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Total organic carbon (TOC) content of shale gas reservoir is an important parameter of shale gas assessment, and how to accurately determine the content of TOC is a key problem of shale gas exploration and development. The author used the Lower Silurian Longmaxi formation in Qianjiang area as the research object. Through the statistical analysis of TOC content testing of shale gas reservoir and drilling and logging data, the log response characteristics of TOC content were analyzed. Furthermore, the four logs which consist of volume density logging(DEN), gamma logging (GR) spontaneous potential logging(SP) and acoustic logging(AC) were selected optimally as the feature vector. Afterwards, the BP neural network prediction model of TOC content was established, the BP neural network algorithm was improved and the TOC content of Lower Silurian Longmaxi formation of two shale gas wells in Qianjiang area were predicted and compared. The results show that the BP neural network model based on logging parameters has strongly approximate nonlinearization, which can reflect the nonlinear relationship between the TOC content of shale gas reservoir and logging parameters. The error between prediction results and measured values is small,and the relative error is less than 10%. [ABSTRACT FROM AUTHOR]

To investigate the influence of drainage strategy on gas production with the coal-bed methane (CBM) reservoir in Southern Qinshui Basin, China as example, based on molecular dynamics and rock mechanics theory, the mechanisms of methane, diffusion and seepage in high rank coal were systematically investigated. Based on a constant permeability soft, a stress-dependent permeability model and a S-D permeability model considering the varying cleat compressibility, respectively, the influence of early drainage rate on gas production potential in high rank coal seams of different coal textures was investigated using a CBM numerical simulation soft called Simed. It demonstrates that the transportation of methane in coal seam involves desorption, diffusion, seepage in natural fractures and hydraulic fractures and the gas production will be sufficient only when all these four steps work coordinately. The diffusivity will increase and the conductivity will decrease due to the increase of effective stress and plugging in hydraulic fracture by pulverized coal during CBM depletion. Coal seam permeability is one of key factors influencing the gas production potential in the study area. High permeability will result in high gas production. Tectonically deformed coals are sensitive to early drainage rate but not sensitive to the fracture conductivity above a certain value. Intact and blocky coal seams are not very sensitive to early drainage rate but sensitive to fracture conductivity. A low rate of early drainage suits low permeability coal seam. A high drainage rate can be used to extract the coal and water continually from a high permeability coal seam during CBM depletion. [ABSTRACT FROM AUTHOR]

At Chengzhuang mine field in Jincheng mining area, the fracture pressure of coal floor rock, in situ stress, the hydraulic pressure of aquifer in the coal floor and the aquifuge thickness were analyzed after coal-bed methane ( CBM) development. The theory and method for evaluating water inrush risk in coal floor after CBM development were developed. The influence mechanism of CBM vertical wells development on coal floor water inrush during coal mining was obtained. The results show that the completion depth of CBM wells, the underground pressure in coal mining and the water pressure make the aquifuge in the coal floor break through rupture. If the minimum horizontal stress in aquifuge is greater than the hydrostatic pressure of confined water, the water inrush would not occur, otherwise, the water inrush would occur. If the completion depth of CBM wells and the underground pressure in mining and the water pressure of confined water do not make coal floor aquifuge break through rupture, the ratio of water pressure of the aquifer in the coal floor to effective aquifuge thickness between coal seam and the aquifer determines the water inrush risk of the coal floor. According to key parameters of the fracture pressure of coal floor rock, water pressure and the ratio of water pressure to aquifuge thickness, the water inrush risk in the coal floor is divided into four types; safety (I) medium safety (II), poor safety or danger (III) and extremely dangerous (IV). The distance between No. 15 coal seam of Taiyuan formation and the Ordovician limestone aquifer is small, and the variation is large. The coal mining after CBM vertical wells development is threatened by the confined water in Ordovician limestone. Water inrush from No. 3 coal floor does not occur in coal mining after CBM development. If the completion depth of CBM wells for No. 9 coal seam and the coal floor damage depth caused by the coal mining are same according to the calculation under coal floor 15 m, the water inrush risk of coal floor is mainly the type of medium safety, however, there is a water inrush risk in the deep area. [ABSTRACT FROM AUTHOR]

