Status and problems of coal cleats′ definition,elements,origin in nearly 200 years are reviewed and summarized.According to the observation in mine,experiment in laboratory,combined with the development engineering practice of coalbed methane,the results show that cleat is a kind of endogenetic fracture with geometric shape and fixed combination style.Endogenous cracks,in random growth with no-direction,generated by coal matrix shrinkage in coal-forming process,are not cleat.Butt cleat and face cleat are accompanied,occurring metabolism with the coalification progress.The cleats are the result of joint action in the vertical stress and coal volume shrinkage.The main direction to reveal the cleat-forming mechanism is to study the change regulation of geochemical parameters of coal macromolecules functional reorganization in coalification,to study the coupling relationship between coal functional restructure and vertical stress,and to study the way and process of cleat metabolism.
Particle size effect,which is the influence of coal particle size on the mercury porosimetry mesurement of coal powder,may lead to incorrect interpretation.It′s necessary to pay attention to particle size effect for recognizing the pore structure of coal.Mercury porosimetry experiment was conducted with two groups of anthracite in five different particle sizes,which were 2-3cm,40-50 mesh,90-100 mesh,140-200 mesh and >200 mesh respectively,combined with particle size measurement and SEM observation.With the decrease of particle size,the total pore volume increases,especially the increment of pore volume and specific surface area between mesopore and macropore.The pore size distribution and specific surface area distribution change from unimodal distribution to double hump distribution.The increased macropore volume is not interior pores.With the decrease of particle size,mercury extrusion efficiency significantly reduces.The smaller the coal samples were crushed,the more closed pores were released.Most mercury is stranded in interparticle voids of coal powder after mercury withdrawal,which leads to the incorrect appearance of low extrusion efficiency,and can′t indicate the connection of pores.When coal particle size is bigger than 3mm,particle size effect of pore structure by mercury porosimetry can be neglected.
Hydraulic fracturing is an essential technology to stimulate shale reservoirs,and accurate prediction of fractured shale gas well productivity is the basic for the efficient development of shale gas reservoir.This paper establishes a gas-water two-phase percolation mathematical model under the condition of desorption and diffusion by means of numerical simulation and calculation method.And the numerical model is given in this paper.By means of the simulator for post-fracturing performance of the shale gas reservoir is developed,and quantitative analysis of the influence of fracture parameters,physical parameters and desorption diffusion parameters to fractured shale gas well production is investigated.The research results show that hydraulic fracturing improves single well production,and it′s an effective measure to develop shale gas.Therefore the induced hydraulic fracture conductivity and natural fracture permeability are the main influence factors on shale gas production,and the higher the induced fracture conductivity and natural fracture permeability,the higher the gas and water production rate.Relatively speaking,the matrix permeability and diffusion coefficients have less effect on production.
This study employed low pressure gas adsorption porosimetry to investigate the evolution of meso-and micro-pores in a suite of 11 New Albany shale samples(the Illinois Basin,Devonian/Mississippian,U.S.)across a wide range of thermal maturity corresponding to vitrinite reflection Ro values from 0.35% to 1.41%.Meso-and micro-pore volumes follow a nonlinear evolutionary path with thermal maturity,which is constricted by the transformation of kerogen during the early mature stage and secondary cracking of bitumen/oil at the late mature stage.Fractal dimensions of pore walls derived from Frenkel-Halsey-Hill(FHH)and thermodynamic models range from 2.47-2.61,indicating that pores in shales express fractal characteristics in terms of shape and size.This study also finds the fractal dimension of pore walls to be positively correlated with meso-and micro-pore volumes,and negatively related to average pore size in shale,with only the least mature and the two most mature samples behaving as outliers.Enhanced fractal dimension leads to growing pore wall heterogeneity and increasing adsorption sites and gas adsorption capacity of shale.Therefore,the fractal dimension of pore walls may serve as a quantification indicator of pore heterogeneity and thus a valuable predictor for the gas adsorption capacity of shale.
