Research On Layer Selection Method For Fracturing Of Multi-layer Superpimposed CBM System In The Southern Junggar Basin

Coalbed methane(CBM)resources are rich in the Southern Junggar Basin,Xinjiang,which is the exploration and development demonstration base for future National CBM Development Plan.Compared with existing CBM industrial bases such as in Qinshui Basin,the coal field in the Southern Junggar Basin is characterized by multi-layers,large thickness and low gas content as well as low coal rank.It's therefore,necessary to make targeted development strategies.For the geological characteristics of multi seam superimposed,the combing exploiting of multi-layer can greatly reduce the cost,improve efficiency and resource recovery in line with the "layer complement" development ideas.Layer selection for fracturing has become the primary problem to be solved for combing exploiting.Based on field investigation,core data,regional and coalfield data,combined with analysis of abundant coal-rock and coal-bed methane,the paper investigates the development characteristics of coal rock in Xishanyao Formation,establishes the sequence stratigraphic framework,identifies the sedimentary facies,restores the lithofacies palaeogeography,figures out reservoir features of coal bed,divides the CBM system,builds the layer selection standard and method for optimizing fracturing.The Xishanyao Formation in the study area is characterized by thick in west and north,thin in east and south,where the sedimentary environments are primarily delta facies-lacustrine facies.The formation can be divided into three third-order sequences from upward to downward,which generally occur lowstand,transgressive and highstand systems with locally forced regressive systems.The coal is mainly accumulated in Sequence?,followed by Sequence ? in delta plains interbay,fluvial lake and shore shallow lake environment,in which the early and late stages of transgressive system tracts are corresponding to shore shallow lake and delta plain environments,respectively.Multi-set of thick coal beds are developed in Xishanyao Formation,mainly composed of primary structural coal and crushed structural coal with low primary permeability but high gas content and Lanshi pressure.The coal is primarily composed of semi-bright to semi-dark vitrinite,followed by interinite and least exinite,characterized by low water,low ash,low sulfur and high volatility.Under microscopic observation,the micropores are developed with few medium to macro pores observed.The pores connectivity is well at moderate to good level with specific surface area generally lower than those of high rank coal.Nuclear Magnetic Resonance(NMR)images reveal that the seepage pores and fractures are most developed in the coal reservoir between Manas River and Hutubi River,where the connectivity is also best.The accumulation and enrichment of coalbed methane in Southern Junggar Basin is controlled by three factors including sedimentary,tectonic and hydrodynamic condition.Three types of hydrocarbon accumulation mode are established including a)Accumulation by blocking reservoir with high angle monoclinal fault,b)Enrichment by blocking multi-coal seam,multi-argillaceous lithology with medium angle monoclinic,c)Residual type by washing coarse detrital lithology of low angle monoclinic.According to the sequence stratigraphy and gas concentration,three separate gas bearing systems are identified in Xishanyao Formation with gas concentration increasing and then decreasing from upper to lower formation,where the relative high concentration of gas occurs in the middle to lower formation.The thick black mudstone layers in gas-bearing systems intervals separate the three gas/water bearing systems,which,therefore,are considered to be the key layers for identifying the superimposed gas-bearing systems in the Southern Junggar area.The parameters indicative of favorable reservoir,including resources,engineering and other related parameters are analyzed.Resources parameter include factors such as coal bed thickness and lateral distribution etc.Engineering parameters include well cementation quality etc.,whereas other related parameters refer to burial depth etc.factors.On the basis of those parameters and the actual implement situation of Xinmacan 1 well,the layer selection standard is established.Two combined layers of 2#+5# and 8#+10# are optimized for exploiting according to the parameter scores in the three gas-bearing systems.