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.