Abstract

The Production estimated from standard porosity logs in carbonate reservoirs has often been greater than expected. This study looks for the correlation between well productivity and high resolution porosity information derived from borehole electrical images. Good production contribution has been obtained from intervals where logs show low porosity (verified by PLT analysis) whereas zones having higher porosity could have major or minor contribution to the well production. Carbonates, in general, have complex porosity systems with widely varying proportions of primary and secondary porosities. The term "secondary porosity" includes vugs, moulds, bioturbation features and open fractures that are often missed on conventional porosity logs. These diagenetic phenomena are not captured on coarser resolution conventional logs (density, neutron and sonic), thus porosity is usually under-estimated or overlooked.

Resistivity images have been known to provide both high resolution and azimuthal borehole coverage to characterize reservoir heterogeneity from a qualitative perspective. New generation LWD borehole imagers record wireline-par resolution data while rotating at full borehole coverage (unlike wireline imagers). This case study sets sight on two horizontals wells – one injector and one producer – targeting Lower Cretaceous carbonates. Production / Injection logging (PLT) data are integrated with high resolution image-derived geological information to explain the nature of flow profile and dominant textural parameters at play at various levels along the borehole-section. Part of this exercise seeks to derive a more quantitative high resolution image-based secondary porosity by solving the porosity from the Archie equation in the flushed zone.

The workflow arrived at, allows one to understand correlations between various carbonate porosity components and well productivity data. In the present case study, it is concluded that not all productivity is derived from primary porosity. This is important in terms of well completion, carrying out optimized completion, design of hydraulic fracturing stages and placement of packers and inflow control devices.

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