oa Reducing uncertainty in water saturation in carbonate reservoirs of southern Iran: A regional correlation for cementation factor

The calculated value of water saturation (Sw) in a field is based on petrophysical parameters combined with a saturation equation. The exactness of calculated water saturation values depends on accuracy of the input parameters. Monte Carlo modeling of water saturation in different wells in southern Iran revealed that the cementation exponent (m) is one of the most important parameters dominating uncertainties in calculated Sw. The cementation exponent is normally measured in laboratory from core experiments. Alternatively it can be estimated from resistivity and porosity logs in the clean water-saturated intervals using Archie’s equation and Pickett plot methods. Applying a constant value for m using above methods resulted in very high uncertainty in calculated water saturation (negative median in oil zones and higher than 100% in water zones). This abnormality suggests that the input parameters in the water saturation evaluation should be reconsidered. The cementation exponent is affected by several factors including<br>lithology, porosity, type of pore system, tortuosity, shapes and sorting and packing of the particulate system. Therefore the m value is not a constant over a well, but varies depending on many physical parameters and lithological attributes of porous media. A practical correlation was developed for m as a function of porosity and lithology using measurement data of several fields in the area and a variable m from this correlation was created for all wells. Applying a variable m successfully reduces the amount of uncertainty in calculated water saturation.