Asphaltene Deposition Prediction and Control in a Venezuelan North Monagas Oil Field

Abstract Asphaltene deposit formation is a severe problem frequently faced during crude oil production and transportation, that affects the reservoir, production tubing and surface facilities, depending on the stage where this critical point is reached. This point corresponds to the pressure at which asphaltene particles start to form. This value is directly related to the oil physical chemical stability. When flocculation pressure is reached into the reservoir, severe formation damages occur due to asphaltene precipitation in the porous media, evidenced as wettability and/or relative permeability alterations. When asphaltenes precipitate in the production tubing, refractory plugs reduce or even stop crude oil production until chemical/mechanical removal is performed. The stability of several crude oils produced in a North Monagas oil field (Carito) from Venezuela, in terms of asphaltene deposition, was evaluated to predict their behavior and prevent future plugging. "Spot Test" and n-heptane titration (asphaltene flocculation onset determination under atmospheric conditions) were used as crude oil stability criteria. Solid deposits, from these wells production tubing and surface facilities, were characterized in order to establish if their origin was organic (asphaltenes or paraffins) or inorganic (scales or sand), and to confirm the stability results assessed in the first set of experiments. Once the asphaltene problem was identified, several commercial asphaltene inhibitors were evaluated, considering their efficiency when a fixed dosage of these products was added to the studied crude oils. Dead oil n-heptane titration of treated crude oils allowed identifying the products that induced higher flocculation onsets when compared with untreated sample results. A second condition that was evaluated for inhibitor selection, was the product thermal stability, considering that downhole treatments would require to face temperatures as high as 150°C. Results of some practical applications derived from this evaluation are presented in this work. Introduction Crude oil is a colloidal system, where the continuous phase is constituted by the maltene fraction (saturates, aromatics and resins), being the asphaltenes the dispersed phase. The asphaltenes are dark brown to black solids with not defined melting point, that decompose under heating, leaving a carbon residue. They constitute the fraction with higher heteroatom and metal concentration, higher polarity and molecular weight. Due to these characteristics, asphaltenes can aggregate through intermolecular associations to form precipitates and, eventually, obstructions in the reservoir, production tubing and surface facilities. Asphaltene deposition can occur in different production stages including production tubing, surface facilities, even in the reservoir. This phenomenon decreases oilfield productivity and generates severe production costs due to frequent treatments for prevention and deposit removal. Additionally, it reduces dramatically rock permeability, causes severe formation damages and generates very difficult to manage well plugging. This problem has been reported in Venezuelan oilfields for a long time, especially in North Monagas area (Eastern Venezuela), where Furrial, Carito, Musipán, Corozo, Santa Bárbara, among others oilfields have been affected. Asphaltene precipitation depends on several factors, such as: flow conditions, pressure differential, rock/fluid system, etc. Crude oil stability can be affected by pressure, temperature and oil composition. At constant temperature, the higher the pressure, the higher the liquid density, and consequently, the more soluble can be the asphaltenes. However, if a vapor phase is present, the lighter components can be dissolved, changing the liquid composition and reducing its solvent power. Disperse phase characteristics and resins peptizing power play an important role in asphaltene precipitation [1–3]. Recent studies have indicated the importance of the asphaltene nature in crude oil stability [4–7]. Three factors have been identified as crucial aspects of colloidal crude oil stability, in terms of asphaltene precipitation:A continuous phase with a high content of aromatic compounds,High concentration of resins that are compatible with the asphaltenes, andHighly soluble asphaltene fractions (low aromaticicy, high hydrogen content and low aromatic condensation degree).