A Thermochemical Process for Wax Damage Removal

Abstract

Abstract Relevant declines in oil production on several Reconcavo Basin wells (NE Brazil) have been observed as a result of wax deposition phenomena. Evidences indicate several points of wax obstruction, such as: reservoir rock, perforations, casing annulus, tubing and production lines. The process consists on the injection of an emulsion formed by an aqueous solution containing nitrogenated salts, an organic non-polar solvent and a viscoelastic polymer. When the mixture is placed in the annulus, a strongly exothermic chemical reaction occurs, generating a high quality stable foam. The penetration radius of the foam into the porous media is enough to assure sufficient wax removal and induce immediate production enhancement. During well depressurizing, nitrogen gas, an aqueous saline phase and an organic mixture containing wax and the solvent are recovered. All the reaction products are environmental friendly. This paper presents lab and field results of this workover method based on in situ thermal generation of foam. Lab studies include foam formulation optimization aiming maximum wax deposit removal in several reservoir conditions. Field implementation includes the analysis of production enhancement in 10 wells in the area where the technique has been applied. Introduction The phenomenon of wax deposits formation is common in petroleum industry and it occurs consequent to modifications in the thermodynamics variables that change the solubility of wax fractions present in petroleum. The paraffining phenomenon involves specially saturated hydrocarbon of linear chain and high molecular weight during production, flow and treatment of petroleum. The deposition in subsea lines, surface equipment, production column, or even in reservoir rock can cause significant and crescent loss of petroleum production. Precipitation and deposition of wax are associated to phase equilibrium of hydrocarbons and to fluid-dynamics conditions of flow, respectively. The paraffining becomes one function of petroleum intrinsic characteristics and temperature, velocity and pressure variations during the production. The appearance of a solid phase in petroleum and the subsequent wax deposition are related to changes in the phase equilibrium, caused by petroleum cooling and/or separation of lighter fractions, originally dissolved in petroleum.1 There are several preventive and/or corrective methods to control wax deposition: use of chemical inhibitors, injection of hot organic solvents and mechanical removal. In Recôncavo Baiano Basin, the wax deposition occurs principally because of high paraffin content in the oil produced together with reservoir low temperature and low flow rate of the production wells. The consequences of the foreseen paraffining are : reduction of internal diameters of production columns and of production lines, which are able to cause overload in pumping units and the consequent reduction in the petroleum production. The wax deposits can in extreme cases lock the rod of the pumping units, making it difficult to carry out safely any wax removal operation through conventional methods.2