Cased Hole Reservoir Evaluation in Old Wells with Complex Completion and Lithology – A Case Study from a Mature Gas Field in the Dnieper-Donets Basin, Ukraine

Abstract The Dnieper-Donets basin (DDb) is the biggest and most explored hydrocarbon region in Ukraine. The DDb is a Late Devonian rift, filled with pre- and syn-rift Devonian sequences and overlaid with post-rift Carboniferous, Permian, Mesozoic and Cenozoic fill. The depth of the basement is found at a depth of approximate 4–5 km in the northwestern part of the basin. However, it dips along its strike southeastward and lies below about 15 km near the Donbas foldbelt (Figure 1). DDb is also a salt basin. While in its axial part salt penetrates over 10 km thick Carboniferous-Permian sequence, this part of the basin accommodates the biggest known hydrocarbon field (Figure 2) (T. Petrovska et al., 2021). The dominant reservoir lithologies that contain most of oil and gas reserves in DDb are clastic rocks. Lower Carboniferous-Lower Permian carbonates and fractured anhydrites near the top of Paleozoic sequence contain comparatively smaller quantities of hydrocarbons. Hydrocarbon accumulations in the DDb are primarily structural traps, however faults and stratigraphic pinchouts usually control outlines of individual pools. Majority of the hydrocarbon fields in Upper Carboniferous-Lower Permian reservoirs are in faulted anticlinal uplifts covered by Permian salt seal, while fields in Lower Carboniferous reservoir rocks are usually associated with salt-assisted structural traps in deep, central basin areas. The first hydrocarbon shows in DDb were first recorded in 1936. The industrial hydrocarbon extraction has begun in 1950s-1960s when several significant gas fields were discovered (Ulmishek, 2001). There are over 200 hydrocarbon accumulations under production in the basin. At present, most of these fields are depleted and the gradual decrease in reserves is pushing the operator companies in Ukraine to look for the bypassed intervals in existing wells or explore new prospects. Another important aspect is understanding the actual formation pressure of each producing zone as it is a crucial piece of information for optimizing production. One of the logging technologies that is widely used to address these challenges is Multidetector Pulsed Neutron Logging (MDPNL). MDPNL has proved its efficiency in locating bypassed reserves that were missed during early open-hole evaluation and as well through monitoring the changes in formation properties that have occurred since the casing was set and production begun.