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

Reservoir Characteristics:

This example characterizes injection into a sandstone with a permeability of ~1500 md. The injection zone thickness was approximately 130 ft. Initially, seawater was injected - from October 26, 1993 until June 19, 1997. The second phase of injection was then initiated and data were provided up to February 28, 1999.

Data Analysis:

Information was avaialble for two wells. After June 20, 1997, both wells were taking large quantities of produced water (and produced water-aquifer blends) at relatively low pressures. The average injection rate was ~80,000 BPWD (13,000 m³/day) at approximately 1,500 psi WHP (100 bars) for Well A-03 and 25,000 BPWD (4,000 m³/day) at 1,200 psi (80 bars) for Well A-07. The produced water was regularly mixed with aquifer water with the amount of AW varying daily. The mixture varied from 100% produced water to 100% aquifer water, with a typical value of 50% produced water. The aquifer water surface temperature was approximately 80°F, while the produced water surface temperature was approximately 130°F. Figures 1 and 2 show temperatures for each well (based on the method of mixtures).

No completion data were available for the well. The Reciprocal Injectivity Index (RII) was calculated from the WHP only, with attendant errors. Plotting RII and the recorded wellhead temperature against time indicates a strong correlation between them. As seen from Figures 3 and 4. As the WHT (shown in blue) increase so does the corresponding RII (shown in red). In fact it would be safe to say that the RII plot is scaled version of the temperature plot.

While it is uncertain that this is fractured injection, these data clearly show a reduction in the injectivity as the temperature increases, likely due to thermal alteration of the in-situ stresses.

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