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AbstractFor many years, unfiltered produced water has been used in the flooding of heavy oil reservoir in Southern Oman for pressure maintenance. Initially it was thought that the water injection was under matrix conditions. However, it is well accepted in the industry that water injection and particularly produced water injection almost, always takes place under fracturing condition. Significant decline in injectivity was repeatedly seen due to downhole blockages. Routine clean-ups and acidizations were carried out without success. Early 2003, a dedicated study was conducted to identify the root cause of this problem and recommend a remedial action. The purpose was to determine if the decline is due to poor water quality, reservoir compartmentalization, matrix/fracture or wellbore plugging.An unconventional approach was used to conduct a comprehensive study using historical data by modifying the Hall-Analysis technique. It was concluded that poor water quality was the problem. The interpretation of the Modified Hall Plot together with Bottomhole Pressure (BHP) vs. Rate plot also provided evidence of wellbore and/or fracture plugging. Both qualitative and quantitative analyses characterized the initial matrix injection period and fracturing injection regime that followed. Fracture plugging, routine well stimulations and fracture propagation trends were interpreted from the plots. A 3D in-house simulation package for waterflood under fracturing conditions was used, independently, to simulate injection conditions and confirm the results. Simulated injection pressure was much lower than actual field data confirming severe plugging. Further analysis showed that the plugging was due to unconventional particulates different from solids and oil contaminants in water. Recommendation was made to identify the contaminant and maintain a maximum suspended solid concentration of 100ppm to contain fracture propagation. A recent filter trial confirmed the presence of FeS contaminant.Successful remedial action was taken followed by recommissioning and water treatment facility design to realize estimated 150m3/d/well and improving ultimate recovery. Recently, another field study has successfully completed adopting same technique. By modifying Hall Analysis technique, it is possible to use field data for diagnosing matrix/fracturing regimes, assessing degree of damage and determining the location of the damage.
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