Novi Sad, Serbia
Novi Sad, Serbia

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Boskovic Z.,NIS Naftagas | Cebasek V.,University of Belgrade | Mitrovic V.,University of Belgrade | Stanic S.,University of Belgrade
Technics Technologies Education Management | Year: 2010

The main function of the cement in the annulus of the well is to prevent any fluid communication between drilled formations to provide long term zonal isolation and to bond and support the casing. In addition to isolating oil - gas - water producing zones, cement also aids in protecting the casing from corrosion, preventing blowout by quickly forming a seal and sealing off zones of lost circulation, or thief zones. However, even when cementing job has been done correctly from technical point of view, changes in downhole conditions can induce sufficient stresses to destroy the integrity of the cement sheath. Mechanical damage is caused by a large increase of wellbore pressure (pressure integrity test, increase of mud weight, casing perforation, stimulation, gas production), a large increase of wellbore temperature (geothermal production, steam injection, HT/HP wells) or the formation loading (creep, faulting, compaction). The weaker the formation, the worse the condition, since a weak formation is not able to mechanically support the cement deformation. The consequence will be loss of zonal isolation which can be detected, in gas wells, by long term gas migration problem. Model for selection adequate cement slurry and analyzing cement sheath integrity during well life is necessary.

Cebasek V.,University of Belgrade | Boskovic Z.,NIS Naftagas | Gojkovic N.,University of Belgrade | Mitrovic V.,University of Belgrade
Technics Technologies Education Management | Year: 2010

Analysis covers results of tangential stress and deformations at the point of contact between casing-cement stone, because both values reach their maximum at this point. Analysis has been conducted for the previously described models, where calculation has been done for each model for both cement stone Class G and Cem. Sistem, therefore calculation for 10 series of most usually used well designs has been done. Such models are easily customized to a well design, whether for analysis of cemented annulus for intermediate or production casing string in various well geometries.

Radivojevic D.,NIS Naftagas | Rundic L.,University of Belgrade | Knezevic S.,University of Belgrade
Geologica Carpathica | Year: 2010

The Čoka structure is a fault-bounded anticline in northern Banat, in the southern part of the Neogene Pannonian Basin. The structure and its vicinity were explored by 24 wells. In addition to well logs, paleontological, sedimentological and petrological analyses of cores and 27 seismic sections with different parameters of acquisition and processing were used for geological investigation of the area. The E-SE dipping pre-Neogene basement consists of Lower Triassic clastics and, in the NW part of the study area, Paleozoic greenschists. Thin Middle Miocene (Badenian) sediments unconformably overlie the basement and pinch out towards the elevated NW part of the study area. They are also missing in some wells on the apex of the Čoka structure, probably due to erosion. Badenian sediments were deposited in a shallow marine environment. The late Middle Miocene (Sarmatian) strata are missing and the Badenian is directly overlain by Upper Miocene (Pannonian) sediments. The latter also pinch out towards the NW but in contrast to Badenian sediments, they are present in all boreholes on the Čoka structure. Pannonian deposition took place in a caspibrackish environment of Lake Pannon, with predominance of marls and fine-grained clastics. Pannonian sediments are conformably overlain by latest Miocene (Pontian) and Pleistocene lacustrine, alluvial and terrestrial sediments.

Ignjatovic S.,University of Belgrade | Vasiljevic I.,University of Belgrade | Burazer M.,NIS Naftagas | Banjesevic M.,University of Belgrade | And 2 more authors.
Swiss Journal of Geosciences | Year: 2014

The study reports new aeromagnetic and gravity data for the northern part of the Timok Magmatic Complex (TMC), East Serbia. The TMC is part of the Tethyan Eurasian metallogenic zone well known for hosting large copper and gold deposits. The complex formed by continuous volcanic activity 90–78 Ma ago, that developed in roughly three phases: Turonian andesites, Santonian–Campanian andesites/basaltic andesites (both mostly volcanic) and Campanian latites/monzonites (mostly shallow intrusive). The aeromagnetic measurements included acquiring total magnetic intensity data that were corrected for diurnal variations, leveling, microleveling, calculated normal field values, calculated anomaly values of total magnetic field intensity and reduction to the pole. The gravity measurements were carried out in an irregular grid with relative gravity values obtained using a Worden gravity meter. 2D modeling reveals that the subsurface extension of the Campanian Valja Strž pluton is ten times larger than it is indicated by its surface outcrops. This implies that the area south and southeast from the pluton can be interesting in terms of finding new porphyry systems. The model indicates that this intrusive body should not be considered as a deeply dissected pluton. This sheds new light onto its potential with respect to epithermal gold mineralization, as well. The model also suggests that there are larger non-exposed bodies of Santonian–Campanian volcanics and near-surface hydrothermally altered rocks than it is inferred from geological maps. The results of our study suggest that further interdisciplinary investigations in the TMC, in particular those integrating geophysics and geology, may have potential of advancing the existing exploration models. © 2014, Swiss Geological Society.

Cebasek V.,Rudarsko geoloski fakultet | Boskovic Z.,NIS Naftagas | Gojkovic N.,Rudarsko geoloski fakultet | Mitrovic V.,Rudarsko geoloski fakultet
Technics Technologies Education Management | Year: 2010

Usage of cement slurry with additives for expansion and flexibility is not always reasonable. In the wells, which shall not undergo cyclic stress or changes in well conditions, it is required to analyze, through application of a model, whether compression strength of cement stone is sufficient to sustain radial σr, tangential σq stress and deformations. If that would be the case, than standard types of cement slurry, with corresponding compression strength of cement stone, assure stability of cemented annulus during production well life.

Pigott J.,University of Oklahoma | Radivojevic D.,NIS Naftagas
Central European Journal of Geosciences | Year: 2010

Seismic stratigraphy based chronostratigraphic (SSBC) analysis of the Serbian Banat region allows the delineation of the spatial and stratigraphic relationships of the generally regressive and shallowing upward Neogene depositional fill of a tectonically unstable central portion of the Pannonian Basin. When geometrically restored in time and space, the sediment dispersal directions, sediment source directions, types of sedimentation breaks and the tectonic events influencing basin evolution can be delineated. For such an analysis the timetransgressive lithostratigraphic units used in the neighbouring Hungarian part of the Pannonian Basin are conveniently introduced based upon their characteristic seismic facies and constrained borehole log records as mappable seismic stratigraphic sequence units, termed "seismic operational sequences". The respective Neogene stage and operational sequence equivalents (Hungarian lithostratigraphic units or formations) are the Middle Miocene (Badenian, Sarmatian), Upper Miocene-Lower Pliocene (Pannonian-Endrod and Szolnok Formations; Pontian- Algyo and Ujfalu Formations and Lower Pliocene- Zagyva Formation) and Upper Pliocene- Quaternary (Nagyalfold Formation).SSBC analysis greatly assists in the geological constraint or " geovalidation" of interpreted seismic stratigraphic relationships and provides potentially critical insight into stratigraphic and structural problems of non-unique interpretations. In the specific case, using such an approach on previously unpublished regional seismic lines, SSBC analysis reveals that the Banat region has undergone structural inversion. This may be related to changes in local stress directions along strike slip faults, which initiated in earliest Late Miocene (Endrod Formation), culminating in the reverse tilting and incipient shortening of the western graben.Therefore during the time interval that the Badenian through Endrod sediments were deposited in the graben, autocyclic progradation initiated from the Kikinda Szeged High in the East followed by Szolnok, Algyo, Ujfalu and younger units prograding from the West as the central high uplifted relative to the graben. Such tectonic inversion has substantial hydrocarbon potential implications for exploration in the region.

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