Contents of Natural Gas Geoscience, No.6, 2017
Horizontal well staged fracturing is the main technical means of shale gas reservoir reformation.The flowback rate of fracturing fluid is low in shale gas reservoir.The retained fracturing fluid in nanopore and microcrack seriously affects the desorption-diffusion-permeability of shale gas.Spontaneous imbibition and flowback experiment swere carried out on the shale of Longmaxi Formation in the eastern Sichuan Basin to reveal the characteristics of fracturing fluid flowing back and the change of aqueous phase trapping damage.The effects of pore structure,fracture,flowback pressure and time on spontaneous imbibition and flowback behavior were evaluated.Results show that,the smaller the pore radius,the greater the capillary force,the higher the spontaneous imbibition rate and the poorer the flowback effect.Aqueous phase spontaneous imbibition can induce cracks in shale,which increased the channel of permeability and promoted aqueous phase flowback.Flowback pressure and flowback time have a positive correlation with the flowback effect of aqueous phase,and after 24 hours of flowback,the maximum flowback rate is 46% and permeability recovery rate is 67%.This paper draws the conclusion that,the potential aqueous phase trapping damage of shale gas reservoirs is severe,and reduce aqueous phase in shale benefits to improve permeability of shale gas.
Compared with the conventional reservoir,the complicated property of gas-shale reservoir has a more significant influence on fracturing fluid recovery efficiency(RE) and production performance(PP).A numerical simulation model about early gas-water two-phase flow is established based on the typical data to study the effects of reservoir properties on RE and PP,such as subirreducible water saturation,relative permeability jail,wettability and matrix capillary force.The results,firstly,show that more fluidsareimbibed into the reservoir because of “subirreduciblewater saturation”,resulting in the larger water saturation increment,the higher initial gas production and the lower RE.However,the water saturation of matrix is still higher after 10 days shut-in if the original water saturation is higher at first causing less gas is produced in late stage.Secondly,the cumulative gas production(CGP) decreases with the severity of the “relative permeability jail”,in which the CGP is only 1.32×104m3 after 3 years production.Fewfluid is recovered(4.74%) if a conventional relative permeability is applied.Therefore,the weak relative permeability jail curve is more suitable for the tight reservoirs than other two types of relative permeability curves.What's more,the weak water wet formation reduces the imbibition and replacement leading to the decrease of CGP and the increase of RE.Finally,with the increase of capillary pressure,the higher gas production in early stage and the lower in late stage are revealed because of gas permeability impaired by high water saturation caused by the strong imbibition,resulting in lower CGP and combined with lower RE.
Field-test data indicate that some fractures have little or no contribution to the total gas production.However,most well-testing models did not consider the effects of unequal gas production of fractures(UGPF) on well testing interpretation.Therefore,this paper establishes a well-testing model with consideration of UGPF based on the source function and Newman‘s product.Then,type curves of multi-fractured horizontal well(MFHW) with UGPF are developed through numerical algorithm to involve the effects of wellbore storage and skin factor.Sensitivity analysis(i.e.UGPF,fracture half-length,number of fractures,fracture spacing) are further discussed.There have big differences among well-testing type curves between equal gas production of fractures(EGPF) and UGPF.The early-radial flow behaves as a horizontal line with the value of nearly 0.5/N in the pseudo-pressure derivative curves when the interferences among fractures are tiny.Permeability of tight gas reservoir is very low so that the field-test data is hard to reflect the characteristics of pseudo-radial flow regime.If the early-radial flow regime is mistaken as the pseudo-radial flow regime,the interpreted permeability will be incorrect.Therefore,the effect of UGPF on well testing interpretation in tight gas reservoir should be investigated.Finally,this paper provides a method to estimate the parameters,which can be regarded as a reference for fracture performance evaluation.
