Yanjun Meng

Geological and hydrological controls on water coproduced with coalbed methane in Liulin, eastern Ordos basin, China

Significant amounts () of water are currently being extracted from coalbed methane (CBM) wells in Permian–Carboniferous coal in the Liulin area of the eastern Ordos basin, China. Waters coproduced with CBM have common chemical characteristics that can be an important exploration tool because they relate to the coal depositional environment and hydrodynamic maturation of groundwater and can be used to guide CBM development strategies. The CBM production targets of the No. 3 and 4 coal seams from sandstone in the Shanxi Formation and No. 8, 9, and 10 coal seams in the karst of the Taiyuan Formation were deposited in fluvial-deltaic and epicontinental-sea environments, respectively. This paper combines CBM geology, hydrogeology, CBM recovery, and laboratory data to define mechanisms of CBM preservation including the important influence of groundwater. Relevant indices include fluid inclusions as an indicator of the hydraulic connection between the coal seam reservoir and the overlaying strata and the ensemble characteristics of total dissolved solids (TDS) contents of water, water production rates, and reservoir temperatures as an indication of the current hydraulic connection. The TDS contents of waters from the No. 3 and 4 and No. 9 and 10 coal seams are double those from the subjacent karst No. 8 coal seam, indicating the important control of fast flow in karst. Low-salinity fluid inclusions from the roof of the subjacent-karst No. 8 coal seam also indicate an enduring hydraulic connection with overlaying strata during its burial history. Relatively low current temperatures in the No. 8 (subjacent-karst) coal seam also infer a strong hydraulic connection and active flow regime. Deuterium concentrations are elevated in the mudstone-bounded No. 9 and 10 coal seams, further confirming low rates of fluid transmission. The gas contents of coal seams from the Taiyuan Formation are higher than those from the sandstone-bounded coal seams in Shanxi Formation, also correlating with low rates of water transmission and low permeability. Conceptual models for these fluvial-deltaic and epicontinental-sea environments that are consistent with geology, gas content, and gas and water production rate histories are of gas-pressure sealing for the Shanxi Formation and hydrostatic-pressure sealing for the Taiyuan Formation. These results confirm the important controls of hydrogeological conditions on the preservation of CBM and the utility of hydrogeological indicators in prospecting for CBM.

Coalbed methane produced water in China: status and environmental issues.

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 271,280xa0L/well/day, and the concentration of total dissolved solids (TDS) ranges from 691 to 93,898xa0mg/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.

Division of coalbed methane desorption stages and its significance

Abstract To quantitatively analyze the effect of desorption characteristics on productivity, a division method of coalbed methane (CBM) desorption stages was established based on the Langmuir adsorption isothermal theory, and the indicating significance of the method was analyzed with cases of CBM production. The desorbed efficiency was used to quantitatively characterize the CBM desorbed rate under different reservoir pressures. On the basis of key nodes on mathematical curves, the starting pressure, turning pressure and sensitive pressure were defined. For different coal samples, the corresponding desorbed efficiencies of the three nodes were always constants, therefore the CBM desorption process was divided into four stages, i.e., inefficient, slow, fast and sensitive desorption stages. Studies reveal that, fast and sensitive desorption stages contribute a lot to CBM productivity, while inefficient and slow desorption stages have very little contribution; absorption capacity, gas-bearing property and reservoir pressure are the key factors for the desorption characteristics. The production status of southern Qinshui Basin and Liulin area in Ordos Basin in Northwest China demonstrates that the method can guide the CBM development effectively.

Distribution of stable carbon isotope in coalbed methane from the east margin of Ordos Basin

The commercial recovery of methane from coal is well established in the coalbed methane (CBM) blocks at the east margin of Ordos Basin, China. CBM forms with various carbon isotopic ratios (δ13CPDB) due to the carbon isotopic fractionation in biogenical or thermogenical processes. Based on the geologic evolution of coalbed reservoir and studies on the characteristics of δ13CPDB values distributed spatially (e.g., horizontal CBM well location area, vertical coal burial zone, coal rank, etc.) and temporally (e.g., geologic evolution history), we explored the formation mechanism of carbon isotopic of methane. The relatively low δ13CPDB values are widely distributed along the research area, indicating a trend of “lighter-heavier-lighter” from north to south. From a combination analysis of the relationship between δ13CPDB and the relative effects, the essential aspects in determining CBM carbon isotope being light in the study area are: the genesis of secondary biogas in the north; water soluble effects in the active hydrodynamic areas in the middle; desorption fractionation effect promoted by tectonic evolution in the south; and the sudden warming hydrocarbon fractionation accelerated by magmatic event in particular areas (e.g., Linxian).

Characteristics and control mechanisms of coalbed permeability change in various gas production stages

According to dimensionless analysis of the coalbed methane (CBM) production data of Fanzhuang block in southern Qinshui basin, the dimensionless gas production rate is calculated to quantitatively divide the CBM well production process into four stages, i.e., drainage stage, unstable gas production stage, stable gas production stage, and gas production decline stage. By the material balance method, the coal reservoir permeability change in different stages is quantitatively characterized. The characteristics and control mechanisms of change in coalbed permeability (CICP) during different production stages are concluded on five aspects, i.e., permeability trend variation, controlling mechanism, system energy, phase state compositions, and production performance. The study reveals that CICP is characterized by first decline, then recovery, and finally by increase and is controlled directly by effective stress and matrix shrinkage effects. Further, the duration and intensity of the matrix shrinkage effect are inherently controlled by adsorption and desorption features.