Coalbed methane carbon isotope is an effective parameter to reflect the coal-bed gas origin and occurrence conditions. Through the analysis of the measured data of coalbed methane carbon isotope (δ13C1) in the Qinnan-East block of Qinshui Basin and its coal reservoir gas-bearing properties, the distribution property of methane carbon isotope (δ13C1) in No. 3 coal seam was analyzed, then the correlations and their models among the coalbed methane carbon isotope (δ13C1) and vitrinite reflectance, burial depth and gas-bearing properties of coal reservoir were established respectively, and its controlling mechanism of coalbed methane carbon isotope distribution was finally revealed. The research results show that the δ13C1, value of naturally desorbed gas of No. 3 coal seam is from -28.89‰ to -53.27‰ and the average value is -36.48‰ in this area. The δ13C1 value is heavier compared with the same evolution degree coalbed methane in other parts of China and shows that the coal seam has a good preservation condition. The δ13C1 value of No. 3 coal seam and vitrinite reflectance, burial depth have a non-linear Logarithmic function relationships, and increases with the increasing of the vitrinite reflectance and burial depth of coal seam, which is suited to the national statistical regularity. Coalbed methane carbon isotope in this area is mainly controlled by the isotopic differentiation effect under the thermodynamical mechanism and the fractionation effect in the process of the coalbed methane desorption-diffusion-migration. Coalbed methane carbon isotope (δ13C1) also has a good correlation with gas-bearing properties of coal reservoir. The δ13C1 value of No. 3 coal seam becomes heavier with increasing of the coalbed methane content, coal reservoir pressure and gas saturation, which best fits the logarithmic relation. And the control factors of gas-bearing properties in coal reservoir are consistent with that of its methane carbon isotope (δ13C1). [ABSTRACT FROM AUTHOR]

The stress sensitivity of the coal reservoir is one of the geological factors affecting the productivity of CBM wells, how to reduce or avoid effect of the stress sensitivity on CBM well production is a question worth considering in CBM wells production process. Through the analysis of the stress sensitivity of coal reservoir, the productivity model of CBM wells considering the permeability stress sensitivity was deduced, then the yield reduced the magnitude of value (β) was put forward to describe the influence degree of the stress sensitivity on the productivity of CBM wells, and the impact of the effective stress on the permeability of coal reservoir and the effect law of CBM wells productivity were finally revealed. Research results show that the permeability of coal reservoir reduces with the effective pressure increases by the negative exponential law and coal reservoir shows obvious stress sensitivity during the development of CBM wells. The productivity of CBM wells which consider the stress sensitivity of the coal reservoir is lower than that don't. With the producing differential pressure increases, CBM wells productivity increases significantly, and gradually tend to be stable, and the yield reduced the magnitude of value(β) increases meanwhile. The value(β) should be the overall increased with increasing the stress sensitive coefficient of coal reservoir. The production of CBM well increases to a lesser extent with the pressure difference increases and gradually tend to be stable. Enlarging the production pressure difference can not get the maximum yield. Therefore CBM development need to be draw up a reasonable production pressure difference and strictly control the production intensity. [ABSTRACT FROM AUTHOR]

Based on the basic conceptions of coalbed methane (CBM)/shale gas, the geological conditions of coalbed methane/shale gas exploitation are systematically analyzed, which mainly includes the following three aspects: the geological conditions of reservoir formation, the conditions of occurrence environment and engineering mechanics conditions of exploitation. Furthermore, the geological conditions for extracting coalbed methane and shale gas are comparatively analyzed. The similarities and differences of the geological conditions between coalbed methane and shale gas are also revealed. Coalbed methane/shale gas is a kind of unconventional natural gas which is self-generated and self-stored in coal seam/shale. Their enrichment accumulations depend primarily on the existence and the quality of basic geological conditions of source-reservoir-preservation as well as their mutual cooperative relations. All coalbed methane/shale gas within a certain burial depth have undergone desorption-diffusion-migration process, and the phenomenon of vertical zoning exists universally. The higher evolution degree of organic matters, the smaller the depth of desorption zone, while the deeper of weathered zone, the greater depth of desorption zone. The enrichment of the coalbed methane/shale gas in the desorption zone to a certain extent complies with the rules of structure controlling gas accumulation in the conventional natural gas accumulation ; whereas the enrichment of the coalbed methane/shale gas in the primary zone is more controlled by the characteristics of coal seam/shale's adsorption. The coal/shale reservoir varies from occurrence environment conditions, on which adsorbed gas and free gas in coal/shale reservoir transforms into each other, which results in the differences of the coalbed methane/shale gas in reservoir type, scale, quality and other aspects. The main geological factors affecting the CBM development are coal seam thickness and its stability, gas content or resource abundance of CBM, structure and fracture, permeability and preservation conditions of CBM, etc. The main geological factors affecting the development of shale gas include shale thickness, organic matter content, thermal maturity, gas content, nature fracture and brittle mineral content, etc. [ABSTRACT FROM AUTHOR]

