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Technical Paper: Identification and Characterization of Transition Zones in Tight Carbonates by Downhole Fluid Analysis

Society: SPE
Paper Number: 101257
Presentation Date: 2006
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Abstract

In tight carbonate reservoirs, several factors make it difficult to estimate reserves in transition zones. In particular, underestimation of reserves sometimes occurs because the formation oil can be more mobile than expected. The measurement of mobility of the different phases throughout the transition zone, which is affected significantly by complex rock heterogeneity, can only be accomplished by selective flow measurements. To use openhole sampling tools for these flow measurements, it is essential to differentiate between water-base mud (WBM) filtrate and connate water thereby avoiding any negative impact on the oil initially in place (OIIP) calculation and on development decisions. In principle, pressure gradients from traditional open-hole point pressure measurements should help define accurate fluid gradients and contacts. However this procedure is inadequate to identify and characterize the transition zones. Supercharging, capillary effects, changing wettability and saturations, and pressure variations from production and/or injection effects all play a role in complicating interpretation of the pressure measurement data. In addition, gauge effects, depth errors, formation thicknesses and fluid density contrasts yield potentially large uncertainties in interpretation of gradient data.

Downhole pH measurement, embodied within Downhole Fluid Analysis (DFA) procedure, has dramatically improved the situation. Downhole pH measurements utilizing robust pH dyes, the principle behind Litmus paper, have been successfully utilized in differentiating between WBM filtrate and formation water. The downhole fluid analyzers LFA and CFA successfully helped determine the level of the anticipated water cut, oil compositions and gravity, and GOR throughout the transition zone allowing a more reliable estimation of hydrocarbon in place and better well placement and production strategies. This process yields considerable cost savings especially by avoiding expensive production tests.

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