Produced water re-injection (PWRI) has been carried out throughout the world to minimize the environmental impact of waste waters. There are a few studies on commingled produced water (PW) with injection water (sea or aquifer water), and still a lower number of reported field cases. This might be the first waterflood operation where incompatible waters with potential iron sulfide precipitation were injected into a tight carbonate reservoir.

Mixing disposal water containing high H2S and aquifer water containing dissolved iron results in the precipitation of iron sulfide particles. Coreflood studies with the mixed waters did not cause any adverse effects on the core permeability. This was attributed to the formation of wormholes created by H2S that is dissolved in the mixed waters. Based on extensive lab studies, a pilot test was conducted on Well B# 215, a water disposal well in Field B. The objective of this study is to confirm the feasibility of injecting mixed water into this well. The results, if successful, can be applied in fields with similar bottom hole conditions.

Injection of the commingled waters started a year ago. Extensive field samples (disposal, aquifer and mixed waters) were collected for geochemical analysis, sulfide and iron content before the onset of the pilot test. The percentage of disposal water in the commingled injected water was calculated based on the concentrations of various key ions (Na, Cl, Ca and Mg). These values were compared with flow rate of various streams measured in the field.

Based on the results obtained, there is no indication of formation damage noted in this well. The percentage of disposal water in the commingled water varied during the test period, but there was no damage noted. In addition, there was no need to flow this well back for cleaning, which resulted in significant cost savings and reduced environmental impact. This paper will discuss lab results and field application in detail. It will also address benefits gained from mixing aquifer water and disposal water.

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