Reservoir souring most times occurs during secondary recovery, after water injection using seawater or produced water from a different reservoir. Water compatibility studies is thus necessary prior to injection to detect potential for souring and to implement preventive measures since souring poses challenges during production. This work stresses the importance of fluid compatibility studies before undertaking water injection projects. Two cases were considered; in the first case, water injection program was implemented using seawater without conducting fluid compatibility studies and serious souring problem was encountered later in the life of the reservoir. In the second case, fluid compatibility study was conducted where produced water from four sources were proposed to be used for water injection in two reservoirs. One reservoir (RW1) had high sulphate content of 20mg/l but did not have Sulfate Reducing Bacteria (SRB) probably because the reservoir temperature was 103oC, well above the limit for the existence of most SRB. The second reservoir (RW2) with a temperature of 75oC had SRB concentration of 845cfu/ml and had a sulfate concentration of less than 0.01mg/l, indicating that souring will only occur if water containing sulfate is injected into it. The study shows that reservoir souring could occur in both reservoirs from external sources. It was concluded that three out of the four proposed produced water cannot be injected into RW1 without treatment since their water samples contain SRB. Reservoir souring and its associated problems were thus prevented from occurring in RW1 due to fluid compatibility studies.