Scales such as aluminum silicates are commonly believed to come from reservoir rocks such as shale and/or their interaction with injected chemicals from EOR (enhanced oil recovery) methods. In hydraulic fractured wells, scales could be a major source of damage to fracture conductivity, which is usually less than 5% of their desired value by fracturing design. Despite much research on conductivity loss, few efforts have been directed at proppants, the number one material by quantity (except for water) used in fracturing operations.
In this research, a brief review is conducted on how proppants might be an active agent in scale-generating reactions. Using a ceramic proppant sample from the Bakken shale oil field as an example, a new method is proposed for characterizing proppant strength and interaction with fracturing fluids, rock, and formation and production fluids under realistic down-hole conditions. It was found that certain ceramic proppants widely used in the Bakken field are vulnerable in weakly acidic fluids (i.e. pH=4). Temperature increases expedite the proppant weakening process. Estimated conductivity loss could be ~50% compared with that treated at neutral pH. This is alarming because many wells produce sweet (carbon dioxide CO2) and/or sour (hydrogen sulfite H2S) gas along with production fluids. Common scale inhibitors such as phosphonate-based acids and polycarboxylic acids (PAA) could also lower the pH of fluids in propped fractures to 3~4, break the ceramic proppant and generate fines and scales.
The first lesson learned is that proppants could be a source of scale. Proppants, especially ceramic ones, should be treated as a chemical and potential source of scale when implementing designs for hydraulic fracturing or gravel packing. Not only will poor quality proppants crush easily, but also they may generate migrating scales, a double inhibition for flow assurance. Extreme care should be exercised when choosing proppants for sour wells. Tighter specification on acid resistance of proppants is proposed. Field quality control and scale management in fractured wells will also be discussed.