Scale formation in oil and gas wells commonly occurs, causing not only pipeline blockage, equipment failure, or formation damage during production, trasnsportation, and treatment, but also premature abandonment of wells in serious cases. Although types of mineral scale occurrence depend on the types of ions in water, sulfate and carbonate scales are the most commonly found scales in oil and gas fields. In this study, we investigated a single approach to prevent complex mineral scales from deposition using water-soluble polymer dispersant or the combination of water-soluble polymer dispersant of carboxymethyl cellulose (CMC) and phosphonate inhibitors of diethylene triamine penta(methylene phosphonic) acid (DTPMP) or hexamethylene diaminetetra (methylene phosphonic) acid (HDTMP) in highly saturated solution or high ionic strength (IS) brines. This study shows that CMC effectively prevents sulfate (barite and gypsum) and carbonate (calcite nd iron carbonate) scales from deposition. The particle size dispersed in the presence of CMC remains in nanosize ranges. When CMC was combined with phosphonate inhibitors of DTPMP or HDTMP, sulfate scales were even more effectively controlled, compare to CMC or phosphonate inhibitors by themselves. In the combination of CMC and DTPMP, the majority of barite (> 90%) remained in a size of smaller than 200 nm and the total mass of barite deposition on 316 stainless steel coupon was negligible, as low as 0.079% of total input mass. Gypsum formation was inhibited for at least 6 hours and gypsum particles remained in a size of smaller than 200 nm for 12 hours in the combination of CMC and HDTPM. For calcite, measured induction time was 134 minutes and calcite particles were dispersed for at least 15 hours with its average particle size of 396 nm in the presence of CMC. Iron carbonate particles were well dispersed for 2 hours in the presence of CMC.