Cementation Exponent Estimates and their Impact on Oil Initially in Place Calculations : A Case Study from Lower Qamchuqa Formation in the Bai Hassan Oilfield, Northern Iraq

Abstract

The effect of Cementation Exponent has been investigated on the calculated water and hydrocarbon saturations and the estimated oil initially in place. The Aptian Lower Qamchuqa (Shua'iba) Formation from the well BH-96, Bai Hassan Oilfield, is used as a case study. Four different values of Cementation Exponent were applied in calculating water saturation using the Archie equation. The static value of 2 commonly used for carbonate reservoirs was one of the cases used to calculate an average of 68% water saturation. In the second case, the value of 1.81 is used for m based on the average m values obtained through testing 22 core samples in the laboratory. The average of 60.5% Sw was calculated for the studied part of the Lower Qamchuqa Formation in this case. The Pickett plot method was used as a third case, and 1.65 was estimated as the m value through which the average of 54% Sw was calculated. In the fourth case, variable values of m were used for the different depth intervals from which core samples were tested in the laboratory. In this case, the calculated average water saturation was equal to 60%. The upper part of the studied section was generally of low porosity and high water saturation, whereas the middle and the lower parts contained hydrocarbons with different saturations. Using shale content, porosity, permeability, and water saturation cutoffs for the four different cases, the net to gross pay ratios were calculated. The ratios ranged between 56.5% in the case of using the 1.65 value of m and 47.6% in the case of using variable core tested m values, and accordingly, the difference of about 4 m in net thickness is expected to be underestimated or overestimated, respectively, when is calculated.