The form and distribution of CO2 under different displacement methods has been studied by visualization experimental equipment with the condition of high pressure and high temperature,and the storage capacity of CO2 has also been investigated by image analysis technology.Experimental results show that,free state CO2 mainly distributed on the top of the model under the immiscible displacement,and the storage mechanism is structure and stratigraphic trapping.While under the miscible displacement it mainly distributed on the middle and upper parts of the model,and the oil and CO2 miscible zone will locate in the lower part of the model,and the storage mechanism is structure and stratigraphic trapping and dissolution trapping in the remaining oil.During the experiment of water alternating gas(WAG) displacement,the sweep area of free state CO2 spreads from the mainstream line to both sides,the diffusion degree gradually decreases from the inject end to the output end,and the CO2 is dissolved in the injected water,and the storage mechanism is structure and stratigraphic trapping,dissolution trapping in the injected water and the remaining oil.The distribution of CO2 under the WAG miscible displacement and CO2 miscible displacement after water displacement is consistent,in which the free state CO2 and miscible CO2 discretely distributed in the model,and the CO2 was dissolved in the injected water,and the storage mechanism is structure and stratigraphic trapping,dissolution trapping in the injected water and the remaining oil.The calculation of storage capacity shows that,the storage capacity under the miscible condition is higher than that under the immiscible condition,and structure trapping is the main form of CO2 storage.The storage capacity of CO2 miscible displacement after water flooding is excellent,which means the reservoir after water flooding is anideal place for CO2 storage.
In order to identify theadsorption/desorption capability of coalbed methane(CBM) of different macro lithotypes in low rank coal,weselected vitrain and durain samples of Dafosi No.4 coal in Binchang Mining Region to conduct the adsorption/desorption experiments,analyzed the adsorption/desorption characteristicsand studied the differences of adsorption/desorption and the nature of desorption from the perspectives of adsorption thermosdynamic.The results showthatcomponents of vitrain and durain mainly consist of the middle and micropores at Dafosi minefield,and the pore connectivity of durain,with the open hole as the main,is better than vitrain,which is mainly dominated by a semi closed flask shaped or ink bottle shaped pores;The adsorption/desorption capacity of different macro lithotypes was affected(from strong to weak) by pressure,temperature,moisture,granularity and pore characteristics;The effects of moisture and granularity on the different macro lithotypes are different in the process of depressurization and desorption,the desorption hysteresis rate ofdurain was larger than vitrain.The effect of differential pressure transfer on the adsorption of coal is not as good as that of competitive adsorption between water molecules and methane in coal.Desorption of CBM is the result of the joint effect of pressure drop(pressure-energytransferfunction) and vaporization of water vapor replacement of methane(displacement effect).
The chemical structure of 19 samples with maturities ranging from 0.35% to 5.32% RO was measured by Fourier transform infrared spectroscopy(FTIR).The results show that with the increase of RO,different functional groups in coal structure have different evolution rules.The C-H bond on the aromatic ring structure increases first,when RO>2.5%,decreases in the infrared spectrum. With the increase of RO,the overall trend of absorption peaks intensity of the oxygen side chain and aliphatic side chain weakened.The content of oxygen-containing functional groups decreased gradually.When RO>2.5%,the coal structure basically no longer contains oxygen side chain;When RO<2.0%,the aliphatic side chain increased rapidly with increasing coalification;when RO>2.0%,the aliphatic side chain in the coal structure slowly reduced;when RO>2.5%,nearly no fat side chain was detected in the coal structure.This demonstrates that coal has different gas generation features at different stages.The rapid reduction of coal content in the oxygen-containing side chain indicates that the formation of CO2 gas mainly occurs in the relatively low evolution stage.The two-stage evolution aliphatic groups indicated that the main gas generation period of coal was at the stage with thermal maturity <2.0%RO,however,the upper limit of thermal maturity for gas generation from coal was >5.0%RO.The Fischer-Tropsch reaction,which happened between hydrogen atom cleaved from aromatic rings at graphitization(RO>3.0%) and CO2 gengerated by oxygen-containing groups cleavage from coal at RO<2.0%,should be one of the important reasons causing carbon isotopic reversal of coal derived gases at over maturity.
