mechanical techniques (pigging, mechanical/manual striping, and mechanical vibration , ), chemical treatments (dispersants, antifoulants, coagulants, and polar co-solvents − ), and thermal techniques (steam injection, hot chemical injection, microwave technique, and in situ combustion). Between all of these techniques, chemical injection is one of the most widely used strategies to tackle the asphaltene precipitation and deposition barrier. , Millions of dollars per year are estimated to be spent each year on installing and servicing asphaltene mitigation equipment and chemicals .…”