Logging Fluid Identification Method of Low Resistivity Contrast Reservoir Invaded by Fresh Water Mud-A Case Study of Well Block YD in Erdos Basin

The freshwater mud invasion caused low resistivity contrast of oil and water zones in well block YD, then the reservoir fluid is very difficult to identify. By simulation study, the authors find freshwater mud invasion made the oil reservoir resistivity increase rate R/R-o from 3.8 similar to 6.8 to 1.5 similar to 2.5, and made a 65% reduction in the reservoir resistivity. But the water layer resistivity rise by 50%. That further reducing the contrast of the reservoir resistivity. Logging fluid identification rate of the reservoir is less than 74%. So the authors partition the well block YD, calculate a mixture resistivity of formation water after freshwater mud invasion, calculate true formation resistivity, then use resistivity increases rate and reservoir quality factor to establish reservoir recognition charts. This method rise the coincidence rate of 17 %.We use the method to deal with 49 wells in well block YD. The results raise the rate of approximately 10% than the original interpretation. It is a powerful direction for low resistivity well log interpretation.