Water Role and Its Influence on Hydrogen Isotopic Composition of Natural Gas during Gas Generation

In order to discuss the role and influence of water during the generation of natural gas,the participation mechanism of water during the evolution of organic matter and its influences were summarized.In addition,we carried out an anhydrous cracking experiment of oil extracted from the Feixianguan Formation source rock in a closed system,which led to the establishment of the kinetic models for describing carbon and hydrogen isotopic fractionation during gas generation from organic matter.The models were calibrated and then applied to the northeastern Sichuan Basin.By combining a series of gas generation experiments from octadecane pyrolysis without water or with distilled water in varying mass proportions,several results were proved:(1) the hydrogen isotopic composition of natural gas becomes lighter with the participation of formation water;(2) we can quantitatively study the hydrogen isotopic fractionation with the kinetic model for describing carbon isotopic fractionation; (3) more abundant and reliable geological information can be obtained through the combined application of carbon and hydrogen isotopic indices.

Spatial-temporal coupling between high-quality source rocks and reservoirs for tight sandstone oil and gas accumulations in the Songliao Basin, China

The spatial-temporal relationship between high-quality source rocks and reservoirs is a key factor when evaluating the formation,occurrence,and prospectivity of tight oil and gas reservoirs.In this study,we analyze the fundamental oil and gas accumulation processes occurring in the Songliao Basin,contrasting tight oil sand reservoirs in the south with tight gas sand reservoirs in the north.This is done using geochemical data,constant-rate and conventional mercury injection experiments,and fluid inclusion analyses.Our results demonstrate that as far as fluid mobility is concerned,the expulsion center coincides with the overpressure zone,and its boundary limits the occurrence of tight oil and gas accumulations.In addition,the lower permeability limit of high-quality reservoirs,controlled by pore-throat structures,is 0.1×10^-3μm^2 in the fourth member of the Lower Cretaceous Quantou Formation(K1q^4)in the southern Songliao Basin,and 0.05×10^-3μm^2 in the Lower Cretaceous Shahezi Formation(K1sh)in the northern Songliao Basin.Furthermore,the results indicate that the formation of tight oil and gas reservoirs requires the densification of reservoirs prior to the main phase of hydrocarbon expulsion from the source rocks.Reservoir“sweet spots”develop at the intersection of high-quality source rocks(with high pore pressure)and reservoirs(with high permeability).

Scale-Dependent Nature of Porosity and Pore Size Distribution in Lacustrine Shales: An Investigation by BIB-SEM and X-Ray CT Methods

Due to heterogeneous pore distributions within shales,petrophysical properties of shales determined by scanning electron microscopy(SEM) and X-ray computed tomography(CT) methods strongly depend on the observed domain size(analysis scale). In this paper,the influence of the analysis scale on areal and bulk porosities and pore size distribution(PSD) for lacustrine shales from the Dongying sag of Bohai Bay Basin,China were investigated using broad ion beam(BIB)-SEM and X-ray CT methods.The BIB-SEM cross-sections with high imaging resolution(10 nm/pixel) and a large field of view(>1 mm2)mainly describe the 2 D nanoscale pore system in the two shales(samples F41#-2 and Y556#-1),while CTbased 3 D reconstructions with resolutions of 0.42(F41#-1) and 0.5 μm/pixel(H172#-1) reflect the 3 D submicron pore system. The results indicate that the areal(bulk) porosity exhibits a multiple power-law distribution with increasing analysis area(volume),which can be used to extrapolate the porosity of a given area(volume). Based on SEM and CT investigations,the sizes of the minimum representative elementary areas(REAs) and volumes(REVs) were determined respectively,which are closely associated with the heterogeneousness of the pore system. Minimum REAs are proposed to be 2.93×10~4(F41#-2) and 0.91×10~4μm2(Y556#-1),and minimum REVs are 0.016(F41#-1) and 0.027 mm^3(H172#-1). As the analyzed areas(volumes) are larger than the minimum REA(REV),obtained 2 D(3 D) PSDs are comparable to each other and can be considered to reflect the shale PSD. These results provide insights into the porosity and PSD characterization of shales by SEM and X-ray CT methods.

