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Tao S.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,Coal Reservoir Laboratory of National Engineering Center | Xu H.,Coal Reservoir Laboratory of National Engineering Center | Li S.,Coal Reservoir Laboratory of National Engineering Center | And 7 more authors.
Journal of Petroleum Science and Engineering | Year: 2017

The fluid velocity sensitivity (FVS) effect in coal reservoirs affects the production of coalbed methane (CBM) wells by generation of coal fines. In this work, coal samples from Baode Block, northeastern Ordos Basin were made into coal cores for FVS analysis under different initial permeabilities, injection volumes and injection intensities. Standard brine with a mineralization degree of 8% (the mass ratio of NaCl: CaCl2: MgCl2·6H2O=7.0:0.6:0.4) was chosen as the driving fluid. Also, the production responses of CBM wells to FVS at different stages of drainage were discussed. The results show that there is a critical flow velocity for the generation of coal fines. Below this critical flow velocity, coal fines can discharge with fluid, which increases the reservoir permeability. Over this critical flow velocity, FVS tends to occur in coal reservoirs, which blocks the effective seepage paths and reduces reservoir permeability. Additionally, the larger the flow velocity and injection volume are, the greater the permeability damage rate is. But the increasing rate of permeability damage slows down, revealing that the damage of FVS to permeability mainly occurs in the initial period of coal fines migration. Moreover, abrupt increase of flow velocity can damage reservoirs and give rise to massive coal fines generation. During the drainage process of CBM wells, the FVS effect is mainly affected by the dropping rate of working fluid level at the stage of single-phase water. There exists a period of massive coal fines generation when the working fluid level first drops near the coal seam. In the stage of two-phase gas and water, the deposition rate of coal fines is faster and the migration distance is shorter than that in the stage of single-phase water. Due to the difficulty of coal fines discharge, the FVS effect tends to occur in this stage. In the stage of single-phase gas, the gas flow can make coal fines agitate partially. But the FVS effect is relatively weak as coal matrix shrinkage can improve the reservoir permeability to a large degree. © 2017 Elsevier B.V.

Fu H.,China University of Geosciences | Fu H.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,China University of Geosciences | Tang D.,Coal Reservoir Laboratory of National Engineering Center | And 11 more authors.
Marine and Petroleum Geology | Year: 2017

To date, prospecting work on low-rank coalbed methane (CBM) resources in the middle of the southern Junggar Basin is still in the primary stage, and only a few CBM exploration wells or pilot wells have been deployed in local regions. Systemic understanding of CBM reservoir-forming conditions and geological controlling factors is lacking in the study area, resulting in the mismatch between CBM well deployment and actual geological conditions, as well as poor exploration efficiency. In this paper, the geological controlling effects of the structure, sedimentation, and hydrogeology on CBM enrichment are systematically discussed for the first time, based on the early CBM exploration achievements. The results show that the Xishanyao coal and the Badaowan coal are developed in the upper and lower part of the neutral surface of a fold, respectively. The reservoir-forming conditions of the Badaowan coal are not discussed in this paper due to its poor development. The Xishanyao coal that developed in the axial part of the syncline is most beneficial to CBM enrichment with concentrated extrusion stress and great methane adsorption capacity, while the axial part of the anticline is not favorable for CBM preservation with large tensional stress. The gas content of the Xishanyao thick seams developed in the syncline is higher (average of 4.63–6.34 m3/t) than that in the monocline (average of 2.84–4.56 m3/t). Reverse faulting is more beneficial to CBM enrichment than normal faulting, due to the better sealing capability. The gas content of the Xishanyao coal is obviously influenced by the coal thickness and its roof lithology. The hydrodynamic conditions and total dissolved solids (TDS) values of coalbed water range greatly on regional scale, which leads to a deeper methane weathering zone in the middle-west areas (>1119.62 m) than the eastern Liu-huanggou areas (<501.71 m) and have an important influence on exploration target optimization of CBM exploration wells. Combined with the geological characteristics of the structure, sedimentation and hydrogeology, three CBM enrichment models are proposed in this paper (i.e., broad fold model, northward monocline model and overlying composite model). The reservoir-forming processes and development positions of these CBM enrichment models are discussed systematically to provide a scientific basis for selecting CBM exploration target zones. © 2017 Elsevier Ltd

Lv Y.,China University of Geosciences | Lv Y.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,China University of Geosciences | Tang D.,Coal Reservoir Laboratory of National Engineering Center | And 4 more authors.
International Journal of Coal Geology | Year: 2012

