Analysis Of The Burst Strength Of Perforated Casing In The Fracturing Process

Fracturing measures are often used to improve reservoir recovery ratio in unconventional wells.These measures have characteristics of high pump pressure,high and large flow rate.Excessive internal pressure will lead to burst and tear along the perforation hole of casing.When reservoir cannot completely be opened by the first perforation,it often requires repeated perforation,which seriously affects the casing strength.Therefore,the paper mainly studies the effects of perforation parameters and repeated perforation on the burst strength of casing.The stress state model of casing is established based on the elastic mechanics and the formula of stress strength under the axial load,external pressure and internal pressure caused by temperature is obtained.According to the characteristics of theoretical,a three-dimensional finite element model of perforated casing is built.Grid division of pipe and the constraint on one end is xU,yU,zU,the other isxU,yU.Under the internal pressure,external pressure and axial load,the influences of perforation parameters on the burst strength of casing are obtained.The result shows that when the aperture and phase angle remain unchanged,the residual burst strength of casing decrease by 10.0% with the increasing of hole density between 8-24 hole / m,and when the hole density increases from 16 holes / m to 20 holes / m,the residual burst strength of casing gets the maximum reduction.When the hole density and phase angle remain unchanged,the aperture between 4mm-16 mm,the residual burst strength of casing decrease by21.0% with the increasing of aperture,and when the aperture increases from 4 mm to 8mm,the residual burst strength of casing gets the maximum reduction.When the aperture and hole density remain unchanged,the residual burst strength of casing decrease by 6.7% with the increasing of phase angle.The way of distributing repeated perforation is divided into the encryption along axial,circumferential and the spiral line of the old hole.The law of the burst strength of casing is obtained by finite element calculation of different holes distribution.The result shows that when the new hole is axial distribution along the old hole,the residual burst strength of the casing will obviously insufficient as the center distance is too small.The residual burst strength of the casing will decrease by 7.7% with the increasing of center distance.When the new hole is circumferential distribution along the old hole,the residual burst strength of the casing will decrease by 31% as the angle is too small,and the residual burst strength of the casing willdecrease by 10.9% smoothly with the increasing of circumferential angle.When the new hole is of encryption along the spiral line of the old one,the burst strength of the casing will obviously insufficient as the distance between two holes is too small.The residual burst strength of the casing will decrease by 2.1% average with the increasing of center distance and circumferential angle.Effects of perforation parameters and repeated perforation on burst strength of perforated casing are obtained according to the study of the paper,and the recommended practice for evaluating the safety of the perforated casing in the fracturing process is given which provide a preference for choosing the optimal perforation parameters and holes distribution of repeated perforation.