Abstract

Waterflood projects in oil fields cause important thermodynamic effects in the aqueous phase, resulting in precipitation of insoluble salts due to changes in physical parameters.

Giant fields offshore Brazil are operated by waterflooding. For some of the fields, injection of seawater, rich in sulphates, is not compatible with formation water that contains barium or strontium. Adding seawater to connate or aquifer water leads, during the field production life cycle, to saturation of certain compounds and hence their consequent deposition.

Decision about prevention or remediation of barium and strontium deposition must be attained during the conceptual phase of the field development plan. So, forecasting of the possibility of occurrence of this problem may have a strong impact in the water injection system design specifications and the well completion and interconnections to the floating production unit.

The scope of this paper is to present the results of the analysis conducted to investigate the scale deposition potential in Marlim Leste field, Campos basin, offshore Brazil.

Estimates of intensity of occurrence of precipitation and the critical locations have been conducted in order to obtain the fraction of seawater in the produced brine, which is an important information for evaluation of the problems associated to scale deposition in producing wells.

Based on these simulations economic feasibility studies (CAPEX versus OPEX) compared the alternatives of sulphate removal plants or well treatments with chemicals in order to obtain the final decision of what to implement in this field.

Introduction

The Marlim Leste field is a deepwater offshore field located in Campos Basin, 10 km East of the Marlim field and 8 km North of Marlim Sul field. The water depths range from 800 to 2000 m. The main reservoir age is Oligo-Miocene, and the depth is about 2700 m. The 3P field reserves are above 500 mmboe (80% proved).

For the Marlim Leste field, the development plan comprises the installation of a FPU (Floating Production Unit) with 180,000 barrels per day processing and treatment capacity. For this unit it is being planned the connection of 23 horizontal wells and one vertical well - 15 wells will be producers and 9 will be water injectors. The water to be injected is seawater. Mixing of seawater and connate or aquifer water will vary in the field production life cycle, and when the mixture becomes saturated in relation to a certain compound, it is expected to occur the process of scale deposition.

Scale formation may impact oil production and the economics of the project therefore this issue has to be addressed as soon as possible, during the conceptual phase of the field development plan.

Laboratory and Simulation Studies
Laboratory

Three water samples have been collected and analysed. The most representative quantification of the ions of interest is shown in Table 1.

Table 2 shows an empirical classification adopted concerning the scale deposition potential according to the barium and strontium contents in formation water.

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