A study on the characteristics of seismic wave propagation in fractured marlstone reservoirs based on mechanical principles

Abstract

Marlstone reservoirs are closely related to the degree of fracture development and their hydrocarbon accumulation are also controlled by the distribution of fractures. Hence, the prediction of fracture development zones is very important to the discovery of fractured oil and gas reservoirs. Tectonic movements and their intensities are one of the key factors affecting the formation of fracture systems and the distribution of tectonic stress field has great impact on the generation of fractures. Therefore, taking marginal subsags in the Jiyang Depression, Eastern China, as an example, this paper studied the characteristics of seismic wave propagation in fractured marlstone reservoirs based on mechanical principles. This study proposed the equivalent geological model and elastic characterization parameters for the fractured marlstone reservoirs in study area, analyzed the seismic response patterns, anisotropy characteristics, and seismic wave-field distribution of the marginal subsags in the Jiyang Depression, discussed the influences of fracture filling fluid, density, inclination, orientation, and layer thickness on the characteristics of seismic wave propagation in fractured marlstone reservoirs, and finally explored the shear wave splitting phenomenon and the exploration and development significance of the seismic wave propagation in fractured marlstone reservoirs. The study results show that the fracture filling fluid can lead to the dispersion and attenuation of seismic wave propagating in the medium, and rock saturated with fluid can exhibit frequency-dependent attenuation and dispersion due to wave-induced fluid flow between pores and fractures. The change of fracture density can make seismic wave appear anisotropic and biphasic and the difference between the high and low frequency of the longitudinal wave phase velocity becomes more and more obvious. The change of fracture inclination and orientation can cause the shear wave splitting phenomenon, in which the fast shear wave polarized parallel to the fracture with a fast velocity and a slow attenuation and the slow shear wave is polarized in the direction perpendicular to the crack with slow velocity and fast attenuation. The change of fracture layer thickness can make because the change of phase velocity and attenuation and the larger the thickness of the fracture layer is, the greater the difference between the reflection amplitude of the bottom interface and that of the non-fracture layer is. The study of characteristics of seismic wave propagation in fractured marlstone reservoirs can effectively obtain the multi-directional shear wave impedance and then fine characterize the fracture strike and density of tight oil fractured reservoir, which has important theoretical research and practical exploration significance. The study results provide a reference for further researches on the characteristics of seismic wave propagation in fractured marlstone reservoirs based on mechanical principles.

Coalbed Methane Reservoir Parameter Prediction and Sweet-Spot Comprehensive Evaluation Based on 3D Seismic Exploration: A Case Study in Western Guizhou Province, China

“…Take the reciprocal of the acoustic time difference and convert it into P-wave velocity (VEL) in m/s, and the second elastic parameter density in g/cm 3 . The interpretation version of the coal structure is as follows: primary structure coal: DEN ≥ 1.5 and VEL ≥ 2438, fragmented coal: 1.3 ≤ DEN < 1.5 and 2257 ≤ VEL < 2438, structure coal: DEN < 1.3 and VEL < 2257.…”

“…The Upper Permian consists of clastic rocks intercalated with carbonate rocks and coal seams, and is rich in coal and CBM resources. According to the Evaluation of CBM Resource Potential in Guizhou Province, the CBM geological resources of the Upper Permian strata in Guizhou Province are 3056.19 billion m 3 , and the average abundance of CBM resources is 112 million m 3 /km 2 . The recoverable geological resources of CBM are 1379.13 billion m 3 , accounting for 45.31% of the total resources.…”

“…According to the Evaluation of CBM Resource Potential in Guizhou Province, the CBM geological resources of the Upper Permian strata in Guizhou Province are 3056.19 billion m 3 , and the average abundance of CBM resources is 112 million m 3 /km 2 . The recoverable geological resources of CBM are 1379.13 billion m 3 , accounting for 45.31% of the total resources. The gas content of 500 m deep coal seams in the Longtan Formation in Western Guizhou is generally between 8.0 and 25.0 m 3 /t, with an average of about 12.0-15.0 m 3 /t.…”

“…Seismic exploration technology [2][3][4] is often used to find out the spatial distribution, structural morphology, fracture development, and other characteristics of coal reservoirs, and to analyze and interpret the fracture distribution and in situ stress characteristics of coal reservoirs. Seismic exploration is a geophysical exploration method to infer the nature and form of underground rock strata by observing and analyzing the propagation law of seismic waves generated by artificial earthquakes based on the difference in elasticity and density of the underground medium.…”