Growing Injection Well Fractures and Their Impact on Waterflood Performance

Abstract

Abstract Most water injection wells in waterflooded reservoirs have fractures that grow with time. These fractures can have a significant impact on the reservoir performance (oil production rates, o/w ratio and ultimate recovery). A single well model that predicts the length of injection well fractures by modeling fracture growth due to fracture face plugging and thermal stresses is coupled with a reservoir simulator to simulate injection wells that have fractures that dynamically grow with time. The relative importance of the injected water quality, formation permeability, injection rate and the temperature of injected water on the rate of fracture growth are demonstrated. The single well model accurately accounts for all the physics of fracture growth at the injector while the reservoir simulator accounts for the large-scale reservoir structure. The presence of high conductivity fractures in the injectors affects the waterflood sweep efficiency. Results indicate that fracture orientation, rate of fracture growth, injection water quality and reservoir heterogeneity play an important role in determining the oil production rates and ultimate recovery. The 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. 1. Introduction Waterflooding is the most widely used improved oil recovery method. Injection of sea and surface water is common in mature fields. Water injection is also used for pressure maintenance. Most water injection wells have fractures that grow with time. Fractures can be initiated in injection wells due to thermal stresses, changes in pore pressure and an increase in injection pressure due to particulate plugging. These fractures may have a significant impact on the reservoir performance. Unlimited fracture growth can have a number of undesirable consequences. The presence of growing high permeability fractures in the injectors distorts the water flood fronts. Depending on the position of the injection wells, this may result in poor sweep efficiency and consequently in premature water breakthrough. Unconstrained vertical growth of fractures can connect water and hydrocarbon bearing zones. An important factor to be considered here is that most injection wells are not fractured at the commencement of injection. Rather, fracturing is induced during the course of injection and these fractures grow with time. Hence, in determining the effect of injection well fractures on the reservoir, these injection well fractures cannot be modeled as static fractures of fixed length and conductivity. The growth of these fractures with time needs to be taken into account. Traditional reservoir simulators allow only for fixed fracture lengths in injectors and producers. Hence, our approach to modeling the effect of injection well fracturing involvesDeveloping a single well model to predict fracture growth in injection wells due to particle plugging, thermal and pore pressure effects.Coupling the single well model to a reservoir simulator to study the effect of injection well fracturing on oil recovery.