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Asphaltene plugging is one of the most frequent causes of production depletion in medium and light crude oil reservoirs, specially when the pressure declines due to its exploitation, and asphaltene solubility reaches a minimum. Other physico-chemical phenomena, like intrinsic unstability of the produced crude oil, incompatibility between "commingled" produced oils, drilling fluids and produced oil, or between streams entering flow stations, as well as inadequate stimulation procedures, can be responsible for asphaltene deposition during crude oil production and transportation, going from the reservoir to surface facilities. Prevention methods oriented to asphaltene deposition control include the injection of chemical additives. These products have the particularity of disperse or inhibit asphaltene aggregation, avoiding the formation of precipitates that eventually deposit on metal or mineral surfaces, causing a severe flow reduction, and even formation or tube plugging. The main purpose of this work is to show the application of a systematic technique for chemical treatment evaluation, oriented towards the production restarting of asphaltene plugged oil wells in a Lake Maracaibo reservoir. Dead oils and deposits from neighbor oil wells of the same reservoir were characterized, along with the evaluation of some commercial additives, in terms of their performance as asphaltene deposition inhibitors, under atmospheric conditions. The results allowed selecting the best product to be evaluated under reservoir conditions. Its addition to live oil samples, at reservoir pressure and temperature, showed a clear reduction of the asphaltene precipitation pressure onsets. A complete chemical treatment, including the selected product, was used for production restarting of the plugged oil wells. Field operation procedures used in these cases are described. Also, production history data of the actual oil wells, comparing their productivity before and after the treatment, are shown in order to demonstrate the efficiency of the methodology. Introduction Organic solid deposition is one of the most serious problems faced in oil production operations. This problem can occur through all the production, transportation and storage steps, depending on the fluid nature and surrounding conditions, affecting the flow behavior of the reservoir through oil wells and pipelines. Asphaltene precipitation and deposition have origin in several physico-chemical phenomena.1–5 These aspects include pressure depletion during production, or tubing restrictions, crude oil intrinsic unstability (chemical characteristics), fluids incompatibility due to commingling production and blends from different wells, inappropriate stimulations and treatments (cleaning and squeeze procedures with highly paraffinic solvents, as gasoil, kerosene, etc.). Generally, asphaltene deposition can be controlled using predictive, corrective or preventing methods. Stability tests, based on an IFP standard method,6 can provide information about the crude oil asphaltene precipitation propensity, and stability of a crude oil blend. Some models, like SPLAH™, allow predicting the asphaltene flocculation pressure and temperature conditions, after the introduction of experimentally determined oil characteristics in this program (fluid composition and physico-chemical properties).7–10 Corrective actions include removal methods with mechanical treatment (scrapping or pigging)6 or chemical treatments (deposit dissolution using aromatic solvents)11–17. These methods (and their combinations) are used when the real problem is already present, and important production looses have occurred.

Asphaltene deposition in oil production constitutes a critical and difficult to control problem, in terms of productivity and production operation costs, going from the reservoir to surface facilities. This phenomenon causes severe plugging and flow reduction due to phase changes in the crude oil through its production. When pressure declines in the reservoir, due to its exploitation, the single phase system may undergo asphaltenes precipitation and deposition, specially near the bubble point, where their solubility reaches a minimum. Severe formation damages occur when this pressure reduction takes place within the reservoir, and asphaltenes deposit in the porous media. The magnitude of the asphaltene precipitation tendency is strongly related with the crude oil characteristics, considering the asphaltene particles stabilization by other components of the oil. Prevention methods oriented to the asphaltenes deposition control include chemical additives injection. These products have the particularity of disperse or inhibit the asphaltenes aggregation, avoiding the formation of precipitates that eventually deposit on metal or mineral surfaces, causing a severe flow reduction. The main purpose of this work is to show an experimental methodology oriented to the selection of appropriated asphaltene inhibitors to be applied under surface, production and reservoir conditions. Dead oils characterization, along with several commercial additives effectiveness evaluation at surface conditions, followed by PVT studies on bottomhole crude oil samples, allowed to design adequate asphaltene prevention treatments for every particular case. Dead oil studies included SARA characterization and asphaltenes precipitation trends. Live oils analysis were performed by means of asphaltenes dispersion vs. pressure curves, asphaltenes flocculation onsets under isothermal pressure depletion, and a selected inhibitor evaluation. Examples from operations carried out in western Venezuelan facilities, illustrate the benefits derived from this methodology. Introduction Asphaltenes are defined as the solid material that precipitates from the crude oil, asphalt or bitumen when an excess of a low molecular weight paraffinic solvent, like propane, n-pentane, n-heptane, etc., is added.1,2 Field experiences3 and experimental laboratory observations4 indicate the asphaltene stability is affected by several factors, like oil composition, pressure and temperature, being the first two properties more important that the last one.5 Asphaltene precipitation and deposition have origin in several physical-chemical phenomena. These include pressure depletion during production, fluids incompatibility due commingling production and blends from different wells, inappropriate stimulations and treatments (cleaning and squeeze procedures with highly paraffinic solvents, as gasoil, kerosene, etc.). Prevention methods can be less expensive and long-lasting than remediation mechanical procedures,6 aromatic solvent treatments,7–11 tailor designed solvent mixtures,7,12,13 and combinations of the above treatments, achieving this way substantial cost savings and considerable production rates. Taking into consideration the pressure factor, procedures oriented towards pressure maintenance have been suggested.14 Some times it is very difficult to keep well pressure high enough to guarantee solubilized asphaltenes, consequence of reservoir depletion and/or inconveniences in gas injection, due to the absence of gas resources near the reservoir. In those cases, methods oriented towards asphaltene precipitation prevention, based in the use of additives that inhibit these deposits built, known as asphaltene dispersants, are used. Recently, new strategies have been introduced, which involve the injection of chemical dispersants as inhibitors directly into the well, to minimize the growing process of the deposit.15