Abstract
Oilfield produced water often contains high levels of total dissolved solids (TDS) in addition to precipitates, suspended particles, and hydrocarbons. The main components of the dissolved solids include sodium, calcium, and magnesium salts. The combination of the high salinity and high hardness in produced water can be very damaging to many types of fracturing fluids that are commonly formulated with fresh and clean water. It is usually costly to treat high-TDS produced water to such an extent that it can be safely and stably used to formulate fracturing fluids. Meanwhile, it is also expensive to dispose of produced water by underground deep well injection or to transport produced water for disposal at a commercial oilfield wastewater disposal facility. On the other hand, fresh water for formulating fracturing fluids is becoming more costly and more difficult to obtain in many oil/gas production areas. Fluid systems have therefore been highly desirable and sought after that can be formulated directly with high-TDS water and perform well at high temperature of 275 °F or above.
A group of fracturing fluids was successfully identified recently that performed very well at high temperatures when formulated with untreated high-TDS produced water. The produced water tested had TDS up to about 330k mg/L and hardness (calcium carbonate equivalent) up to about 64k mg/L. The fracturing fluids comprised of organometallic-crosslinked derivatized polysaccharides. In one typical example, the viscosity stayed above 100 cP (at 100/s shear rate) for about 80 minutes at 275 °F recorded for a fracturing fluid prepared with untreated produced water with TDS of about 300k mg/L and hardness of about 50k mg/L. The magnitude and lifetime of the fluid viscosity were dependent on a number of factors such as the produced water TDS and hardness, test temperature, polymer loading, fluid pH, etc. The fracturing fluids also showed minimum damage to the formation or proppant pack tested. In a typical proppant pack conductivity test at 250°F, the retained conductivity was about 90% for the fracturing fluid mixed with appropriate amount of oxidative breaker.
The outstanding performances and the underlying mechanisms of the high-temperature fracturing fluids prepared with untreated extremely high-TDS and hard produced water will be discussed, and the field-related laboratory test results will be presented.
