Uncertainty of Porosity Measurement Correlations using NMR and Conventional Logging in Carbonate Reservoirs

Porosity is a dimensionless parameter, defined as the ratio of pore volume filled with fluid to the bulk volume. Porosity is a critical volumetric parameter used to estimate the reserve for a given reservoir and it can be used as an input for reservoir simulation as well. In addition, porosity can be classified into two types, effective porosity (interconnected pores) and total porosity (connected and isolated pores). Total porosity is defined as the sum of effective porosity and clay bound water (CBW). In other words, total porosity obtained from conventional logging tools will be equal to effective porosity in the absence of clay and while it is not the case when clay is present. There are several methods used to estimate porosity of the formation. These include: measuring actual porosity in the core laboratory, computerized tomography (CT) scan, neutron-density logging, sonic tools, and NMR logging tools. All conventional logging tools (neutron-density and sonic logging tools) are strongly dependent on lithology, whereas NMR logging tool is independent of lithology. The NMR logging the most accurate 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 2 IPTC 17260 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, more than hundred NMR reading tool were used to develop empirical correlations to estimate the free fluid (FFI) and Clay bound Water (CBW) for Arab D reservoir. This can be used as a checking parameter for the used cutoff values by the service company to ensure full compliance with the measured values in the laboratory. The correlations also will optimize the logging tool time and reduce the operation cost. Results of pre-study (SPE-168110) 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, correlations for CBW showed R of 0.96 and for FFI R is 0.99.In addition, in clean formation, correlations showed for CBW is R of 0.98 and for FFI R is 0.99.