Physical Properties And Comprehensive Evaluation Ofcoalbed Methane Reservoirs Of Jurassic In The Northern Qaidam Basin

Coalbed methane(CBM) resources of low rank coals are abundant in northwestern China, characterized by thick coal seams, with high porosity, high permeability and low gas content. However, there has been inadequate exploration and inaccurate testing of coal seam contents in this area, and therefore a study into the characteristics and a comprehensive evaluation of the coal reservoirs is necessary. Such a study would be effective in making up for deficiency in the process of CBM exploration and development. Additionally, a new comprehensive evaluation model proposed oriented by CBM reservoirs for the selection of favourable areas would provide immediate theory and technical guidance for CBM exploration and development in the northwestern China.This paper focuses on the Jurassic coal reservoir at the northern margin of Qaidam Basin, and applies theories from the field of petrology, coal geology, sedimentology, gas geology and coalbed methane geology, based on geological surveys and sampling from underground mines and drilling field, in addition to a variety experimental tests and analysis, including coal petrology identification, macrocracks and microfractures analyses, mercury porosimetries, N2 adsorption, porosity and permeability measurements, scanning electron microscopy and CH4 isothermal adsorption, to study the detailed CBM petrophysical characteristics in the research region. On the basis of these studies, a comprehensive evaluation model for CBM reservoirs has been founded and the main research achievements are acquired as follows.1. The adsorption characteristics of the CBM reservoirs and their influence factors were elucidated based on CH4 adsorption of the coal samples from the research area under different experimental conditions.(a) The Langmuir volume of the coal samples in the northern Qaidam Basin lies within the range 26.51-42.19 m3/t with an average of 33.19 m3/t under an experimental temperature of 25℃. Accordingly, the values of Langmuir pressure lies within the range 0.83-1.59 MPa with an average of 1.47 MPa. This indicates that it is relatively easy for coal reservoir to adsorb methane under low pressure conditions, but the adsorption capacity drops off notably with increasing experimental pressure under higher pressure conditions.(b) The Langmuir volume and Langmuir pressure of all coal samples are decreased with the increase of experimental temperature. Based on the CH4 adsorption results, adsorption ability of Wanggaxiu coalfield is the lowest, whereas the Langmuir pressures of Dameigou and Lvcaogou coal mines are relatively high, which means they are more easy to desorb coalbed methane during exploitation.(c) The CH4 adsorption ability shows a significant positive correlation with vitrinite content, and negative correlations with inertinite content, moisture, ash content, coal structure index and experimental temperature, and there is no obvious correlation with coal rank. According to an integrated study, the dominant factors influencing adsorption ability are coal quality, temperature, and coal maceral content.2. Structural feature of seepage pores and adsorption pores was described in detail and their influence on the occurrence of CBM was studied through the integration of two analytical techniques: mercury porosimetry and N2 adsorption.(a) The porosity of coal samples in the research area decreased with an increase in vitrinite reflectance and ash content: the higher the ash content of coal, the lower the porosity and the poorer the reservoir. Additionally, we found the greater the damage to the coal structure, the higher porosity of the coal, therefore, in general the porosity of deformed coal is higher than that of indigenous coal.(b) Based on the results from the low temperature liquid nitrogen adsorption analysis, the specific surface area and total pore volume of the coal samples in the northern Qaidam Basin lie between 0.843 m2/g and 55.12 m2/g with an average of 23.34 m2/g, and within the range 2.46-50.43×10-3ml/g with an average of 25.17×10-3ml/g respectively. There is a positive correlation between specific surface area and total pore volume, and the volume ratio of the different pore types are, in descending order, micropores(<10 nm), keypores, and macropores(>100 nm), for all samples except Dameigou coal mine.(c) According to different curves of capillary pressure, seepage pores structures(>100 nm) can be divided into 2 types, typeⅠ1 and typeⅠ2. For typeⅠ1 pores, the mercury ejection curve is non-parallel with the mercury withdrawal curve, with a lower withdrawal efficiency, which shows that the pore structure of typeⅠ1 is not homogeneous and is characterised by poor connection of pores. Whereas for typeⅠ2 pores, the mercury ejection curve is almost parallel with the mercury withdrawal curve, indicating a higher withdrawal efficiency of more than 80%, which suggests that there is a good connection between the pores, and it is favorable for CBM accumulation and exploitation.(d) Based on the different characteristics of low temperature liquid nitrogen adsorption curves, adsorption pores structures(<100 nm) were divided to 3 types. Type Ⅰ coal structure has good adsorption and reservoir ability, but its desorption is poor, making exploitation relatively difficult. Type Ⅱ coal structure is typically characterized by micropores with good permeability, and is favorable for coal adsorption, desorption and diffusion. Type Ⅲ coal structure has a “double-peak” system which is unfavorable for effective CBM diffusion.3. The heterogeneities of seepage and adsorption pores were quantitatively characterized using fractal analyses. The relationships between pore surface fractals(D1), pore structure fractals(D2), and adsorption ability, coal quality, pore structure parameters and coal structure were investigated, and the influences of the fractals on CH4 adsorption feature and permeability were discussed.(a) According to fractals calculation of adsorption pores, the pore surface fractals were found to have relatively low values, between 2.001 and 2.345, whereas the pore structure fractals have higher values ranging between 2.641 and 2.917. This demonstrates that the difference in adsorption pore structure is not obvious among the low adsorption pores stage(rk<1.38 nm), and that the pore structure of coal samples with extraction isotherms was more complex than those without among the high adsorption pores stage(rk>1.38 nm).