Carbonate reservoirs are usually characterized by extremely variable pore space structure, which has to be taken into account during formation evaluation from the well-logging data. The most efficient tool for studying such reservoirs is the implementation of specially-dedicated core studies with simulation of formation conditions. Such studies take a long time. As an alternative, a reliable estimation of reservoir poroperm properties and saturation may be obtained with the use of extended logging program which includes special wireline log-suite.
The subject of study in this paper is Devonian carbonate formation of the Kharyaga field, represented by products of reef decay and characterized by mostly intergranular porosity. In one of the wells drilled within a poorly-studied block of the field to estimate its prospects and identify potential water encroach, in addition to standard log-suite, special wireline tools were used: Multifrequency Dielectric Logging (MDL) DielectricScanner and High Resolution Nuclear Magnetic Resonance (NMR) CMR Plus.
As result of MDL data processing, the rock water capacity and the MN exponent (coincides with the exponents of Archie Low at m=n) were obtained. Based on the difference of porosity and water capacity the residual oil saturation was determined, whose values are confirmed by core-data from other wells. The MN exponent turned out to be substantially higher than the saturation exponent n from core analysis data. Lower values of saturation exponent n from core data may be related to changes in rock wettability occurred during extraction of core plugs before the measurements. As result of interpretation, oil saturation obtained by using the adopted exponents from core analysis turned out to be overstated compared to oil saturation values calculated with use of MN exponent. Irreducible water saturation based on NMR data was obtained with use of relaxation time cut-off estimated through factor analysis of wireline NMR log data (since there were no special core studies done for this purpose). The total sum of oil saturation values from MDL data and residual water saturation values from NMR is practically equal to 100% of the rock pore space within studied interval, and, consequently, there is no mobile water in the formation. The expected inflow is defined as ‘oil’ – it was confirmed by formation sampling in open hole (by MDT tool) later on. Thus, the use of special wireline tools allowed obtaining the reliable estimates of oil saturation and accurate determination of expected fluid inflow.
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