The evaluation of shale gas content has a great significance on resource prediction,reserve calculation,and exploration and development planning.Desorption method is one of the methods which is widely applied to the evaluation of shale gas content.The present shale gas desorption method has mainly followed coalbed methane technologies.After the core or cutting shale sample is sealed in the desorption canister,desorbed gas and residual gas are measured,and the lost gas is fitted using the measured data.Their sum is the total gas content.After years of development,desorption apparatuses as well as data analytical methods have been improved continuously.USBM method,polynomial method,Smith-Williams method,decline curve method and Amoco method have been successively developed to analyze desorption process.In actual applications,temperature and other environmental factors,lost time,fitting method and desorption apparatuses will influence the result of desorption.Due to the differences in diffusion mechanism and desorption process between shale gas and coalbed methane,evaluation of shale gas content based on methods from coalbed methane may lead to a significant error.Future research will target to improve desorption apparatuses,develop shale gas desorption models,standardize desorption methods as well as set up a relevant industry standard.
Through detailed studies of marine shale gas in China and investigation of typical shale gas zones in the United States,similarities and differences of geological features of the shale gas in USA and China were summarized.The comparative study is of important theoretical and practical significance for marine shale gas in Sichuan Basin.(1)Barnett,Marcellus and Haynesville shale basins are foreland basin,while Sichuan basin in Paleozoic is intra-craton basin.Their depositional environments are deep water shelf,mainly siliceous and calcareous shale,with high brittleness mineral content.(2)TOC of Barnett shale is 3%-13%，averaged at 4.5%;Marcellus is 3%-12%,averaged at 4.0%;and Haynesville shale is 0.5%-4%.TOC of Wufeng-Longmaxi and Qiongzhusi is 1.5%-3% and 2.5%-4.5%,respectively.(3)Maturity of three major shale formations is moderate,while that of Sichuan Basin is at over-mature stage.The RO values of the Barnett,Marcellus and Haynesville formations are 0.5%-2.1%，1.2%-3.5%，and 1.2%-3%,respectively.However,the ROvalues of the Longmaxi and Qiongzhusi formations are 2.5%-4.5% and 1.5%-3%,respectively.(4)Barnett shale has thickness of 30-180m,porosity of 4%-5%,and permeability less than 1×10-3μm2;Marcellus shale has thickness of 15-60m,porosity averagely of 10%,and permeability less than 1×10-3μm2;Haynesville shale has thickness of 70-100m,porosity of 8%-9%,and permeability less than 5×10-3μm2.Longmaxi shale has thickness of 25-120m,porosity of 3%-10%,and permeability of(0.01-1)×10-3μm2;Qiongzhuxi shale has thickness of 40-100m,porosity of 0.1%-3%,and permeability of(0.01-42)×10-3μm2.(5)Gas contents of Barnett,Marcellus and Haynesville shale are 4.2-9.9,1.70-2.83 and 2.5-9m3/t,respectively,and that of Longmaxi and Qiongzhusi shale are 1.7-4.5 and 0.55-1.2 m3/t,respectively.(6)Except Qiongzhuxi shale,the other four shale formations have good sedimentary cover,which is conducive to gas preservation.(7)American shales have more favorable surface conditions,mostly plains,while mostly hills in Sichuan Basin.(8)Except Haynesville shale,fractures are well developed in other shale formations.(9)Shale gases from both the USA and China have rich resource potential.The Geological and technically recoverable resources of Longmaxi Formation are 17.5×1012m3 and 1.77×1012m3,respectively while that of Qiongzhusi Formation are 8.86×1012m3 and 0.886×1012m3,respectively.
It is urgent to understand natural gas accumulation model from the accumulation process in order to provide guidance for further exploration of deep-layers with high temperature and overpressure in South China Sea.The accumulation process and accumulation model of overpressured low permeability gas pool of the 1st member of Huangliu Formation of Upper Miocene of XF13-1 in XF area,Yinggehai Basin,have been studied by applying natural gas geochemistry and natural gas accumulation process research method in light of well data and 3D seismic data.Two kinds of accumulation models were constructed for XF13-1 gas field: One model is characterized by the feature that gas charged firstly,then reservoir changed tight and overpressured.The Ⅰ gas unit of XF1-1-14 well was regarded as an example of this model.This model with good natural gas component,was thought of as the most favorable natural gas accumulation model of deep strata in Yinggehai Basin.Another model had the characteristic that gas charged,synchronized by reservoir changing tight and overpressured.The Ⅱ gas unit of XF13-1-4 well was an example.Two favorable areas with the first gas accumulation model were proposed.