Adopted hydraulic fracturing method to measure in-situ stress, obtained in-situ stress of 26 coalbed methane wells in Southeast margin of Ordos Basin. Through statistical analysis, the correlations and their models between in-situ stress of No. 2 coal seam in Shanxi Formation of Permian and the burial depth of coal seam were established, the distribution law of present stress and its controlled mechanism were revealed. The results show that the average value of the fracture pressure gradients, closure pressure gradients and coalbed reservoir pressure gradients of the No. 2 coal seam in Shanxi Formation of Permian in study area are 1.96, 1.69, 0.71 MPa/100 m, respectively. Coalbed reservoir stress (the maximum horizontal principal stress, the minimum horizontal principal stress, the vertical principal stress ) and the coalbed reservoir pressure both increases linearly with the increasing of the burial depth. The coalbed reservoirs with a burial depth lower than 1 000 m, in-situ stress state principally shows σv > σ >hmax > σhmin, the minimum horizontal principal stress is below 16 MPa, the recent in-situ stress state is tension; the coal reservoir effective stress coefficient equals to 0. 48, lower than the parameter of shales in oil gas basin, which is 0. 80 generally. In-situ stress state of coalbed reservoirs with a burial depth deeper than 1 000 m transforms into σhmax ≥ σv ≥ σhmin, the minimum horizontal principal stress is bigger than 16 MPa,the in-situ stress state transforms into compression. The present in-situ stress is controlled by the regional tectonic stress field of North China, the maximum horizontal principal stress direction is mainly in the NEE-SWW direction. The coal reservoir pressure in the study area is lower, coal reservoir pressure under the conditions of the same depth in southeast margin of Erdos basin is lower 0. 73 -0. 93 MPa than southern Qinshui basin. Coal reservoir stress has positive correlation to the recent stress. As stress increases, the coal reservoir stress increases. [ABSTRACT FROM AUTHOR]

By the mechanical tests of the coal samples, the physical and mechanical properties of coal and the law of permeability during the complete stress-strain process were studied. It is shown that, compared with the rocks from coal roof and floor, the coal is more prone to plastic deformation with the property of lower mechanical strength, lower elastic modulus and higher Poisson' s ratio. For the coal samples which have obvious strain-softening character during the complete stress-strain process, first, the coal sample volume is compressed and the permeability of coal with stress increases slightly or permeability changes little in the micro-cracks closure and elastic deformation stage. After the stress increases higher than the elastic limit, the coal sample gets into the crack propagation stage. The volume strain of coal changs from compression to expansion. The permeability of coal first slowly and then sharply increases with an increasing of crack extension. During the strain-softening stage, the coal permeability achieves the maximum and follows by a sharp decrease. The coal permeability after the peak strength is generally greater than that before the peak. For coal samples which have little strain-softening character or strain-hardening during the complete stress-strain process, the maximum permeability is mainly achieved in the plastic deformation stage before the peak. During the strain-hardening stage after the peak strength, the permeability decreases with an increasing of stress. Commonly, the coal permeability after the peak strength is generally less than that before the peak. [ABSTRACT FROM AUTHOR]

Adopted samples from the high-rank coal reservoirs in Southeast margin of Ordos Basin, through the stress sensitivity experiment of coal samples, stress sensitivity of coal reservoirs and the effects of effective confining pressure, fractures inside the coal and water condition on stress sensitivity were analyzed. The results show that, the permeability decreases according to negative exponential function with the increase of effective stress. When effective stress changes from 2.5 MPa to 10 MPa, dimensionless permeability of the coal samples is 0. 10 ~ 0. 28 ,and the average value is less than 0. 15; the damage rates of permeability are 71.92% ~ 90. 14%, and the average value is 84.59%. When effective stress is less than 5 MPa, the coal reservoir permeability decreases rapidly with the increase of the effective stress, the stress sensitivity is the highest; when effective stress is between 5 MPa and 10 MPa, the stress sensitivity is medium; when effective stress bigger than 10 MPa, the decline of the permeability slow down, and the stress sensitivity is weak. The initial permeability of coal samples with fractures inside is higher, and the stress sensitivity is relatively small; the irreversible plastic deformation generated larger in the lifting-pressure process results in relatively high irreversible damage rates after the pressure decreas. The permeability of wet coal samples reduces rapidly with the increase of effective pressure, with the increase of moisture content, the stress sensitivity is more obvious. [ABSTRACT FROM AUTHOR]