Study On Evaluation Method Of Coal Reservoir Sensitivity

Sensitivity evaluation of coal reservoir is very important for the development of coalbed methane.Sensitivity research can provide basis for damage mechanism analysis of coal seam,in order to prevent damage,optimize the follow-up operation measures,design the system engineering technology scheme to protect CBM development and improve the output of single well.Unlike conventional sandstone reservoirs,coal and rock have unique physical and chemical properties,and CBM development has unique drainage and production characteristics.At present,conventional sensitivity evaluation methods of oil and gas reservoirs are not completely applicable to coal reservoirs,so it is necessary to establish a technical method suitable for sensitivity evaluation of coal reservoirs according to the characteristics of coal reservoirs.In this paper,the sensitivity of some coal reservoirs in eastern Yunnan and Western Guizhou is evaluated by conventional core flow experiments,and the sensitivity of the formation is obtained.Combining with the study of physical and chemical properties of coal,the damage mechanism of different sensitivity is revealed.Based on the sensitivity damage mechanism of coal and rock and the characteristics of coal and rock itself,the improved recovery rate experimental method was established to evaluate the acid sensitivity,alkali sensitivity and salt water sensitivity of fractured coal and rock which could not obtain standard core,and its applicability was evaluated.In addition,considering that there are two processes of drainage and gas recovery in the process of coalbed methane exploitation and the permeability of some coal reservoirs is very low,the sensitivity of gas measurement method to evaluate the flow rate of coal reservoirs is established,and the conversion formula between gas measurement method and liquid measurement method is given.The results show that the main reason for the flow rate sensitivity is that the fine pore structure of coal seam is blocked by the dispersive migration of clay minerals and matrix particles with loose junction at high flow rate;the acid sensitivity is mainly caused by the dissolution of calcite and quartz in acid solution and the increase of the volume of seepage channel;the alkali sensitivity is due to the double electric layer repulsion of kaolinite and illite at high p H value.The increase of salt sensitivity is due to the cation exchange of clay minerals such as kaolinite,illite and ammonium mica,which results in the hydration,dispersion and migration,and blockage of migration channels.There is a linear correlation between relative recovery rate and permeability change rate.In acid,alkali and brine,the correlation coefficients of fitting curves are 0.87,0.84 and 0.80,respectively.The sensitivity of fragmented coal can be evaluated by using the improved recovery rate method.Based on the kinetic energy theorem and considering the slippage effect of coal reservoir,a method for evaluating the sensitivity of gas velocity measurement is established.The theoretical basis is sufficient,which is a useful supplement to the sensitivity of coal reservoir.The experimental results of two formations show that the errors of the conversion formulas between gas velocity measurement and liquid velocity measurement are 20% and 13.6%,respectively.