CRITERIA FOR REASSESSMENT OF OIL AND GAS POTENTIAL OF RESERVOIR ZONES IN SOUTHEASTERN WESTERN SIBERIA BASEMENT

Abstract

Relevance. The Paleozoic formations of Western Siberia are the main object of hydrocarbon potential further increase of the considered territory. The geological structure complexity of the basement rocks, the unsatisfactory quality of drilling, casing and testing of wells caused low efficiency of oil prospecting in the Paleozoic sediments, where all the identified hydrocarbon deposits and accumulations were discovered concurrently with the exploration of the sedimentary cover objects. One of the promising directions for increasing the prospecting efficiency in basement rocks is related to the analysis of drilling features and test results of previously drilled wells, where we can distinguish the geological and technological factors. The geological factor considers the results of drilling and well testing from the position of a block, folded, layered model of the structure of a fractured-porous-cavernous reservoir zone allocated as part of the Paleozoic complex. The analyzed model evaluates the changing possibility of the compressible fluid (oil, gas) volume during drilling and well testing. During drilling, as a result of increased pressure at the bottomhole relative to the reservoir pressure in a productive formation with abnormally high reservoir properties, there is a decrease in the volume of hydrocarbon fluid, which leads to the absorption of drilling fluid, sometimes to a complete loss of circulation. This effect can be observed when drilling a well both in the contour of the reservoir and in the aquifer in the immediate vicinity of it. When well testing and decreasing pressure during its development, the expansion (relaxation) of previously compressed volumes of hydrocarbon fluid occurs. In the case of drilling a well in the water-oil zone, alternating water-oil and oil-water inflows are observed, and if the well is located in the water-bearing part of the reservoir, the spontaneous outflow or high flow rates at low dynamic levels of highly mineralized formation water with increased values of the gas factor relative to the limit saturation values are noted. The technological factor assesses the quality of the production casing cementing, which affects the final test results and, as a consequence, the assessment of the oil and gas potential of the analyzed well section. The systematization of the drilling and testing results allows us to determine the criteria for assessing the oil and gas potential of the studied wells and areas, in order to resume prospecting for the discovery of hydrocarbon deposits in the rocks of the pre-Jurassic complex. The purpose of the research is to analyze the results of drilling and testing wells carried out in the basement rocks of the southeastern part of the West-Siberian Plate in order to identify oil-promising areas and resume prospecting operations in previously studied areas. The object of the study is the Paleozoic basement sediments of the southeastern part of the West Siberian Plate, which are represented by rocks of different material composition, including carbonate varieties. The authors have carried out the analysis of the results of drilling and testing wells in the Paleozoic complex, and determined the criteria of the obtained data for the possible resumption of oil prospecting in areas previously studied by deep drilling.

Keywords

• pre-jurassic sedimentary complex
• basement rocks
• pre-jurassic sedimentary complex test results
• drilling fluid absorption
• formation water overflows