Shale pore structure,especially nano-scale pore,can affect the prospect of shale gas resources and even later development engineering.Material composition and pore structures (size<100nm) were investigated via 23 black shale samples from different layers.The results showed that the average content of the total organic carbon (TOC) of samples from Carboniferous-Permian Shanxi and Taiyuan formations are lower than that from Upper Proterozoic Hongshuizhuang and Xiamaling formations.However,the clay mineral content of samples from Carboniferous-Permian is higher than those from Upper Proterozoic,especially kaolinite.Descending order of the average pore size and average pore volume are Hongshuizhuang Formation,Xiamaling Formation,Shanxi Formation and Taiyuan Formation.The clay mineral content,type and TOC can significantly affect the pore structure of shale.There are three segments of TOC content,<2%,2%-4.58% and >4.58%.The specific surface area and pore volume have a positive correlation with clay minerals content,but have a negative correlation with TOC,respectively.When TOC content is >4.58%,the change of TOC content has more obvious effects on the pore structure.When TOC content is <2%,the clay mineral contents and types have obvious effects on the pore structure,and effects of illite is larger than that of kaolinite.The ratio of TOC content to clay mineral content has a good correlation with specific surface area and pore volume,which indicates that compared to the changing degree of the specific surface area and pore volume caused by the organic matter abundance,the change caused by clay minerals content increasing is more obvious.Both distributions are also the important instructions and characterization of the pore structure.In a word,the characteristics of clay minerals have great influence on the pore structure of shale from Shanxi and Taiyuan formations,and TOC content has great influence on the pore structure of shale from Xiamaling and Hongshuizhuang formations.
The Carboniferous of Zhongguai Uplift develops igneous rock.High velocity igneous rocks have a shielding effect on underlying formations,which leads to poor imaging quality and is not conducive to fine exploration.The distribution rule of oil and gas in Carboniferous reservoirs is complex,and each trap has different oil-water interfaces.The distribution of oil and gas is controlled by fault,and the fine description of faults is affected by the quality of seismic data,and the minor fault is very difficult to identify.The conventional high resolution processing techniques always compensates the high frequency components while the signal-to-noise ratio decreases;in consequence,it makes the structure interpretation difficult,especially for the small fault.To satisfy the exploration demand in following stage,a high resolution technique based on Generalized S-transform is introduced in this paper.The proposed method is characterized by high fidelity and preserved-amplitude;it can compensate the high frequency contents on the basis of high signal-to-noise ratio.This method makes the seismic reflection continuous,and has the potential in the detailed seismic interpretation and seeking the structural trap.
This study focuses on the exploration of Gaojiahe 3-D area in the lower part (C2b2) Benxi Formation of Upper Paleozoic,Ordos Basin.The method using seismic information to study sedimentary system distribution characteristics was expounded.Based on 90 degree phase seismic sets,using frequency analytical technology,sand seismic reflection frequency is analyzed.Combined with seismic filtering,the sand seismic interference effect was pressed.On the basis of high resolution sequence stratigraphy analysis and the stratigraphic framework of seismic sedimentology study of technology,the high frequency seismic profile was established.Finally,different frequency strata slice was interpreted by combining the geological,well logging and seismic sedimentology.Combined with mixing display,principal component analysis (PCA) and space carving technique,the spatial distribution of sedimentary system was determined.The method is helpful to improve the accuracy of the prediction system.The study shows that the Lower Benxi Formation (C2b2) is mainly developed with barrier-shoreline depositional system.A large-scale tidal channel developed in the east area.The branches of the tidal channel or tidal canal are also developed in the S228-S207,S209-S229,Y287-S224 and the Y123 area,which mainly extend from north to west or north to south.Tidal channel performed as the main sand rich region and the logging performed as bell or dentate bell shape,and the strata slice performed linear strong energy distribution pattern.Sand flat are developed in the S208-S231 area and distributed along the edge of the tidal channel interbedded with mud layer.Sand flat show a variety types of logging facies,for example,the funnel shape,box or composite funnel shape.Sand flat or sand spots in the strata slice performed as mid-strong energy reflection mode with porphyritic-vermicular shape.
The study of microscopic pore structure is of very important significance to the classified evaluation of tight sandstone reservoirs of Shahezi Formation in Xujiaweizi Fault Depression,Songliao Basin.By use of nitrogen adsorption experiment,microscopic pore structure of tight sandstone reservoirs was studied.A discussion about controlling factors of microscopic pore structure development was made.The results show that microscopic pore structure of tight sandstone reservoirs of Shahezi Formation can be subdivided into three types according to the conformation of nitrogen adsorption-desorption curves.The first type is similar to H2 of ICPAC,the second type is similar to H3 of ICPAC and the third type is similar to H4 of ICPAC.The micro-pore（＜10nm）and the transition pore（10-50nm）provide the dominant specific surface and pore volume and are the major space of tight sandstone gas accumulation of Shahezi Formation.The pore diameter of tight sandstone of Shahezi Formation can be subdivided into the bimodal distribution types and the three peak distribution types.The main peak is in a range of 30-60nm and the secondary peak is in a range of 7-10nm and 100-110nm.There are various types of micropores in the Shahezi Formation,including intergranular pore,innergranular pore and intercrystal pore.The influence of sedimentary environment to the pore structure is related with the difference of lithology and rock texture of different sedimentary environment.Mineralogical composition has a big effect on microscopic pore structure.Specific surface and total pore volume show a positive correlation with the clay contents and a negative correlation with the quartz contents.However,average pore diameter shows a negative correlation with the clay contents and a positive correlation with the quartz contents.
