IN-SITU STRESS CHARACTERISTICS AND FRACTURE ZONE ACTIVATION ANALYSIS OF CHANGNING SHALE GAS IN SICHUAN PROVINCE

In the Changning shale gas development blocks in Sichuan, casing deformation occurred in more than 30% of horizontal wells during hydraulic fracturing, which in turn seriously restricted the efficient development of shale gas. Based on the principle of geomechanics, this paper analyzed the mechanical conditions of casing deformation caused by fracture zone sliding induced by fracturing in Changning shale gas area. Firstly, we used the logging and testing data of well Ning 201 to establish the in-situ stress model in Changning area. We used density logging to determine vertical stress, pressure build-up test to determine pore pressure, rock mechanics test to determine rock mechanical parameters, stress quadrilateral to determine the range of the horizontal minimum stress and the horizontal maximum stress, and used in-situ stress profile and focal mechanism solution to determine stress state. Secondly, based on the seismic and microseismic data, we identify the fracture zones of H19 platform, and counted the strike and dip characteristics of the fracture zones. Thirdly, based on the mechanical model of fault activation, we analyzed the activation of fracture zones of H19 platform. According to the location of fracture zones on Mohr circle and the order of activation, we divided the fracture zones into six groups. We analyzed the influence of pore pressure and stress mode on the activation of fracture zones, and pointed out the internal and external causes of fault slip induced by hydraulic fracturing in Changning block. Finally, we compared the characteristics of in-situ stress, fracture zone development and fracture zone activity in Changning and Fuling blocks. The results showed that:(1)The vertical stress in Changning area is 2.6 SG,the pore pressure is 2.0 SG,the horizontal minimum stress is 2.4~2.7 SG, and the horizontal maximum stress is 3.4G~3.7 SG, and Changning area is a strike-slip/reverse stress state. (2)The dips of fracture zones of H19 platform are in the range of 71°~90°. (3)The bottom-hole pressure required for the activation of all fracture zones of H19 platform is 3.0 SG. During hydraulic fracturing, the average value of bottom hole pressure is 2.8 SG, and the maximum value of bottom hole pressure is 3.1 SG,almost all fracture zones are activated. The difference between the shear stress and sliding resistance of fracture zone gets the minimum value in the strike-slip/reverse stress state, which is the internal cause that the fracture zones are relatively easy to be activated in Changning area. Hydraulic fracturing reduces the sliding resistance greatly, which is the external cause of fault slip induced by fracturing in Changning area. (4)The magnitude of three-dimensional stress in Fuling block is similar to that in Changning block, which is also a strike-slip/reverse stress state. But the fracture zone is not developed, or develop only some fractures with low dip angle. When hydraulic fracturing, the construction pressure is smaller. This is the main reason why the casing deformation in Fuling block is not as frequent as that in Changning block. This result states the geomechanical reasons of casing deformation in Changning area, which provides theoretical significance to solve the problem of casing deformation in Changning area.