Flow Channel Mode In Coal Reservoir And Its Significance For Quantitative Drainage Of Coalbed Methane Wells

As the dominant channel of fluid migration in the hydraulic fracturing reconstruction of coal reservoirs,natural fractures and fracturing fractures differ in development,spatial distribution,morphological characteristics,and mutual configuration relations and combination modes in different locations within the coal reservoirs.In the process of CBM well drainage and depressurization,this difference will lead to the inconsistency of pressure drop and flow velocity in different parts of coal reservoir.The pressure drop and flow rate are closely related to the increase of effective stress and the migration and settlement of fines.Therefore,it is of great significance to define the main channels of flow in coal reservoir during the process of drainage and pressure lowering of coal-bed methane wells,analyze the difficulty degree and pressure distribution characteristics of fluid flow in different channels,and identify the most vulnerable part of permeability of flow channels.This paper discusses the above problems from three aspects: fluid flow channel,fluid flow and pressure distribution,coal fines migration and settlement.(1)Establishment of typical fluid flow channel mode and induction of combined flow channel mode.Field dissection and observation were carried out on seven fractured CBM Wells in Sihe mining area in the south of Qinshui basin.Hole as the starting point,tracking the advantage of flow migration channels in the coal reservoir,knowing fracturing cracks in coal reservoir development regularities and distribution characteristics of various fractures in coal reservoir configuration and combination,the morphological characteristics,open width of fracture,degree of internal filling,diverting capacity,development scale.Based on channel's characteristic,two typical flow channel modes and three kinds of flow channel combinations are proposed.Two typical fluid flow channel modes: the first type combination mode,in which the cleat is directly in vertical direction or indirectly connected with the fracturing sand spread fracture through the gas-expansion joint;the second combination mode,in which exogenous fractures cut through multiple layers of coal rock vertically and upward,connectes cleats or gas expansion joints in different layers.The three combined modes are as follows: 1.Large scale cracks cut through small scale cracks;2.Small scale fissures connect head to tail;3.The abnormal permeability inter-layer is cut horizontally and extends vertically to the flow channel.On the basis of this understanding,the fluid flow channel model of well FSL-2 in Fukang mining area and well PQT-1 in Zhijin mining area are constructed.(2)Fluid flow and pressure distribution in the flow channel.Based on the observation of the roadway wall,the flow channel is set as a rectangular tube.According to the Darcy-Weisbach formula,the formula of fluid flow rate and pressure drop in the parallel channel is derived.Combined with the formula of local energy loss,the influence of two combined modes of channel crossing and head-to-tail connection on fluid flow is discussed.Based on Darcy's law and the continuity equation of fluid flow,the pressure distribution in non-parallel wedge channels is derived.Based on the assumption of incompressibility of liquid and the conservation of fluid volume,the influence degree of the flow pressure change in the bottom of well on the pressure change in the channel was derived.(3)Distribution of materials in the channel and migration of coal fines.Based on the theory of multi-phase flow pattern and mineral fine physics separation principle,the material distribution in the flow channel of different occurrence and flow direction is discussed.The in-situ coal fine in different parts of underground mine was collected for carrying out tests,including immersion test of activated water,initiation and fluid velocity test of coal fine,and sedimentation test of suspended coal fine liquid.The main source of pulverized coal,the particle size value of transported pulverized coal fines corresponding to different fluid flow rates,the form of aggregation of fines,and the easy settlement of pulverized coal in the channel were recognized.The critical pressure gradient value of coal dust migration in channel with different width is calculated.Through the study of the flow channel morphological characteristics,the flow channel and the coal matrix block diversion capacity difference,the sensitivity of fluid pressure inside the channel to pressure changes at the outflow end of the channel,the critical pressure gradient and minimum migration velocity of fines in different width channels.It can reveal the part of fluid flow channel that permeability is easily damaged.The fluid pressure distribution,fluid velocity and critical pressure gradient associated with the permeability damage of the channel are obtained to diagnose the cause of the attenuation of the flow conductivity of the flow channel,that not only provides specific data support for the establishment of CBM well drainage and mining system,but also provides a basic understanding and a new idea for the quantitative calculation of the influence of bottom-hole flow pressure change on the coal reservoir diversion capacity.