Carbonate and sulfate scales are two typical scales formed in oil fields, which can cause tubing blockage, reduce production, and interfere with well intervention. Phosphonate-based scale inhibitors are widely applied to inhibit scale deposition.
In this study, the performance of four different phosphonate-based scale inhibitors (SI-T1, SI-T2, SI-T3, SI-T4) was evaluated at high temperatures like 105~135°C in high total dissolved solids (TDS) brines (>50,000 ppm). A series of tests was applied to evaluate the performance of the scale inhibitors under test conditions, including the brine compatibility test, the inhibition efficiency against three typical scales (CaCO3, CaSO4 and SrSO4), thermal stability, and rock interaction test.
Scale inhibitor SI-T2 demonstrated the highest inhibition efficiency against the three types of scales at 105°C in the high TDS test brines. SI-T2 was thermally stable at a higher temperature of 135°C. It had a strong adsorption capability on the limestone. SI-T3 also presented good inhibition performance, brine compatibility and thermal stability. Interestingly, the thermally aged SI-T2 presented a different inhibition efficiency for CaCO3 and CaSO4. The aged SI-T2 kept the same level of inhibition performance for CaCO3 as the non-aged inhibitor, but lost inhibition performance against CaSO4 after thermally aged at 135°C. This study provides a systematic evaluation of phosphonate-based scale inhibitors under high temperatures in high TDS scaling brines. It will benefit inhibitor selection for field application in high temperature and high hardness carbonate formations.
Scale is an adherent deposit of inorganic compounds precipitated from water onto surfaces. Most oilfield waters contain certain amounts of dissolved calcium, barium or strontium salts. The mineral scale can be formed by chemical reactions in the formation water itself, by mixing of formation water with injected seawater, or by mixing of the well streams of two incompatible oilfield waters. In carbonate reservoirs, when calcium is deposited as calcium sulfate or calcium carbonate scale, a loss of production and increased maintenance expenses can result. Therefore, effective mitigation of scaling potential is of importance to the oil producers.1