Effect of single straight, intersecting and T-shaped natural fractures on horizontal wellbore stability in shale reservoirs

Abstract The development of natural fractures can easily lead to horizontal wellbore instability caused by stress concentration after drilling in shale reservoirs, which is not conducive to the safe development of shale gas, but how different natural fractures impact the horizontal wellbore stability in shale reservoirs is still unclear. Therefore, the effect of single straight, intersecting, and T-shaped natural fractures on horizontal wellbore stability in shale reservoirs is studied. Then a stress distribution model around the horizontal wellbore in shale reservoirs considering natural fractures is established based on linear elasticity theory, the stress distribution around horizontal wellbores in shale reservoirs with single straight, intersecting, and T-shaped natural fractures is acquired through the finite element method (FEM), and the effect of different natural fractures on horizontal wellbore stability is analyzed finally. Results show that among three types of natural fractures, intersecting natural fractures relieve the maximum stress of 1.676 × 106Pa, and the horizontal wellbore is the most stable; T-shaped natural fractures have the largest effect on total stress around the horizontal wellbore, which is the largest and can reach 68.105 × 106Pa, which makes the wellbore the most unstable; single straight natural fractures and intersecting natural fractures can cause the maximum circumferential stress on the wellbore wall respectively in the two directions of the maximum horizontal principal stress and the vertical principal stress, and wellbore wall is prone to compression and tensile failure respectively; circumferential stress and total stress around horizontal wellbore are the largest in the direction of the maximum horizontal principal stress, the wellbore is the most unstable. This work will be helpful for a more reliable assessment of borehole instability and benefit the drilling design in shale reservoirs with single straight, intersecting, and T-shaped natural fractures.