Carbon isotope fractionation during shale gas transport: Mechanism, characterization and significance

The gas in-place(GIP)content and the ratio of adsorbed/free gas are two key parameters for the assessment of shale gas resources and have thus received extensive attention.A variety of methods have been proposed to solve these issues,however none have gained widespread acceptance.Carbon isotope fractionation during the methane transport process provides abundant information,serving as an effective method for differentiating the gas transport processes of adsorbed gas and free gas and ultimately evaluating the two key parameters.In this study,four stages of methane carbon isotope fractionation were documented during a laboratory experiment that simulated gas transport through shale.The four stages reflect different transport processes:the free gas seepage stage(Ⅰ),transition stage(Ⅱ),adsorbed gas desorption stage(Ⅲ)and concentration diffusion stage(Ⅳ).Combined with the results of decoupling experiments,the isotope fractionation characteristics donated by the single effect(seepage,adsorption-desorption and diffusion)were clearly revealed.We further propose a technique integrating the Amoco curve fit(ACF)method and carbon isotope fractionation(CIF)to determine the dynamic change in adsorbed and free gas ratios during gas production.We find that the gases produced in stage Ⅰ are primarily composed of free gas and that carbon isotope ratios of methane(δ13C1)are stable and equal to the ratios of source gas(13C 10).In stage Ⅱ,the contribution of free gas decreases,while the proportion of adsorbed gas increases,and the δ13C1 gradually becomes lighter.With the depletion of free gas,the adsorbed gas contribution in stage Ⅲ reaches 100%,and the δ13C1 becomes heavier.Finally,in stage Ⅳ,the desorbed gas remaining in the pore spaces diffuses out under the concentration difference,and the δ13C1 becomes lighter again and finally stabilizes.In addition,a kinetic model for the quantitative description of isotope fractionation during desorption and diffusion was established.

Lacustrine Shale Oil Resource Potential of Es3L Sub-Member of Bonan Sag, Bohai Bay Basin, Eastern China

Following shale gas, shale oil has become another highlight in unconventional hydrocarbon exploration and development. A large amount of shale oil has been produced from a host of marine shale in North America in recent years. In China, lacustrine shale, as the main source rock of conventional oil and gas, should also have abundant oil retained in place. In this study, geochemical and geologic characteristics of lacustrine shale from Es3L sub-member in Bonan sag were characterized by using total organic carbon（TOC）, Rock-Eval pyrolysis, X-ray diffraction, and ？log R method. The results show that the Es3L sub-member shale have TOC contents ranging from 0.5 wt.% to 9.3 wt.%, with an average of 2.9 wt.%. The organic matter is predominantly Type I kerogen, with minor amounts of Type II1 kerogen. The temperature of maximum yield of pyrolysate（Tmax） values ranges from 424 to 447 ℃, with an average of 440 ℃, and vitrinite reflectance（Ro%） ranges from 0.7% to 0.9%, indicating most of shales are thermally mature. The dominant minerals of Es3L shale in Bonan sag are carbonates（including calcite and dolomite）, averaging 51.82 wt.%, and the second minerals are clay（mostly are montmorillonite-illite-mixed layer and illite） and quartz, averaging about 18 wt.%. Finally, its shale oil resources were evaluated by using the volumetric method, and the evaluation result shows that the shale oil resource is up to 5.94 billion tons, and mostly Class I resource. Therefore, the exploration of the lacustrine shale oil of Es3L in Bonan sag should be strengthened.

