The Occurrence Mode And Movability Of Residual Fluid In Anthracite

The effective release of residual coalbed methane(CBM)relies on the occurrence mode and movability of residual fluid within coal reservoirs.Therefore,this thesis focused on investigating the 3~#coal of the Shanxi Group in the southern Zhengzhuang Block of the Qinshui Basin.To examine the multi-scale fracture network,pore-fracture connectivity,occurrence mode,formation mechanisms,and movability of residual fluid,the Micro-CT,microfluid displacement,SEM,and numerical simulation experiments were utilized in this research.The study reveals the development pattern and connectivity of multi-scale fractures in coal reservoirs.CT scans identify three categories of fractures—medium,small,and micro—in the 3~#coal.The research identifies the multi-scale fracture development characteristics in different coals and their variations.It delineates fracture types and constructs a map of the distribution model for multi-scale fractures.The interconnected exogenous fractures,dewatering fractures,intercrystalline pores,and gas pores in coal reservoirs display significant heterogeneity.The study identifies the types and formation mechanisms of residual water in coal reservoirs.Based on water volume,shape factor,and location,single residual water clusters are classified into five shapes:network shape,patchy shape,column shape,corner shape,and isolated shape.As the displacement pressure increases,the saturation of network-shape residual water decreases,while the saturations of patchy-,corner-,and isolated-shape residual water increase.The decrease in residual water saturation due to increasing displacement pressure is more pronounced in semi-bright coal compared to semi-dull coal.The primary causes of residual water in coal reservoirs are non-simultaneous displacement,deflection flow rate,dead-end fractures,and by-passing.The displacement pattern from unsteady to steady-state displacement significantly reduces the residual water saturation resulting from non-uniform displacement,deflection flow rate,and by-passing in coal.The study establishes critical mathematical models for the movability of typical residual water in coal reservoirs.The critical mobilization condition for isolated-shaped residual water is Lg Ca>-2.89.The movability of isolated-shaped residual water depends on gas velocity and fracture aperture.The movability of column-shaped residual water increases gradually with increasing gas velocity and decreasing fracture intersection.For corner-shaped residual water,the critical condition for mobilization is Lg Ca>-0.89.Corner-shaped residual water is generally difficult to mobilize.Network-shape residual water can be mobilized when the displacement pressure exceeds the capillary force at the gas-liquid interface.The study describes the occurrence mode and mobilization potential of residual fluid in coal reservoirs.The occurrence pattern of residual fluid in the study area is influenced by macro-lithotype,porosity,permeability,gas content,structural position,and bottom hole pressure.Gas-liquid migration in the high-yield well of the study area predominantly exhibits capillary fingering,with some instances of transitional displacement patterns.Fluid flow in the medium and low-producing wells of the study area is controlled by capillary fingering.The formed residual water mainly consists of network-shape residual water caused by by-passing in bright coal.Semi-bright coal contains primarily network-shape and corner-shape residual water,while corner-shape residual water characterizes the residual water formed in semi-dull coal.With increasing Ca,CBM wells display high fluid mobilization,shorter gas production time,and increased cumulative production.Low-producing wells with favorable reservoir properties and high fluid production potential exhibit medium-production well criteria under high Ca displacement patterns.There are 158 figures,22 tables and 215 references in this thesis.