Formation Damage Due to Scale Formation in Porous Media Resulting From Water Injection

Abstract

Abstract Due to the extensive use of water injection for oil displacement and pressure maintenance in oilfields, many reservoirs experience the problem of scale deposition when injection water starts to break through. In most cases the scaled-up wells are caused by the formation of sulfate and carbonate scales of calcium and strontium. Due to their relative hardness and low solubility, there are limited processes available for their removal and preventive measures such as the ‘squeeze’ inhibitor treatment have to be taken. It is therefore important to gain a proper understanding of the kinetics of scale formation and its detrimental effects on formation damage under both inhibited and uninhibited conditions. This paper presents the results of an experimental and theoretical study of permeability reduction of porous media caused by scaling. Two incompatible solutions of calcium and /carbonate ions were injected into the porous medium, where calcium or calcium carbonate was generated by chemical reaction. Mechanisms by which a precipitate reduces permeability include solids deposition on the pore walls due to attractive forces between the particles and the surfaces of the pores, individual particles blocking pore throats, 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, change in temperature, and rate of mixing controls 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 [14, 17, 20, 25] 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 [19–20, 27–30, 32, 36, 39–44]. Whilst the formation of carbonate scale [3, 42–43] 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, [21–23, 25–26, 43–44] 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, [25–26] 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 [25] 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 [25] with a drop in injectivity of approximately 7000 bbl/day over a period of six years.