In order to characterize the pore structure of Chang 6-Chang 8 members tight sandstone reservoirs in the southwestern part of Ordos Basin accurately and visually,thin-section,Scanning Electron Microscope (SEM) and high pressure mercury injection techniques were used to analyze pore types and reservoir characteristics. Micro-nano CT imaging technology is adopted to give three-dimensional (3D) representation of pore spatial morphology,size and connection,and effect of pore type on porosity is further analyzed in this paper. The results indicate that Chang 6-Chang 8 members sandstones have four major types of pore textures,including inter-granular pores,intra-granular pores,inter-crystalline pores,and micro-fractures. Pore spatial morphology of 3D is divided into three types by the ratio of length to width（l/d）. Inter-granular pores are irregularly shaped with 3D imaging,whereas intra-granular and inter-crystalline pores,and microfractures have spherical and pipe shape,respectively. Irregular-shaped and pipe-shaped pores are predominated at micro-scale,the pore connectivity is better. The pore volume ranges mostly from 20 to 100μm3,and the average pore radius is commonly less than 5μm. The spherical pores are predominated at nano-scale,the pore volume ranges mainly from 104 to 107nm3,with the average pore radius of less than 0.1μm. The correlation between porosity and pore abundance is poor. Pipe-shaped pore quantity,average radius of pipe-shaped and irregular-shaped pores have significant effect on sample porosity. Pipe-shaped and irregular-shaped pores at micro-scale have significant effect on porosity of Chang 6-Chang 8 members tight sandstone reservoirs.
In this paper,we investigate the coupling relationship between tight oil and gas accumulation and densification in the Upper Triassic Yanchang Formation in the southwestern part of Ordos Basin.Based on the analyses of diagenetic sequences and inclusion homogenization temperature,hydrocarbon inclusions in quartz over growth were trapped in the peak of hydrocarbon generation and expulsion in the middle and late periods of Early Cretaceous,while those in ferruginous carbonates were formed by hydrocarbon charging in the late period of Early Cretaceous.Original reservoir porosity,permeability,the capillary force and the excee pressure and their relationship with reservoirs in hydrocarbon accumulation periods were studied based on the present pore characteristics and the capillary force of reservoirs.The results show that hydrocarbon mainly accumulate in reservoirs with good porosity and permeability.However,there are differences in controlling factors of porosity and permeability in two accumulation periods.It is proposed that the super low permeability reservoirs had not been developed in the middle and late periods of Early Cretaceous,the ironed carbonate cement in the reservoir compacted eventually with poor property of reservoirs in the late period of Early Cretaceous.The characteristics of abnormal pressure showed that the abnormal high pressure,which was generated by under-compaction and increasing pressure during maturation of excellent source rocks in Chang 7 sub-layer,was the principal force of large-scale hydrocarbon migration in low permeability reservoir.
Based on three groups of elemental geochemical indicators(Sr/V,Cd/Ca,Zr/Rb),the paper analyzed the paleoclimate,paleoproductivity and hydrodynamic condition,and explained genetic mechanism of organic matter within the sequence stratigraphic framework of Jurassic Dongyuemiao Member of the Ziliujin Formation in Jiannan area of the Upper and Middle Yangtze Region.The study results show that it has own unique characteristics compared with other lacustrine basin.The organic matter of the low-stand system tract(LST) was the richest,that of the lake expand system tract(EST) was relatively rich,while that of the high-stand systems tract(HST) was poor.The organic matter of LST shallow lake facies is richer than that of LST lakeshore.The main controlling factors of the differential characteristics are paleoproductivity and preservation environment of organic matter.
