Water injection for pressure maintenance, waterflooding or disposal of produced water has become an important issue in the management of produced fluids. However, suspended particles in the injected water cause formation damage and can lead to fracture initiation due to an increase in injection pressure. Injectivity decline in injection or disposal wells can have a large impact on the economic feasibility of water disposal operations. Fracture propagation could potentially result in undesirable environmental and reservoir engineering consequences.
The paper presents results of a study conducted to understand the mechanisms of injectivity decline and fracture propagation during water injection. Large-scale injection tests were conducted under simulated in-situ stress and pore pressure conditions, using rock blocks 27"×27"×32", representative of a consolidated reservoir rock and laboratory-made produced-water.
Results indicate that the rate of fracture growth is strongly dependent on the type and concentration of particles in the injected fluid. The shape, size and compressibility of the injected particles play an important role. Our results clearly demonstrate that the injectivity remains almost constant once a fracture is initiated. The injected particles are filtered largely near the tip of the fracture where the leakoff is the highest. The depth of penetration of the particles into the matrix appears to be small. Analyses using optical microscopy observations on thin sections show that the filter cake is not a continuous film but rather a region (from 3 to 10 grains deep) where the connected flow paths have been effectively blocked by solid particle deposition. Results from these experiments provide a basis for developing analytical and numerical modeling tools for injectivity decline and fracture propagation in fractured injectors.
Water re-injection for disposal of produced water is an increasingly important issue in the management of produced fluids. Water production of the order of 1 million barrels a day for Prudhoe Bay field and hundred of thousands barrels a day for North Sea fields were reported1. Other mature oil and gas production regions of the world exhibit similarly high volumes of produced water. Surface disposal of produced water is particularly problematic in offshore operations, thus disposal by re-injection is increasingly becoming common. This means that large volumes of produced water must be disposed under sustained rates of injection. Injectivity decline in water injection or disposal wells can have a dominant impact on the economic feasibility of offshore water disposal operations. Laboratory experiments2 show that matrix injectivity is strongly dependent on water quality. The authors showed that even for relatively clean injection water, injectivity declines rapidly due to the plugging of the near wellbore region by particles present in the injection water (the half-lives of these unfractured injectors were thirty to sixty days even for a good water quality). In the field, such rapid declines in injectivity are not observed, indicating that wells are fractured. This is confirmed by recent evaluations of field operations suggesting that water is injected above fracturing pressures in a large proportion of injection wells3. Field observations also suggest that injecting above fracturing pressures better sustains long-term injectivity4,5. This indicates a weaker dependence to water quality during fracture injectivity.