The huge shale gas potential in the Paleozoic strata in South China region has attracted a widespread attention in recent years.This paper focused on the study of adsorption characteristics and controlling factors on the Paleozoic shale.A total 19 shale samples were obtained,their mineral compositions were analyzed,and their isothermal adsorption experiments were carried out.A thermal simulation experiment was conducted for a selected sample to obtain a suite of sub-samples with different maturities,their isothermal adsorption capacities were measured and the adsorption characteristics of shale with very high maturities were discussed.The result shows that the maximum adsorption capacity is between 0.21cm3/g and 5.63cm3/g for the Upper Permian shales and 1.66-3.32cm3/g for the Lower Cambrian-Lower Silurian shales.TOC content of shale is the main factor to control the adsorption capacity.The higher the content of TOC,the greater the adsorption capacity of the shale.The adsorption ability of clay minerals is weak for the studied shale and their contribution to the adsorption capacity is generally covered by the associated organic matter.Maturity is another key factor to influence the adsorption capacity of the shale.For samples with an EqRO or RO<3.6%,the adsorption capacity increased with increasing maturity,but when EqROor RO>3.6%,the adsorption capacity of the shale decreased with further increasing maturity.The present result is greatly significant to the evaluation of shale gas in the Paleozoic strata in South China region.
Burial history,tectonic-sedimentary history of Panxie mining area in Huaina area,thermal evolution history of Permian mud shale since Late Paleozoic are reconstructed by the vitrinite reflectance geothermometer and paleo-heat flow method.The geological significance of hydrocarbon generation from Permian mud shale is discussed.The results show that the Permian experienced the stable sedimentary stage in the Early Permian-late stage of Middle Jurassic,the rapid uplift from the early stage of Late Jurassic to Late Cretaceous,the re-deposit from the Early Paleocene to the Middle-Late Paleocene,the stable uplift from the Middle-Late Paleocene to the Late Neogene,and the slow deposit since Early Pleistocene in the study area.The geothermal evolution of basin is consistent with the course of the burial-sedimentary history,the paleo-heat flow experienced stable stage from Permian to Late-Jurassic,the drastic decrease from the end of Late Jurassic to the end of Paleogene and the subsequent smoothly stabilization.Permian source rocks enter the early to middle-late stage at the mature stage from the Middle-Late Permian to the Middle Jurassic,and the maturation ended in the end of Late Permian.The formation of shale gas includes four stages such as the generation of bacterial gas and thermogenic gas from Early Permian to Late Jurassic,the replenishment and the whole tectonic destruction of gas reservoir from Late Jurassic to Late Cretaceous,the generation of bacterial gas and thermogenic gas in some Paleogene strata,the second tectonic destruction of gas reservoir from Late Paleogene to Early Quaternary.The burial and uplift periods of Permian from different boreholes are consistent.There is a little difference in the sedimentary and uplift degree.The thermal evolution degree is caused by the subtle difference from the sedimentary degree and the uplift-erosion rate,but the hydrocarbon generation from different boreholes is generally consistent.
Longmaxi Formation shales in Chongqing and its neighbor area have high TOC values and are over-mature.Samples at different tectonic locations in the study area were selected,and their geochemistry,mineral components and reservoir character were investigated.The result suggests tectonic character is the main controlling factor on the shale reservoir space.Shales collected at Liangcun and Qiliao profiles that located at stable tectonic zone have higher porosity and 1.7-300nm pore volume compared with shales from active tectonic zones.Also,high brittle mineral and low chlorite contents are beneficial to shale reservoir space.The abundance of organic matter shows little relation to reservoir space.BET specific surface area and t-Plot micropore area show positive correlation with the abundance of organic matter,and show negative correlation with the content of chlorite,and the result suggests the surface area of shale is mostly composed of organic pores,and chlorite shows apparent block effect on reservoir space.The result also suggests tectonic impact shows little influence on shale surface area.
