Growing Injection Well Fractures and Their Impact on Waterflood Performance

by
Phani Bhushan Gadde, MSE

University of Texas at Austin, 2000
Supervisor: Mukul M. Sharma

Water flooding is the most widely used improved oil recovery method. Most water injection wells have fractures that grow with time. These fractures can have a significant impact on reservoir performance. This thesis presents a model to predict the initiation and growth of injection well fractures in the near wellbore region and to study the effect of these growing fractures on oil recovery.

Fractures can be initiated in injection wells due to the combined influence of thermal stresses, changes in pore pressure and an increase in injection pressure. Fracture growth and fracture face plugging are modeled as dynamically coupled phenomena. The relative importance of injection water quality, thermal stresses and changes in pore pressure are clearly delineated. The results indicate that wellbore plugging can significantly affect the initiation of a fracture. Further, plugging of the fracture face itself can have a major impact on the growth of the fracture, and the overall injectivity of the well. It is found that at a given injection rate, an increase in plugging leads to significant increases in fracture half-length. In the absence of thermal effects the injectivity of fractured injectors is shown to be independent of the water quality and the permeability of the formation. Results also demonstrate that a longer fracture does not necessarily imply an increase in injectivity, if there is significant permeability impairment due to plugging of the fracture face. The combined effects of particle plugging, phase mobilities and thermal stresses need to be properly accounted for in predicting the performance of injectors.

This single well model is coupled to a reservoir simulator to undertake field scale studies. The reservoir simulator passes variables such as flow rate, cumulative injected volume and the average reservoir pressure to the single-well model at selected time steps. The single-well model returns the fracture length, permeability in the damaged zone and the skin factor for each injection well. These are then used to reset the permeability field in the reservoir simulator. The presence of growing high permeability fractures in the injectors affects the waterflood sweep efficiency. Results indicate that, the oil recovery can increase or decrease depending on the fracture orientation and the rate of fracture growth. The effect of heterogeneity is also shown. Results of the simulations can be used to set injection well pressures and rates, specify water quality and to select injection well patterns to maximize oil recovery.

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