The temporal and spatial production characteristics of coalbed methane (CBM) wells in the Fanzhuang Block of the Southern Qinshui Basin, the first commercial CBM-producing basin in China, were studied to determine the dominant factors. The study indicates that gas production differs significantly in different wells adjacent to or located in different tectonic areas. In combination with the production characteristics in the temporal and spatial variations, the impact of seven factors (burial depth, thickness of coal, gas content, porosity/permeability, effect of fracturing, structural setting, and hydrogeological conditions) on the gas production was analyzed based on bivariate correlation analysis and gray system theory, which are suitable for solving the complex interrelationships between multiple factors and variables. The results indicate that hydraulic fracturing is an effective guarantee of high-production CBM wells in the first several months during a high-rank CBM field development. However, the effect of hydraulic fracturing on enhancing the gas production of CBM wells decreases with time because the initial reservoir permeability, lower than the hydraulic fracture permeability, limits the adsorption, diffusion and flow of CBM. This limitation inevitably leads to a slow gas production rate when the adsorbed CBM from the initial reservoir becomes the main gas source instead of the fracturing-affected zone. Gas content and permeability are two key factors that dominate CBM well productivity, because they perform elementary roles in controlling the volume of gas sources and conductivity, respectively. The structural setting and hydrogeological condition are also two important factors that dominate CBM well productivity because they can influence the spatial distribution of permeability and gas content as well as other factors (such as aquifers). © 2012 Elsevier B.V.

Fu H.,China University of Geosciences | Fu H.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,China University of Geosciences | Tang D.,Coal Reservoir Laboratory of National Engineering Center | And 10 more authors.
Journal of Natural Gas Science and Engineering | Year: 2016

Junggar basin is one of the most typical low-rank coal-bearing basins in china. The Xishanyao coal developed in the southern Junggar basin contains plenty of coalbed methane (CBM) resources, with suitable burial depth and great coal thickness. To date, the researches about CBM geological characteristics, controlling factors and exploration potential are still lacking in the western of Urumchi River, which severely obstacle for further development of low-rank CBM. Firstly, CBM reservoir-forming conditions of Xishanyao coal in the middle of the southern Junggar basin are investigated in this paper. The results show that total coal thickness varies greatly from 18.58 to 66.34 m with some thick seams (>5 m). Coal rank ranges from 0.50 to 0.88%, with an average value of 0.65%. Coal macerals are dominated by vitrinite (45.6-66.4%, avg. 55.6), followed by inertinite (29.7-55.7%, avg. 42.8) and liptinite (0.5-4.7%, avg. 1.7). Coal reservoir property is relatively poor, with permeability ranging from 0.002 to 1.220mD (avg. 0.193) and porosity changing from 0.56 to 9.73% (avg. 5.99). Controlled by coal rank and coal macerals, pore type is dominated by micropore, followed by mesopore-macropore and little microfracture. Micro-fractures are mostly developed in bright coal and semi-bright coal. Methane adsorption isothermal measurement reveals that the maximum adsorption capacity (VL) varies from 6.73 to 14.14 m3/t. The in-place gas content ranges from 0.29 to 7.72 m3/t (avg. 5.1) and increases with the increasing of burial depth. Secondly, geological controls about CBM accumulation are analyzed in this paper, i.e., structure, sedimentation and hydrogeology play "the most important", "the most basic" and "a supporting" role in the process of CBM accumulation, respectively. Finally, analytical hierarchy (AHP) fuzzy prediction method and optimal segmentation method are used to carve out the grade of exploration potential, and the best prospective target areas for low-rank CBM production are forecasted. © 2016 Elsevier B.V.

Fu H.,China University of Geosciences | Fu H.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,China University of Geosciences | Tang D.,Coal Reservoir Laboratory of National Engineering Center | And 9 more authors.
Energy and Fuels | Year: 2016

The coalbed methane (CBM) resources are abundant in low-rank coal (Ro < 0.70%), which has raised widespread concerns within domestic geologists. In brief, low-rank CBM resources are still at an early stage of exploration in China, and the study on adsorption mechanisms and control factors of low-rank CBM is not systematic. As an important geological event of dividing brown coal and bituminous coal, "first coalification jump" has an obvious control action to the coal composition and pore structure of coal reservoirs and the occurrence and migration modes of methane gas, and the targeted research remains lacking. In this paper, the control function of coal rank to reservoir properties was analyzed first and the role of "first coalification jump" was emphasized in the discussion. In addition, in combination with methane adsorption isotherm experiments under equilibrium moisture (30 °C), internal factors (e.g., coal rank, specific surface, macerals, volatiles, and moisture) that control methane adsorbability of the coal were discussed systematically. Studies have shown that methane adsorbability of low-rank coal was mainly affected by adsorption space along with moisture sharp decreases but not Brunauer-Emmett-Teller (BET) specific surface increases under the dominant control function of coal rank. © 2016 American Chemical Society.