(b) The pore surface fractals show a significant binomial correlation with Langmuir volume, a positive correlation with vitrinite reflectance and specific surface area, but no correlation with moisture, ash content, volatiles, average pore diameter, total pore volume, and coal structure. The pore structure fractals do not have an obvious correlation with Langmuir volume or coal structure, have a positive correlation with vitrinite reflectance, specific surface area, micropore content, and total pore volume, and a negative correlation with moisture, ash content, volatiles, and average pore diameter.(c) The seepage pore fractals from YQ-1(2.9586) and Wucai coal mine(2.8816) are relatively high, which indicates that the seepage pore structure of this region is complex. Furthermore, these fractals show a negative correlation with vitrinite reflectance.4. The characteristics of macrocracks and microfractures in the coal reservoirs of the northern Qaidam Basin were studied, and the effects of the petrological composition of the coal on its microfractures were discussed, based on the experimental results from geological surveys, optical microscopy and scanning electron microscopy.(a) The coal macrocraks of Jurassic in the northern of Qaidam Basin were characterized as follows. Fracture types are dominated by vertical fractures, while bedding and oblique fractures are less. Macroscopic petrographic types are dominated by bright and semi-bright coals, while dull coals are less frequent. Fracture surface types are dominated by stepladder-like and fracture scale is middle-large type. The fractures are slightly dense to dense, range in height from 3 to 8 cm, and are typically less than 0.5 cm in width. Connectivity of the fractures is moderate, and they are commonly either filled with calcite or are hollow.(b) The microfractures are dominated by C and D types, while the percentages of A and B types are obvious lower. The total percentage of A+B types is high at ether end of the research area and lower within the central region.(c) The density of microfractures in the Jurassic of the northern of Qaidam Basin is not clearly correlated with vitrinite reflectance and vitrodetrinite, but has a positive correlation with vitrinite content, telocollinite content, vitrite percentage, vitrinertite percentage, gelification index(GI), and tissure preservation index(TPI), while having a negative correlation with inertinite content, desmocollinite content, semifusinite content, inertodetrinite content, inertite percentage, inerivitrite percentage, oxidation index(OI) and breakage index(BI).5. Taking the Yuqia coalfield, as an example, based on a description of the coal structure and lithology, low temperature liquid nitrogen desorption analysis, micro-fracture statistics, X-ray diffraction, and rock mechanics of coal deformation, the distribution of the coal structure were predicted and the pore-fracture structure and XRD structure of different coal types were compared.(a) The coal structure in the Gaoquan coal mine comprises mainly indigenous coal and fragmented coal, with a minor amount of granulated coal and mylonitised coal. The macrolithotypes of these coals are dominantly semibright and semidurain coals, with secondary bright coal and durain coal. In the Yuqia coalfield, the macrolithotype is mainly semibright coal, with secondary semidurain coal, while petrographically it is composed of dominantly bright coal and durain coal. In the Quanji coalfield, fragmented coal and granulated coal dominate, with a minor amount of indigenous coal. The macrolithotypes of these coals are mainly semibright and semidurain coals, and the petrographic composition is mainly bright coal and durain coal, with a small amount of vitrain and fusain. In the Delingha coalfield, the coal structures are mainly indigenous coal and fragmented coal, with macrolithotypes of mainly bright coal and semibright coal, and a petrographic composition dominated by vitrain, with secondary amounts of bright coal, and minor durain coal.(b) Considering the mechanics of coal deformation, in the Yuqia coalfield of northern Qaidam, tectonically deformed coals are developed across the whole of the Yangshuihe prospect area, in the southern portion and the fault zone of the northern Yudong prospect area, in the northern Gaxiu prospect area, within the fault zone of the northern No.2 coalmine area, and in the south-eastern region and fault zone of the north-western Beishan prospect area. In contrast, in the central Yudong prospect area, the southeastern Gaxiu prospect area, and the south-central No.2 coalmine area, tectonically deformed coal is not well developed.(c) Coal samples from the lower coal measures of the Wucai coalmine, Wanggaxiu, and in Lvcaogou, show stronger deformation with more complexity to their reservoir pore structure, which produces problems for methane desorption and CBM exploitation. The specific surface areas and pore volumes of the coals increase with a decrease in the comprehensive index that represents coal structure, resulting in an increase in methane desorption ability.(d) The average lamellae distance, stacking height and degree of extension for the coals studied here are 0.4088 nm, 8.0241 nm and 18.8497 nm, respectively. The comprehensive index for coal structure shows a positive correlation with lamellae distance, and a negative correlation with stacking height and the degree of extension, which suggests that the lamellae distance decreases while the extension and stacking height increase in response to the increasing destruction of the coal structure.6. By analysing the various evaluation factors and principal parameters of the coal reservoir in the northern Qaidam Basin, we establish a comprehensive evaluation model for Jurassic coalbed methane in this area, applying a multi-layer fuzzy synthetic evaluation model to optimise selection of favourable areas for CBM exploration and development.(a) Following exploration of CBM in the northern Qaidam Basin, an evaluation index system is established for CBM exploration and development, which takes into consideration a combination of the physical properties of the coal reservoir, the characteristics of the coal geology, resource and preservation conditions, and 12 additional secondary influential factors, such as permeability.(b) The influential factors of the CBM evaluation index system are quantitatively sorted using a multi-hierarchy analysis method. Combining this with a membership function, established within a real situation, can optimise the system for selecting favourable areas. Application of this method indicates that the Yuqia Coalfield is the most suitable for CBM exploration and development, followed by the Quanji Coalfield, Saishiteng Coalfield, and the Delingha Coalfield.