Two stages of hydrothermal occurred in the Cambrian carbonate of Gucheng area,Tarim Basin and thepetrologic and geochemical characteristics of the dolomite indicate that it was influenced by hydrothermal;however,the relationship between hydrothermal activities and the formation of dolostone reservoir and the distribution of the reservoir were not well studied yet.According to the characteristics of Cambrianlithofacies paleogeography and platform margin petrology,it suggests that the dolomitization occurred during shallow burialstage,the action mode of hydrothermal is late-stage reformation,further indicating that the type of reservoir is hydrothermal reformed reservoir.Based on the ways of hydrothermal influence on reservoir,it shows that the reservoir type can be named as hydrothermal-reformed dolostone reservoir.Magnesium isotope analysis shows that the magnesium ions needed for saddle dolomite are mainly from Cambrian itself.Hydrothermal first dissolved the Cambrian dolostone and enriched magnesium ions and then precipitated dolomite locally.Based on logging and core data,with the calculation of covariance distances between logging values,the hydrothermal reforming intensity of Well CT 1 was predicted.Combined with 3D seismic data,the distributions of hydrothermal reforming intensity in plane were obtained.Combined with the differences of different hydrothermal reformation,the distribution of hydrothermal reformed reservoir with different qualities was predicted.
Statistical definition of “tight gas” is proposed based on the probabilistic characteristics of the conventional porosity（2 100 points） and permeability（2 073 points）in seven tight sand gas basins,Western Rocky Mountain area,USA（Washakie,Uinta,Piceance,Greater Green River,Wind River,Powder River,Sand Wash）.It can be verified by analyzing the porosity and permeability data of Sulige tight sand gas field,Ordos Basin: “Reservoir is tight if 80% accumulation probability of conventional permeability is less than 0.1×10-3μm2,corresponded with the porosity of 10%”.Based on this,44 typical tight sand gas basins are selected for the data-mining of the basin parameters in global range,which are graded as the sequence of “global”-“continental”-“national”-“basinal”,aiming to analyze the probability distribution rule of the tight sand gas basin parameters.
Located in the Kuqa Depression of Tarim Basin,the Dawanqi and Dabei areas are important oil and gas exploration areas.Light oil takes the predominant composition of the hydrocarbons while the further exploration was impeded by the weakness research of the geochemical characteristics.Gas chromatographic method was taken to analyze the composition and geochemical features of the light oils in Dawanqi and Dabei areas.The C4-C7 composition and methylcyclohexane index of light hydrocarbons suggest the oils were derived from terrigenous humic source.K1 of Mango parameter is largely in line with the transition metal catalysis genesis model showing a preferably coherences of similar origins.Heptane value and isoheptane values show the oils bears with relatively high maturity,which is well conformed to the maturity (RO=0.8%-0.93%) calculated based on the relevance to oil generation temperatures.The results based on the aromaticity/ paraffin indexes and alteration parameters of C7 suggest that several types of alterations including biodegradation,evaporation fractionation and water washing have affected the light hydrocarbons.The study on the characteristic analysis of light hydrocarbon will be of great importance to identify the original source,maturity and alterations.
Based on gold-tube pyrolysis experiments and quantitative determination of light hydrocarbon(C6-14) products,the yields and composition characteristics of light hydrocarbons derived from different types of organic matters were studied in the paper.The quantification of light hydrocarbon products by PY-GC-MS,shows that C6-14 yieldsfrom lacustrine Type-Ⅰ and marine Type-Ⅱ1 organic matters with high IH areapparently higher than that from marine Type-Ⅱ2 organic matter.The maximum light hydrocarbon yields of Cretaceous lacustrine type I organic matter,Xiamaling type Ⅱ1 organic matter and Permian typeⅡ2 organic matter are 33.56,39.58 and 10.08mg/gTOC,respectively.Relatively,the content of aromatic hydrocarbons in light hydrocarbon products is obviously higher than that of saturated hydrocarbons.Meanwhile,the corresponding pyrolysis temperature or maturity reaching the maximum C6-14 yield for Type-Ⅰ kerogen is lower than that forType- Ⅱ kerogen.The evolution of the yields of normal alkanes(n-alkanes) with different carbon number during gold-tube pyrolysis,indicates that the equivalent vitrinite reflectance(Easy% RO) with the maximum yield forn-alkanes with lower carbon number is much higher than that with higher carbon number.This is attributed to the higher generation and cracking activation energy of lower carbon alkanes.By analysis of the light hydrocarbon compositions,we think that several parameters including the content of naphthalene,benzene/hexane and toluene/heptane are well relevant with maturity,and can be used to indicate the degree of thermal evolution of organic matters.
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