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.
