A new technique for electrical rock typing and estimation of cementation factor in carbonate rocks

Abstract

Natural heterogeneity of carbonate rocks causes difficulties in estimation of their petrophysical properties such as cementation factor. These problems are mainly caused by complex pore structure of carbonate rocks and presence of various types of pore in them. Generally, when analyzed versus porosity, formation resistivity factor (F) data are highly scattered in carbonate reservoirs. Several methods have been reported in literature to decrease such data scattering such as data classification by permeability, pore types and reservoir rock types. However, outcomes of these techniques have exhibited still some degree of the data scattering. Besides, many correlations have been developed to relate cementation factor to porosity such as Borai's and Shell correlations. Indeed, these correlations take into account the porosity rather than further considering rock quality in terms of electrical conductivity.In this paper, Electrical Quality Index (EQI) is proposed as a new parameter for classification of F data in an attempt to improve estimation of cementation factor. Rock samples with similar EQIs have similar electrical behavior. In other words, EQI can be used to group rock samples into distinctive groups with specific equations of formation resistivity and cementation factors versus porosity. The proposed technique was applied to the 112 carbonate rock samples reported in Ragland's work. EQI divided these samples into 9 groups which have different equations of F versus porosity with high values of determination coefficients. Also, cementation factor versus porosity plots showed linear trends with high determination coefficients for all of the EQI groups.