Hydrocarbon gas huff-n-puff optimization of multiple horizontal wells with complex fracture networks in the M unconventional reservoir

The oil production of the multi-fractured horizontal wells(MFHWs) declines quickly in unconventional oil reservoirs due to the fast depletion of natural energy. Gas injection has been acknowledged as an effective method to improve oil recovery factor from unconventional oil reservoirs. Hydrocarbon gas huff-n-puff becomes preferable when the CO_(2) source is limited. However, the impact of complex fracture networks and well interference on the EOR performance of multiple MFHWs is still unclear. The optimal gas huff-n-puff parameters are significant for enhancing oil recovery. This work aims to optimize the hydrocarbon gas injection and production parameters for multiple MFHWs with complex fracture networks in unconventional oil reservoirs. Firstly, the numerical model based on unstructured grids is developed to characterize the complex fracture networks and capture the dynamic fracture features.Secondly, the PVT phase behavior simulation was carried out to provide the fluid model for numerical simulation. Thirdly, the optimal parameters for hydrocarbon gas huff-n-puff were obtained. Finally, the dominant factors of hydrocarbon gas huff-n-puff under complex fracture networks are obtained by fuzzy mathematical method. Results reveal that the current pressure of hydrocarbon gas injection can achieve miscible displacement. The optimal injection and production parameters are obtained by single-factor analysis to analyze the effect of individual parameter. Gas injection time is the dominant factor of hydrocarbon gas huff-n-puff in unconventional oil reservoirs with complex fracture networks. This work can offer engineers guidance for hydrocarbon gas huff-n-puff of multiple MFHWs considering the complex fracture networks.

Metal-organic cages containing two types of binding sites:trapping hydrocarbon gas in solution

The design and synthesis of artificial molecular containers for the encapsulation of hydrocarbon gases to study their host-guest chemistry are highly important for potential application in gas storage,separation,and understanding of their biological functions.In this work,we report the subcomponent self-assembly of four cubic Zn_(8)L_(12)Br_(4)(HL=N-(4-R)-1-(5-methyl-1Himidazole-4-yl)methanimine)cages with good solubility in chloroform,which are capable of binding hydrocarbon gases including methane,ethane,and ethene in solution at ambient temperature.Two types of gas binding sites(one is in the cavity,and the other is at the window)are discovered in these cages,which are documented by nuclear magnetic resonance(NMR)spectra and density functional theory(DFT)calculations.Their performance of encapsulation of hydrocarbon gases can be tuned by carefully adjusting substituent groups.These metal-organic cages containing two types of binding sites provide new artificial models to mimic the structures and functions of biological systems in binding and transforming hydrocarbon gases.

A Study of the Migration and Accumulation Efficiency and the Genesis of Hydrocarbon Natural Gas in the Xujiaweizi Fault Depression

In order to investigate the migration and accumulation efficiency of hydrocarbon natural gas in the Xujiaweizi fault depression, and to provide new evidence for the classification of its genesis, a source rock pyrolysis experiment in a closed system was designed and carried out. Based on this, kinetic models for describing gas generation from organic matter and carbon isotope fractionation during this process were established, calibrated and then extrapolated to geologic conditions by combining the thermal history data of the Xushen-1 Well. The results indicate that the coal measures in the Xujiaweizi fault depression are typical ＂high-efficiency gas sources＂, the natural gas generated from them has a high migration and accumulation efficiency, and consequently a large-scale natural gas accumulation occurred in the area. The highly/over matured coal measures in the Xujiaweizi fault depression generate coaliferous gas with a high δ^13C1 value （〉 -20‰） at the late stage, making the carbon isotope composition of organic alkane gases abnormally heavy. In addition, the mixing and dissipation through the caprock of natural gas can result in the negative carbon isotope sequence （δ^13C1 〉δ^13C2 〉δ^13C3 〉δ^13C4） of organic alkane gases, and the dissipation can also lead to the abnormally heavy carbon isotope composition of organic alkane gases. As for the discovery of inorganic nonhydrocarbon gas reservoirs, it can only serve as an accessorial evidence rather than a direct evidence that the hydrocarbon gas is inorganic. As a result, it needs stronger evidence to classify the hydrocarbon natural gas in the Xujiaweizi fault depression as inorganic gas.

Geochemical Characteristics of C_5–C_7 Light Hydrocarbons in Gas Hydrates from the Permafrost Region of Qilian Mountains

As an important part of gas hydrates, light hydrocarbons （LHs）, especially C5 to C7 hydrocarbons with various monomer compounds, provide a wide variety of geological and geochemical information, which have received much attention from organic geochemists and petroleum geologists.

