Thermochemical Process To Remove Paraffin Deposits In Subsea Production Lines
Authors: Khalil C.N. , Neumann L.F. , Linard C.A. , Santos I.G. 
Published: October 2008

The production of several wells from the Albacora ield (Campos Basin, Rio de Janeiro, Brazil) is being regressively lowered due to a paraffin deposition rows in the subsea production flowlines. The subsea low temperature is considered to be the leading factor in the paraffin deposition or waxing process of deep water flowlines.

The PETROBRÁS Production Department at Campos and it's Research Center are developing techniques to adapt the Nitrogen Generating System (NGS) to operations designed to clean the subsea flowlines of the Campos Basin, mainly those serving the Albacora Field. This process comprises the irreversible fluidization of the paraffin deposit through combined thermal, chemical and mechanical effects obtained in situ by the controlled generation of nitrogen gas, according to the chemical reaction:

NH4C1+ NaNO2= N2+ 2H20 + NaCl + HEAT

A field operation program involves, at the start, selection of an organic solvent the assessment of the effective internal volume of the stretch to be treated and numerical simulation of the process.

The various dewaxing operations effected at the Albacora field have confirmed the good performance of the NGS process, the benefits of which comprise preservation of the environment, since there is no discharge of by-products to land or sea; also, the increase in oil production, which is of the order of 32%, representing USD 200,000 a day.

INTRODUCTION

Generation, deposition and removal of organic precipitates known as paraffins or waxes is a well-known problem of the petroleum industry. In the Campos Basin, the paraffin deposits have been removed chiefly by circulating cold organic solvent. However, in the Albacora field, this technique has proved inefficient, and flexible flowlines had to be replaced periodically in order to bring to acceptable levels the oil production in this field. The overall arrangement of flowlines of the Albacora field is illustrated in FIGURE 1. The thermochemical process to remove paraffin deposits in subsea production flowlines, developed at the Research Center of PETROBRÁS, has brought a new means to cope with this problem by is significantly reducing oil production losses.

THEORY AND DEFINITIONS

The paraffin deposition phenomenon occurring in subsea production flowlines means basically the gradual and progressive deposition of organic compounds throughout the whole extension of the flowlines, caused by the flow of oils, chiefly those oils showing high contents of high molecular weight paraffin molecules. Of course the deposition process is closely associated to the low temperature of the seabottom, to the temperature gradient established between the oil and the seawater throughout the subsea flowline, to the oil and gas flowrates, to the shear dispersion and to a lesser extent to the gravitational segregation and the Brownian movement (1, 2, 3). The generation of paraffin crystals is associated to the lTCG (Initial Temperature of Crystal Generation) which is defined as"... the highest temperature in which is initiated the generation of a solid phase in oil, whenever oil is submitted to a previous thermal treatment and a controlled cooling rate, under isobaric conditions..."