Advanced Formation Evaluation Approaches in Complex Low-Resistivity Thin Shale Sand Laminations: Success Case Histories from Western Ukraine

Abstract

Several gas fields from the Carpathian Foredeep basin are characterized by high heterogeneity of rock quality. It is critical to understand the characteristics of pore architecture and mineralogy to quantify the rock's storage capacity and productivity. Field "A" is characterized by thin low-resistivity shale-sand laminations, which poses technical challenges to conventional evaluation methods. Until recently, only conventional local logging suites were deployed, and cutoffs-based interpretation was applied. Core analysis was not done. The Ukrainian segment of the Carpathian fold belt and foredeep is located in the westernmost part of the country, bordering Poland, Slovakia, and Romania. A few fields are situated in the foreland basin but most of the production comes from the fold belt, where complex structural traps are in a series of nappe units. Many of the fields were found based on an understanding of the surface geology alone. The presence of anisotropic layers with a predominance of very thin beds and intercalation of shale, siltstone, and sands with low resistivity contrast between water and gas significantly affects the definition of the reservoir properties and potential. Recently, the use of modern logs was mandated to obtain reliable information. In this study, we analyze and discuss the applicability and results of using advanced technology and tailored logs interpretation methods adapted for the local conditions. These methods were applied in different cases in 2019-2021 and enabled building the first robust petrophysical model for these types of reservoirs. Tri-axial resistivity measurements combined with high resolution density and neutron porosity logs optimally defined the porosity and saturations within the thin bedded sequences. Water volumes and textural parameters were computed from dielectric dispersion measurements. Pores system's heterogeneity and grain sorting, free fluid content and downhole testing optimization was performed using high-resolution nuclear magnetic resonance logs. The ability to measure formation pressure in the thin layers help understanding connectivity and deliverability of the reservoirs. The integration of these log measurements enabled unlocking the true properties of the anisotropic layers and quantify the hydrocarbons in place. High-definition borehole imager and dipole sonic logs complemented the petrophysical logs analysis and assisted the geomechanics and geophysics modeling. The addition of pulsed neutron spectroscopy logging further reduced the evaluation uncertainties providing an independent assessment of gas presence and proper control on mineralogy and matrix effects on the log responses to further refine the computation of total and effective porosity, and volumes within the thin sands. Finally, accurate reservoir summations were calculated and used together with producibility estimates and rock mechanical properties to guide the completion and production strategy. This paper presents examples of fit-to-purpose evaluation programs being deployed in such complex scenarios. In addition, it describes key information used to define a future field development management strategy and to optimize the petrophysical analysis. A comprehensive evaluation program and logs analysis can also be used as data calibration for other offset wells and nearby fields with similar properties and evaluation challenges.