Salih Muhammad Awadh - Professor, Department of Geology, College of Science, University of Baghdad, Baghdad, Iraq

The oilfield water in the upper member of Zubair reservoir (Barriemian-Hauterivian) in Rumaila North oil field is investigated for the interpretation of salinity and geochemical evolution of brine compositions. The interaction of the oilfield water with reservoir rocks resulted in a brine water, which is derived from the marine water and is partially mixed with meteoric water; the composition of the water is similar to that of formation water from the offshore/onshore Mesozoic reservoirs of the Gulf of Mexico. The high average of total dissolved solid (TDS) (215625 mg/l) is consistent with the average electrical conductivity (351024 μs), and is predominantly represented by Cl (123679 mg/l), Na (29200 mg/l), and Ca (14674 mg/l). The main contributing cations (epm%) are Na (70.2), Ca (18.9), Mg (8.1), and K (1.7). The main anions include Cl (99.7), SO4 (0.25), HCO3 (0.07), and CO3 (0.005). The sodium content is six times greater than that of seawater; calcium and magnesium contents are three times greater, and chloride is 6.5 times greater. The sulfate is depleted to one sixth, due to a sulfur release from sulfates into the hydrocarbon phase. Zubair oilfield water is characterized by an acidic pH (pH=5.2- 5.77), high specific gravity (1.228), high fluid pressure (4866 psi), hydrocarbon saturation of 43%, water saturation of 57%, and porosity of 12.7%. The mineral saturation model indicates that Zubair oilfield water is unsaturated water with respect to all the suggested minerals at a pH of 5.45, but at a simulated pH of 9.12, brucite is at equilibrium; brucite and portlandite are supersaturated at a pH of 11.9. The mineral solubility responses to the changes in temperature, pressure, pH, oxidation potential (Eh), and ionic strength, thereby proportionally developing formation damages.

Oilfield Water, Specific Gravity, TDS, Water Resistivity, Saturation Index, Salinity