In the past, high-volume water-shutoff treatments have not been applied because of the economic burdens they incur. Today, many operators have reconsidered high-volume water-shutoff treatments because these treatments make oil production from mature reservoirs more economically feasible. Many wells in mature North Sea reservoirs produce a large amount of water. Consequently, these wells often produce less oil than they potentially could.
The evolution of diagnostic and interpretation techniques has significantly enhanced the degree of accuracy and completeness of production problem diagnoses. Reservoir models can be used to identify and design effective water-shutoff treatments. This paper describes how a production operation simulator is used with an advanced-processes reservoir simulator to design water-shutoff treatments. The following reservoir simulator options were necessary to design the treatments properly.
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thermal options
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chemical-injection options
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chemical-reaction options
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flexible-gridding options
The technique was recently used to design water-shutoff treatments and their placement for jobs in the North Sea area. The results of the simulations were used to predict the effects of the following factors.
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interval permeability distribution
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treatment rate
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reaction rates of treatment fluids
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resulting gel strength
The technique also allows treatments to be designed on the basis of realistic treatment temperatures rather than bottomhole static temperatures. The results of the simulations were used to optimize treatment placement rates, fluid composition, and shut-in times of jobs pumped in the Norwegian sector of the North Sea.
One case shows how cooldown inside the reservoir can be used to place a treatment that would have otherwise spontaneously gelled at reservoir temperature. Another case shows how temperature histories for different stages of the treatments were constructed from the simulation results. These temperature histories showed that different activator compositions and/or concentrations were required for early, intermediate and final treatment stages.
Water production can seriously compromise the profitability of oil- or gas-producing wells. The cost of produced-water disposal is becoming a major burden for many operators. Although prevention is usually more effective than treatment, excessive water production is most often treated rather than prevented. The keys to the success of shutting off or preventing excessive water are:
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proper identification of the water-production mechanism
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appropriate design of the treatment
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effective placement of the treatment
Figure 1 illustrates the process of a typical water-shutoff (or conformance) treatment. Each step of the process will now be discussed.
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