The Lower Paleozoic shale from the South China region has very high maturity and is lack of vitrinite.Although solid bitumen usually occurs in shale,its measuring reflectance value varies widely,thus there is a big challenge to determine the exact maturity of the shale,which puzzles the conventional and unconventional petroleum exploration conducted in this area for many years.In this study,the Raman spectroscopic characteristics of solid bitumen in shale samples taken from the Yangtze region with different maturities(BRO =0.43%-5.51%)were investigated.The results indicate that the Raman spectral parameters of the solid bitumen,including band position(WD and WG),band separation(RBS),full width at half maximum(FWHM-D and FWHM-G),and band intensity ratio (ID/IG) are all related to the bitumen reflectance,but with considerably different correlations,constrained apparently by maturity.Linear regressions were performed between these parameters and bitumen reflectance,and some parameters with higher correlation were selected.They are RBS(within 1.5%-3.5% of BROand ID/IG (with BRO>3.5%),showing a correlation coefficient as high as 0.95.It is believed that the two Raman spectral parameters of solid bitumen will be of significant practical use for the maturity assessment of the Lower Paleozoic shale when standard measurement and curve fitting procedures are utilized.
The pores in shales are mainly of nanometer-scale,and their pore size distribution is very important for the preservation and exploitation of shale gas.This study is focused on the pore structures of Upper Ordovician-Lower Silurian black shales sampled from well YC7 in southeastern Chongqing area.Nitrogen and carbon dioxide gas adsorption were conducted at 77.4K and 273.15K,respectively,and the pore structures were characterized by modified BET,BJH,DFT and Stoeckli methods.The results show that:(1)All the micropores,mesopores and macropores are well developed in both Lower Silurian and Upper Ordovician shales,and their relative proportion is almost identical;(2)The mean diameter of micropore for our samples is about 1.26nm;(3)The micropore size distribution curves from the Stoeckli method using CO2 adsorption at 273.15K can be well correlated with that calculated by DFT method using nitrogen adsorption at 77.4K.This indicates that a continuous pore size distribution could be achieved for micropores,mesopores and part of macropores in shales by combing together the N2 and CO2adsorption data.
Shale with a maturity within “oil window” contains a certain amount of residual soluble organic matter.This soluble organic matter will have an important influence on the characterization of shale reservoir.In this study,two shale samples were collected from the Upper Permian Dalong Formation in the northwestern margin of Sichuan Basin.Their geochemistry,mineral composition,pore structure(surface area and pore volume)were investigated before and after removing soluble organic matter(SOM)by the extraction with dichloromethane or trichloromethane.The results show that the TOC,S1,S2,and IH of the extracted samples decrease obviously,but the mineral composition has no significant change as compared with their raw samples,thus the original pore structure is thought to be unaltered from the extraction.SOM occupies pore volume and hinders pore connectivity,and the extraction greatly increases the surface area and pore volume of the samples.The residual SOM in the shale samples occur mainly in the micropore and small mesopore,and their occupied pore size range seems being constrained by the maturity.For the shale samples with lower maturity,SOM is mainly hosted in organic pores with diameter less than 5nm.For the shale samples with moderate maturity,micropores and some mesopores ranging between 2nm and 20nm are the main storage space for the SOM.
X-ray diffraction is a basic method for determination of shale mineral composition， quantitative accuracy by diffraction by mineral composition,particle size,the diffraction peaks overlap,and other factors.Choose well QQ1# drilling wells hole samples in Sichuan Basin shale mineral components determined,by comparison with two kinds of experimental results,find out the methods of improving the precision for quantitative shale mineral components.Method 1: Whole rock direct determination and quantitative XRD;Method 2: The phase after dissolving stokes gravity separation of different graded mineral XRD measurement and quantitative respectively.Results showed that through two experimental methods have a greater difference between the percentage of carbonate minerals,directly reflects the XRD quantitative methods in the content of mineral component is not fully trusted.The phase after dissolving and gravity separation XRD test can largely improve the test precision and shale in the mineral content of different granularity of shale diagenesis and shale properties than the single mineral content has more practical significance.
