Discussion on the efficient exploitation method of deep coalbed methane with pressure relief by ground directional well+hydraulic slotting

In order to improve the efficiency of reservoir pressure relief of deep coalbed methane(CBM),as the deep coal reservoir is characterized by high reservoir pressure,high in-situ stress and low permeability,based on the depressurization principle by cutting to improve reservoir permeability,combining underground gas extraction and surface development of unconventional natural gas technology,the efficient exploitation method of deep CBM development with pressure relief by surface directional well+hydraulic slotting is put forward.The surface directional well+hydraulic slotting methods include two processes of surface directional drilling and staged hy-draulic slotting. The principle of this method for increasing permeability and increasing production is as follows:(1) Directional wellbore and hydraulic slots communicate the natural fracture system,and new fractures can be induced during high-pressure hydraulic slotting,which increase the number and connectivity of migration channels;(2) The pressure relief space is formed by multiple groups of slots,and the fracture opening increases with in-situ stress releasing to accelerate reservoir pressure drop.Compared with conventional hydraulic fracturing,this method is more beneficial to the formation of meshed fluid migration channel,the expansion of coal seam pressure relief range and degree,and the enhancement of CBM desorption and migration.Moreover,this method can avoid reservoir damage caused by crustal stress transfer to the deep coal seam and fracturing fluid injection,which is applicable to the stimulation of deep CBM reservoirs with complex geological conditions.With the consideration of the working conditions of ground and underground,the key technical problems are put forward for the pressure relief method of deep CBM exploitation by ground directional well +hydraulic slotting,including the controlling of hydraulic slot parameters,the flowback of solid phase particles,the matching of directional well completion and hydraulic slotting,as well as the power loss of high-pressure fluid transmission. The pressure relief method of surface directional well+hydraulic slotting stimulation is promising for its application in unconventional natural gas development,prevention and control of high pressure and gas disaster for deep coal,and fluidized intelligent mining of deep coal resources.