Abstract

Inorganic scaling is a predominant formation damage mechanism in most producing and injection wells worldwide. Scales are deposited from oilfield brines when there is a disturbance in thermodynamic and chemical equilibrium. The degree of super saturation caused by the disturbance in equilibrium can be increased to result in scaling by changes in fluid composition through adverse mixing and by changes in temperature and pressure.

Although a few cases have been reported of non-water producing oil wells that produced scales, typically oil field scaling accompanies water production. Scaling problems during well development and supplemental fluid injection are primarily due to mixing of two incompatible fluids, whereas production related scaling is typically controlled by pressure and temperature. Acid or any common solvent cannot easily remove some types of scales like barium sulfate (BaSO4), which readily incorporates radioactive radium in its structure.

Therefore detection and prevention is critical wherever any precipitation or co-precipitation that includes BaSO4 may be taking place. As severity of scaling is controlled by temperature, pressure and composition, the key to the detection, prevention and effective treatment is a methodology to determine at what conditions in the reservoir or at what point in the well the scales are precipitated and deposited and also determine the type of scale. This is important because sometime scales can be forming at a point in the well bore without showing up at neither the surface or at the sand face.

Near infrared (NIR) technology has been extensively used in detecting the organic scales such as asphaltene deposits. To the best of our knowledge, this study presents the first extensive use of NIR technology to diagnose inorganic scaling problems.

This paper presents several examples where NIR technology has been used to detect the onset of inorganic scale as a function of pressure depletion at reservoir temperature for water from different fields. We also show how the NIR technology has been used to determine the effectiveness of various commercial inhibitors at reservoir conditions. We also present further screening of the inhibitors (selected based on the results of NIR tests) to determine their effective use in the field and the design of the inhibitor treatment for the producing wells.

Introduction

There is a multitude of technical papers1–6 published on the application of near infrared technology for asphaltene studies. To the best of our knowledge, not much has been published on the use of this technology for inorganic scaling.

The principle of NIR spectrography used for inorganic scale onset measurements is based on Rayleigh scattering which is the phenomenon in which light is scattered by objects small in relation to the wavelength of the light. NIR light energy includes the wavelengths from 800 to 2000 nm. This principle is used as the basis for detecting the formation of scale particles as they first crystallize (i.e., scale on-set) from the brine as a function of changes in thermodynamic equilibrium. The NIR cell provides data from minus 15 to 400°F, and at pressures from 0–15000 psig, using live-charged brines. The NIR system allows brine samples to be evaluated under production (formation, tubing, screens, pipelines), injection (surface facilities, down hole mixing), and defined mixing conditions (e.g. commingled water flooding, waste disposal).

The two main types of scale which are commonly found in the oilfield are carbonate and sulphate scales. Whilst carbonate scale is due to changes in the pressure and pH changes of the production fluid and sulphate scale is mainly associated with the mixing of incompatible brines, i.e. formation water and injection water. The use of seawater for pressure maintenance and oil recovery typically results in the problems with sulphate scale deposition, which are mostly calcium and strontium. Removal of these scales are in most cases complicated because the deposits are rarely pure calcium carbonate, calcium sulfate, barium sulfate or strontium sulfate but are usually a mixture of two or more of the inorganic components, corrosion products, organic scales, silica and other impurities trapped in the inorganic lattice.

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