The Influence of Gas Invasion on the Composition of Crude Oil and the Controlling Factors for the Reservoir Fluid Phase

Abstract

Oil and gas in deep strata is complex in property and variable in phase type, which imposes great challenges to exploration. This is particularly true in the deep Ordovician of Tarim Basin (NW China) where abundant oil and gas resources were discovered with various phase types including gas, condensate, volatile oil, normal oil, and heavy oil. The prediction of the composition and phase type of deep fluid is a crucial issue in future development. In this study, typical oils from different types of reservoirs from the East Lungu area (north Tarim Basin) were sampled and analyzed with two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) for the reconstruction of gas invasion on its impact oil compositions and reservoir phase type. The concentration of diamondoids in crude oil and the loss of n-alkane decreased gradually along the westward direction of gas invasion. Coupled with several other parameters, the quantitative evaluation of the gas invasion intensity indicated that gas invasion occurs from east to west in the late period (12–2 Ma) in the study site and altered the paleo reservoirs formed in the Late Hercynian (293–255 Ma), and thus formed the condensate (strong gas invasion), volatile oil (weak gas invasion), and normal oil reservoir (no gas invasion), successively. The prediction of the phase type distribution provides a scientific basis for deep oil and gas exploration and resource assessment.

Significant Control of Gas Invasion to the Phase Behavior of Deep-Seated Hydrocarbon Fluids in Western Junggar Basin, NW China

“…When natural gas periodically infiltrates and mixes with the crude oil in a trap, it leads to the occurrence of fractional distillation, which is referred to as “gas invasion” fractional distillation. , Gas invasion enhances the distillation of crude oil in the reservoir, resulting in the removal of gas-soluble components and depletion of lighter normal alkane fractions. , The distribution of normal alkanes in crude oil that has not undergone gas-invasion fractional distillation follows the relationship described in eq . In gas-invaded crude oil, lower carbon number normal alkanes dissolve into the gas phase and are carried away, while higher carbon number normal alkanes are minimally affected, which leads the distribution of normal alkanes with lower carbon number deviates from the linear relationship described by eq . ,, There exists a linear relationship between the logarithm of the molar concentration of normal alkanes and their carbon number in unaltered crude oil. This relationship can be described by Ln false[ MC false( n false) false] = a · n + Lg false( A false) …”

“…When natural gas periodically infiltrates and mixes with the crude oil in a trap, it leads to the occurrence of fractional distillation, which is referred to as “gas invasion” fractional distillation. 10 , 28 Gas invasion enhances the distillation of crude oil in the reservoir, resulting in the removal of gas-soluble components and depletion of lighter normal alkane fractions. 29 , 30 The distribution of normal alkanes in crude oil that has not undergone gas-invasion fractional distillation follows the relationship described in eq 1 .…”

“…In gas-invaded crude oil, lower carbon number normal alkanes dissolve into the gas phase and are carried away, while higher carbon number normal alkanes are minimally affected, which leads the distribution of normal alkanes with lower carbon number deviates from the linear relationship described by eq 1 . 28 , 29 , 31 There exists a linear relationship between the logarithm of the molar concentration of normal alkanes and their carbon number in unaltered crude oil. This relationship can be described by …”

Hydrocarbon Phase State Evolution and Accumulation Process of Ultradeep Permian Reservoirs in Shawan Sag, Junggar Basin, NW China

“…The type of hydrocarbon phase in the reservoir not only affects resource evaluation, field development, and facilities design but also has different economic benefits. ,, In addition, there are differences in the physical properties (e.g., density, viscosity, interfacial tension) of hydrocarbons with different phase states, resulting in significant differences in the dynamics of hydrocarbon charging, migration, and accumulation. − It is of great importance for understanding the current compositional distributions in the reservoir, the mechanism of the migration and accumulation of hydrocarbons in ultradeep reservoirs, and explaining the distribution of hydrocarbons in different phase states in a basin. − …”