Abstract

The SCALE2000 (V2.0) software (under Windows 95/98/NT) was specifically developed to predict quantitatively scale deposits induced by oil production operations. The conceptual model is based on the Pitzer's specific ion interaction formalism for Na-K-Ca-Mg-Sr-Ba-Fe-Cl-SO4-OH-H-Ac-CO3-HCO3-CO2-AcH-SiO2-H2O system under 25 - 250°C and 1 - 1000 bar conditions. SCALE2000 enables to perform four main kinds of calculations including solubility of the main known minerals responsible for scale problems in geothermal energy and oil productions (carbonates, sulphates, silicates, &), effect of temperature and pressure on the scale precipitations, effect of binary mixing of physico-chemical contrasted waters, quantitative predictions of scale formation based on a detailed kinetic formalism. As it was demonstrated by petroleum engineers, carbonate system is crucial for precise scale risk predictions. The reliability of SCALE2000 results was checked against measurements of individual mineral solubilities and kinetics found in the literature.

In this study, SCALE2000 was applied to the actual high temperature (164°C), high salinity (~ 84 g/l) and high pressure (320 bar) data of the Block 3 (Angola) oil-field. In agreement with observations, the precipitation of barite was predicted by SCALE2000. The mineralogical nature of deposits was intimately dependent on the reservoir water/seawater ratio. SCALE2000 simulations of the sulphate depositions on the tubing wall of the PAC 302B Pacassa well (block 3, Angola), as a function of the water produced, has permitted to predict the kinetics of barite precipitation for production P-T conditions (180 bar; 140°C). Simulation results were in good agreement with field observations.

Introduction

Scale can impair productivity dramatically, thereby increasing hydrocarbon production costs. Solid scale formation mainly results from changes in physical-chemical properties of fluids (i.e., pH, pCO2, T and P) during production or from chemical incompatibility between injected water and formation water. However, prediction remains notoriously difficult, mainly due to the complexity of modelling both thermodynamic and kinetic behaviour of carbonate and sulphate systems in high salinity solutions at high temperature and high pressure (HS/HT/HP). True activities of dissolved species, accounting for all specific interactions between ions, must be known. The most comprehensive model to calculate activities in waters is the specific ion interaction model of Pitzer (1). The thermo-kinetic software SCALE2000, which is based on Pitzer's approach, was specifically developed in order to address the problem of scale deposit prediction. The basics of this simulator were first established in the framework of an EC-funded project (Qc-Scale; contract JOF3-CT95-0009); the development of the finalized improved version (2.0) was funded by BRGM and, partly, by TotalFinaElf.

In this study, SCALE2000 was used to evaluate and quantify scaling deposits of the high temperature (164°C), high salinity (~ 84 g/l) and high pressure (320 bar) Block 3 (Angola) oil-field.

The thermo-kinetic software SCALE2000 (V2.0)

SCALE2000 (V2.0) calculates aqueous speciation, mineral saturation indices, mineral solubilities, reaction paths and the effect of mixing between contrasted fluids. It takes into account the effect of total pressure from 1 to 1000 bar on thermodynamic constants, on the first term of activity and osmotic coefficients in Pitzer equations (2). The model for calculation of equilibrium constants of aqueous, mineral, and gaseous species reactions is based on Helgeson-Kirkham-Flowers (3) modified (4) equations referred to as the HKF-revised model.

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