Asphaltene Deposition and Chemical Control in CO2 Floods

Abstract

Abstract CO2 miscible gas flooding is a popular method of improved oil recovery. Asphaltene deposition in many of these systems is a costly operational problem associated with the utilization of this method of recovery. A CO2 flood in West Texas was experiencing major problems with asphaltene deposition related ESP failures and tubing plugging. Operational procedures, field experience and a successful chemical program have greatly reduced the cost of these problems over the last 3 years. A new inhibitor for continuous capillary injection has been recently developed and used in successful field trials. To evaluate the field performance of this new asphaltene inhibitor, a wellhead side stream filter loop was constructed to monitor asphaltene precipitation/deposition. The objective was to measure and compare the time needed to plug a given size filter by untreated and chemical treated produced fluids. Pressure data at the filter loop and wellhead was also recorded to monitor the effectiveness of the treatments. The field trial results proved the use of a filter-plugging side stream was an effective way of assessing asphaltene inhibitor performance. Based on the filter plugging and wellhead pressure data, it was concluded that asphaltene precipitation/deposition occured if the produced fluid was not chemically treated. The currently used product with successful prior field results was able to reduce asphaltene deposition and doubled the filter plugging time. The new asphaltene inhibitor was shown to be more effective for stabilizing the asphaltenes in the CO2 flooded produced fluids. The new inhibitor extended the time needed to plug a filter by more than four times as compared to untreated fluids. Case history information, laboratory tests results, side stream configuration, field test procedure and field test results are presented. Introduction Asphaltenes are heterocyclic unsaturated macromolecules consisting primarily of carbon, hydrogen, and minor proportion of heteroelements such as oxygen, sulfur, nitrogen etc1,2. The amounts of carbon and hydrogen in asphaltenes vary over a very small range and indicate that the H/C ratio is fairly constant at about 1.1–1.2. The constant H/C ratio suggests that asphaltenes have a strong aromatic composition. Asphaltenes are typically defined by solubility tests as those components in crude oil that are soluble in benzene but insoluble in low molecular weight n-alkanes1,2,3. The asphaltenes are believed to exist in the oil as a colloidal suspension, and are stabilized by resins adsorbed on their surface1. These higher molecular weight components of crude oil are normally in equilibrium at reservoir condition. As crude oil is produced this equilibrium may be disrupted by a number of factors including temperature declines, pressure reductions, crude oil chemical composition changes, introduction of miscible gases and liquids, mixing with diluents and other oils, and during acid stimulation, hot oiling and other oilfield operations. The upset of the colloidal system may result in irreversible flocculation of asphaltenes. The deposition and precipitation of flocculated asphaltenes can severely reduce the permeability of the reservoir, cause formation damage and can plug-up the wellbore and tubing. The two primary mechanisms for asphaltene flocculation and deposition are depressurizing the oil and mixing of solvents with reservoir oil during enhanced oil recovery (EOR).