Prediction of decline in shale gas well production using stable carbon isotope technique

Prediction of shale gas production is a challenging task because of the complex fracture-pore networks and gas flow mechanisms in shale reservoirs.Empirical methods,which are used in the industry to forecast the future production of shale gas,have not been assessed sufficiently to warrant high confidence in their results.Methane carbon isotopic signals have been used for producing gas wells,and are controlled by physical properties and physics-controlling production;they serve as a unique indicator of the gas production status.Here,a workable process,which is combined with a gas isotope interpretation tool(also known as a numerical simulator),has been implemented in Longrnaxi shale gas wells to predict the production decline curves.The numerical simulator,which takes into account a convection-diffu-sion-adsorption model for the matrix and a convection model for fractures in^(13)CH_(4) and ^(12)CH_(4) isotopologues,was used to stabilize the carbon isotope variation in the produced gas to elucidate gas recovery.Combined with the production rates of the four developing wells,the total reserves ranged from 1.72×10^(8) to 2.02×10^(8) m^(3),which were used to constrain the trend of two-segment produc-tion decline curves that exhibited a transition from a hyperbolic equation to an exponential one within 0.82-0.89 year.Two-segment production decline curves were used to forecast future production and estimate ultimate recovery.

Quantitative evaluation of adsorbed and free water in deep shales:a case study on the Wufeng-Longmaxi Formations from the Luzhou area,southern Sichuan Basin,China

Deep shale gas reservoirs commonly contain connate water, which affects the enrichment and migration of shale gas and has attracted the attention of many scholars. It is significant to quantitatively estimate the amounts of adsorbed and free water in shale matrix pores, considering the different impacts of pore water (adsorbed water and free water) on shale gas. In this paper, pore water in six deep shale samples from the Wufeng-Longmaxi Formations in the Luzhou area, southern Sichuan Basin, China, was quantitatively evaluated by saturation-centrifugation experiments. Further, the impact of shale material composition and microstructure on the pore water occurrence was analyzed. The results show that amounts of adsorbed and free water are respectively 1.7967–9.8218 mg/g (mean 6.4501 mg/g) and 9.5511–19.802 mg/g (mean 13.9541 mg/g) under the experimental conditions (30℃, distilled water). The ratio of adsorbed water to total water is 15.83%–42.61% (mean 30.45%). The amounts of adsorbed and free water are related to the pore microstructure and material compositions of shale. The specific surface area of shale controls the amount of adsorbed water, and the pore volume controls the amount of free water;organic pores developed in shale solid asphalt contribute specific surface area and pore volume, and inorganic pores developed in clay mineral contribute pore volume. Therefore, the pores of shale solid asphalt accumulate the adsorbed water and free water, and the pores of clay minerals mainly accumulate the free water.

Methane adsorption effected by pore structure of overmature continental shale:Lower Cretaceous Shahezi Formation,Xujiaweizi Fault,Songliao Basin

Overmature continental shale is commonly developed,but few studies have given insight into its pore structure and sorption capacity.Various techniques,including SEM,helium porosity and permeability,N_(2)/CO_(2)adsorption,MICP,and NMR,were used to detect the pore structure of shale from the Shahezi Formation,Xujiaweizi Fault,Songliao Basin.The excess methane adsorption volumes were measured by the volumetric method and modeled by the Langmuir model.Based on the findings,the most developed pores are intraparticle pores in clay minerals,followed by the dissolution pores in feldspar,but organic pores are uncommon.The selected shales have low helium porosity(mean 1.66%)and ultralow permeability(mean 0.0498×10^(−3)μm^(2)).The pore throats are at the nanoscale,and the pore-throat size distributions are unimodal,with most less than 50 nm.The studied shales are characterized by the lower specific surface area(SSA)and pore volume(PV)but the larger average pore diameter.The total SSA is contributed by the micro-and mesopores,while the PV is dominated by meso-and macropores.The pore structures are more complex and controlled by multiple factors,such as mineral compositions and diagenesis,but organic matter is not critical.The maximum absolute adsorption methane volume(VL)is 0.97−3.58 cm^(3)/g(mean 1.90 cm^(3)/g),correlating well with the total SSA,SSA,and pore volume of micropores,which indicates that methane is mainly adsorbed and stored in micropores,followed by mesopores.