Abstract

This paper deals with the removal of formation damage caused by paraffin precipitation by injecting solvents in the nearbore of production wells. Depending on the produced crude composition, the production process itself may enhance this organic damage build-up around the bottomhole. The chemical treatment is a profitable alternative to restore production even in wells previously considered as depleted by means of this organic deposition. Some care must be taken while selecting both candidate wells to be treated and solvents for use. Bad choices in both cases may lead to undesired results. The lack of a standard procedure for sizing these treating jobs does not prevent their accomplishment. In order to provide the means some thermodynamic trends and field and lab observations upon the paraffin formation process must be followed. Several field operations were carried out using this approach and indicate an overall success was attained.

Introduction

Crude oil production from different locations may be affected by the precipitation and deposition of paraffin. Paraffin is the solid phase of a crude mixture formed by its heavier hydrocarbon components, which may become insoluble under prevailing conditions. Temperature is the main factor involved in the paraffin precipitation from crude oils, besides composition.

At low temperatures, the content on components above C20 normally supply enough solidifying material for the paraffin occurrence, no matter their structures are neither linear nor saturated in carbon bonds. These components are those which become unstable in the liquid mixture at higher temperatures. The paste made by liquid oil trapped by an agglomerate of paraffin crystals plus several other solid materials found on the production stream is also called wax by most field operators. Undesirable effects like formation pores, tubing and flow lines plugging, besides sharp increases on the crude viscosity, are caused by the paraffin precipitation. Costs frequently rise as preventive or corrective measures are taken to control the losses caused by the paraffin appearance on a crude production system. These losses include reduction on well productivity and premature well abandonment. Formation damage is probably the most severe problem caused by paraffin to the oil production industry. Petroleum technicians may be deceived by the paraffin formation damage because on most occasions there is not a clear perception of its occurrence. Production decreases provided by the this organic damage occurrence are often imputted only to natural depletion. This work is the result of the experience gathered after several field operations were carried out in order to restore production on wells thought to be nearly depleted after operating for some years. These wells faced the worst consequences of the paraffin formation damage occurrence: sharp declining production, low recovery and high costs.

The approach used here differs a little from a thermochemical method called SGN (stands for Nitrogen Generating System, in Portuguese), developed by PETROBRAS in recent years. The solution adopted on this work employs a solvent as the main agent with the help provided by higher bottomhole temperature - may be a lower cost alternative. The solvents used consist of petroleum fractions taken from native crudes at nearby processing units. For the removal of paraffin damage on affected wells the solvents were squeezed into their formations.

Results provided by this work show that the paraffin damage removal may be economically achieved by the suggested approach.

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