Example - Injectivity as a Function of Temperature - Thermal Stress Alteration versus Viscosity

Reservoir Characteristics:

Although this is a weak sand, it was indicated that injection was under fracturing conditions. "Early" (pre-1997) PWRI experience indicated that this reservoir experienced a significant loss of injectivity, believed to be caused by BaSO₄ scale carry-over.

Competing Mechanisms:

• Since the produced water is hotter, you would expect higher in-stu stresses, but,
• It would be anticipated that the thermally induced stress changes would be smaller because this is a material with a small Young's Modulus.
• On the other hand, in this soft formation, it was expected that injectivity would increase because of reductions in viscosity; i.e., in the absence of plugging and scaling, it was expected that this hot, and fairly clean PW should have had a higher injectivity than cold seawater.
• Figures 1 and 2 show that injectivity did not increase with increasing temperature, suggesting the BaSO₄ scale carry-over mechanism.
• The cause of the fluctuations in injectivity are not specifically known - they were not reported. They might be attribuatable to:
• Periodic stimulation not reported to the analysts, although it is felt that this may not be likely because of difficulties in removing BaSO₄ and it might be anticipated that this would be more of a problem for seawater.
• Periodic shutdowns that allowed formation pressure to decrease. This is not known.
• Characteristic plugging/propagation cycles that occur during fracturing produced water injection.

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