The organic carbon content,mineral compositions,pore structures and sorption capacity of core samples from the Lower Cambrian Niutitang Formation and Palang Formation in northern Guizhou Province were analyzed to obtain the pore structure characteristics,pore structure controlling factors and the impact of organic matter on methane sorption capacity.The results show that the study section is a tight reservoir with characteristics of low porosity and permeability.The samples have a BET surface area ranging from 5.64m2/g to 28.29m2/g and the NLDFT micropore volume and mesopore volume in the range of 0.02-0.54cm3/100g and 0.53-3.38cm3/100g,respectively.The positive relationships between porosity,specific surface area,micropore volume and TOC content indicate that organic matter controls the pore structures.However,shales with high TOC content may have certain limitation on the development of organic matter pores.In addition,clays also contribute a portion of total porosity.The methane excess sorption at 12MPa(nex12MPa)and Langmuir sorption capacity(nL)of the Lower Cambrian shales range from 0.30cm3/g to 3.71cm3/g rock and 0.41cm3/g to 4.22cm3/g rock,respectively.The methane sorption capacity shows a positive correlation with TOC but a decrease in high TOC samples because of low micropore volume and specific surface area.
Pore size distribution for 23 fresh outcrop shale samples collected from Shuanghe Town,Changning County and 14 core samples collected from the well Qianqian 1 in southeast Chongqing,Sichuan Basin were investigated by low pressure nitrogen adsorption.The main factors controlling pore development and gas accumulation in shales were discussed by integrating total organic carbon(TOC),mineralogy and shale gas content.The results show that open slit-like and parallel plate structure are major pore types with an average pore diameter of 3.76-8.53nm， and 2-30nm mesopores are dominated.The BET surface area and total pore volume are high in the Wufeng Formation and lower part of the Longmaxi Formation,and the decrease in the upper part of the Longmaxi Formation,in consistent with the trends of TOC,suggesting that organic matter is the key controlling factor on the shale pore development.In addition,samples with higher content of clay minerals but similar TOC content have larger specific surface area,and the clay-mineral-hosted pores are present.The Wufeng Formation and lower part of the Longmaxi Formation in the Sichuan Basin are preferred layers of shale reservoir fracturing because of high TOC,high rock brittleness and high gas content.
To understand resource potential of tight oil in small fault-subsided lake basins of China,this paper mainly takes the Lower Xinhe Formation of the Middle Jurassic in Northwest China as an example.According to the data of geochemistry，reservoir physical property，thin section,electron microscope scanning，energy spectrum,etc.,this study systematically investigated geological characteristics of tight oil in the Lower Xinhe Formation of the basin.The result indicated that(1)Depocenter and subsidence center are not overlapped in the source rocks of the Lower Xinhe Formation.The high abundance of source rock(TOC>1%)in the Lower Xinhe Formation is mainly distributed in the middle and eastern sag of the basin.While,mature source rocks are mainly distributed in the middle and western sag of the basin.(2)The mineral composition of sandstones in the Lower Xinhe Formation are predominantly quartz,feldspar,carbonate rocks and clay minerals.Quartz and feldspar contents are relatively high,with an average of 60%.The reservoirs are dense and poriferous,permeability is generally less than 10% and 1×10-3μm2,but oil saturation distribution is at 10%-60%.(3)The oil-bearing tight reservoirs are located in the middle and bottom of Lower Xinhe Formation,and the plane is mainly distributed in Xiaohuzi Sag.Four types of pores are presented in the tight sandstones of the Lower Xinhe Formation,such as interparticle pores,intraparticle pores,fracture pores and organic-matter pores.The first three are more common in minerals.(4)In the Lower Xinhe Formation,oil reservoir has high pressure coefficient,while the density of crude oil is not heavy.The oil and gas has four ways of occurrence manners in the tight reservoirs,such as interparticle pores,dissolved pores,intercrystalline pores and micro-fracture.The Yabulai Basin has two tight oil exploration targets,the slope of Yanchang Sag and deep depression zones of Xiaohuzi Sag.Industrial oil flow was obtained in deep depression zones of Xiaohuzi Sag,suggesting a great resource potential and favorable targets for future exploration.
In the process of gas reservoir development,a composite reservoir may be formed due to acidification,formation pollution or reservoir geological characteristics.An unsteady flow mathematical model is established for horizontal well passing through the inner region of the composite gas reservoir,and the rate transient analysis(RTA)is also conducted.In order to solve the problem,Laplace transform,Muskat method,the superposition of point source and the principle of Duhamal are used to get the transient pseudopressure of horizontal well.On the basis of the transient pseudopressure,the transient rate of horizontal well in real space is obtained by the Stehfest numerical inversion method.The dimensionless rate curves are plotted,and the effects of the dimensionless inner area radius,skin factor,the mobility ratio,the storativity ratio on the dimensionless rate of horizontal well passing through the inner region are analyzed.
