One of the most effective methods for increasing the productivity of oil and gas resources is multi-stage hydraulic fracturing. However, casing deformation challenges associated with multi-stage hydraulic fracturing notably compromise the integrity of the wellbore. Many field data analysis shows that more than half of casing deformation is strong related to the faults. According to the classical fault reactivation theory, the increase of pore pressure will lead to fault slip. However, some studies shows pore pressure rising time by diffusion or formation rock volume changes do not match the seismic data; other studies suggest natural fractures between faults and the last fracturing stage can be the communication channels. And fracturing fluid distribution from Guandong block show the fluid cannot reach the fault hundreds of meters away. For these cases, local geostress change caused by fracture operations seems to be a more reasonable mechanism leading to fault movement hundreds of meters away.
In this paper a finite element model was established to simulate the multi-stage fracturing process in a fault-bearing formation and a parametric study to investigate the influence of rock friction, fault length, and multistage fracturing on the fault slip.The results shows that local geostress change caused by fracture operations is the main mechanism leading to formation deformation or fault movement. Without the fault, the fracturing operation will cause the casing deviation; if with a fault, the displacement of the formation is discontinuous, which leads to sheering deformation of the casing. The fault relative slip is affected by the formation friction coefficient and fault length. Moreover, multistage fracturing operation has a superimposed effect on faults, increasing the time interval between fracturing operations is an effective method.
Casing deformation is a common problem during large-scale volume fracturing operations [1-2]. Severe casing deformation can prevent proper positioning of tools such as plugs or perforating guns at the design position during subsequent fracturing operations. It will decrease the effectiveness of the large-scale volume fracturing of the reservoir, and thus reduce the economic benefit of the oil-field. At present, studies show that the main causes of casing deformation during large-scale volume fracturing are (1) geological [3-6] and engineering [7-8] factors which cause the local stress on the casing large enough to exceed its yield strength, eventually causing the casing deformation. (2) Reactivation of the natural fractures and faults in these formations due to large-scale volume fracturing fluid injection, which causes the fault slip and leads to the casing deformation [9-11]. Under the initial far-field geostress condition the fault in the formation is in equilibrium.
The fracturing operation increases the pore pressure subsequently decreasing the effective stress on the fault plane, and when the frictional force is less than the shear stress on the fault plane, the fault is reactivated and slip. The reactivation criterion of the fault plane is usually the classical Mohr-Coulomb criterion .