Experimental Study On Gas-water Relative Permeability Characteristics Of Coal Reservoir Under Thermal Coupling Based On NMR

With the increase of coal bed methane mining depth,deep coal bed methane resources have become an important basis for the large-scale development of coal bed methane industry.Conditions of the coal bed methane in our country and the environment itself is relatively complex,low pressure,low permeability,low saturation and heterogeneity,etc,and compared with shallow and deep ground stress and formation temperature rise obviously,combined with coal is made up of coal matrix,micro pore and fracture of coal/gas/water three-phase coexistence of double porous media.At the same time,the gas/water two-phase flow stage is the key stage in the whole process of"drainage gas recovery".Therefore,it is of great significance to carry out experimental research on gas/water permeability characteristics of coal reservoir under thermal-thermal coupling based on nuclear magnetic resonance（NMR）.This paper mainly uses Macro MR12-150H-I low-field nuclear magnetic resonance instrument,high temperature and high pressure displacement system for nuclear magnetism（high temperature and high pressure probe for nuclear magnetism,high temperature and high pressure displacement circulation control system）,and takes coal samples from Zhaogu No.1 Mine of Jiaozuo Coal Industry Group as the research object.The dynamic evolution characteristics of gas/water permeability in coal reservoirs under different temperatures and confining pressures were tested by NMR.Firstly,the NMRT₂spectrum of coal core was tested under different temperatures and confining pressures,and the corresponding characteristics of movable water saturation,bound water saturation and total porosity in coal on temperature and confining pressure were studied,as well as the influence law of temperature and confining pressure on coal core permeability.Then,the gas/water two-phase seepage law of coal reservoir and its corresponding characteristics on temperature and confining pressure are studied by carrying out coal core gas flooding and water flooding tests under different temperatures and confining pressure conditions and real-time dynamic monitoring of NMRT₂spectrum in the test process.The research conclusions are as follows:（1）Limited pressure has a great effect on the fracture structure of coal pores.With increasing thermal insulation pressure,the pores in the coal sample are squeezed to increase the number of small pores,the overall pore volume decreases,the porosity decreases,some pores are narrowed,and the throat becomes smaller,resulting in an increase in irreducible water saturation.The proportion of small pore throats increases,the proportion of medium and large pore throats decreases,and the permeability gradually decreases.There will be an inflection point in the decline process.If the applied confining pressure is lower than the inflection point,the decline of permeability is steep.If it is higher than the inflection point,the magnitude decreases and tends to be gentle,and the connectivity between pores gradually becomes worse.（2）The pore structure of coal is affected by temperature.When the applied temperature increases,the porosity decreases,the pore structure is redistributed,Smaller and semi-closed bottles decreased,the proportion of closed bags increased,and the proportion of pouch tubes increased,leads to an increase in the imperceptible saturation of water and a decrease in permeability.There is an inflection point in the process of changing permeability with temperature.When the temperature is below the inflection point,the reduction is greater.When the inflection point is larger,the reduction is smaller.Compared with the effect of limited pressure on permeability,temperature has less influence on it.（3）Gas-water relative permeability of coal is affected by confining pressure.When the confining pressure increases,the pores and pore throats are deformed,the pore volume decreases,gas and penetrates and resurrection waters.The relative penettion of the first gas continues to decrease,while the relative and first acute penettion is the opposite.The permeability curve of the two phases in relation to gas and water is generally a concave curve.At the same time,the slope of the relative permeability curve for the carbon-like aqueous phase is also different,meaning that the relative permeability of the aqueous phase increases with increasing end pressure.（4）The change of temperature will change the gas-water relative permeability of coal.When the temperature increases,the T₂spectrum pores gradually obviously,the pore volume decreases,The shape of the relative permeability curve does not change significantly,completely to the left.Relative permeability decreases in the gaseous phase and increases in the water phase.（5）The pressure difference has a significant effect on the gas-water relative permeability of coal rock.When the pressure difference increases,the pore volume gradually decreases,the amount of water in relative water decreases gradually.The relative order of the water levels is reversed.The relative permeability curve also decreases with increasing pressure differentials.The degree of crusher is lighter,the rotation slowly moves to the left.Isotonic water and fenced water spin gradually.The research results of this paper not only provide a theoretical basis for scientifically formulating the development and deployment of coal bed methane and effectively eliminating the blindness of drainage control,but also enrich the basic theory of gas-water two-phase seepage in coal reservoirs.