Effect Of Unloading Process On Mechanical Properties Of Shale

Eastern Sichuan is the main area of shale gas enrichment in China.It has undergone obvious tectonic uplift process in geological history.The change of shale mechanical properties and its dissipation ability during tectonic uplift are the difficult problems that puzzle the evaluation of shale gas resources.This paper first establishes an experimental model according to the actual tectonic uplift process in eastern Sichuan.Secondly,it uses the siliceous shale of Longmaxi Formation in the field of Eastern Sichuan to simulate shale rock mechanics during unloading process in the laboratory,and uses pulse attenuation method to test gas permeability.Finally,combining with various geological data,it focuses on the analysis of Dingshan Mountain in eastern Sichuan from the perspective of rock mechanics.Reasons for the difference of gas content in regional shale gas.The results show that the rock deformation stage during unloading can be divided into three stages: plastic deformation stage,fracture deformation stage and sliding stage along the friction surface after the peak;the deformation characteristics are reflected by both volume strain and lateral expansion coefficient.In the case of slow uplift,the axial compression is the main compression before fracture,the axial compression after fracture,and the lateral expansion is the main compression after fracture,and in the case of fast uplift,before fracture,axial compression is dominant,while after fracture,lateral expansion is dominant;deformation resistance decreases slowly,damage degree increases slowly,deformation resistance decreases rapidly and damage degree increases rapidly before fracture through the joint reaction of secant modulus and damage coefficient;the severity of fracture decreases with the increase of maximum burial depth,and cracks extend along bedding.The direction of extension is easy to produce.The main factors affecting the fracture depth during shale unloading are the maximum burial depth,while the uplift process and the uplift speed have little influence on the fracture depth;the main factors affecting the fracture denudation amount are the maximum burial depth,followed by the uplift process,and the uplift speed has little influence on the fracture denudation amount.The influence of maximum burial depth on the lost capacity is complex;the dissipation ability of shale decreases in turn during the early rapid-late slow uplift,uniform uplift and early slow-late rapid uplift;the dissipation ability of shale uplift isweaker than that of uplift,and there is little difference between the rapid uplift ability and the slow uplift ability with the increase of maximum burial depth.The poor gas-bearing property of Well DY 1 is due to the rupture of the Longmaxi Formation shale during the Mesozoic-Cenozoic uplift,which forms evident escape channels,and the well is close to the large faults and outcrop areas,so shale gas is more easily lost;The rupture depth of the Longmaxi Formation shale in Well DY 2 and Well DY 5 is not different during the Mesozoic-Cenozoic uplift,but because Well DY 5 is far from the large faults and outcrop Recently,the amount of denudation is larger than that of well DY 2,which indicates that well DY 5 has a larger amount of shale gas loss than well DY 2,so the gas content of well DY 2 is better than that of well DY 5.