Abstract

In the oilfield, due to the extensive use of water injection for oil displacement and pressure maintenance, many reservoirs experience the problem of scale deposition when injection water starts to breakthrough. In most cases the scaled-up wells are caused by the formation of sulphate and carbonate scales of calcium and Strontium. Due to their relative hardness and low solubility, there are limited processes available for their removal and the preventive measure such as the ‘squeeze’ inhibitor treatment has to be taken. It is therefore important to have a proper understanding of the kinetics of scale formation and its detrimental effect on formation damage under both inhibited and uninhibited environment.

This paper presents an experimental and theoretical study of permeability reduction of porous medium caused by scaling. Two incompatible solutions, calcium and sulfate/carbonate rich ions were injected into and calcium sulphate or carbonate generated within the porous medium by chemical reaction. Mechanisms by which a precipitate reduces permeability include solids depositing on the pore walls because of attractive forces between the particles and the surface of the pore, a single particle blocking a pore throat, and several particles bridging across a pore throat. The characteristics of the precipitate influence the extent of formation damage. Conditions such as large degree of supersaturation, presence of impurities, a change in temperature, and the rate of mixing control the quantity and morphology of the precipitating crystals.

Introduction

The formation of mineral scale associated with the production of hydrocarbon has been a concern in oilfield operation. Depending on the nature of the scale and the fluid composition, the deposition can take place within the reservoir which causes formation damage [10, 13, 15, 20] or in the production facilities where blockage can cause severe operational problems. The two main types of scale which are commonly found in the oilfield are carbonate and sulphate scales [14–15, 22–24, 27–28, 31–35]. Whilst the formation of carbonate scale [2, 34–35] is associated with the pressure and pH changes of the production fluid, the occurrence of sulphate scale is mainly due to the mixing of incompatible brines, [16–18, 20–21, 35–36] i.e. formation water and injection water. In the oilfield, the universal use of sea water injection as the primary oil recovery mechanism and for pressure maintenance means that problems with sulphate scale deposition, [20–21] mainly calcium and strontium, are likely to be present at some stage during the production life of the field. A field example is the Iranian offshore of Siri field [20] in the southern Persian Gulf. The oil in this field comes from a formation called Mishrif; this formation is common between Iran and the United Arab Emirates. Water injection into the Siri field was started in 1984 with 9100 bbl/day in order to maintain the pressure and to increase the oil recovery. But the injectivity decreased rapidly by 1990 the water injection was only 2200 bbl/day, and subsequently the water injection was stopped. The history of water injection in the Siri field is shown in Fig.1 [20] with a drop in injectivity of approximately 7000 bbl/day over a period of six years. Table 1 shows the various types of scales that are commonly found in oilfield [21]. Table 2 shows the major components of scales found in the Iranian oilfield [21]. The deposits are seldom pure calcium sulphate or calcium carbonate, but are usually a mixture of two or more of the inorganic components plus corrosion products, congealed oil, paraffin, silica and other impurities. These materials are trapped in the inorganic lattice and frequently complicate the removal of the deposit. Of all the scales, calcium sulphate and calcium carbonate have been singled out for study in this work.

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