Genesis of the Sinian Solid Bitumen and its Significance for Hydrocarbon Accumulation in the Anyue Gas Field of the Sichuan Basin

Objective Bitumen is generally associated with oil and gas, which was originally used as an indicator of hydrocarbon reservoirs. With the progress of organic geochemical measuring and testing techniques, bitumen, especially solid bitumen sampled from reservoirs, has been proved to be closely related to the evolution of hydrocarbon reservoirs. The Sinian cores collected from the Anyue gas field contain abundant pores, vugs and fractures, which are filled with a mass of solid bitumen of two epochs and dolomite in between. This work focused on the characteristics of different generations of bitumen and the genesis, in an effort to better understand the process of the Sinian gas accumulation in the Anyue gas field.

Geochemical Characteristics and Gas-to-Gas Correlation of Two Leakage-type Gas Hydrate Accumulations in the Western Qiongdongnan Basin, South China Sea

In recent years,a series of highly saturated leakage-type gas hydrates have been discovered in the western Qiongdongnan Basin(QDNB),South China Sea.Based on the molecular compositional and isotopic characteristics of the gas samples relevant to the gas hydrates collected from the two leakage-type gas hydrate accumulations in the GMGS5 and GMGS6 drilling zones,a detailed geochemical gas-to-gas correlation was conducted in this study,in order to further understand the geochemical characteristics and possible hydrocarbon sources of these gas hydrates.The natural gas relevant to the gas hydrates in the GMGS5 block is characterized by wet gas(67.96%<%C_(1)<98.58%,C_(1)/C_(1+)<0.9)and significant molecular and carbon isotope fractionation within the depth profile,whereas the gas samples from the GMGS6 block exhibit the characteristics of dry gas(99.25%<%C_(1)<99.81%,C_(1)/C_(1+)>0.9)and lack molecular and carbon isotope fractionation.Approximately 40%‒60%of the methane within the gas hydrate is of microbial origin,while the C^(2+)gas components are typical coal-type gas that are derived from thermogenic source rocks or deeply-buried natural gas fields.In addition,typical in situ primary microbial methane(−80.6‰<δ^(13)C-C_(1)<−67.7‰)was discovered in well W b,which was applied to estimate the contribution of the microbial gas to the gas hydrates.The gas-to-gas correlation results show that the hydrate gases within the two leakage-type gas hydrate accumulations in the GMGS5 and GMGS6 blocks are geochemically different,suggesting that they may have been derived from different source kitchens.Our results further indicate that the deeply-buried thermogenic gas significantly contributed to the shallowly-buried gas hydrates in the western QDNB and multiple effective thermogenic source kitchens provided the hydrocarbon gas in the gas hydrate accumulations.

North slope transition zone of Songnan-Baodao sag in Qiongdongnan Basin and its control on medium and large gas fields,South China Sea

Based on analysis of newly collected 3D seismic and drilled well data,the geological structure and fault system of Baodao sag have been systematically examined to figure out characteristics of the transition fault terrace belt and its control on the formation of natural gas reservoirs.The research results show that the Baodao sag has the northern fault terrace belt,central depression belt and southern slope belt developed,among them,the northern fault terrace belt consists of multiple transition fault terrace belts such as Baodao B,A and C from west to east which control the source rocks,traps,reservoirs,oil and gas migration and hydrocarbon enrichment in the Baodao sag.The activity of the main fault of the transition belt in the sedimentary period of Yacheng Formation in the Early Oligocene controlled the hydrocarbon generation kitchen and hydrocarbon generation potential.From west to east,getting closer to the provenance,the transition belt increased in activity strength,thickness of source rock and scale of delta,and had multiple hydrocarbon generation depressions developed.The main fault had local compression under the background of tension and torsion,giving rise to composite traps under the background of large nose structure,and the Baodao A and Baodao C traps to the east are larger than Baodao B trap.Multiple fault terraces controlled the material source input from the uplift area to form large delta sand bodies,and the synthetic transition belt of the west and middle sections and the gentle slope of the east section of the F12 fault in the Baodao A transition belt controlled the input of two major material sources,giving rise to a number of delta lobes in the west and east branches.The large structural ridge formed under the control of the main fault close to the hydrocarbon generation center allows efficient migration and accumulation of oil and gas.The combination mode and active time of the main faults matched well with the natural gas charging period,resulting in the hydrocarbon gas enrichment.Baodao A transition belt is adjacent to Baodao 27,25 and 21 lows,where large braided river delta deposits supplied by Shenhu uplift provenance develop,and it is characterized by large structural ridges allowing high efficient hydrocarbon accumulation,parallel combination of main faults and early cessation of faulting activity,so it is a favorable area for hydrocarbon gas accumulation.Thick high-quality gas reservoirs have been revealed through drilling,leading to the discovery of the first large-scale gas field in Baodo 21-1 of Baodao sag.This discovery also confirms that the north transition zone of Songnan-Baodao sag has good reservoir forming conditions,and the transition fault terrace belt has great exploration potential eastward.

