The majority of injectors are likely to be fractured (intentionally or unintentionally) during their life cycles. Assessment of facture conductivity damage and associated surrounding formation damage is an essential step for maximizing and maintaining injector performance during produced water injection for water flooding. Fractured injectors experience less injectivity decline over time in comparison to matrix injectors.
The current paper will present a mechanistic model for the plugged fracture behavior and discuss its applications to field cases. In addition, implications, recommendations and best practices for optimized produced water injection will be addressed within this context.
The well undergoes two distinct processes that alternate over its life to create the sustained injectivity behavior associated with this scheme. Injectivity damage can occur particularly in pseudo-matrix conditions, where a stationary fracture progressively plugs (in addition to permeability impairment of the surrounding reservoir rock), resulting in pressure buildup and injectivity decline. As solids are deposited in the fracture, the "effective" exposed surface area is gradually reduced, at least partially, as a function of the injected amount of solids. Furthermore, fracture face damage (external filter cake build-up) and accumulated surrounding formation damage (internal filter cake build-up) progress as the volume of injected water increases. This plugging process continues until the injection pressure required for the designated injection rate reaches a critical value, which is greater than the pressure required to re-fracture the formation and/or to extend or propagate the plugged fracture. At such pressure a sudden fracture propagation or further breakdown of the formation is evidenced by a sudden increase in the Injectivity Index or similarly, a sudden drop in the Reciprocal Injectivity Index – RII - (unit injection pressure per unit constant injection rate). Thereafter, the required pumping pressure drops and/or the injection rate increases with the newly created surface area that has been caused by fracture propagation or initiation. The process then repeats itself. As the pressure rises it approaches and finally exceeds the fracture pressure required for further propagation, resulting in a saw-toothed shape pressure-time (or rate-time) behavior. The slopes of the bounding lines are controlled by the damaged formation and fracture characteristics, as well as the injected water quality. The slopes can be made to diverge, converge or remain almost parallel. The third option indicates a well optimized injection scheme.
The final section of this paper will imply lessons learned, mitigation strategies, conclusions and recommendations based on analyses of field data from a number of produced water injectors. Pressure transient analysis and results from pressure fall-off test will be presented to verify the study results.