Study On The Influence Of Stress Sensitivity On The Liquid Retention Effect In The Middle-high Rank Coal Reservoirs

China's coalbed methane development started in the middle and high-level coal seams,which are rich in resources,accounting for 68% of the total resources.In recent years,medium and high rank coal seams in Qinshui Basin and eastern Ordos Basin have entered the stage of large-scale development of coalbed methane.The pore size of coalbed methane reservoirs is mostly nanometer and micron,with small throat,extremely complex pore structure and high capillary pressure.With the increase of buried depth,coal reservoir bears greater overlying pressure and stronger mechanical compaction.Under the action of stress sensitivity,the pore structure and seepage capacity of coal reservoir change,which affects liquid phase retention,and ultimately affects gas reservoir production and recovery.The existing achievements focus on the study of liquid retention effect or stress sensitivity of coal reservoir alone,and few scholars combine the two damage factors.In this paper,the basic physical property experiment,stress sensitivity experiment,microscopic pore structure experiment and gas-water permeability measurement experiment of reservoir core are carried out by combining laboratory experiment with theoretical model,and the stress sensitivity degree and liquid phase retention degree of medium-high rank coal under different water-cut conditions are studied.The main results are as follows:1.With the increase of axial load,the original pores and fissures in coal begin to close,and the new fissures on the weak surface are connected and expanded;Most pores are transformed into micropores,the average pore diameter is greatly reduced,the specific surface area and total pore volume are increased,and the stress sensitivity is enhanced.Isothermal adsorption curves are changed from L3 type to L1 type before deformation,and pore morphology is changed from bottle-shaped pores(ink bottle-shaped pores)to impermeable pores with one end closed.2.The irreducible water saturation of deformed coal samples increases,and the average permeability damage rate is 66.32%,so the liquid phase retention degree of coal samples in the study area is moderately strong.In the saturated state,the migration efficiency of water in the pores of coal samples treated under different loads and different water-bearing conditions is obviously different,while the water occurrence state is close.With the increase of centrifugal rotation speed,some micropores with pore size less than 1nm will communicate with each other to form micropores with pore size ranging from 1 nm to 10 nm,while water in some micropores with better connectivity will be discharged.3.The irreducible water saturation of coal sample is high,the relative permeability of gas phase is small,the span of two-phase flow is narrow,the gas saturation at isotonic point is low,and the relative permeability of gas at isotonic point is low.The greater the axial load,the wetter the coal sample and the higher the retention degree of liquid phase.4.The productivity of gas wells decreases linearly with the increase of liquid phase retention.Factors such as reservoir conditions,construction difficulties and economic conditions should be taken into consideration when mining coalbed methane on site,so as to adjust measures to local conditions,remove liquid phase retention in a balanced and efficient manner,and improve the drainage and production efficiency of coalbed methane.