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 sulfate/carbonate ions were injected into the porous medium, where calcium sulfate 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 control the quantity and morphology of the precipitating crystals.
