Study On Acoustic Response Characteristics Of Fluids In Coal Reservoirs

This dissertation adopted the experimental methods of coal industrial analysis,vitrinite reflectance measurement,maceral analysis,scanning electron microscope observation,uniaxial compression experiment,coal ultrasonic P-wave and S-wave testing experiment,gas displacement experiment,etc,and studied the basic characteristics of coals,the changes in acoustic response of coals with different metamorphic degree as well as various saturation of mixed phase fluids.Taking Fanzhuang block in Qinshui Basin as an example,the characteristics of various logging parameters,especially acoustic time difference logging,were analyzed,a multi-logging parameter prediction model for gas content was proposed,a gas saturation prediction model was obtained based on acoustic experiments,and the gas content and gas saturation distribution of the 3#coal seam in Fanzhuang block were further predicted.The detailed discussions are as follows:(1)The basic characteristics of 9 groups of coal samples of different coal ranks were analyzed,and the vitrinite reflectance(R_o)was between 0.34-2.54%.For the low rank coals,the pores are small and the connectivity is poor,cracks like shrinkage cracks can be seen,and some pores are filled with kaolinite.As the coal rank increases,connected pores,short linear pores and slit-type pores appear,and a few pores are filled with clay minerals.For the high rank coals,a large number of metamorphic pores appear,and mineral filling is common.(2)The P-wave velocity(V_p)and the S-wave velocity(V_s)of coals are both positively correlated with density and R_o.In terms of acoustic anisotropy,V_p and V_s change as a cosine function with a period of 180°with the pseudo azimuth.The relative anisotropy of each coal's V_pand V_s is relatively discrete,ranging from 7%-21%.The relative anisotropy of V_s varies more than V_p.(3)Based on the static method and the dynamic method,the mechanical properties of coals were studied,and it was revealed that the uniaxial compressive strength,elastic modulus of water-saturated coals are decreased compared with dry coals,while the Poisson's ratio of water-saturated coals are increased.The decrease and increase of various parameters of water-saturated tectonically deformed coal are greater than those of primary coal.The dynamic elastic modulus of coals has a linear positive correlation with V_p and V_s,while the dynamic Poisson's ratio has no obvious relationship.The dynamic elastic modulus is greater than the static,while the dynamic Poisson's ratio is less than the static.(4)With the continuous increase of water saturation(S_w),V_p and V_s show nearly exponential growth,and the change of V_p with S_w is more significant.As the degree of coal rank rises,the increase in V_p or V_s decreases to a certain extent.With the continuous increase of gas saturation(S_g),V_p and V_s decrease exponentially,and the change of V_p with S_g is more significant.The relative anisotropy of V_p and V_s of coals is linearly positively correlated with S_w,and the relative anisotropy of V_p is more obvious.(5)Take Fanzhuang block in Qinshui Basin as an example,it was clarified that the existence of organic matter and fluids in coals make the logging curve behave as high acoustic time difference.The acoustic time difference in Fanzhuang block is distributed between 311.14-532.07?s/m,and the average is 399.54?s/m.A correlation analysis model between the gas content of coal reservoirs and the acoustic time difference is established.Acoustic time difference,density,natural gamma,compensated neutron and depth data were selected,and a multi-logging parameter gas content prediction model was further established using SPSS.The predicted gas content was calculated,and the gas content is distributed in the range of 12.80-22.57 m~3/t,showing the characteristics of"high in the west,low in the east,the highest in the northwest".Based on the acoustic experiments,the gas saturation prediction model for high-rank coal reservoirs is derived,and the predicted gas saturation is calculated,mainly distributed in 61.56-83.25%,showing the trend of"high in the northwest and low in the southeast".