The fracture developed sections are the dominant storage spaces for the buried hill inside reservoirs.In the exploration process of Xinglongtai buried hill inside reservoir in the Liaohe Oilfield,great attention has been paid in order to find out the correlation between the well bore information response characteristics and various information in the fracture developed section.It has been found out that in the fracture developed section,the drilling fluid tends to lose during the drilling process,the logging presentation gets higher,gas logging data gets unusual,the geochemical logging and light hydrocarbon logging data show that the oil saturation gets higher,logging information has obvious characteristics,and various data are well correlated.So the reliability of the recognition and prediction is greatly improved in fractured developed sections.It has built a reliable geological basis for perforation interval optimization,the rational classification of effective thickness in reserve calculation and the establishment of dimensional model of development in buried hill inside reservoir.The method has effectively guided the exploration and development practices of buried hill inside reservoirs in the Liaohe Oilfield.By using this method,we have defined OOIP of hundreds million tons in the Xinglongtai Buired Hill,and an annual million ton production capacity has been established efficiently,so the effective development of the reservoir is achieved.
Adopting potential theory,the superposition principle and the related principle of fluid mechanics,a steady mathematical model for fractured horizontal well is established considering interactive disturbance among fractures,pollution skin,non-uniform distribution of fracture,finite conductivity flow in fracture and wellbore,and fracture-wellbore convergent flows,non-Darcy flow in fracture.Then the numerical solution to the model is derived.The model is applied to predict an actual productivity of horizontal wells and to analyze the influence factors of productivity.Result shows that the model can be used for horizontal well productivity prediction with various complicated situations and it shows high accuracy.Due to the fracture′s mutual interference,the production rate of fracture differs.Fractures on both ends of the horizontal wellbore have high yield,while fractures in middle have low yield.Horizontal well productivity increases with the growth of horizontal interval length,fracture half-length,and fracture conductivity.Pollution skin of fracture has significant effects on productivity.The increase of it leads to a sharp decline of productivity.Thus it is important to reduce the damage of fracturing operation on formation as much as possible.Within relatively reasonable fracture spacing scope,the effect of fracture distribution pattern on productivity is not obvious.Since wellbore radius affects wellbore pressure drop,its design should be in accordance with the productivity of the horizontal wells.
The viewpoints on the source rock of the NGH(natural gas hydrate)are controversial because of the limited researches on hydrocarbon potential of the source rocks in Muli area of Qilian Mountain.The coal samples from the main coal formation(J1-2y,T3g and C2y)were collected.Based on a comprehensive evaluation on the hydrocarbon potential of coal rocks,the characteristics of biomarkers in saturates and aromatics were studied.Coals from 3 formations in the Muli Area all belong to humic type.The coal samples in J1-2y belong to poor source rocks(The coal samples in Dongku coal mine belong to non-hydrocarbon source rocks),which has some hydrocarbon potential.However,the samples from T3g and C2y are not source rocks.The coal of J1-2y in Jiangcang and Hushan coal mines can even generate small amount of coal-derived oil.The coal in J1-2y from Juhugeng and Dongku coal mines belong to type A,and the coal-forming environment is weakly oxidizing marsh which can provide a large number of terrestrial organic matters.The coal samples in J1-2y from Hushan and Jiangcang coal mines and the coal samples in T3g from Shaliuhe profile belong to type B,the sedimentary environment is reductive shore-lacustrine covered by certain depth of water which can provide a large number of low hydrobiont organic matters.Both Type A and Type B are found in C2y of Kekeli village profile.
Eleven Ordovician oil samples from Halahatang Depression were investigated by 2-D gas chromatography(GC×GC)and conventional gas chromatography to contrast their C5-C7 light hydrocarbons compositions.The results indicate that Halahatang oils are typical mature-high mature marine oils,deposited in a brackish-salty reducing environment with similar source input.The C6 light hydrocarbon and Mango parameters obtained from the conventional gas chromatography correlate well with those from the GC×GC,however,the C7 light hydrocarbon from the GC×GC is characterized by lower heptane and isoheptane values,decrease of paraffin and increase of cycloalkane abundance.Due to no need for pretreatment,oil direct injection for the GC×GC can provide more real information associated with its higher separation.
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