Experimental Study On Rock Physics Of Low Rank Coal In Shenfu Region

Rock physics experiment is an important method to study physical property parameters,pore characteristics and adsorption mechanism of coal.However,because of its compactness,poor correlation of pore permeability and complex pore structure,the effect of conventional petrophysical experiment method applied to coal is not satisfactory.Based on this,this paper relies on laboratory core analysis technology and takes the coal and rock in the Shenfu area of the Ordos Basin as the research object to carry out the exploration of the coal rock porosity,permeability,adsorption capacity and pore characteristics,aimed at solving problems in the experiment and building a laboratory core analysis methods that match the CBM reservoirs.In the current rock physics experiment,there is a great error and low accuracy in the porosity test of coal rock.In this study,the porosity test of columnar coal was carried out using NMR,liquid measurement and gas measurement.The results found that the coal gas porosity measurement is influenced by gas injection pressure,molecular sieve effect,the air component.But when the effect of the above factors will be minimized,the coal gas porosity should be preferred.because on the one hand,the helium molecule diameter minimum,can more fully spread into nano pore network of coal,and the test state of overburden pressure simulated the formation pressure of coal reservoir,to a certain extent,restore the true porosity of coal rock.After washing oil and salt,general core is hydrophilic which makes the fluid can completely saturate into the pore network of coal,so the experiment of Liquid measuring method tend to be real values,as a result that Liquid porosity can be used as a secondary selection.The interlayer water in coal rock clay minerals can produce nuclear magnetic signal,which leads to the general deviation of nuclear magnetic porosity.The nuclear magnetic signal of the skeleton needs to be deducted as background value to correct the nuclear magnetic porosity.To explore the NMR evaluation of coal and rock's permeability,based on the unsteady method of coal and rock's permeability,the four kinds of nuclear magnetic permeability model?SDR,SDR extension model,Coates,Coates extension model?is applied to the accuracy of the coal and rock,it is found that only the SDR extension model can more accurately evaluate the coal and rock's permeability,but it still has large error.on the basis of this puts forward,using only"T_2g"build independent parameters suitable for nuclear magnetic permeability model of coal and rock,the result was very similar to unsteady permeability,which shows that using nuclear magnetic resonance?NMR?could accurately evaluate the coal and rock permeability.Integrated liquid nitrogen adsorption and NMR to develop coal rock pore characteristics,it is found that isothermof coal and rock belongs to?curve.According to the different pore diameter distribution characteristics of the liquid nitrogen adsorption curve,the adsorption curves were divided into three types:S1,S2 and S3.At the same time,based on the difference of the pore structure of the coal rock,the desorption line is divided into Y1,Y2 and Y3.Based on the nuclear magnetic T2 spectrum and the liquid nitrogen curve of coal rock,two methods of pore size conversion were applied to coal rock are proposed:inflection point method,surface relaxation rate method.Using aperture conversion results,select 10 nm as the boundary to accumulate coal pore volume of less than 10 nm,based on the theory of micro hole filling,the seam gas content of coal is predicted.It is found that thet relative error is 16.75%,further applying this method to the NMR logging.In view of the quantitative characterization of coal adsorption capacity,the conventional isothermal adsorption volume method is always used,but its test cycle is long which lacks coal and rock dynamic measurement in the process of adsorption,and it is unable to distinguish between the adsorption gas and free gas.On this basis,the traditional isothermal adsorption device is combined with nuclear magnetic resonance instrument to simulate the methane concentration process of coal and rock reservoir.based on the adsorption on coal gas and free gas lateral relaxed time T₂,partition adsorption peak and free adsorption peak in T₂spectrum distribution range,including methane adsorption of methane,free distribution respectively in 0.01¹.49 ms,T2=1.86²8.12 ms.At the same time,on the basis of NMR T2 free methane spectrum of the linear relationship between the area amount of gas content,the total amount of free methane gas is obtained when the coal rock reaches the adsorption equilibrium at different pressure.thus putting forward the adsorption capacity using NMR method for quantitative evaluation of coal rock,the results with the conventional isothermal adsorption volume method were highly consistent,and the influence of temperature,moisture,pore volume and industrial group on the adsorption capacity of coal and rock is studied on this basis.