Fracture Propagation and Injector Performance Predictive Model During Produced (Dirty) Water Injection

Abstract

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 well-injectivity 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 parameters[3-4] 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.

Injector Fracturing Concepts

The objective of this paper is to illustrate how fractures propagate during produced water reinjection. The role of porous formation mechanics on the interaction between a permeability-damaged zone around the fracture and a plug at the fracture tip is specifically explored. This paper also discusses the following four concepts:

Concept 1 The application of fracture mechanics techniques to hydraulic fracturing during the initiation of clean water reinjection and continuing throughout the life of the reservoir.  Fracture mechanics can be used to predict the relationships between the injection rate, the size of the hydraulic fracture and the required injection pressure for clean water.

Concept 2 As injection proceeds, particles in the produced water are deposited in the injection formation horizon and a "damaged" zone forms around the hydraulic fracture surface. These deposits decrease the permeability of the zone and tend to increase the required injection pressure (for a fixed injection rate). Considerations must be given to the water quality (concentrations and characteristics of the damage-causing contaminants) and its relationship to formation damage.

Concept 3 Another significant characteristic of the continuing injection process is that a plug of produced water particles can collect at the tip of the hydraulic fracture. This plug restricts flow at the crack tip and also tends to increase the injection pressure which is required to dispose water at a given rate and also cause the fracture to propagate.

Concept 4 The two combined phenomena (indicated in Concepts 2 and 3 noted above) affect fracture propagation. Although both phenomena tend to increase the required injection pressure for a given injection rate, their influences on the local stress state and their impact on the criteria for crack propagation differ greatly.