Investigation of Cementation Factor in Iranian Carbonate Reservoirs

Abstract

Cementation factor (cementation exponent) is one of the most important parameters in saturation equation to determine the water or hydrocarbon saturations. It acts as a power of porosity in the most of saturation equations which increases the importance of this parameter. The best cementation factor data is based on laboratory measurements and log interpretation results, but sometimes it is considered as a constant value in oil and gas reservoirs which causes large errors in saturation calculations. Using a constant value for cementation factor in saturation equations may cause large errors and uncertainties in hydrocarbon saturation determination because it is dependent to porosity value, type of porosity and pore types, pressure and lithology. Correlations are the best alternatives when there are not sufficient laboratory measurements or accurate log interpretation values. The effect of cementation factor on saturation was investigated. It was shown that cementation factor has almost the same values in porosities more than 5 percent but there are completely different values in porosities less than 5 percent. Investigation of pressure, porosity and lithology effects on cementation factor from the laboratory measurements and log interpretation in Iranian carbonate reservoirs was resulted in a new correlation for Sarvak, Kangan and Dalan formations. In the new correlation, cementation factor was plotted versus porosity to compare with the most famous existing correlations like Shell and Borai. The most common correlation which is being used in Iranian reservoirs is Shell formula whereas it has completely different trend compared to the new correlation in low porosities. Also a couple of Monte Carlo simulations were used to investigate the effect of cementation factor on water saturation uncertainty in an Iranian carbonate reservoir. The first Monte Carlo simulation was done based on the existing laboratory and log data and the second one based on the new correlation for cementation factor. Results showed that application of appropriate correlations was resulted in decreasing saturation uncertainty. The accurate determination of cementation factor improves the saturation determination and consequently OOIP calculations. Also, applying the correlations decreases the uncertainty of saturations in the reservoir zones to increase the accuracy level of possible, probable and proven reserves in a hydrocarbon reservoir.