Meng Y.,China University of Geosciences | Meng Y.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,China University of Geosciences | Tang D.,Coal Reservoir Laboratory of National Engineering Center | And 6 more authors.
Environmental Science and Pollution Research | Year: 2014

As one of the unconventional natural gas family members, coalbed methane (CBM) receives great attention throughout the world. The major associated problem of CBM production is the management of produced water. In the USA, Canada, and Australia, much research has been done on the effects and management of coalbed methane produced water (CMPW). However, in China, the environmental effects of CMPW were overlooked. The quantity and the quality of CMPW both vary enormously between coal basins or stratigraphic units in China. The unit produced water volume of CBM wells in China ranges from 10 to 271,280 L/well/day, and the concentration of total dissolved solids (TDS) ranges from 691 to 93,898 mg/L. Most pH values of CMPW are more than 7.0, showing the alkaline feature, and the Na-HCO3 and Na-HCO3-Cl are typical types of CMPW in China. Treatment and utilization of CMPW in China lag far behind the USA and Australia, and CMPW is mainly managed by surface impoundments and evaporation. Currently, the core environmental issues associated with CMPW in China are that the potential environmental problems of CMPW have not been given enough attention, and relevant regulations as well as environmental impact assessment (EIA) guidelines for CMPW are still lacking. Other potential issues in China includes (1) water quality monitoring issues for CMPW with special components in special areas, (2) groundwater level decline issues associated with the dewatering process, and (3) potential environmental issues of groundwater pollution associated with hydraulic fracturing. © 2014 Springer-Verlag Berlin Heidelberg.

YanJun M.,China University of Geosciences | YanJun M.,Coal Reservoir Laboratory of National Engineering Center | DaZhen T.,China University of Geosciences | DaZhen T.,Coal Reservoir Laboratory of National Engineering Center | And 5 more authors.
Arabian Journal of Geosciences | Year: 2013

An analytical theory of stages division of coalbed methane (CBM) desorption is proposed and established based on the Langmuir adsorption isothermal model. The concept of desorbed efficiency (η) is proposed to quantitatively characterize the CBM desorbed rate. According to key nodes on mathematical curves, the starting pressure (Pst), turning pressure (Ptu), and sensitive pressure (Pse) are defined. The Langmuir desorption isotherm is divided into four stages, i.e., inefficient, slow, fast, and sensitive desorbed stages. Then, the theory is tested by methane adsorption-desorption isothermal experiments. It is noted that inefficient and slow desorbed stages have little contribution to CBM productivity, while fast and sensitive desorbed stages are very significant to CBM productivity. The adsorbed capacity and the reservoir pressure are two key factors for CBM desorption. Higher adsorbed capacity and relatively shallower burial depths are more favorable to CBM development. © 2013 Saudi Society for Geosciences.

Meng Y.,China University of Geosciences | Meng Y.,Coal Reservoir Laboratory of National Engineering Center | Tang D.,China University of Geosciences | Tang D.,Coal Reservoir Laboratory of National Engineering Center | And 6 more authors.
Journal of Natural Gas Science and Engineering | Year: 2014

The Liulin area in the eastern Ordos Basin is one of the optimum areas for medium-rank coalbed methane (CBM) exploration and development in China. This work investigated the CBM geology and accumulation characteristics of No.4 coal in the Permian Shanxi Formation in the Liulin area based on data from 29 coal mining wells, 25 CBM wells and 14 coal samples. The results show that coal rank (medium volatile bituminous and low volatile bituminous) varies laterally, with the maximum vitrinite reflectance (Ro, max) ranges from 1.23 to 1.90%. Coals are dominated by vitrinite (47.9-90.5%); followed by inertinite (7.7-50.5%) and liptinite (0-0.6%). Minerals account for only a small proportion (0.4%-16.5%, avg. 4.7%). The results of well tests show that the coal permeability ranges from 0.02 to 3.44mD, and the reservoir pressure ranges from 2.58 to 9.26MPa. Most coal pores are less than 100nm in diameter, and are favorable for gas adsorption but unfavorable for gas flow. Microfractures of coals are characterized by dendritic and step-shaped structures, relatively good connectivity, directionality and infrequent mineralization. The dominated types of microfractures are type C (less than 5μm wide and more than 300μm long) and type D (less than 5μm wide and 300μm long), with a large span of frequency ranging from 18 to 78 per 9cm2. Methane adsorption isothermal measurements reveal that their maximum adsorption capacity (dry ash-free) vary from 21.36 to 26.38m3/t. The in-place gas content is generally 4.93-14.96m3/t. In combination with the geological information, the data reveals that structure properties, coal roof lithology and hydrogeology conditions have important influences on gas accumulation, preservation and enrichment. Furthermore, integrated geographical information system (GIS) and analytical hierarchy fuzzy prediction method (AHP) are used to evaluate the CBM production potential in the Liulin area. The best prospective target area for CBM production is forecasted to be located in the central slope belt (near southern Beilijiyuan and Yangjiayu areas) of the Liulin area. © 2014 Elsevier B.V.

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