Microscopic production characteristics of tight oil in the nanopores of different CO2-affected areas from molecular dynamics simulations

Understanding the mechanisms of CO2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO2 EOR mechanism in the displacement-affected area (DPAA) and diffusionaffected area (DFAA) of quartz nanopores were thoroughly investigated using molecular dynamics simulation techniques. First, the following two contents were mainly simulated, namely CO2 flooding oil in the single/ double nanopores of DPAA and CO2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO2 on oil transport. Moreover, different CO2 injection/ flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO2 solubility and the oil swelling factor gradually decreased with distance from the CO2-oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO2 dissolved in the oil. However, the injection rate of CO2 was inversely proportional to oil recovery. In addition, it took longer for the displacement efficiency in the 6 nm pore of double pores to reach the same displacement efficiency as in the single 6 nm pore. In the DFAA, the effect of flowback rate on the displacement efficiency of oil was relatively low. However, the thickness of the water film was a key factor that affected the oil displacement efficiency in the DFAA

Microscopic production characteristics of tight oil in the nanopores of different CO2-affected areas from molecular dynamics simulations

Understanding the mechanisms of CO2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO2 EOR mechanism in the displacement-affected area (DPAA) and diffusionaffected area (DFAA) of quartz nanopores were thoroughly investigated using molecular dynamics simulation techniques. First, the following two contents were mainly simulated, namely CO2 flooding oil in the single/ double nanopores of DPAA and CO2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO2 on oil transport. Moreover, different CO2 injection/ flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO2 solubility and the oil swelling factor gradually decreased with distance from the CO2-oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO2 dissolved in the oil. However, the injection rate of CO2 was inversely proportional to oil recovery. In addition, it took longer for the displacement efficiency in the 6 nm pore of double pores to reach the same displacement efficiency as in the single 6 nm pore. In the DFAA, the effect of flowback rate on the displacement efficiency of oil was relatively low. However, the thickness of the water film was a key factor that affected the oil displacement efficiency in the DFAA

Oxygen isotope composition of meltwater from a Neoproterozoic glaciation in South China

The water cycle is an integral part of Earth surface dynamics, and water\u27s oxygen-isotope composition retains information about the forcing and response of Earth\u27s local and global climate. Water isotope signals of the recent geological past can be directly obtained from archives such as ice cores, groundwater, or pore fluid. For the more distant past, mineral proxies have to be used. Multiple episodes of global glaciation may have occurred in the Neoproterozoic Era, of which the record of oxygen-isotope composition of glacial meltwater is sparse; the few records that are derived from carbonate minerals are prone to late-burial and metamorphic alteration, and therefore subject to alternative explanations. Here we present a case in which meltwater δ18O is retrieved from barite (BaSO4) and malachite (Cu2CO3(OH)2)-associated sulfate (MAS) in a diamictite in Kaiyang, Guizhou, South China. The core of our argument is based on the lowest-ever-published sulfate δ18O values found in the barite and MAS, reaching as low as -20.3‰ (Vienna standard mean ocean water, VSMOW). These data suggest that the water involved in the oxidative weathering of these chalcocite clasts had a δ18O value of -34‰ ± 10‰, similar to that of polar glaciers today. Excluding the possibility of glacier meltwater alteration during the past 600 m.y. after the deposition of the diamictite, the sulfate mineral assemblage reported here provides an important constraint on the nature of the Neoproterozoic glaciation that the Kaiyang diamictite represents. © 2013 Geological Society of America

Nanoscale Pore Characteristics and Influential Factors of Niutitang Formation Shale Reservoir in Guizhou Province

Nanoscale pore characteristics are crucial in assessing the resource potential of gas shales. Although the Niutitang formation was widely deposited in the upper Yantze Platform, South China and has been recognized as a promising shale gas reservoir, there lacks substantial breakthrough in the exploitation of shale gas from the Niutitang formation. Aiming at better understanding the reservoir properties and corresponding influential factors, 14 core samples from the Lower Cambrian Niutitang formation locating in the central Guizhou province were investigated in the current study to characterize the nanoscale pore system in the shale. Organic geochemical analyses (i.e., total organic carbon content and thermal maturity), X-ray diffraction, low pressure nitrogen adsorption, and field emission scanning electron microscopy were employed to obtain complementary information of the pore system. Measured TOC in this study is generally > 1.50% and averages 3.35%. All of the samples are in the over-maturity stage with R-o ranging from 2.39% to 3.29%. X-ray diffraction shows that quartz, clay minerals and plagioclase are the dominant minerals. Nitrogen adsorption results indicate that all of samples show type IIb nitrogen adsorption isotherms with type H3 hysteresis loops, which imply the coexistence of micropores, mesopores and macropores in the shale. The mesopores account for 60-70% of total pore volume, and are likely contributed by clay minerals and quartz. Organic matter appears to be the major contributor of the micropores and specific surface area, and is closely linked to the rapid decrease of average pore size with increasing burial depth. The field emission scanning electron microscopy reveals abundant organic matter pores in the middle-upper Niutitang formation, but lesser or smaller in the bottom of Niutitang formation. The lower Niutitang formation seems to develop substantial amounts of organic-clay aggregates, which preferentially lie parallel to the shale bedding and contain lots of nanoscale pores. The perpendicular variation of pore structure features is explained with multiple mechanisms, including thermal maturation of organic matter, compaction by strata pressure, dissipation of shale gas, etc. The results of our study have emphasized the interesting and complex features of the nanoscale pore structures in the gas shales, which may facilitate future assessment and exploitation of shale gas resources

Identification of distinctions of immiscible CO2 huff and puff performance in Chang-7 tight sandstone oil reservoir by applying NMR, microscope and reservoir simulation

CO2 huff and puff (HNP) is one of the most effective methods to improve tight oil recovery after the primary depletion process. The seepage mechanisms between CO2 and crude oil are complicated in porous media during CO2 HNP process. Therefore, in this paper, the CO2 HNP process of Chang-7 tight oil reservoir, Ordos Basin, China, was studied by nuclear magnetic resonance (NMR) technology, microscopic observation and numerical simulation. Experimentally, using NMR technology and microscopy methods, the distribution characteristics of residual oil during CO2 HNP process were measured intuitively. Numerically, a group of core-scale and field-scale simulations considering molecular diffusion and asphaltene precipitation were established to further verify and elongate the experimental results. The results show that at the initial state, the crude oil in the tight core was mainly distributed in nanopores, sub-micro-nanopores and sub-micropores, where the oil content exceeded at least 73% in these pores. During CO2 HNP process, the oil recovery was more pronounced for the 1st and 2nd rounds than for 3rd to 5th rounds. Notably, even if the cores with more nano-pores were more favorable for the 4-5th CO2 HNP rounds, the oil molecules in nanopores were still difficult to be available. Moreover, the CO2 sweep scope could be divided into displacement affected region and diffusion affected region. CO2 could effectively drive the crude oil in the displacement affected region. While the oil could be successfully displaced by dissolved gas flooding in the diffusion affected region only under the appropriate conditions. Meanwhile, the core-scale numerical models confirmed that it was necessary to consider molecular diffusion and asphaltene precipitation factors, which would make the simulation results in line with the experiment. In terms of the ultimate oil recovery, the field-scale model only considering the diffusion (2.456%) > the model both considering the diffusion and asphaltene (2.436%) > the model without considering the diffusion and asphaltene deposition (2.412%) > the model only considering the asphaltene deposition (2.388%)