Porosity is an important volumetric parameter used to estimate the reserves for oil and gas reservoirs and as an input for reservoir simulation. Porosity can be classified into effective porosity (interconnected pores) and ineffective porosity (isolated pores), while total porosity (ϕT) is defined as the sum of effective porosity and the porosity associated with Clay Bound Water. In clean formations, total porosity is equal to effective porosity, whereas in shaly formations it must be corrected for clay effect. Formation porosity can be determined using several methods. These methods include: measuring actual porosity in the core laboratory, Computerized Tomography (CT) scan, neutron-density logging, sonic tools and Nuclear Magnetic Resonance NMR logging tools.

The NMR logging is unique compared to all other methods since it is independent of the reservoir lithology. It can be used to estimate the reservoir porosity directly without the knowledge of matrix lithology. On the other hand, conventional logging such as neutron-density and acoustic depend strongly on lithology which might yield incorrect porosity measurement. Several studies have been conducted to estimate porosity for both sandstone and carbonate reservoirs using different logging tools, however, determining porosity is a challenge in complex and unconventional lithologies. In sandstone, the presence of shale and clay minerals will affect the response of all porosity tools. Carbonate is even more complicated than sandstone due to its heterogeneity and triple porosity system (pores, vugs, and fractures). In addition, the assessment of porosity measurements accuracy using NMR logging will be considered in this study. An attempt will be made to develop an empirical correlation from NMR data to obtain reliable porosity estimation.

In this work, case studies will be presented using NMR logging tool to show how NMR reduces the uncertainty of porosity measurements in carbonate reservoirs compared to other conventional logging tools, which will improve reserves estimation. The study will also show the effect of impurities in the carbonate formation porosity measurements by conventional and NMR logging methods compared to stress core porosity by graphical means. Results of this study showed that a clear criterion to divide the formations into dolomitic and clean formation (pure limestone) should be established to get more accurate result. In the dolomitic formation, Neutron-Density showed the least AARE of 40.2% compared to 84% for the NMR tool and 174% for the sonic tool. However, for clean formation NMR tool was the most accurate tool with AARE of 10% compared to 14.3% for N-D tool and 42.3% for sonic tool.

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