Advanced Reservoir Characterization Using Novel NMR Technology Secures Complex Carbonate Gas Condensate Pay, A Case Study Onshore Ukraine

Abstract

Formation evaluation in a gas condensate carbonates reservoir with high temperature and pressure is very challenging: low porosity and gas have an effect on reserve estimation and fluid typing identification. A complex of or state-of-the-art petrophysical studies were implemented for the first time in Europe in the Machukhske field in Ukraine, which helped to estimate the reservoir properties, rock quality, permeability and fluid typing of the main challenging productive carbonate reservoir of the Tournasian formation at a qualitatively new level.

The 8.5" section was drilled through the Tournasian formation with oil-based mud and a composite logging suite with high pressure and temperature (P, T) ratings was deployed. Gamma Ray, Neutron, Resistivity, Density and Formation Testing tools were run along with latest generation of multifrequency, focused Nuclear Magnetic Resonance (NMR) wireline tool. Longitudinal (T1 ) and transversal relaxation time (T2) distributions were calculated from multifrequency echo trains of raw NMR data to evaluate hydrocarbon porosity and saturations. The evaluation of T2 spectra used blind source separation driven by statistical independent component analysis (BSS-ICA), a machine learning algorithm. These results were then compared against those obtained from traditional two-dimensional NMR (2D-NMR) maps, specifically the T1T2 maps, that rely on the simultaneous inversion for T1 and T2. An adequate data acquisition sequences or logging activations ensured a suitable magnitude of the borehole signal, which enabled tool to apply long polarization times needed to detect volatile fluids. Conventional logs and core data were integrated with NMR results to minimize uncertainties, mathematical artifacts, and different effects. Rock quality indicators based on NMR porosity fractions and acoustic velocities were calculated and revealed some rock heterogeneities or porosity-lithology facies.

In challenging borehole condition with high P & T, high quality composite logging suite data was successfully obtained. An advanced reservoir characterization study was performed by integrating the NMR data with conventional logs which also helped to reduce the uncertainty in formation evaluation by clearly identifying pay and shale zones, deeper understanding of the storage and flow capacity of reservoir and the furthermore, providing necessary parameters for optimizing completion design.

An innovative study was carried out which helped not only meet objective of the well, but also results became reference for detailing the geological and hydrodynamic models of Machukske gas condensate field. The geological and technological model of the field was updated, and further field development strategies were optimized.