Study On Controlling Factors Of Marine Shale Gas Dispersion In South China

Shale of Wufeng-Longmaxi Formation in Sichuan Basin is an important layer for shale gas production in China.Exploration practice has proved that shale gas production varies greatly in different blocks,which is closely related to shale gas loss.Therefore,this paper studied the controlling factors of marine shale gas dispersion in the south of Sichuan Basin.Taking the Wufeng-Longmaxi Formation shale in the south of Sichuan Basin as the research object,the main controlling factors of shale gas dispersion are studied by combining experimental and theoretical analysis,static and dynamic analysis,macro and micro analysis,and the corresponding shale gas dissipation mechanisms were revealed.The conclusions are as follows:(1)Structural activity and shale sealing effect are the main controlling factors of shale gas dispersion.Shale pore-fracture structure and shale gas dispersion interact each other.(2)Shale sealing(self-sealing and roof-floor sealing)plays an important role in controlling shale gas loss.When the pressure of shale gas reservoirs in different research areas exceeds 60-70 MPa,the gas reservoirs begin to dissipate strongly;Comparing the distribution patterns of roof and floor in different areas(Fushun-Yongchuan,Changning,Southeast Chongqing),it is found that the distribution pattern of roof and floor in Fushun-Yongchuan is the best,shale gas dissipation is relatively weak,Changning is next,Southeast Chongqing is worse,and shale gas dissipation is relatively strong;The second,third and LM5 shale beds of the first member of the Longmaxi Formation have self-sealing effect on the shale gas producing layers of the Wufeng-Longmaxi Formation(the lower part of the Wufeng-Longmaxi Formation).Shale gas adsorption can reduce the permeability of shale under formation conditions and established a three-stage self-sealing model of shale.(3)Shale gas dissipation is strong in areas with early uplift time and large uplift range.During the uplift process of Wufeng-Longmaxi Formation,the gas content of shale decreases as a whole.Among them,the proportion of adsorbed gas increases first and then decreases.The critical depth is 1000 m.It also confirms that the content of adsorbed gas in Changning area is higher than that in Weiyuan area and the content of free gas is lower.Taking Jiaoshiba and Changning area as examples,the passage and power of shale free gas dissipation in anticline and syncline structures are analyzed respectively.It is considered that shale gas dissipates from low part of structure to high part of structure in anticline and syncline structures.At the same time,this also leads to the gas-bearing property of syncline structural style being more lower than that of anticline structural style.(4)The pore-fracture structure of shale is systematically characterized,and the internal relationship between pore-fracture structure and shale gas loss was analyzed.It is considered that the correlation between shale porosity and TOC in different blocks can be used to qualitatively evaluate shale gas loss.That is to say,when the correlation between porosity and TOC is good,shale gas dissipation is weak,on the contrary,shale gas dissipation is relatively strong.The slope of porosity and TOC can semi-quantitatively reflect the gas-bearing property of shale.The micro-retention mechanism of inorganic minerals on organic pore in shale was pointed out.Based on the above results,the typical pore-fracture characteristics of Wufeng-Longmaxi Formation shale in different blocks were identified,and it is pointed out that the organic pore is greatly affected by shale gas dissipation.