The Impact of Ultra-Low Temperature Sandstone Reservoirs on Scale Inhibitor Retention

Jordan, Myles (Nalco Champion) | Johnston, Clare (Nalco Champion)


ABSTRACTThis study looks at the issues faced by operators with low temperature sandstone reservoirs of only 40°C and 54°C and the challenges these low temperatures brought which include high MIC for sulphate scale control and poor chemical retention & release properties observed during the reservoir condition corefloods. These findings will be compared and contrasted with two other higher temperature (71°C and 95°C) sandstone reservoirs where phosphonates and phosphate ester chemicals have been evaluated and deployed in the field.The findings from this detailed coreflood study and review of previous experimental/field deployed scale squeeze treatment data shows that phosphonates work very well at elevated temperatures; at and above 70°C where their stronger retention and excellent release profiles makes them a favored chemical for such treatments. However at lower temperatures these molecules are not well retained on the rock and it is the phosphate ester chemicals that are more effective and provided the longer squeeze life to its respective MIC value. Comments on the interaction/performance of polymer scale inhibitors will also be made for these low temperature conditions.The implication of these findings show that phosphate esters offer the potential for extended squeeze lifetime in the <50°C sandstone reservoirs that are being developed in Northern Norway (Barents Sea) and the shallow subsurface depth, cool reservoirs being developed in offshore Brazil.INTRODUCTIONOilfield scales are inorganic crystalline deposits that precipitate from brines present in the reservoir and production flow system.1,2,3 Precipitation occurs as the result of changes in the ionic composition, pH, pressure and temperature. The primary scale formation mechanisms and the scale resulting from these mechanisms are detailed in Table 1.Scaling Tendency and Scale Mass as a Function of TemperatureSulphate scale (particularly barite) forms as produced water (a mixture of Ba-, Sr-, Ca-rich formation water and sulphate-rich seawater) cools. Barite scale tendency increases with decreasing temperature, because barite is less soluble at lower temperatures. This is shown in Figure 1 where the barite scale tendency for a formation water (barium = 110ppm) and seawater (sulphate = 2900ppm) blend is tested between 5 and 75°C.1 The barium sulphate scaling tendency (thermodynamic driving force for precipitation) is 5-6 times higher at 5°C than at 75°C (Figure 1). The challenge of controlling scale at low temperature is therefore well-recognised.