Factors of gas accumulations formation in oil-bearing sediments and in casing annulus of wells

Abstract

Gas component study is one of the important tasks of petroleum geology. Gas component can exist in various forms in sedimentary rocks. Of great interest is nitrogen, the gaseous accumulations of which are formed in oil-bearing strata, causing complications during the oilfield development. The problem of abnormal nitrogen accumulations had great relevance in the fields of the Volga-Ural petroleum province, which is one of the long-term developed with a large stock of wells for various purposes. This article discusses possible sources of gaseous nitrogen and the reasons for its accumulations in oil-bearing reservoirs. The main purpose of the article is to clarify the reasons for the gaseous nitrogen and its deposits formation. The main patterns of the areal distribution of nitrogen gas accumulations in oil-bearing strata are revealed on the basis of field, hydrogeological, geological and geophysical researches data analysis. It has been established that during the gas caps formation, the source of gaseous nitrogen is its dissolved compounds in groundwater and oil, biochemical decomposition of which leads to the dissolved molecular nitrogen accumulation in a liquid medium. The release of free gaseous nitrogen and the formation of its accumulations is associated with the decompression of formation waters for natural (geological) or man-made reasons (hydrocarbons extraction). Disturbance of the natural hydrodynamic regime in oil-bearing formations leads to the release of gaseous nitrogen and the formation of its accumulations under favorable conditions (the presence of reservoirs, structures and impermeable rocks in the top of the formation).