Modes of Shale-Gas Enrichment Controlled by Tectonic Evolution

The typical characteristics of shale gas and the enrichment differences show that some shale gases are insufficiently explained by the existing continuous enrichment mode. These shale gases include the Wufeng–Longmaxi shale gas in the Jiaoshiba and Youyang Blocks, the Lewis shale gas in the San Juan Basin. Further analysis reveals three static subsystems（hydrocarbon source rock, gas reservoirs and seal formations） and four dynamic subsystems（tectonic evolution, sedimentary sequence, diagenetic evolution and hydrocarbon-generation history） in shale-gas enrichment systems. Tectonic evolution drives the dynamic operation of the whole shale-gas enrichment system. The shale-gas enrichment modes controlled by tectonic evolution are classifiable into three groups and six subgroups. Group I modes are characterized by tectonically controlled hydrocarbon source rock, and include continuous in-situ biogenic shale gas（Ⅰ_1） and continuous in-situ thermogenic shale gas（Ⅰ_2）. Group Ⅱ modes are characterized by tectonically controlled gas reservoirs, and include anticline-controlled reservoir enrichment（Ⅱ_1） and fracture-controlled reservoir enrichment（Ⅱ_2）. Group Ⅲ modes possess tectonically controlled seal formations, and include faulted leakage enrichment（Ⅲ_1） and eroded residual enrichment（Ⅲ_2）. In terms of quantity and exploitation potential, Ⅰ_1 and Ⅰ_2 are the best shale-gas enrichment modes, followed by Ⅱ_1 and Ⅱ_2. The least effective modes are Ⅲ_1 and Ⅲ_2. The categorization provides a different perspective for deep shale-gas exploration.

Surface geochemical data evaluation and integration with geophysical observations for hydrocarbon prospecting,Tapti graben,Deccan Syneclise,India

The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area. Soil samples were collected from favorable areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons （methane through butane） were used as surface indicators of pe- troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons （C_1-C_4） and their carbon isotopes from the western part of Tapti graben, Deccan Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane （C1 518 ppb） and ethane plus higher hydrocarbons （∑C2＋ = 977 ppb） were observed. Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. The carbon isotopic composition of methane, ethane and propane ranges are from 22.5‰to -30.2‰ PDB, -18.0‰ to 27.1‰,, PDB and 16.9‰-32.1‰ PDB respectively, which are in thermogenic source. Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentra- tions were associated with dykes, lineaments and presented on thinner basaltic cover in the study area, which probably acts as channel for the micro-seepage of hydrocarbons.

Group Contribution based Flammability Limit Estimation of Hydrocarbon-Inert Gas Mixture

As group contribution method is easy to apply and has a wide application range,current study has developed this model to predict flammability limit of hydrocarbons mixed with inert gas using the Marrero/Gani group contribution method,which is significative to the safe application of hydrocarbons in the ORC system.The whole modeling process is divided into two parts:pure compound prediction and mixture prediction.The contribution factors of inert gases and dilute concentration were first introduced in the group contribution method.Moreover,the respective 95%-confidence interval of the mixture based on linear superposition method has been proposed in the developed group contribution model to improve the safety coefficient.For CO2 as inert gas,the average relative error and correlation coefficient are 5.34%and 0.88 for lower flammability limit while 6.99%and 0.95 for upper flammability limit.For N2 as inert gas,the average relative error and correlation coefficient are 7.47%and 0.84 for lower flammability limit while 6.68%and 0.97 for upper flammability limit.Most importantly,this group contribution method has extended the application range to make up the shortcomings of other flammability limit prediction methods aiming at hydrocarbon and inert gas mixtures and proposed the uncertainty analysis to provide reliable prediction range.

