Research On Coalbed Methane Enrichment And Productivity In Liulin Area

Liulin area is located in the middle of the eastern margin of the Ordos Basin,where bituminous coal No.8+9,10 of Upper Paleozoic Carboniferous and coal No.3+4,5 of Lower Permian are developed,and it is a southwest dipping monoclinal structure with a gentle angle.The cumulative coal thickness is about 10 meters and is distributed regionally.Most of the coal seam depth is less than 1000m,but not uplift above the belt of weathering and oxidation,and the thickness of the overlying strata and root lithology has little effect on coalbed methane enrichment.Coal gas content in Taiyuan Formation is higher than that in Shanxi Formation,which is positively correlated with depth and negatively correlated with ash yield.The coal of Taiyuan Formation is characteristics with relatively high CO2 content and low δ C13,compared with Shanxi Formation,and it can be resulted from hydrology dynamics,which leads to low gas saturation in coal of Taiyuan Formation.CBM desorption efficiency(α/β)2、starting pressure(C1-PL)、transition pressure(α-PL)and sensitive pressure are defined first in this paper.The desorption efficiency is 2.59m3/MPa,1.0 m3/MPaand 0.55 m3/MPa at the point of starting pressure,transition pressure and sensitive pressure separately,and four desorption stages are divided accordingly,which are low efficiency desorption stage,slow efficiency desorption stage,fast desorption stage and sensitive desorption stage.It is little contribution to CBM productivity in the stage of low efficiency desorption and slow desorption,while it is import to CBM productivity in the stage of fast desorption and sensitive desorption.Water production amount from coalbed in Taiyuan Formation is larger than that of Shanxi Formation,and that in the north is larger than that in the south.Generally speaking,it is good to produce the single coalbed from Shanxi Formation,and the gas production is varied producing two sets of coal both Taiyuan and Shanxi Formation together.Coal cleats can joint to a greater extent on the condition of multi—branch horizontal wells,so it relatively easy to form regional pressure drop funnel and high gas production to 9744m3/d.It should be noted that there may be dramatically different between two nearby horizontal wells,and many things must be considered including the orientation of horizontal branches and coal reservoir protection.High CBM resources is a basic parameter for high production,and high gas saturation,high desorption efficiency,high reservoir permeability with appropriate matching hydrodynamic conditions are key geological factors controlling CBM wells productivity.CBM wells with high gas production are parallel with coal roof structure contour,which is the result of superposition of the above main controlling factors of regional changes integrated.There is abundance CBM resources and good permeability in coal of Taiyuan Formation compared with the Shanxi Formation,but the former is characteristics with low gas saturation and long dewatering time,coupled with high reservoir pressure,low desorption efficiency and high water content in the roof limestone,resulting in bad developing condition.It is strongly suggested that development focus on Shanxi Formation at present,step by step from northeast to southwest.