Geological Controls And High Pruductivity Model Of Low-rank Coalbed Methane Reservoir In The Huanglong Coalfied,Shaanxi,China

The exploration and development of low-rank coalbed methane(CBM)has achieved commerical success in partial region of the Huanglong Jurassic coalfield of southwestern Ordos Basin.Nevertheless,the large-scale development of CBM is faced with various technical challenges in terms of geological conditions.In this study,the formatting mechanism of highly permeability coal reservoir and the geological factors which controlled the accumulation for low-rank CBM in the coalfield were investigated systematically.Finally,geological models of high CBM productivity were established.The innovative findings in the thesis are summarized in the following.(1)The formatting mechanism of highly permeability of low-rank coal reservoirs was revealed in Huanglong coalfield.The parameters of tectonic stress field were extracted according to the well test data.The coal reservoir with maximum value of permeability was around the buried depth of 600 meter,which was corresponding to lowest values of the lateral pressure coefficient,horizontal principal stress difference and effective stress.A two-stage reverse coupling“<?D”shaped causality model of in-situ stress—permeability was proposed,which reveals the development characteristics and geological control mechanism of highly permeability.(2)The control characteristics of the first coalification jump(FCJ)on the pore structure and the adsorptivity of coal in early coal coalification stage were revealed.The FCJ of the coal samples in the range R_r,0.60—0.65%,which had a significant impact on coal adsorptivity.Before FCJ,the Langmuir volume and free hydrocarbon yield of the coal samples decreased following an increase of R_r,and the main controlling factor was the microscopic composition of the inert-rich coal.After FCJ,values of these two parameters increased significantly with the R_r,and the influence of coalification on the adsorption capacity of coal was more significant,but the influence of macerals weakened dramatically.The results suggested that the bitumen produced by coalification could be adsorbed by vitrinite,and thus blocking or filling the pores of vitrinite.This elucidated the fundamental reason for the abrupt changes of coal adsorption capacity before and after FCJ.(3)Accumulation models of low-rank CBM were established in Huanglong coalfield.In the study area,the CBM enrichment areas were mainly distributed in the north of Huangling mining area,the east of Jiaoping mining area,the south central of Binchang mining area and the north central of Yonglong mining area.It was found that the buried depth of 300—800m was the best interval for CBM enrichment in the coalfield.The identification result of CBM stable isotope composition suggested that CBM was original from secondary biogenic gas(SBG)at the Binchang,Yonglong and Jiaoping mining area,and from the mixture of SBG and thermogenic gas at the Huangling mining area.Two types of CBM accumulation models,including dual accumulation of hydraulic plugging and SBG generation at the gentle slope of basin margin and multi-source CBM accumulation model,were proposed.The first type is caused by the generation and preservation of SBG,which depended on the favorable permeability and hydrogeological conditions for the low-rank coal reservoirs at the edge of the basin.The second type was derived from the vertical migration of gas along the structural fractures from the underlying oil and gas resources to the strata of coal measures,resulting in the co-presence of CBM in the coal measures strata.(4)High productivity geological models for gas production capacity of low-rank CBM well were established.Test data from the regional exploration and development indicated that high permeability of coal reservoir with the weak hydrodynamic conditions,resulting a high gas productivity.The difference of resource conditions in the CBM enrichment area had limited effect on the productivity of CBM well.Vertical well and multilateral horizontal well have good applicability for low-rank CBM development,but U-shaped well did not.Combining the accumulation models of CBM,high productivity models for gas production capacity of low-rank CBM well,including anticline wing region with high position,axis of the anticline and the syncline axis region with high gas content,were established.Vertical wells were expected to be arranged in the high-position areas of geological structure(e.g.,anticlines),and multi-branch horizontal wells were more favorable in the low-position areas of synclines.Finally,a technology system with complementary advantages of the two well types for low-rank CBM development was proposed.There are 114 figures,29 tables and 240 references in this thesis.