Abstract

Carbonate reservoirs show huge variations in petrophysical properties resulting from leaching/dissolution, fracturation, cementation and or dolotomization. Permeability anisotropy is one property which if understood properly, helps at different stages of reservoir development especially in cases of substantial density difference between fluids; primary production below the bubble point, gas cycling, gas/or water coning etc. Where the anisotropy is severe, it can also influence fluid injection and production rates. For carbonates, microporosity and fractures make it very challenging to relate porosity and lithofacies to permeability. In addition, thin bedded layers that can enhance production maybe unidentifiable because of the coarse nature of conventional logs, at least using Logging While Drilling (LWD) in wells drilled with Oil base muds (OBM) until recently.

The recently introduced LWD dual physics (ultrasonic and resistivity) imaging service with a resolution of 0.2 inch now opens the door to capture heterogeneity using high definition images at very fine scales in OBM that was not possible earlier. High definition borehole images are used to isolate and quantify secondary porosity features like vugs. The density of the vugs per unit length is plotted and correlated to pretest mobility which is a direct indicator to flowrate.

The vug density shows a positive correlation with pretest mobility. The high-resolution images also show intervals where the mobility is enhanced as a result of the influence of thin bed boundaries or where the mobility is lowered because of dense features like stylolytes. The vug density-mobility (vd-m) plot completes a picture of pretest mobility trends limited by the distance between pressure points stations. With the vd-m, more refined facie analyses are possible, together with a better understanding of the permeability distribution across a well.

The effectiveness of a more refined picture of the permeability heterogeneity is seen in the Lower Cretaceous carbonates in an offshore Middle East well. Known porosity variations from core analysis and petrographic images attest to the heterogeneity seen in the vd-m correlation logs. The resolution of vd-m is helpful in understanding the productivity and injection capabilities of such carbonate reservoirs, for example, for optimizing ICD design. Further integration with other data (advanced mud gas analysis) and work processes (geosteering) provide a way of identifying fluid changes in wells, identifying different litho-types and aid in justifying critical geosteering decisions.

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