Presented at 2006 SPE International Symposium and Exhibition on Formation Damage Control held in Lafayette, LA, 15-17 February 2006.
Fracture Propagation, Filter-Cake Buildup and Formation Plugging During PWRI
Produced water reinjection (PWRI) offers an efficient and effective means of disposing of the PW waste stream and provides an opportunity for a water drive when applied during waterflooding. The required rate of produced water reinjection can be anticipated using the expected pore volume replacement ratio and water-cut estimated from the production forecast. Fracturing is likely to occur during produced water reinjection at voidage replacement rates. The extent (size) of the induced fracture(s) will significantly impact the waste disposal process. It is, therefore, necessary for well injectivity planning and fracture sizing to have an accurate estimate of pore pressure, the rock’s mechanical properties and the minimum in-situ stress in the injection horizon. This collective information can be used to estimate the required injection pressure and the number of injectors throughout the production period. In addition, well planning and design will also benefit from predictions concerning the injector performance histories – and the length of the created fracture. Overall, the waterflood planning-cycle efficiency will be increased. It is generally accepted that PWRI leads to plugging of fractures and damage of injection zone permeability 1-2. The engineering issue faced by the operator is reduced to establishing the balance between two (2) competing mechanisms. The first mechanism is related to the wellinjectivity improvement that may result from any fracturing associated with produced water reinjection. The competing mechanism results from plugging of the near crack-tip region and the impairment of reservoir performance (permeability) around the fracture caused by water contaminant invasion of the injection horizon. The interplay between the reservoir rock properties and water quality parameters3-4 has been qualitatively discussed in some detail in the past. However, this paper will confine discussions to fracture propagation and its impact on well injectivity, under conditions of produced water reinjection in permeable reservoirs. Results of such analyses can provide estimates of filter-cake permeability and thickness, as well as the magnitude of permeability impairment around the fracture and the extent of the impairment zone.