Olivine versus peridotite during serpentinization:Gas formation

The dependence of starting materials and their initial grain sizes on the formation of gases （H2, CH4, C2H6 and C3Hs） during serpentinization was investigated by conducting hydrothermal experiments at 311℃ and 3 kbar on olivine and peridotite with initial grain sizes ranging from 〈30 to 177 μm. Hydrocarbons （CH4, C2H6 and C3H8） were produced from reaction between dissolved CO2 in the starting fluids and HE formed during serpentinization, which were analyzed by Gas Chromatography. It was found that olivine serpentinization produced much less H2 and CH4 compared with those after peridotite alteration, while their C2H6 and C3H8 were identical. For example, for olivine with initial grain sizes of 〈30 μm, the amounts of HE and CH4 were 79.6 mmol/kg and 460 μmol/kg after 27 days, respectively. By contrast, the quantities of H2 and CH4 produced in experiment on peridotite with the same run duration were much larger, 119 mmol/kg and 1300 μmol/kg, respectively. This indicates that spinel and pyroxene in peridotite may increase the amounts of HE and hydrocarbons, possibly due to the catalytic effect of aluminum released by spinel and pyroxene during serpentinization. Moreover, the production of H2 and hydrocarbons is negatively correlated with initial grain sizes of the starting material, with smaller amounts of HE and hydrocarbons for larger initial grain sizes, indicating that the kinetics of serpentinization influences the formation of HE and hydrocarbons, possibly because of the lack of catalytic minerals for the starting material with larger grain sizes. This study suggests that olivine cannot completely represent peridotite during serpentinization, and that H2 and hydrocarbons in hydrothermal fields near the mid-ocean ridge may be produced in a very long period of serpentinization or the presence of catalytic minerals due to large grain sizes of ultramafic rocks.

A nanocomposite consisting of silica-coated magnetite and phenyl-functionalized graphene oxide for extraction of polycyclic aromatic hydrocarbon from aqueous matrices

In this study,graphene oxide was covalently immobilized on silica-coated magnetite and then modified with 2-phenylethylamine to give a nanocomposite of type Fe3O4@SiO2@GO-PEA that can be applied to the magnetic solid-phase extraction of polycyclic aromatic hydrocarbons（PAHs） from water samples.The resulting microspheres（Fe3O4@SiO2@GO-PEA） were characterized by Fourier transform-infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,CHNS elemental analysis,and vibrating sample magnetometry（VSM） techniques.The adsorbent possesses the magnetic properties of Fe3O4 nanoparticles that allow them easily to be separated by an external magnetic field.They also have the high specific surface area of graphene oxide which improves adsorption capacity.Desorption conditions,extraction time,amount of adsorbent,salt concentration,and pH were investigated and optimized.Following desorption,the PAHs were quantified by gas chromatography with flame ionization detection（GC-FID）.The limits of detection（at an S/N ratio of 3） were achieved from 0.005 to0.1 μg/L with regression coefficients（R2） higher than 0.9954.The relative standard deviations（RSDs） were below 5.8%（intraday） and 6.2%（inter-day）,respectively.The method was successfully applied to the analysis of PAHs in environmental water samples where it showed recoveries in the range between 71.7%and 106.7%（with RSDs of 1.6%to 8.4%,for n = 3）.The results indicated that the Fe3O4@SiO2@GO-PEA microspheres had a great promise to extraction of PAHs from different water samples.

An isotope study of the accumulation mechanisms of high-sulfur gas from the Sichuan Basin, southwestern China

The mechanism of hydrogen sulfide(H_2S) generation plays a key role in the exploration and development of marine high-sulfur natural gas, of which the major targets are the composition and isotope characteristics of sulfur-containing compounds.Hydrocarbon source rocks, reservoir rocks, natural gases and water-soluble gases from Sichuan Basin have been analyzed with an online method for the content of H_2S and isotopic composition of different sulfur-containing compounds. The results of comparative analysis show that the sulfur-containing compounds in the source rocks are mainly formed by bacterial sulfate reduction(BSR), and the sulfur compounds in natural gas, water and reservoir are mainly formed by thermal sulfate reduction(TSR). Moreover, it has been shown that the isotopically reversion for methane and ethane in high sulfur content gas is caused by TSR. The sulfur isotopic composition of H_2S in natural gas is inherited from the gypsum or brine of the same or adjacent layer,indicating that the generation and accumulation of H_2S have the characteristics of either a self-generated source or a near-source.